Ovl Qc Manual-2012

QUALITY MANUAL

2012

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SEISMIC DATA ACQUISITION QUALITY MANUAL 2012

O N G C V i d e s h L i m i t e d Page 2

Preface

ONGC Videsh Ltd. (OVL) has been routinely acquiring seismic data in its areas of

operation in many countries and over widely varying geographic locales and

subsurface geology. Over all these years a need has been felt to evolve and

implement standard guidelines for ensuring quality seismic data meeting all

safety and environmental obligations. This need became acute in the wake of

spurt in activities of OVL in the light of challenges to produce 20 MMT of oil

equivalent by 2015. A decision was thus taken to review the existing practices

and compile them into a quality assurance manual for company wide use. A

team of geo-scientists was put in place and entrusted with this job.

The obvious and easiest option available the team was to have a look at the

similar guidelines compiled by parent company ONGC. The quality assurance

manual drafted by Chief Geophysical Services (CGS) thus became the starting

and rallying point for this exercise. Some of best practices gleaned through

partnership with other exploration companies were also invoked wherever

deemed fit. Although it was attempted to adhere as closely as possible to the

parent company policies, the changes were required in the light of realities of

working in multiple countries and with multiple partners. An important aspect of

this is reflected in the need to carry out Environment Impact Studies (EIS) prior to

seismic campaign and attending to indigenous community issues in a great

many countries. These issues have proved to be most important factor affecting

acquisition costs and schedules.

Eventually we now have in our hands a manual of comprehensive quality

assurance guidelines in following pages that should make the task of seismic

data acquisition sustainable and productive (qualitatively and quantitatively)

for OVL geo-scientists.

We thank foremost the office of CGS, ONGC and all colleagues, partners etc

who helped with their suggestions and discussions.

Deepak Sareen

Rajiv Srivastava

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CONTENTS

CHAPTER-1 INTRODUCTION

4

CHAPTER-2 PRE-SURVEY STUDIES

6

CHAPTER-3 TOPOGRAPHIC SURVEY

10

CHAPTER-4 LAND SEISMIC DATA ACQUISITION

17

CHAPTER-5 VERTICAL SEISMIC PROFILING SURVEYS

25

CHAPTER-6 MARINE SEISMIC DATA ACQUISTION

29

CHAPTER-7 INFIELD / ONBOARD DATA PROCESSING

42

APPENDIX-A SURVEY DESIGN AND ACQUISITION

PARAMETERS

44

APPENDIX-B EXPERIMENTAL WORKS FOR LAND SURVEYS

49

APPENDIX-C GENERAL SEISMIC DATA PROCESSING

STEPS

56

APPENDIX-D GLOSSARY 59

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CHAPTER – 1

INTRODUCTION

1.1 GENERAL

The single most important purpose of seismic data acquisition is to meet the exploration

objectives of the prospect area. With the focus of geological objectives shifting to more

complex and subtle features, the need to sharpen the survey parameter design process and

meticulous monitoring becomes extremely demanding.

In view of the state of art telemetry systems with online quality monitoring capabilities and infield

QC processing units for the land and marine crews, it was felt to revise/ review the QC manual

to meet objectives of the seismic surveys.

The economic values of the Seismic API (Acquisition, Processing & Interpretation) input are

achieved through one or both of the following ways:

Savings accrued through high-resolution appreciation of the drillability of locations thereby

resulting in reduction in Dry Wells.

Gains accrued through expansion of Reserves (assets) through identification of New Prospect /

New Pay and also through enhancing Recoverability.

The raw seismic data is baked through Seismic Processing and Interpretation to achieve the

above-mentioned ultimate values of seismic exploration.

As per principle of quality, control and checks during the processes involved in the data

acquisition stage will ensure a better product.

The aim of this manual is to bring out work standards and quality control norms for land and

marine seismic data acquisition. The selection of survey parameters, control and checks are

required during data acquisition and processing to give the best possible data as per the

exploration requirement.

The quality (of seismic data) amounts to conformance to specified requirements of exploration

targets agreed with the client. The quality is a dynamic perception based on utility of a product

and its evolution with time, technological changes and the requirements of the Client.

1.2 ELEMENTS OF A GOOD SIGNAL

What makes a good seismic signal? Processing specialists list three vital requirements:

Good signal-to-noise ratio (S/N)

High resolving power and

Adequate spatial coverage of the target.

High S/N means that the seismic trace has high amplitudes at time that correspond to reflections

and little or no amplitude at other times. During acquisition, high S/N is achieved by maximizing

signal with a seismic source of sufficient power and directivity, and by minimizing noise. Noises

can either be generated by seismic source (coherent noise) or may be cultural noise, sometimes

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orders of magnitude stronger than deep seismic reflections – or be random. Limitations in the

dynamic range of acquisition equipment require that shot-generated noise be minimized with

proper source and receiver geometry. Proper geometry avoids spatial aliasing of the signal,

attenuates noise and obtains signals that can benefit from subsequent processing.

Noise and signal cannot be distinguished when they are aliased due to inadequate sampling. A

common type of coherent noise, which can be aliased, comes from low-frequency waves

trapped near the surface, called surface waves. Planners always try to design surveys so that

surface waves do not contaminate the signal. But if this is not possible, the surface waves must

be adequately sampled spatially so that they can be removed.

The second characteristic of a good seismic signal is high resolution, or resolution power – the

ability to distinguish the top and bottom of the reflectors and quantify the strength of the

reflection. This is achieved by recording a high bandwidth, or wide range of frequencies. The

greater the bandwidth, the greater the resolving power of the seismic wave. The target thickness

determines the minimum wavelength required in the survey, generally considered to be four

times the thickness. That wavelength is used to calculate the maximum frequency in seismic

bandwidth – average seismic velocity to the target by minimum wavelength equals maximum

frequency.

Another variable influencing resolution is source and receiver depth on land, the depth of the

hole containing the explosive source (receivers are usually on the surface). The source-receiver

geometry may produce short-path multiple between the sources, receivers and the earth or sea

surface. If the path of the multiple is short enough, the multiple – sometimes called a ghost – will

closely trail the direct signal, affecting the signal’s frequency content. The two-way travel time of

the ghost is associated with a frequency, called the ghost notch, at which signals cancel out.

This leaves the seismic record virtually devoid of signal amplitude at the notch frequency. The

shorter the distance between the source or receiver and the reflector generating the multiple,

the higher the notch frequency.

The third requirement for good seismic data is adequate subsurface coverage.

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CHAPTER – 2

PRE-SURVEY STUDIES

2.1 GENERAL

Pre- survey studies are essential for effective data acquisition in order to meet exploration

objectives in the survey area. Before the start of any data acquisition project, a database is to

be built up regarding the area.

The following information needs to be gathered prior to the survey operations in the area:

Precise topographic/bathymetry maps from authorized agency in the country/area showing

amongst other things any protected flora & fauna in the area as well as habitats of

indigenous communities. This should have important landmarks, natural geographic features,

permits and administrative & legislative boundaries clearly marked therein.

Report on Environmental Impact Studies.

Satellite imagery for the area and Drainage pattern maps.

Land use maps for ascertaining types of crops etc. in the area for purpose of payment of

compensation.

Geological maps showing various formations exposed and their dip-strikes.

Representative seismic sections in the area and information on previous seismic campaigns

viz. survey objectives, geometries, shooting direction and line locations etc.

Structure maps at levels of interest obtained from previous work in the area.

Near surface and deeper velocity structure gleaned from previous seismic API campaigns.

Well logs and well testing and completion reports for any wells drilled inside/around the

permit.

Information from existing VSP/WSP carried out in/around the area.

Bouger gravity maps and Magnetic (total intensity) maps for the area.

Information on general weather and climate in the area; particularly the information on rainy

season and storm season.

Information on general security situation in the area, no-go areas if any; this can be critical in

certain parts of world.

A reconnaissance map showing location of possible camping and potential obstacles in the

area.

A copy of all mandatory permissions, exploration licenses and other stipulations by

regulatory, socialization, environment and military agencies in the area.

The database gathered above is to be analyzed critically to study the nature of the area with

respect to geological, environmental and logistical complexity. Areas with geological

complexity like highly folded and faulted beds, thrust belts, sub-trap, sub-thrust mapping,

mapping under exposed anticlines, etc need detailed modeling studies to analyze the

subsurface illumination to arrive at the acquisition parameters and design the spread

configuration.

The analysis of all aspects of the surveys including expected data quality, need to be done prior

to experimental and regular seismic work.

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Apart from the above, the following also need to be preferably carried out:

An extensive interaction with processing geophysicists and interpreters during pre-survey

studies and continuously thereafter.

Analysis of the problems faced while processing / interpreting the data of earlier vintages to

plan the necessary strategies.

The quality and quantity, status/ performance, make and specifications of all available

inputs like seismic instruments, ground electronics, shallow refraction/Uphole survey

instrument, shooting systems, geophone strings, topographic survey equipment and

communication equipment need to be analyzed / ascertained. Any additional requirement

of equipment and accessories and their availability to be analysed.

After carrying out the pre-survey studies, the project reports are to be presented in a technical

forum, which comprises geoscientists of the concerned Basin and RCC for improving the

technologies, methodologies and refining the strategies by discussions, suggestions and

constructive criticisms.

2.2 ILLUMINATION MODELING

Subsurface illumination modeling is a valuable tool to analyse the effect of different survey

parameters. It is used to draw conclusions with respect to optimal acquisition geometry, shooting

direction and sub-surface imaging.

3D ray tracing followed by target-oriented binning methods are extensively applied to simulate

the target illumination, which would be obtained by different seismic survey configurations. The

aim is to predict the target illumination and relate this to the image quality of the target

structures after seismic processing. A uniform fold or illumination is usually the ideal result, and

deviations (e.g. shadow zones and variations) indicate regions where imaging problems or

acquisition related amplitude variations (i.e. "footprint") might be expected. Once the

understanding is obtained on how the acquisition configuration influences the imaging, it is

possible to optimize the acquisition geometry and imaging strategy for the best result.

The modeling consists of calculating the reflection points on the target horizon for a specific

survey configuration. The "events" are subsequently binned on a regular grid according to the

spatial sampling in the 3D processed cube. Finally, various attributes or distribution of attributes

are calculated. The results can then be displayed colour-coded on the target horizon or back-

projected onto a horizontal plane. Typical attributes for analysis and display are minimum offset,

maximum offset, total number of hits (fold), offset distribution and azimuth distribution and

reflection amplitude density.

In the pre-survey design phase various acquisition configurations are deployed in complex 3D

models to optimize illumination properties in relation to the survey objective and target

horizon(s). The modeling system is developed to handle all acquisition geometries and all wave

modes.

The availability of modeling software’s in ONGC like NORSAR and MESA should be extensively

used for specialized studies to find solutions for geophysical problems especially in complex

geological setups.

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The steps followed in model based seismic data acquisition are:

2.2.1 Modeling Studies

The starting point of model-based approach is preparation of subsurface geological model

using the available seismic data, well data, surface geological data, etc.

In case of a proposed 2D campaign, the model is a two-dimensional one. However, in case of

proposed 3D campaign the model should be preferably three-dimensional, either generated

from earlier 3D seismic data or by combining a couple of 2D seismic section in the dip and strike

directions. The model thus generated should provide clearly defined reflector boundaries along

with the layer velocities.

2.2.2 Ray Trace Modeling Studies

The two main parametric analysis of the model for designing the acquisition parameter are

offset and group interval analysis.

Ray tracing on the given model is conducted to study the illumination of the various reflectors in

three modes. The highest value of offset and the optimum value of group interval as computed

from theoretical formulae are used for ray tracing.

Shot gather ray tracing: Shot gathers are generated in the desired mode like split spread or end-

on to study the illumination of the reflectors. Any gap in the illumination can be easily seen on

the ray trace diagram. The gap can also be seen in the synthesized shot gather.

CMP ray tracing: The subsurface illumination is to be analysed to see the expected effect after

stacking. Normal incidence/Oblique incidence ray tracing studies need to be carried out to see

the expected effect on CMP gather. Any gap in illumination caused due to structural

complexity or heterogeneity in layer may be seen in trace gather.

Ray tracing with shot placed on the reflector: Here the ray tracing is done by placing the shot

point on the target reflector and allowing the rays to come up to the surface along the paths

guided by Snell’s law. This is very useful to study the ray bending at various interfaces and can

throw light to find out the reasons for difference between expected output and actual output

on a seismic section of a previously acquired data.

Normal incidence ray tracing: Rays that are normally incident on reflector are traced back into

surface using Snell’s law of reflection. These rays represent stacked traces. This ray tracing helps

in understanding the difference between stacked field section and synthetic section that is

generated from assumed geological model. This will lead to realistic geological model through

iteration.

Model initialization: This is generating synthetic seismic data along the geological model for a

desired spread configuration using offsets and group interval computed earlier. First synthetic

shot gathers are generated and then it is subjected to regular data processing sequence to

arrive at the synthetic stack section. This is then subjected to PSDM to match the original model.

This stack section becomes standard for reference for all further studies.

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Far offset analysis: A range of offsets is tested, i.e. stack sections are generated with different

offsets and compared. The offset, which provides most optimum subsurface mapping is taken as

optimum.

Group interval analysis: In this case, stack section with optimum offset is generated for lower

group interval successively to study the improvement in resolution in subsurface mapping.

Line length analysis: Normally, the image line/area needs to be extended by migration aperture

and Fresnel zone for proper subsurface mapping using computed values from formula and

tested by actually generating the various line lengths and comparing them. The minimum line

length exhibiting proper subsurface mapping is taken as optimum.

Type of spread configuration: With the increasing channel capacity of seismic recording

instrument, there is little binding on the number of active channels laid in the spread. Based on

the channel availability, End-On and Split-Spread type of spread are implemented. Nowadays

mostly split spread configurations are used with longer arms on either side to meet the far offsets

requirements for mapping deeper targets, velocity analysis as well as mapping geologically

complex subsurface structures.

Acquisition parameters: The spread configuration and the acquisition parameters obtained by

the modeling studies are confirmed / refined after the field experimental work.

2.3 ENVIRONMENTAL IMPACT STUDIES

The whole world is getting increasingly environmentally conscious and green in its approach to

all outdoor activities. Today it is more or less universal requirement for an operator to carry out/

commission an Environmental Impact Study consisting of doing a baseline survey documenting

existing flora & fauna in the area. The sensitive species are outlined for paying special attention.

The probable impact of seismic acquisition campaign on these is next studied and a set of

guidelines are evolved to ensure no/little damage to these environmental concerns.

Completion of these environmental studies and submission of reports are mandatory prior to

receipt of governmental go ahead for seismic campaigns. This also commonly involves carrying

out afforestation efforts as a compensating mechanism under supervision of local environmental

agencies.

2.4 CONSULTATION WITH INDDIGENOUS COMMUNITIES

It is very common in many countries for an operator to obtain information on presence of native

communities who along with their native environment are protected under local law. The only

way to carry out seismic survey is with cooperation/permission from these communities by

carrying out suitable socialization work.

A detailed protracted set of negotiations through suitable agency are normally required to

arrive at a settlement allowing start of work. These activities need to be started immediately on

issue of PSC as they are very time consuming and eat into the exploration period.

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CHAPTER-3

TOPOGRAPHIC SURVEY

3.1 GENERAL

1. All survey Instruments are to be calibrated, tested and adjusted as per the manufacturer’s

guidelines before deploying them for fieldwork once in every month. Records of these tests

are to be preserved.

2. Angles / Distances / level differences are to be measured to the least count of the instrument

only. No interpolation / approximation to be made.

3. If there is any deviation between staked and actual receiver / source locations, the actual

location coordinates to be obtained by connecting with GNSS and no interpolation /

approximation to be made.

4. In areas of thick canopy / buildings etc, where GNSS signals are not available, Total stations

and Auto levels are to be used for infilling gaps.

5. Source locations are staked only after the formation of road / path for vibrators, to avoid the

change in elevation of pickets after road / path making.

6. All the GTS, BM, Wells falling in the working area to be connected.

3.2 PRE PLANNING.

1. Complete details of logistics, topography of the terrain, vegetation type, statistics of all

source/ receiver locations falling on each land cover, likely skips due to logistics and possible

recovery locations can be obtained well in advance for proper planning / execution of

seismic survey.

2. Source and receiver locations may be plotted on satellite maps. Skip analysis and possible

recovery locations should to be planned before staking with GNSS.

3. Geographic Information Software (GIS) packages if available should be used for effective

database management and for doing various analysis by integrating Remote Sensing (RS),

Differential Global Positioning System (DGPS), Digital Elevation Models (DEMs) and other

data.

4. If required a complete reconnaissance survey / detailed surveying shall be carried out with

GPS to bring out the actual logistic & cultural features for planning, before start of regular

survey.

3.2.1 Connecting Regional GPS network to ITRF (IGS) Stations.

To acquire a good quality data with GNSS, it is mandatory to establish a big GPS network

covering the entire basin. Also it is very much required to tie few stations of the network with

at least 3 or 4 ITRF (IGS) stations, to express the regional network in ITRF reference frame.

1. The number of network stations to be connected with ITRF stations depends on

The size of the network covering the entire block / basin.

Normally atleast one GPS network station to be connected for every 500 km network.

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Relative accuracy of the network in the local datum

Availability of prime or second order survey of India GTS and BM stations.

2. Atleast 72 hours of continuous data at 15 sec epoch to be collected / recorded at the GPS

station which has to be connected with ITRF.

3. The very long baselines connecting GPS network station and ITRF (IGS) station, in the order of

1000 to 2000 Kms are to be processed using advance GNSS processing software like Bernese

/ Gambit.

4. The GPS network stations connected to ITRF stations will have a positioning accuracy of

around 1 to 2 cm. Thus the accuracy of the entire big network covering the whole basin will

have an accuracy of 1 to 2 cm with respect to ITRF reference frame. It is mandatory to build

the network from the stations which have been connected to ITRF stations.

3.2.2 Coordinate Reference System (CRS)

Each country has one/more Standard Coordinate Reference System consisting of a set of

ellipsoid and projection system parameters and all work has to be mandatorily carried out

using the same. So it is of primary importance to ascertain these parameters. Additionally it is

essential to find out /calculate the following.

1. Accurate Transformation parameters between WGS84 and local CRS shall be derived by

establishing GPS network covering the entire basin and then connecting with at least 3 or 4

International GNSS Service (IGS.

2. Classical 3D method of deriving transformation parameters namely, Molodenski-Badekas 10

parameter or Bursa-Wolf 7- parameter methods are preferable over 3 parameter methods.

3. Use of ready built transformation parameters, which is supplied with most of the acquisition,

processing and interpretation software, need to be used only after verification and

recommendation of local surveying authorities.

3.2.3 Orthometric Heights with GNSS

1. GNSS system can be used for obtaining orthometric heights (H), heights above mean sea

level, by replacing the cumbersome process of spirit leveling.

2. EGM 96 (or) EGM 2008 geoid model to be used for obtaining geoidal heights / undulation.

3. RTK range to be restricted to less than 5 km with PDOP of less than 4, to ensure elevation

integrity.

4. In case sufficient GPS / GNSS signals are not available, then conventional spirit leveling to be

carried out for getting orthometric heights. The following norms to be followed in case of

conventional leveling.

Main Loop error shall not exceed 0.025* SQRT (k), where k is the loop distance in Km.

All the BMs falling in the area should be used to derive the elevation of the pickets.

Loop chart showing loop closure and GPS, BM and profile connections etc. shall be

displayed and updated daily.

Leveling loop register showing all information shall be maintained.

Elevation of the line/swath should be available within 3 days of completion of shooting.

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3.3 GPS FIELD PROCEDURES AND GUIDELINES

3.3.1 GPS Instrument Verification

1. Regular verification checks and adjustments shall be carried out for all instruments requiring

and the responsibility for same is that of the survey in-charge of the party. Such verification

checks shall be well documented and accessible to the crew. If necessary. They will form

part of the survey final report.

2. GPS equipment cannot be calibrated in the field but acceptable repeat baselines are to be

regularly performed to ensure instrumental accuracy. The following checks for GPS Static /

Fast Static / RTK equipment may be required at the start-up of a project.

Zero-Baseline check consisting of multiple GPS receivers connected to one antenna through

an antenna splitter. The results of this will be a series of computed base lines of 0.000m to

0.005m in length. It is recommended that these “short baselines are computed using the L1

frequency only.”

Minimum Baseline check consisting of a series of GPS receiver, each with their own antenna,

set up in a straight line with one meter spacing. On computing this small network the sums of

the internal distances should equal to external distance +/- 0.005m in length. It is

recommended that these “short baselines are computed using the L1 frequency only.”

GPS antenna should be checked against different GPS receiver to ensure correct logging of

satellites.

Tribatch used for GPS survey shall also be checked and adjusted for centering errors and

bubble level errors on a monthly basis.

Tripod used for base station occupation shall be well maintained. Regular checks for loose

hinges and feet shall be carried out and bolt tightened as necessary. Tripod Head bushings

shall be checked for excessive wear and shall be replaced as required.

All cable shall be examined for breaks and tears. Such damage shall be repaired and the

cables tested for continuity and leakage

All radio modems and repeaters shall be checked for range and connectivity.

3.3.2 Real Time GPS Survey

1. Real Time Differential GPS (RTDGPS) surveys utilizing Radio Technical Committee Marine Sub

Committee (RTCM) differential corrections will give results in the 1-2 meter range and Real

Time Kinematic (RTK) will give results in the accuracy range of 1-5 centimeter.

2. For all seismic survey operations RTK method shall be used with dual frequency receivers, to

obtain a positional accuracy of 1-5 cm.

3.3.2.1 GPS Base Station Setting Out

GPS Base stations will be used to increase the density of the control points by establishing a

network of stations points placed conveniently for the surveyors to access for base stations for

real time surveys.

When using dual frequency receivers the minimum observation for fast static observation

shall not be less than 20 minutes regardless of the number of satellites available. The

minimum number of satellites shall be five, the elevation mask shall be set to 15 degrees and

the measure sync time shall be 15 seconds. The maximum PDOP value shall be less than 5.0.

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The position of this new Base station shall be determined by using differential techniques from

a minimum of two network control stations. A non- trivial closed loop shall be run linking the

two network control stations and the newly established base station. This closed loop must

close better than 1: 50,000.

The maximum baseline length for fast static shall not be more than 20 km.

Base stations sites should be scouted, built and surveyed before start of the layout / stakeout

survey. This allows for the network to be computed and adjusted without delaying layout.

Occasionally it may be found that radio cover is not available in an area or extra control is

required. It is expedient to set out a new base station using RTK observations and then

occupy it with the base station. Such stations shall be tied to the existing control network as

soon as possible with static / fast static survey.

All Base station positioning shall employ relative differential positioning carrier phase

techniques and baseline ambiguity be resolved using standard software.

A master base station shall be established at each base camp using fast static survey, for

verification / testing of GPS instruments.

3.3.3.2 Antenna Measurement Specification

Usually the greatest source of error in GPS comes from poor antenna measurements and poor

antenna measurement documentation. The utmost care shall be taken in measuring and

verifying antenna heights and entering these measurements into the receiver and the operator’s

field notebook.

3.3.3.3 Static / Fast Static Survey

GPS antenna measurements for all static and fast static surveys, including ITRF determination,

shall be made to 0.001 meters using manufacture specified tape / rod

All static antenna measures will be taken at the start of the observation session and checked

at the middle and at the end of the observation session.

All measurement shall be recorded in the operator’s field notebook

3.3.3.4 Real Time Kinematic (RTK) Survey

RTK Master station antenna height measurements shall be taken

All the station antenna heights shall be taken with a manufactured specified tape / rod to

0.001 meters.

All rover antennas mounted on survey poles shall be checked for the standard height as

specified by the manufacture.

All measurement shall be recorded in the operator’s field notebook. This record shall include

all instances when the rover antenna height changed because of obstruction etc

3.3.3.5 RTK Specifications

Real Time Kinematic (RTK) GPS observation using approved receivers shall be conducted

with the elevation mask at the base and rover set to 15 degrees, the maximum PDOP shall

be 5.0 and measure sync time 1.0 seconds. The minimum number of GPS epochs observed

shall be 4 with minimum number of 5 satellites.

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If periods with only 4 satellites available are encountered then a minimum of 10 GPS epoch

will be observed with a maximum PDOP of 4.

During periods when RTK Fixed mode (Ambiguity resolved / phase solutions) is not available

then RTK Float (Ambiguity not resolved / Float solutions) may be acceptable with the

permission of survey incharge provided that a minimum of 5 satellites are available with a

maximum PDOP of 4 and a minimum of 10 GPS epochs observed. Data collected during

such periods must be scrutinized for elevation spikes.

Prior to start of daily operations a calibration or check shot shall be carried out at a known

control point, in order to check the integrity of the system.

During RTK survey a check shot or re-occupation of a previously surveyed position shall be

carried out after any re-initialization of a receiver. This is a check against a poor initialization

If an RTK initialization takes place under circumstances of high RMS error then the elevations

of stations surveyed after that initialization shall be scrutinized.

Where possible, it is recommended that a PDOP of less than 4 and a baseline less than 5 km

be used for all RTK observations, this ensures elevation integrity.

At no time shall the maximum PDOP exceed 5.0

3.3.3.6 Combined GPS / Conventional surveys.

Occasions will arise where conventional methods are required. These will occur in areas with

mixed / dense vegetation causing blockage of GPS signals and require conventional surveys.

Similar options may be required in industrial / residential areas where construction causes GPS

blockages.

In such cases GPS control stations may be established at the beginning and end of GPS

blockage areas as per the following standards.

RTK measurement of 5 seconds shall be considered acceptable for setting out Back station

or Tie stations for conventional traverses that are required in a GPS survey. Such GPS control

stations are to be a minimum of 250 meters apart.

GPS data gaps to be infilled with the convention method of staking and leveling. Proper records

to be maintained for the same and accuracy of staking and leveling loops to be checked by

the survey in-charge of the party.

3.3.3.7 GPS Quality Control

Quality control of GPS filed data, particularly RTK data of entire swath to be meticulously done

using GP Seismic software, wherever available. The software is meant exclusively for QC of GPS

survey field data in seismic data acquisition.

3.4 TOTAL STATION

Total stations must be used only in the areas where sufficient GPS / GNSS signals are not

available for RTK surveying.

GPS control stations to be used as starting and closing station for traversing with Total station.

Range and baseline accuracy test of the ETS / EDM etc. has to be checked against known

base stations only. Inter visible GPS stations of high accuracy may be used for this purpose.

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ETS data should be downloaded on a daily basis and their acceptability is to be checked by

plotting with suitable software.

3.5 ACCURACIES

1. In terms of WGS 84 spheroid : ( GNSS Baseline Accuracies )

Static / Fast Static

Horizontal : 5mm + 0.5ppm

Vertical 10mm + 1ppm (Ellipsoidal Heights )

Real Time Kinematic ( RTK )

Horizontal : 10mm + 1ppm

Vertical 20mm + 2ppm (Ellipsoidal Heights)

2. Absolute Orthometric Heights (Heights above MSL)

a. Using GNSS & Geoid Model

Absolute vertical accuracy should be of the following

Flat areas : better than 20 cm

Mountainous areas : better than 30 cm

b. Using conventional Levels

Loop error shall not exceed 0.025* SQRT (k) in meters, where k is the loop distance in Km.

3.6 PILLARING

The minimum number of pillars to be established shall depend on the size, location and

logistics of the survey area. Pillars to be erected for establishing GPS network in inaccessible

areas

All pillars are to be connected with the nearest GPS station and tied to the nearby line picket

and also shall have at least 3 permanent reference objects.

All pillars shall be made as per the specifications and engravings / inscriptions shall be made

incorporating Pillar no., SIG no., Field season, company Logo etc.

3.7 MAPS

All maps to be prepared using the standard mapping software and the maps to be submitted in

DWG format. GIS and Remote sensing output maps if any may be submitted in DGN / SHP / IMG

etc .formats.

1. Grid and Spherical coordinates are to be incorporated in the border of the map.

2. Grid crossing shall be marked with crosses only.

3. Wells are to be plotted as per ONGC symbol standards.

4. Seismic Location Maps shall have river course, coast lines, main towns, in addition to all actual

receiver and source locations.

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5. Topographic survey location map shall contain river course, coast lines, main towns, Pillars,

GPS stations etc, in addition to actual receiver and source locations. Cultural features shall be

shown with Standard ONGC conventions (SOI guidelines on symbols)or regulator specifications.

6. Hard copy location maps of following scales shall be submitted for future reference on colour

plots (Soft copy to be submitted in DWG / DGN/ Geo Tiff or similar formats). Some of suggested

map scales are as below:

1: 20000 (3D survey) with scheme of lines.

1: 20000 (3D survey) with Uphole and experimental locations.

1: 50000 (2D survey) with scheme of lines.

1: 50000 (2D survey) with up hole and experimental locations.

1: 50000 (3D & 2D survey) With all topographic and survey details

7. Lines and Picket annotations are to be done with uniformity i.e. Line annotations should be

along the line at both the ends and picket annotations at regular interval should be on the right

side of the line.

8. All the maps shall have proper legends, scale, toposheets index, north arrow etc. Spheroid

and projection parameters used also should be mentioned in the map as mentioned below.

Spheroid : As per country standards in area

Projection : As per country standards in area

Grid (as actually used)

Scale 1: 20,000 (3D survey / 1: 50,000 ( 2D survey )

3.8 SURVEY DATA DELIVERABLES

All the restricted data, manuscripts, maps, publications are to be handled, stored, and

transported as per the guidelines laid down by the regulator/other authorized agency in the

area.

All GNSS raw data to be submitted in both native and RINEX formats

Electronic Total Station raw data to be submitted in both native and ASCII format

The following data should be examined on regular basis for Quality Control after completion of

each swath

Copies of original raw satellite data in the native format for each site occupied.

Copies of baseline computation for each new base station established / occupied.

Operation report including methodology equipment used and problems encountered.

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CHAPTER – 4

LAND SEISMIC DATA ACQUISITION

4.1 RECORDING SYSTEM

1. A complete set of instrument tests, as per manufacturer’s specifications and procedures,

should be before field deployment.

2. In order to ensure that the data is being recorded properly, test data should be recorded as

per the OEM recommendation. The data should be processed on field processing system or

at the Regional Computer Centre

3. All the recommended instrument tests should be carried out as per manufacturer’s

specification, procedures and schedule to monitor the instrument performance during field

operations.

4. The seismic data shall be digitally recorded in on tape cartridges/LTO/ / hard disk in the field.

In case of recording on hard disk, the data shall be copied on to Data Cartridges in the

camp/ computer centre for archival.

5. The cartridges used for recording shall be new and of the best-known brands. The recorded

tapes / cartridges shall be properly annotated and due precautions shall be observed in the

storage and transportation.

6. Air Conditioning units should be maintained for regulating temperature and humidity inside

instrument cabin.

7. The repairs and maintenance of the recording systems including instrument test results should

be properly documented for reference.

4.2 GEOPHONES, CABLES AND GROUND ELECTRONICS

4.2.1 Geophones

a) Geophone Testing:

1. All the geophone strings should be tested as per the standard procedure with a Geophone

Analyser. Each geophone string should have unique identification no.

2. The test results should be documented and record should be available in hard & soft copy.

3. All the receivers should be checked and verified for recommended response (as per

manufactures specification) before start of fieldwork and regularly at least once in a month

during the field season.

4. The defective receivers should be removed from operations and rectified before re-

deployment.

5. The first arrival of energy or a tap on the underside of geophones should produce a negative

number on tape and a down-going deflection on a paper monitor.

6. Tap tests, if necessary, may be carried out to check the general response of the string.

7. Worn out/ broken connectors should be replaced prior to deployment.

b) Geophone Plantation

1. Geophones / Digital Sensors should be planted into the earth firmly and vertically to ensure

good coupling with ground.

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2. Receivers should be planted as close as possible to the staked position. In case of deviation,

actual coordinates of the new location should be provided

3. Plantation near trees, power lines, houses, roads, cultivated fields, wind noise conditions etc.

needs more care and pits of suitable size need to be dug for plantation .

4. If the geophone array is to be adopted, and if it is not possible to spread the whole array,

the array length may be suitably changed depending on the elevation variation or else

geophones may be bunched at the picket.

5. Digital sensors should be planted vertically and tightly coupled to the ground and be buried.

6. Every 3C digital sensor should be oriented manually along the line bearing with the special

auger tool provided for the purpose.

4.2.2 Cables

1. All cables for field use shall be subjected to rigorous checking/repair/maintenance on

continual basis to conform to the manufacturer’s specifications.

2. The end-connectors and the take-out connectors are to be maintained / cleaned regularly

so as to remove the dust. Dust caps are to be used to protect the connector pins / sockets.

3. Worn-out / broken connectors of the cables should be replaced

4. Appropriate Cable Repair Kits should be used for repairing the cable cuts.

5. Each cable should have an identification number and the repairs and maintenance carried

out on it should be documented / logged for reference.

4.3 GROUND ELECTRONICS

1. All ground electronics for field use shall be subjected to rigorous checking / repair /

maintenance before deployment in the field as well as on continual basis in the field so as to

ensure that the parameters conform to the manufacturer’s specifications.

2. The connectors etc. should be cleaned daily after the fieldwork.

3. If necessary, suitable protective cases may be made for handling in the field.

4. Proper documentation of the repairs/ problems and test results should be maintained.

4.4 ENERGY SOURCE

4.4.1 General

Consistency and repeatability of input pulse into the earth ensures a consistent reflection quality.

Variation in amplitude, phase and frequency of reflected signal can be attributed to subsurface

only when input pulse has minimal variation from shot to shot.

1. Source position should be as close to staked position as possible. Deviations shall be

recorded in observer's log with bearing and distance from staked position.

2. The actual position of blasted hole should be located and the information should be sent for

processing along with the acquired data.

3. In case of any shot position errors, the shot should be repeated at the correct position or

resurveyed to obtain the correct position.

4. Each source position will be placed so as to minimize damage to man-made structures/

buildings etc. and the environment.

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5. Designing of source array (Vibrators/ Airgun /Explosive (pattern holes)) may also be

experimented.

6. In case formation does not allow the drilling of up hole, Shallow refraction survey can be

carried out after necessary approval.

4.4.2 Explosives

1. Dynamite (Special Gelatine Class- II or III) along with electric detonators (Class VI) is generally

used as the energy source in areas where shot hole drilling is possible.

2. Optimum depth for shot holes on each line shall be decided on the basis of up-hole surveys

along each 2D line at distance of about 2 km. and 1 Uphole per SKM in 3D surveys, if there is

not much variation in near surface layer, interval may be reduced depending on degree of

variation.

3. While plotting the Uphole data litho-log and pulse shape/amplitude are to be integrated.

4. Near surface model along inline and cross lines should be prepared incorporating actual

elevations.

5. The shot holes should be well tamped. If damage to nearby buildings, structures, etc. is

expected then appropriate preventive steps either by increasing the shot hole depth or

reducing the charge size

6. If desired shot hole depth could not be drilled due to hard formation, then appropriate steps

such as tamping the hole with cement slurry / Bentonite should be taken so as to improve

the data quality.

7. All loaded holes are to be blasted on the same day. Exceptions to this may have to be

made e.g. in case of areas of high static etc. after due approval of QC representative.

8. Partial blast or misfires should be repeated.

9. Blasted shot holes shall be filled with earth immediately after firing of shots.

10. Shooting system after interfacing with recording system should be checked for field time

break and Uphole time. The difference in the Uphole reading in the monitor plot should be

within +/- 2 milliseconds (one sample interval) with respect to the reading at shooters end.

4.4.3 Vibrators

1. Vibrators should be placed in pre-decided pattern as close to the staked position as

possible.

2. The total number of sweeps per Vibro point (= Predetermined No. of Vibrators/ Actual No.

deployed} 2 x Predetermined No. of Sweeps in no case should be less than minimum

decided by experimental work.

3. The Sweep and other vibrator parameters decided after experimental work should be

followed strictly, except in case of logistic obstructions like power lines, villages, hutments,

buildings industrial areas, where number of vibrators and the drive force may be kept within

safer limits.

4. Proper contact of vibrator plate with ground should be ensured.

5. In order to ensure good coupling, a single local sweep should be taken at each Vibro-point

before start of data recording.

6. The on-line QC system available in the Vibrator Control Electronics (e.g. Post Sweep Service)

should be used as per manufacturer’s specifications. Proper fault matrix should be designed

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and followed for each sweep. Corrective measures should be taken whenever a particular

vibrator performance falls out of specifications. Complete record of QC data should be

available in the soft / hard form.

7. An on-line vibrator performance monitoring system should monitor critical performance

parameters during actual sweep, viz. Maximum peak force generated during sweep,

Average force generated during the sweep, Maximum phase error over entire sweep,

Average phase error over entire sweep, Distortion etc.

8. Wireline similarity test should be taken every month or after major repairs and the test results

preserved.

9. If number of Vibrators utilized is less than optimum, then number of stack should be increased

accordingly.

10. Radio similarity test should be taken daily in addition to monthly Wire line similarity test. Wire

line similarity should also be taken on the day of beginning a new line or after repair of

vibrator electronics.

11. Source position location system (through DGPS), which is available with vibrator electronics,

is to be checked as per accuracy limits specified by manufacturer. Position- location given

by DGPS is to be supplied in SPS format to processing centers.

4.5 SEISMIC DATA RECORDING

4.5.1 General

1. Prior to start of recording, all daily tests as specified by the manufacturer should be recorded

and analyzed to ensure that the recording instrument is functioning within specifications.

2. The set of such relevant tests may be defined and documented whenever new equipment is

introduced or equipment is repaired for data acquisition.

3. Except under severe environmental conditions / in difficult terrain conditions (to be duly

supported by observer note) there shall be no defective traces at the start of daily work.

4. Traces are defined as defective under following conditions:

a. If the instrument / field test results of the channel are beyond specified limit.

b. Reverse traces (if unavoidable, they should be logged in the observer report).

c. Dead traces (Dead traces due to natural and cultural obstructions are not counted as

defective).

d. Mono frequency traces like 50/60 Hz due to electrical pick up etc.

5. There shall not be more than three consecutive shots / records / pops with same

defective/noisy traces during production work.

6. A shot / record shall be defined as misfire if no detonation or partial detonation of charge

has taken place, if the number of sweeps has reduced / drive force is below prescribed

limits.

7. Every shot should be monitored using the online QC display, if available, and the problems, if

any, should be logged in the Observer’s log for detailed analysis / corrective action with field

processing unit / computer centre.

4.5.2 Field Operation Planning

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The field operation is very much affected by the natural/ ground conditions of the field. Hence, it

is very essential to study the following data to plan a successful seismic operation.

a) Weather patterns

1. The temperature and rainfall pattern for the proposed period of seismic operation can be

prepared by studying the past data from meteorological department and then

extrapolating them. Weather forecasts obtained from the Meteorological department can

also be used.

2. The rainfall data shall indicate the likely production day loss due to rain and hence can be

taken as a clue to plan daily production rate so as to complete the project within time.

3. The diurnal temperature scale variation shall help in planning the daily operation time in

case of very high or low temperature to optimise crew efficiency.

4. The knowledge of wind conditions prevailing in the survey area shall help in planning the

crew/equipment movement along river, lakes etc.

b) Topography, Vegetation and Logistics

Though there is a general increase in man-made logistics due to natural civilization process, the

natural topography and vegetation is a characteristic of any area. This includes jungles, bushes,

mountains, and low lying areas prone to water logging, approach roads in remote areas, lakes /

ponds, rivers, and cultivated lands. Tackling each of these logistics requires critical field planning.

The survey needs to be planned taking into consideration all these logistics.

c) Line Clearance

This is the one of the most important part of any field operations as the efficiency of the field

operation depends on the pathways for movement of all the manpower and equipment

necessary for seismic work.

The various features of line clearance are:

a) Building steps on slopes for tough terrain

b) Using rope for climbing/alighting in rough and very steep terrain

c) Making tracks along the profile

d) Tight control on maximum permissible cutting

e) Using appropriate measures like rubber mats, bamboos, floaters, boats, buoys, rubber tubes

etc for safe passage of cables across various surface obstacles like national highways, water

bodies etc.

4.5.3 Drilling Operation

In plain operational areas the drilling of requisite number of shot holes are easier and planned

usually on day-to-day basis. However, if the area is logistically difficult and with hard formations

in the near surface, advance planning is required for drilling in such areas.

In some areas, the drilled holes need to be cased with a PVC pipe to avoid collapse of shot

holes.

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4.5.4 Recovery Planning

1. During 3D surveys, necessary recovery plans are to be made for data gaps generated due

to villages, ponds, rivers, oil installations etc., and data is to be acquired in the gaps as per

the optimized recovery plan using appropriate 3D survey design software. Subsequently it is

to be verified whether the data gap is filled as planned.

2. When the source points / receiver locations encounter an obstacle requiring few source

points / receiver stations to be moved to some other nearby locations from their pre-plot

location, it is essential to know how to offset the source or receiver points perpendicular or

along the direction of the line.

3. Fold variations are introduced when more number of source or receivers are moved away

from their pre-plot locations; however, these variations are less important than the fold loss

because of the missed source points or receiver locations.

4. The priorities for moving source and receiver locations during recovery of missed source /

receivers are as follows:

5. Move the source / receiver locations by less than half of the source / receiver interval in

each direction- maintaining fold in each bin.

6. Offsetting upto half the line interval in the perpendicular direction introduces the least

amount of fold striping.

7. Offsetting in the inline direction is discouraged as it introduces more fold striping than

offsetting in the perpendicular direction (unless moved by half the line interval).

8. Do not reoccupy source or receiver locations, as duplicate raypaths do not add any

valuable information.

9. If one or two isolated source points are skipped, their recoveries may be avoided as they

cause discontinuities in the common receiver gathers.

10. However, when there are several source points / receiver locations to offset from the preplot

line, it is preferable to offset them in a continuous smooth line (e.g., in an arc of a circle)

rather than as large sudden changes in the offsets. This will produce a smoother change in

the shot and receiver gathers in addition to the midpoint distribution in the subsurface and,

hence, improve noise cancellation, produce less acquisition footprint, and enhance

imaging.

11. For 2D survey, the full fold at start and end of line should be maintained unless terrain

conditions prohibit it. Efforts should be made to provide full fold coverage all along line by

reducing the number of skips. Also, in case of break in line or re-occupying a line of earlier

vintage, sufficient full fold overlap shall be ensured.

12. For 2D survey, no recovery shots shall be taken for skips if group interval is equal to shot

interval (e.g. For 96 Channel & 48 fold no recovery is allowed). However, if the shot interval is

twice the group interval then recovery is allowed (e.g. For 96 Channel & 24 fold with

alternate shot point shooting). In case, the number of skips is large, the loss in fold may be

compensated by adopting a suitable recovery plan, e.g. reverse shooting etc.

13. In case of change in acquisition parameters like swath geometry, group interval, foldage,

offset, etc., which is likely to affect data quality, sufficient overlap shall be ensured.

4.5.5 Noise

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1. Controllable noises such as those due to movement of men, vehicles, equipment, cultural

noises and source-generated noise should be reduced as much as practicable.

2. Efforts should be made to keep ambient noise (noise due to wind, power line, surface

logistics etc.) to minimum.

3. Noise strips should be recorded at start and end of the day to evaluate the ambient noise

level in the survey area.

4.6 PROJECT DELIVERABLES

These are the following minimum deliverables (in hard & soft copy):

1. Project Report

2. Raw Data Tapes / Cartridges and their logs

3. Observer Logs, Source & Receiver Edit Logs for every line/ Swath

4. Raw data, hodograph, survey coordinates of all the Upholes Surveys

5. Near Surface Model

6. Optimum depth / Weathering velocity and sub-weathering velocity contour map

7. System Logs

8. SPS Data including up-hole times and field Statics.

9. Raw & Processed Topographic survey data (DGPS, Staking, Leveling etc.)

10. Infield Field processing upto Brute Stacks

4.7 WORK STANDARDS

4.7.1 Instruments

All instruments settings should be as per decided parameters for all channels.

Monitor records should be taken at regular intervals. Length of monitor be such that deepest

/ target event can be seen clearly.

4.7.2 Receivers

Receivers should be planted as close as possible to the staked position. In case the deviation

due to obstacle etc, coordinates of a new location should be supplied to the processors.

Bunching / receiver array is to be judiciously employed depending on the noise

characteristics. If elevation variation within the array lengths is more than one metre,

geophone strings may be bunched.

Receivers outside specifications should not be deployed.

4.7.3 Energy Source

Near-surface modeling for delineation of proper shooting medium should be done by

regular compilation and analysis of the Uphole / Shallow Refraction Surveys in a database for

placement of charge at optimum depth.

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Sources should be planted as close as possible to the staked position. In case the deviation

due to obstacle etc, coordinates of a new location should be supplied to the processors.

Uphole time as given by shooting system and as recorded on Monitor should not vary by

more than ± 2ms.

The skips should be kept to a minimum on a line/swath. In case of obstacles wherein large

number of skips cannot be avoided, suitable recovery technique should be adopted.

4.7.4 Topographic Survey

If the accuracies are not met for a certain part of the area, the topographic survey work

(Staking, Levelling etc.) should be repeated for thate portion of the errors.

4.7.5 Defective traces

A trace shall be considered defective if:

1. A trace is dead. Dead traces, due to natural or man-made obstacles to geophone

plantation will not be considered as defective traces. Reasons for not planting the

geophones at such places may be recorded on observer’s log. Hydrophones should

preferably be deployed in water covered areas / swamps without leaving data gaps

2. The recording system does not meet the manufacturer’s specifications.

3. Its polarity is reversed and not logged / corrected.

4. If the instrument/field test results of the channel is outside specifications.

5. In case of multi-component surveys, all components of a receiver at a location will constitute

as one group. Any component of the group is defective will make the group as defective.

4.7.6 Defective recording

A record shall be considered defective if:

1. Data is recorded without performing periodic instruments tests.

2. Data are recorded with incorrect instrument settings / wrong spread geometry definition.

3. Data recorded with only internal time break

4. Data was not identifiable to the recorded shot or not retrievable from magnetic tape

cartridges

5. No detonation of charge / misfires occurs

6. Shot with charge at outside +/- 2m of the predecided optimum depth.

7. Partial detonation / Floating of explosive in a hole.

8. The number of defective traces exceeds 2% of the active traces in a record.

9. Loss of magnetic recording occurs during designated record length for more than 5% of the

active channels due to Transmission Errors

4.7.7 Acceptable recording conditions

Recording shall commence only when the following conditions are present.

1. The number of defective traces are within the defined limit.

2. Prior to each shot, all reasonable and prudent measures shall be taken to ensure that:

a) The recording system is in proper working order.

b) The source and detectors are properly placed.

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c) All monitoring devices are functioning.

d) Prescribed system tests have been conducted.

e) Shot holes are drilled to +/- 2m of required optimum depth.

3. Sufficient personnel are present to conduct the survey operations efficiently and safely.

4.7.8 Work shall not continue on any day if:

1. More than 2% of active traces are defective.

2. Five consecutive records are defective.

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CHAPTER - 5

VERTICAL SEISMIC PROFILING SURVEYS (VSP)

5.1 PRE-SURVEY STUDIES

Before the commencement of VSP operation, it is essential to generate a database as detailed

in Chapter-2.

In addition, the following may also be studied:

Core analysis results

GTO and casing policy

Sonic & Caliper Logs

Cement Bond Logs

Geology / Stratigraphy at the well

Temperature & Pressure conditions of the well

It is very important to be clear about the objectives of a VSP survey and necessary pre-survey

modeling studies should be done so that the survey objectives can be achieved.

Although it is always preferable to carry out VSP operations in open hole there may be instances

when these operations are to be effected in cased hole.

Pre-survey modeling study based on the geological model is essential to give the idea about the

lateral coverage and help in optimizing of source position in an Offset VSP and the sampling

requirement of the survey. Offset VSP / Walkaway VSP helps image the extension of geological

features of interest around the well.

Designing of survey geometry for offset and walk away VSP surveys should be carried out

through modeling exercise. Input geological model is necessary for performing ray tracing and

generation of synthetic VSP response. This exercise gives idea of lateral coverage of the

reflections, mode conversions, amplitude of UPGOING waves with respect to DOWNGOING

wave etc. which are very crucial for optimizing the depth to be logged, optimisation of source

offset, approximate fold distribution etc.

If shear wave / mode converted wave is also of interest then the record length of the survey

should be decided in keeping in the velocity of shear wave.

Whenever feasible a three component sonde should be used for recording three component

VSP data.

5.2 DATA ACQUISITION

The following are the main requirements for a VSP survey and each of them in turn has to be

quality assured.

Equipment

Well or bore hole

Energy source

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5.2.1 Equipment

The essential equipment use for VSP operations are

1) Winch Cable

2) Downhole Tools

3) Recording system

a. Winch Cable

Suitable winch cable for Multi Level Tool should be available along with cable sections of

requisite inter-shuttle interval to join / cascade the various shuttles of the Multi Level Tool. A

fishing tool is also required in case of any downhole eventuality.

For conventional analog tools, a standard 7 core logging cable is required with one end

connected to cable head and the other end compatible with the down hole tool. The insulation

resistance between the cable conductors should be as per manufacturer’s specifications under

specified conditions. There should not be any kinks (which may result in leakages) in the cable

and should be of sufficient length upto 7000m. There should be a provision to measure depth

and tension accurately. Depth to be checked by duplication of levels while lowering in and

pulling out the downhole tool.

b. Downhole tools

The downhole tool consists of seismic sensors and tool anchoring mechanism and provision of

monitoring the degree of anchoring from the surface control panel.

The desirable attributes of geophone system are as follows:

Tool ends : Tapered

Diameter : Suitable for open hole and casing diameter

Temp. Limit : Within manufacturer’s specifications

Locking arm system : Retractable

Geophone arrangement: Triaxial and Gimbal mounted.

Downhole digitization is desirable to ensure a large number of simultaneous and

independent recordings (Upto 6- 12 levels).

The selection of downhole tool and its anchor size depends on hole diameter, depth to be

logged, casing policy, well conditions, cavities, temperature inside the well etc.

c. Recording system

The recording system should be capable of recording minimum 36 digital channels of multi-level

tools with tri-axial three component geophones.

Also, the recording system should be capable of monitoring the quality of the VSP data in real

time. The system should be capable of graphical monitoring of raw data of each geophone,

ambient noise levels, noisy and leaky traces, first break picking, transit time, identify tube waves,

refractions, casing noise, tool slip, electrical noise, poor coupling and environmental noise etc.

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5.2.2 Well Conditions

The condition of the well has a strong bearing ion VSP operations and the decision have to be

taken accordingly is as follows:

Open hole Analyze caliper log for cavings and shale zones

Deviated hole Need Gimbal mounted Geophones

Multiple casings Note casing points and check VSP tool diameter for compatibility.

Cementation Analyze the cement tops and cement quality behind casings using

CBL/VDL logs

Temperature & Pressure Check the downhole tool limits

5.2.3 Energy source

The selection of seismic source depends on the following factors:

Optimisation of source strength.

Horizontal and vertical resolution aspects

Condition of well (casing, mud density, cementing and cavings).

Well site operating time.

Surface Logistics

For land VSP surveys, the main energy main sources are Explosives, air-gun and Vibroseis.

For sophisticated on-land VSP surveys the usage of Vibroseis and Land Air-guns are practiced in

the industry especially for the study of anisotropy, AVO etc from three-component survey

The source parameter optimization is done as given in Appendix-B.

5.3 PRE-SURVEY CHECKS

The following checks are to be done at the well site before rigging up the tool:

1. Testing of Recording System, as per the manufacturer’s prescribed procedures.

2. Testing of Shooting System

3. Testing of downhole equipment

4. Tap Test of Geophones

5. Geophones Continuity and Leakage Tests

6. Polarity checks of the all the three components

7. Ascertain the clamping pressure by opening & closing of the tool arms.

8. Testing of winch unit

Cable Head insulation for continuity & leakage

Calibration of depth counter and tension meter

5.4 WORK STANDARDS

5.4.1 General

The drilling rig time being very cost intensive is critical, hence the VSP operation should be

conducted in minimum time.

1. All equipment settings should be as per decided parameters for all channels.

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2. VSP operation should preferably be carried out in uncased well or in single casing. However,

the cementation quality behind multiple casings should be studied and depth levels at poor

cementation should be avoided.

3. Observations should be taken at 5/10/20m interval depending on the well condition and

casing policy. Spatial Aliasing criterion should be considered while deciding depth interval

between consecutive depth levels.

4. Check shots should be taken at least 10% of the total number of depth levels envisaged

while lowering and the transit times should be compared with the shots taken at the same

depth levels while pulling out the tool.

5. Optimisation of shot hole depth and charge size and shooting media should be done for

dynamite source

6. Shots with floated charge/partial detonation are to be invariably repeated.

7. Consistent source signature for Airguns and Vibrator sources.

8. Uphole time as given by shooting system and as recorded on monitor should not vary by

more than ± 2 ms.

9. If any leakage either in Geophones or in cable head is detected, the tool should be pulled

out and repaired. Observations with faulty tool are to be repeated.

10. Coupling of the tool to the formation should be perfect as would be evident from the

characteristic exponential decay of the waveform amplitude.

11. Calibration of observed transit times with the sonic logs

12. Accuracy of the transit times of check shots and regular shots should be within +/-1 ms.

13. Monitoring the percentage of anchoring pressure for consistent to avoid tool creep.

14. Cable should be slackened after perfect coupling to avoid cable waves. Normally 2-3m of

cable slack is given after anchoring the tool.

15. Minimisation of generation of tube wave noise by

Offseting the source by 50m to 100m from the wellhead for zero-offset VSP.

Lowering the fluid level in the well by at least 70m to 80m.

Increasing in mud density (depending on the tool capacity to withstand high

pressure)

Clamping of cable at wellhead to avoid vibrations.

5.4.2 Data Quality

1. The data quality should be ascertained by the quality of the first breaks; and the first break

should be clear and precisely detectable occurring at consistent time after shot initiation to

ensure precise transit time computation and successful stacking.

2. First-break (for down-hole geophones) of all the three component geophones should be

clear, precisely detectable and the entire waveform is noise free.

3. No shot should be accepted with dead/reverse channel.

4. Amplifier gains for downhole sensors and recording systems should be monitored to avoid

clipping of signal.

5. If possible, pre-shot noises should be minimised by closure of rig machinery like Generators /

Mud agitators etc.

6. Periodical laboratory testing of downhole geophones with Geophone analyser.

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7. The tool should be moved to avoid the caved borehole wall to ensure proper anchoring for

better data quality.

8. Uphole surveys (if earlier data is not available) may have to be conducted at the shot point

location (near well or at the Offset location) for statics.

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CHAPTER - 6

MARINE GEOPHYSICAL SURVEYS

6.1 PRE-PLANNING

Same as in Chapter-2

6.2 SELECTION OF ACQUISITION PARAMETERS

Same as in Appendix-A.

6.3 2D SURVEYS

6.3.1 Energy source

1. Marine Energy source should be a powerful tuned airgun array with suitable frequency

range for achieving the survey objectives.

2. The suitability of different gun configurations with varying characteristics for the selection of

the best gun arrays should be based after gun signature studies.

3. Digital copy of source signature shall be provided along with the operational report (on CD)

and raw data (on LTO/ 3592 cartridge) and the same be preserved in tape library for future

reference.

4. The nominal pressure and towing apparatus must sustain full array operation at speeds upto

6 knots during routine data acquisition.

5. Nominal tow depth shall be commensurate with the desired frequency band and sea state.

6. The far field signature parameters viz., Strength (Peak – Peak), Primary / Bubble ratio and flat

frequency range shall be survey specific and shall be based on past experience / modeling

studies.

7. Specifications of Far-field source signature shall be in accordance with the SEG

recommendations as published in the Special Report of the SEG Technical Standards

Committee “SEG standards for specifying marine seismic energy sources” Geophysics, Vol.

53, No. 4 (April 1988), pp. 556-575. Specifications and drop-out criteria shall be referenced to

the response of a DFS-V recording system with 128 Hz hi-cut filter at 72 dB/octave, slope.

8. Gun depth, volume and operating pressure shall be recorded in the observer's log at the

start and at the end of each line and at intervals not greater than (forty) shot points. All

deviations shall be recorded in the observer’s log.

9. Prior to commencement of regular production work, the air gun array must be charged to

working pressure and individual gauge readings at the output side of the distribution will be

recorded. The air supply must be shut off and each gauge monitored over a ten minutes

period. Pressure loss in excess of 10% for any circuit must be corrected.

10. These tests must be repeated during the course of the production work on a daily basis and

whenever maintenance has been performed or whenever any gun is suspected for

malfunctioning.

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6.3.2 Gun control system

1. Computer controlled precise energy source synchronisation system is required to control

firing of gun arrays and the performance parameters of individual guns are to be

continuously monitored.

2. The summary of the parameters should provide listings of the offset, delay and error value in

milliseconds for all the guns in the system at regular intervals.

3. The system shall be capable of identifying the firing array and unambiguously tagging

correct shot / file number.

6.3.3 Seismic streamer

1. The streamer should be digital type and meet the standards of the Industry.

2. The in-water streamer electronic modules should be capable of digitizing the analogue data

into 24 bits.

3. All streamers' sensors should be in good electrical and mechanical condition throughout the

period of survey and also have sufficient spares for normal operation.

4. The hydrophones in the streamer as well as individual channel response should conform to

the manufacturer's specification.

5. The streamer should be neutrally buoyant and should be towed at a depth of 6 to 7m (or at

specified depth) commensurate with desired frequency band-width keeping the streamer

noise within the specifications. Depth readings will be recorded every kilometer or less.

6. The streamer shall be fitted with acceleration canceling type of hydrophones and should

have adequate number of calibrated depth transducers (at discrete interval of not more

than 300 m). It should also have lead-in and stretch sections as per standard practice in the

industry.

7. The receiver locations along the streamer shall be monitored using the state of art cable

compasses (placed at discrete interval not more than 300m along the streamer). The data

from these compasses will be integrated to provide exact location of each and every

receiver groups on the streamer to determine the streamer position / shape. Manufacturer’s

specifications of all cable compasses shall be verified prior to and on completion of the

survey to establish individual compass bias.

8. Acoustic sensors network should be provided at front end, mid portion and tail end of the

streamer.

9. Provision should be made to monitor the feathering either by cable compasses or by tail

buoy RGPS.

6.3.4 Recording instruments

1. Before commencement of the survey, one test cartridge shall be recorded and processed

onboard. Deficiencies in the test reports, if any, shall be corrected and verified before work

commences. During the survey, all the tests shall be performed and analysed once a month

for satisfactory recording of data.

2. Standard instrument tests as specified by manufacturers shall be carried out daily.

Deficiencies must be corrected and verified before work commences.

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3. The seismic data shall be digitally recorded in SEG-D demultiplexed format on new IBM 3592

/ LTO of good quality & brand.

4. Sufficient Numbers of cartridge drives should be available for uninterrupted data recording.

5. For monitoring quality of seismic data acquisition, continuous display and recording of

seismic data of at least 1 trace at selectable offset through onboard QC system in near real

time mode.

6. Also a paper recording and Oscillograph should be provided for monitoring of all the traces.

7. Noise records shall be taken with lowest low cut filter "in", both on cartridge and monitor

record before start of line and after end of line.

8. For OBC surveys with dual sensors, raw hydrophone and geophone data from each receiver

station shall be recorded before summation irrespective of whether automatic summation of

the data is proposed. The recorded cartridges of such raw and summed data in SEGD

format shall be maintained.

6.3.5 Data format

The seismic data should be recorded in standard SEG-D format, (version 8015/8048/8058). The

tape should not have any labels preceding the 32 byte Signal Header. The byte locations of all

relevant information (including SP no and FFID) written in external and extended header must be

provided in a four-column ASCII file. The serial number of the word should be reckoned from

byte 1 of the general header.

6.3.6 Fathometer

The precision Fathometer shall be operated throughout the course of survey and the data

should be recorded along with the navigation data. All Fathometer data will be corrected for

draft, velocity and tides. Draft and velocity corrections will be clearly mentioned on logs,

required for seismic processing.

6.3.5 Navigation

1. Differential Global Positioning System (DGPS) should be used for uninterrupted location

positioning of the shot points, receiver location, over the entire prospect. DGPS data shall be

input to the Integrated Navigation System (INS) incorporating statistical/quality control

capabilities.

2. DGPS should consist of compatible GPS hardware and software, both at reference stations

and the recording vessel along with other associated equipment etc.

3. DGPS receivers of minimum 8 channels are required at the reference stations and the survey

vessel. A DGPS "health check" should be undertaken prior to commencement of the survey

to prove the integrity of the system.

4. Differentially corrected positions are to be derived by correcting the mobile receiver

computed positions using differential correction data received from reference stations.

5. DGPS Receivers should use Inmarsat Satellite Communications network or V-sat as the data

link to relay the differential corrections. DGPS should have an accuracy of +/- 5m (95%

confidence level).

6. Multiple reference stations are required with at least one reference station using Inmarsat

satellite communication or HF link should be available within the range of 1000 kms from the

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prospect (survey area) in case of a single frequency reference station or within the range of

2000 kms from the prospect in case of a dual frequency reference station.

7. For uninterrupted signal, an alternative monitoring station should be available for transmitting

differential corrections to the recording vessel.

8. Satellite prediction software with latest updated almanac shall be available throughout the

survey and such software shall be used to identify any time periods of poor coverage and

same shall be informed to OVL QC representative in advance.

9. Sufficient back up of all hardware and software, in order to maintain operational integrity,

should be available.

10. All the recordings (seismic & navigation) should have DGPS time tagged.

6.3.6 Positioning data

1. All the marine streamer positioning data must be in the following formats.

a) Post processed navigation data: UKOOA P1/90

b) Raw navigation data: UKOOA P2/94 or P2/91

2. All navigation edits carried out onboard should be provided as ASCII files on a LTO /3592

cartridge as a text or Microsoft Excel (.xls) files.

3. To facilitate the application of tidal corrections during data processing Contractor will need

to provide tide data from nearest station along with other navigation data. This data is

normally provided by national marine boards/coast authorities/ weather stations.

6.3.7 Gyro compass

1. A survey Gyrocompass or equivalent heading system shall be provided on the recording

vessel. The unit shall be calibrated against a known azimuth whilst the vessel is stationary at

the quayside. The heading system’s reading shall agree with the surveyed azimuth to within

0.5 degrees. Wherever, possible, routine checks (e.g. rig transit bearings) shall be undertaken

offshore to verify gyro compass stability.

2. Gyrocompass or the heading system shall be recorded on the vessel integrated navigation

system (INS).

3. No seismic/Gravity/Magnetic data will be accepted without accurate heading data being

recorded and work shall not commence or continue on any line if the heading system is not

operational or the heading data are unacceptable.

6.3.8 Magnetic data acquisition

1. The Magnetometer shall be a Marine Proton Precession Magnetometer. Sampling rate shall

be determined prior to the survey.

2. Magnetic data shall be digitally recorded along with navigation data. It will also be

recorded on strip chart for quality control purpose.

3. The magnetometer sensor shall be towed at a position approximately two to three times the

vessel(s)'s length. This distance shall be recorded on all data forms

4. Strip chart shall be clearly marked at every 1000m showing total intensity in gammas, day,

time, shotpoint and line numbers.

6.3.9 Gravity data acquisition

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1. The Gravimeter shall be similar to a latest model La Coste Romberg Air-Sea established

platform gravimeter. The periods of the L and R platform shall be set at 4 minutes or

synchronised with the shooting system.

2. All gravity readings should be recorded digitally along with navigation data. It will also be

recorded analog on a rectilinear chart paper for quality control purpose.

3. One base station tie at port shall be made. The location of the vessel(s) at dock shall be

clearly documented.

4. The gravimeter shall be kept on heat continuously during the entire duration of the survey, as

well as during port calls or at anchor.

5. Routine checks and calibration shall be carried out as specified by the manufacturers.

6. Strip charts shall be clearly marked at every 1000m showing gravity value, day, time,

shotpoint and line numbers.

7. Non working of the Gravity meter and/or Magnetometer shall not be a cause to delay in

commencing or to suspend seismic and/or gravity /magnetic operation on any line.

6.4 3D SURVEYS

1. The requirement of energy source, streamers, seismic recording and navigation systems for

3D surveys are same/ similar to the 2D surveys described above, however, for monitoring of

the 3D coverage in the area, additionally the 3D binning system is required.

2. The 3D binning system should be capable of displaying the bin coverage in real time and

producing high-resolution colour displays with a provision for hard copy.

3. The system must have the features (but not limited to)such as:

a) Ability for bin editing

b) Display the operator defined offset ranges and analyse fold contributions

c) Accept source-receiver data auxiliary inputs

d) Provide accurate real time steering information, for optimum trace

e) Steering for any user defined input.

f) Duplicate offset reduction as a user selectable option

g) Apply operator selectable flex criteria in either the cross line or in line directions as a

function of offset.

4. All the navigation data should be in UKOOA format. `Raw Data' and `Bin data' (but not

limited to) in UKOOA P1/P2 formats should be generated/recorded separately on cartridges.

5. The Bin data cartridges should, where possible, record minimum four sets of information (but

not limited to) as listed below:

a) Survey set up parameters

b) Survey lines and grids

c) Bin database contents

d) Source-Receiver data

6. Sufficient disc memory and capable of dividing the survey area into the requisite storage

bins, and store all offset coverage in a permanent grid database.

7. Capable of providing hard copies of partial and total grid coverage plots or listings in order

to determine whether or not sufficient offset coverage has been obtained. The system shall

provide a track plot of cable midpoint angles throughout a line.

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8. Ability to recalculate the grid data if cable compass or navigation data is found to be below

specifications or if particular lines are to be added or subtracted from the database.

9. Allow for dividing the streamer into a minimum of 4 segments and shall have the capability

of verifying that the specified offset distribution has been achieved.

10. Facility for automatic rejection of bad compasses in real time should be available.

11. Capable of providing a display in real time of the midpoint distribution of reflection points

from each streamer segments.

12. The data shall be collected utilizing fixed sub-surface gather bins to obtain a nominal fold

per CMP coverage as per foldage requirement. Each fixed sub-surface area gather bin shall

contain a minimum number of traces in four segments. The percentage of acceptability of

the coverage as above shall be achieved after removal of all bad traces/ channels; bad

shot points, duplicate offsets.

13. In some cases, the vessel may have steeedr off the pre-plotted line to compensate for

streamer feather due to tides and currents to provide optimum midpoint coverage. In areas

having obstructions like Rig and Platforms etc., hole of not more than 500m radius of the

obstruction in the 3D mosaic is allowed. However, every effort shall be made to reduce the

coverage hole and comply with the coverage specifications with infill lines if required.

14. Any line re-shoot shall be recorded in the same direction as the original line. All efforts will be

made to ensure that infill recording is acquired in the same direction as the primary line,

unless operationally not.

15. Raw data tape/cartridge of 3D binning system as per industry practice and other data

should be generated for 3D survey including operator entries and bin data information.

6.4 Post Acquisition QC of Navigation/ Positioning data

6.4.1 QC Checks

1. After a line is acquired, following QC checks should be performed on the navigation/

positioning data:

2. All survey parameters are required to be verified.

3. Shot point edits should be completed.

4. Gun edits (like mask error, firing out of sequence etc.) should be completed.

5. Statistical QC Checks

6. Precision analysis in terms of 95% Error ellipses for measurements like GPS positions and

Kalman filtered tracking nodes. A semi major axis (SMA) of maximum 5meters with maximum

5% outliers may be taken as acceptance criteria for GPS position. Similarly an SMA of 15m

with 0% outliers and SMA of 12m with 10% outliers may be taken for tracking nodes.

7. Standardised residuals of Gyro, Acoustic ranges, compasses, base lines, float positions etc.,

to ensure the reliability of network solution.

8. The acceptable outliers against different values of standardised residual are suggested as

below:

Standardised residual + 0.67, permitted outliers 50%

Standardised residual + 1.64, permitted outliers 10%.

Standardised residual + 1.96, permitted outliers 5%.

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9. Estimate of Bias in respect of Gyro, compass, Baseline, float position: The weighted mean of

estimated bias for all shots in respect of these measurements should be examined. The

pooled bias in respect of a particular measurement should be less than the standard

deviation assigned to that measurement.

10. The Actual Detectable Error (ADE) indicates the maximum size of any possible remaining

error in an observation with 90% probability. The ADE for Acoustic ranges, Compass and Float

positions may be tested for all shots against a maximum limit for each type of observation (5

x observational standard error). However, the magnitude of ADE does not indicate that an

outlier exists.

11. Streamer rotation (Estimated Rotation Bias): The ‘rotation’ is an important QC indicator. Plot

of rotation values for all shots of a line for all streamers should be visually examined. The

rotation plot of for all streamers should have similar size and shape. The ‘rotation’ between

streamers should not vary by more than 0.5 degrees.

12. Streamer radial misclosure (Tail tracking node misclosure along): The mismatch between

compass derived tail end positions and the tail-end positions found from acoustic network is

to be checked for all streamers. The minimum, maximum and average values are examined

for all shots in a line. About 90% values indicating a misclosure of less than 5 meters may be

taken as acceptable norm.

13. Accelerations source and streamers (in line & cross line): These indicators are measures of

smoothness of key node positions (tracking nodes) throughout the line in network solution.

High acceleration values indicate uneven and sudden jumps in node positions, which are

practically unlikely (except rough sea conditions). A value within + 0.05 m/sec2 may be

taken as accepted norm for accelerations.

14. The minimum maximum and average values of Separations (in line, Cross line and radial)

between sources, vessel and streamers should be examined. The average values should

match the nominal values used for actual source streamer configuration or the survey

design.

6.4.2 QC of P1/90 Navigation Data

1. The header data of P1/90 must be checked thoroughly to confirm:

2. Line name and sublines are correct.

3. First and last shots area accuracy.

4. Any NTBP sections area accuracy.

5. Misfires and firing sequence changes agree with Observer’s log.

6. Comments or any other error messages have been followed up.

6.5 OBC SURVEYS

6.5.1 Recording instruments

Same as Land & Marine Surveys

6.5.2 Cables & Receivers

1. The dual sensors shall be of digital telemetry type consisting of geophones and hydrophones

and should meet the standards of the Industry. The cable electronic modules should have

24-bit capability. Dual sensor recording at each receiver location is required at all times.

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2. All cables and sensors shall be in good electrical and mechanical condition throughout the

period of survey. Also each hydrophone and geophone in the dual sensors as well as

individual channel responses conform to the manufacturer's specification A minimum of 50%

spare cables with required sensors are required above a requirement of 2 full swaths (for 3D

survey) of cables being deployed at any one time.

3. Polarity for all units shall meet SEG standards such that positive pressure and upward

geophone case motion produces a negative number on tape.

4. Each receiver channel location shall be positioned using state of art acoustic positioning

system with required accuracy.

5. At time of acquisition every effort shall be made to ensure that the location of each receiver

does not differ by more than 6m in-line and 10m cross-line from its pre-plot location.

6. Onboard QC processing facilities to evaluate the quality of data and acceptable noise

levels for any coherent noise trends should be provided. For ambient noise, hydrophone

noise levels shall not exceed 5 µbars RMS for water depths more than 10m and 10 microbars

RMS for water depths 10m to 5m.

6.5.3 Energy Source

Same as Marine Surveys

6.5.4 Navigation

Same as in Marine Surveys

6.6 WORK STANDARDS

6.6.1 General

Prior to commencement of data acquisition the following must be satisfied:

1. DGPS positioning health check and gyro or heading system calibration are done.

2. All recording instruments to be proven to be functioning to manufacturer’s specifications.

3. Prior to commencement of regular production work and at regular intervals all equipment

tests and procedures specified by the manufacturer are to be performed. Also, the following

tests are to be done before the start of survey or whenever some streamer sections/

electronic modules are replaced.

4. All instrument tests as specified by the manufacturer viz. Noise and Polarity test, Streamer

continuity and leakage tests.

5. Before commencement of the survey, one test cartridge shall be recorded and processed

onboard. Deficiencies in the test reports, if any, shall be corrected and verified before work

commences. During the Survey, all the tests shall be performed and analysed regularly/

once in a month for satisfactory recording of data. The test cartridge and processed results

so generated shall be supplied to base office.

6. Adequate stretch section will be provided at the front and rear end of streamer to minimise

cable jerk. Prior to start of survey, tests shall be performed to determine the best combination

of propeller pitch and RPM to minimise ship's induced noise.

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7. Cable noise will be monitored and recorded on the cartridge of the start and end of each

line or at any other time. These noise records will be taken with the instrument switches set as

for production.

8. At the operating depth (7 to 8m) the ambient cable noise will not exceed 5 µbar (RMS) for a

group length of 25m. Groups closer than 250m from the vessel and the three groups nearest

the tail buoy the ambient cable noise, but should be within 10 µbar (RMS). The maximum

noise levels for groups adjacent to depth controllers shall be within 10 microbars rms. All

groups that fail to meet the above specifications will be regarded as bad and will be

marked as such on the observer’s log. Higher noise levels due to factors beyond control (e.g.

nearby seismic operations, drilling activity or adverse weather conditions) will be fully

documented on all relevant observers’ logs.

9. At the specified operating depth the ambient cable noise levels shall not exceed 50/L µbar,

for traces near the vessel (up to 250 m from the vessel), tail buoy, depth controllers and

modules and 25/L µbar , for all other traces where L= group interval. All groups that fail to

meet the above specifications will be regarded as bad and will be marked as such on the

observer’s log

10. The streamer will be balanced for neutral buoyancy. This will be checked prior to the start of

survey to ensure that it runs at the specified operating depth with the boat steaming at

normal shooting speed.

11. If possible, the depth controller (birds) will be situated mid way between depth Transducer

sections. The birds will be displaced as far as possible from hydrophone locations to minimise

noise. The operation of the birds will be checked on deck before deployment

12. Noise record shall be taken both on cartridge and monitor record before start of line and

after end of line.

13. Standard instrument tests as specified by manufacturers shall be carried out daily.

Deficiencies must be corrected and verified before work commences.

14. Streamer continuity and leakage tests shall be done before start of each line.

15. The track of the vessel shall be maintained in such a way that the cross distance from the line

remains within +/- 10m.

16. Operating depth is maintained with the boat steaming at normal shooting speed.

17. Compasses exhibiting dynamic biases in excess of 0.5 degrees shall be considered bad and

shall not be used. A cable compass shall be considered bad if it gives more than five

successive bad values or more than 15% of its readings are bad on any one line.

Malfunctioning compasses will be replaced before next line start.

6.6.2 Work shall not commence on any line if:

1. More than 2 adjacent groups or more than 2% of random groups per streamer are bad.

2. More than 3 non-adjacent or 2 adjacent depth detectors not functioning. Depth detectors

are to be placed at every 300m interval

3. Streamer depth varies more than 1 meter from the chosen depth

4. Streamer drift exceeds 10 degrees unless authorized by onboard QC person.

5. Ambient noise is not reduced to the lowest level consistent with existing sea conditions

6. Instrument noise exceeds manufacturer’s specifications.

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7. Noise due to electrical source (e.g. 50/60 Hz. picked up from ship's generator) is present. This

should be eliminated without the help of notch filter of the recording unit

8. Energy source is operating below 90% of the normal operating pressure (2000 psi or as

agreed with the Client / Basin) or array volume.

9. DGPS is not providing desired accuracy.

10. Onboard single trace recorder or equivalent system is inoperative

11. Monitor Camera is inoperative.

12. Fathometer is inoperative.

6.6.3 Work shall not continue on any line if:

1. More than 2 adjacent groups or more than 2% random groups are bad.

2. More than 3 non-adjacent (or more than 2 adjacent) depth detectors are not functioning

3. Reduction by more than 10% of array volume or 10% of normal operating pressure of airgun

system.

4. More than 2 adjacent guns of similar volume or any critical gun are not operating.

5. Auto firing of any gun occurs or the gun controller is malfunctioning.

6. Four monitor records are missed consecutively.

7. Ten consecutive recordings are bad.

8. More than 5% cumulative shots are bad on any line.

9. Fathometer is inoperative for more than one hour.

10. The onboard single trace recorder or equivalent system becomes inoperative for 30 minutes.

11. Differential Global Positioning System not functioning properly and providing the desired

accuracy.

12. The streamer depth varies more than 1m from the chosen depth.

13. Streamer drift exceeds 10 degrees relative to line heading.

6.6.4 Definition of bad shots:

1. Auto firing / Misfiring of the guns.

2. Firing time of guns varying by more than ±1 ms.

3. Loss of more than 10% of array volume.

4. Pressure drop of more than 10% of normal operating pressure.

5. More than 3 parity errors and if sync errors are present.

6. Non-recording of positioning data.

7. Non-recording or loss of of data on cartridge for any reason whatsoever.

8. Data recorded with incorrect instrument settings.

9. Loss of positioning data.

10. Poor correlation between signatures from individual source array.

6.6.5 Definition of bad groups:

1. Groups having leakage value less than 500 K.

2. Groups, which are dead/no response.

3. Groups, which are intermittent/sluggish/spiky response.

4. Groups for which noise exceeds the units specified.

5. Groups not showing coherency w.r.t. adjacent trace (greater than +/- 6db variation).

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6. Groups showing reverse polarity.

6.6.6 Work Standards: 2D Survey

1. Adequate stretch section will be provided at the front and rear end of streamer to minimize

cable jerk. Prior to start of survey, tests shall be performed to determine the best combination

of propeller pitch and RPM to minimize ship's induced noise. However, the near offset should

be kept as smaller as possible.

2. Streamer noise will be monitored in microbars RMS and measured over the full recording

cycle with recording filters and with the streamer at the specified tow depth.

3. Coherent noise sources should be identified and logged. Action should be taken to ensure

shot to shot timing differentials of the noise do not exceed 500 msec. Coherent noises that

consists of horizontal reflected energy from the acquisition vessel is to be exempted from the

specification.

4. Noise coming from the beam (or side) can be particularly harmful and difficult to deal within

processing. In general terms only low levels of interference from the beam may be tolerated

for short duration within a record. In these situations the use of onboard QC/processing

systems are necessary to fully evaluate the effects on the data.

5. As a general rule, coherent noise should not exceed the following limits for the noise coming

from astern and ahead of the streamer:

Slope Max. Noise (µbar RMS)

GI 12.5 m GI 25.0 m

> 300 msec/km 28 µbar 20 µbar

150-300 msec/km 14 µbar 10 µbar

0-150 msec/km 6 µbar 4 µbar

6. As a guide, average swell noise of up to 25 µbar on 5% of shots per line for less than 2

seconds duration will be tolerated. Effects of swell noise will be evaluated using onboard QC

system. Higher noise levels may be acceptable subject to sea condition and QC

Representative's approval.

7. Coherent noise shall be assessed according to the following:

Amplitude of the interfering signal

Duration of the noise

Repetition and synchronization of the interference

Move-out of the interference

Constant interference upto 10 µbar will only be tolerated up to a maximum duration of 4

seconds.

Acquisition Procedures and Minimum Line Lengths

1. Line run-in distance shall be long enough to ensure there are no residual noise or turn effects

present in the streamer shape and that the gyro compass has had time to settle. In any case

in the absence of physical constraints the minimum run in distance shall not be less than 1.0

times the length of tow (unless constraints by physical obstacles e.g. bathymetry). Note that

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long offsets will be required at the edge of the survey full fold area and binning criteria must

be met right up to the edge of the survey.

2. Guns will be fired in sufficient time prior to start of line to allow for warming up and tuning.

3. Environmental guidelines issued from time to time should be strictly adhered to while

acquiring data.

4. Line run outs to achieve full fold coverage are required at the end of each line and will be

not less than ½ active streamer length plus ½ source to near trace offset.

5. As far as is possible all lines shall be recorded in one pass.

6. Any line terminated within a distance equivalent to the streamer length from the start point

shall be re-acquired in its entirety (except for in-fill lines).

7. Unless otherwise dictated by operational constraints the maximum number of allowable line

segments per line shall be :

Line Length Max. segments allowed

<20 Km 2

20-35 Km 3

35-50 Km 4

Over 50 Km By agreement with QC Representative

6.6.7 Work standards: 3D Surveys

Work shall not commence if:

1. Less than 15 depth detectors functioning per 6000 m long streamer, prorated for shorter

cable configurations

2. Less than 15 compasses functioning per 6000 m long streamer, prorated for shorter cable

configurations


4. Any of the acoustic networks for showing streamer separation not working.

5. All the standards mentioned above and in previous sections.

Work shall not continue if:


2. More than 2 non-adjacent compasses are not functioning.

3. All the standards mentioned

Definition of bad shots

All the standards mentioned above and in previous sections.

Definition of bad groups

All the standards mentioned above and in previous sections.

3D midpoint coverage specifications

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The data shall be collected into fixed sub-surface gather bins to obtain a nominal fold per CMP

coverage as per the foldage requirement below. Each fixed sub-surface area gathers bin shall

contain a minimum number of traces in four segments. The percentage of acceptability of the

coverage in each of the four segments after removal of all bad traces/ channels, rejected shot

points, duplicate offsets etc is as follows:

For Shallow Water Areas:

75% of all midpoints from the near quarter of the streamer

75% of all midpoints from the second quarter of the streamer

75% of all midpoints from the third quarter of the streamer

70% of all midpoints from the fourth quarter of the streamer

For Other Areas:

90% of all midpoints from the near quarter of the streamer

90% of all midpoints from the second quarter of the streamer

90% of all midpoints from the third quarter of the streamer

70% of all midpoints from the fourth quarter of the streamer

6.6.8 OBC SEISMIC SURVEYS

Work shall not commence if:

1. There are more than 2% bad groups per spread.

2. The ambient noise for hydrophone shall not exceed 10 µbars RMS and for geophone 25

µbars RMS with the exception of the short duration noise bursts. However, in regions with

large tidal variations (>2 meters) and/or high currents (>2 knots), Geophone noise will be

accepted “as it is” provided it is not due to a consistent problem that is within the crew’s

control and it is established that the noise can be removed in processing.

3. Acoustic Range Positioning System for receivers not operative.

4. All the standards mentioned in above

Work shall not continue if:

All the standards mentioned above and previously are not met.

Definition of bad shots:

All the standards mentioned above and previously are not met.

Definition of bad groups

1. Groups showing poor coupling.

2. A bad group is defined as a pair of receivers in which either geophone or hydrophones or

both detectors are bad.

3. Any of the standards mentioned above and previously are not met.

3D midpoint coverage specifications

The data should be acquired in such a way as to maintain a minimum of 75% of the nominal

maximum fold in a bin for all the offsets after removal from fold consideration of all bad groups,

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bad shot points, duplicate offsets (at QC representative discretion), and receiver / shots out of

navigation specifications. However, total fold should be 100% of the nominal fold.

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CHAPTER – 7

INFIELD / ONBOARD DATA PROCESSING

7.1 GENERAL

The seismic crew should monitor the data quality on a real time basis. Also, the data should be

analyzed and processed on the Field-processing unit with suitable hardware and software.

Proper documentation of the online / offline data analysis and processing should be made.

7.2 INFIELD QC PROCESSING

The infield processing can establish:

The correctness of the Source- Receiver positions and survey geometry

The signal content of the data.

List of Dead / Reverse traces (Shots, Receivers, Channels) and unusual aspects

Pre-process data

Pick velocities and build database

Processing trials - establish sequence and parameters etc.

Additionally, the following jobs should also be carried out:

Processing of Experimental data

Processing and Analysis of Instrument test data.

Data Quality analysis

Monitor records shall be taken regularly up to the objective zone and will be evaluated

in field for online corrective measures. All the monitors should be analyzed and studied

carefully in camp

The seismic data that is being recorded daily should be recorded as per the quality

norms.

The data should invariably be processed in the camp or onboard the vessel. The following

processed outputs (but limited to) should be generated

Amplitude spectra to ensure proper resolution at the target zone,

Brute stacks to verify the mapping of the target reflectors,

Inline & Crossline displays

Time Slice (if possible)

If any shot or a group of shots is found to have sub-optimal data quality in terms of reflector

mappability and resolution, the source parameters should be reviewed immediately and the

necessary experimental work should be repeated to optimize it again before resuming further

acquisition of data.

7.2 PROCESSING

Once recording of the first line/ Swath or the pilot line/ swath is complete, it should be subjected

to full processing sequence to study the fulfillment of objectives of the seismic survey.

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The processed line / swath should be compared with the earlier seismic data available in the

area. If the processed section indicates acceptable data quality in terms of target reflector

mappability, then subsequent lines/swath can be shot with the same acquisition parameters.

However, if the processed section indicates a major deterioration in the data quality as

compared to the earlier data or the objectives are not mapped satisfactorily then the process

of acquisition parameter designing needs to be reviewed and revised.

It may also be needed to revise the initial model in the light of the presently acquired pilot line.

Using these revised data, the acquisition geometry and parameters should be optimized again

to continue the seismic survey.

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Appendix-A

SURVEY DESIGN AND ACQUISITION PARAMETERS

A. TYPE OF SPREADS

The most common and widely accepted spreads are Split spread and End on.

A.1 Split-spread configuration

Objectives

If random noise is the only problem and the area is free of multiples, the method is well suited.

The method is suitable for shallow targets and variable dip directions.

If the interest is in the shallow as well as deep target, asymmetrical split spread is a better choice.

It reduces the NMO stretch.

The spread is suitable in horst / graben set up.

A.2 End-on configuration

Objectives

It gives a longer spread for same no. of geophones, which enables us to look deeper and is very

cost effective.

It is suitable for better multiple suppression and better velocity analysis.

It provides a better velocity analysis.

It is convenient for field operations. Since the shot points and the corresponding spread are

separated, any activity at the corresponding and successive shot points does not hinder the

active spread.

With the increasing channel capacity of present day seismic recording instrument, there is little

binding on the number of active channels laid in the spread. That is why nowadays mostly split

spread configurations are used with longer arms on either side, which serves all the purpose of

mapping both shallow and deeper targets, velocity analysis as well as mapping geologically

complex subsurface structures.

A.3 Direction of shooting

It is the direction in which the seismic ray travels from the source to the receiver. It has

significance only in case of end-on spread and to some extent in asymmetrical split spread.

A wave traveling updip suffers less scattering and arrives at all the receivers within a given array

at approximately the same time resulting in constructive interference especially at higher

frequencies.

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In the case of updip shooting, the total surface coverage to map a steeply dipping reflector is

less as compared to that of downdip shooting. Hence, up-dip shooting is usually preferred in 2D

Surveys.

In 3 D survey the direction of shooting does not have much significance since the reflected

energy is recorded from all direction. However the spread Geometry/direction of shooting may

be fixed in such a way that majority of reflected energy is recorded from updip side.

In marine surveys the direction of shooting depends on the logistics and the sub-surface

geology. The longer side of the survey area is usually the direction of shooting.

A.4 Acquisition Parameters

The seismic survey design is a process of selecting geometrical parameters that define where

seismic sources and receivers should be positioned and the recording instrument parameters so

that the reflection wave fields generated and recorded to create a 2-D / 3-D image of a

specific target at a specific depth.

The key parameters required to be specified for design of acquisition parameters are:

Narrowest lateral dimension of the geological targets

Depth of the shallowest target

Depth of the deepest target

Foldage required imaging the target at the required depth

Frequency of the signal at target level

Dip and thickness of the target horizon

Representative Velocity function of the area

The basic geometry of a 2D seismic survey is described in terms of:

Shot Interval

Group Interval

Far Offset

Foldage required at the target level

The basic geometry of a 3D seismic survey is described as:

Source-station spacing

Receiver station spacing

Source Line spacing

Receiver Line spacing

Recording swath size

The horizontal resolution provided by 3D seismic image is function of the trace spacing within the

3D data volume. As the trace spacing decreases the horizontal resolution increase. The

dimension of the inline and cross line spacing in a 3D data volume defines the size of the

stacking bin. Bin size in cross line is half of either shot interval or receiver line interval spacing

whichever is less. For a 3D volume the bin size controls the horizontal resolution.

As a general rule there should be preferably four bins and minimum of three stacking bins, across

the narrowest stratigraphic feature that needs to be resolved.

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Bin Size

Inline & Cross Line Bin dimension

Bx = ½ Group Interval (or Shot Line spacing whichever is less)

By = ½ Shot Interval (or Receiver Line Spacing whichever is less)

Spatial aliasing criterion

Bx or By = Vav / (4 fmax * sinθ)

fmax=(Vav/4sin)*1/Bx

also fmax 1/Bx i.e. requirement of high frequency requires smaller bin size

Fresnel zone criterion

Bx or By = (2/3) R

where R = (Vav/2)√ (Tο/fmax),

Tο = two-way-time of the shallowest target

Vav = average velocity,

fmax = max. Frequency,

To = zero offset two-way-time,

Θ = dip, R = radius of first Fresnel zone

The source station and receiver-station/array spacing should be twice the horizontal dimension

of the bin that is required in the source-line and receiver-line direction respectively.

The depth of the shallowest target that must be imaged with 3D data is related to the distance

between the source-line and receiver -lines. The 3D design must ensure that everywhere within

the 3D grid there are always several source receiver pairs that are separated by a distance that

does not exceed the depth to the shallowest target.

When a seismic wave field is generated at a particular source station, the 3D recording swath is

defined as that area spanned by the active receivers in the recording grid.

The active receiver stations should form a continuous areal coverage completely around the

source point and extend at least the distance equal to the depth of the deepest target of

interest.

Foldag:

The foldage optimization is done using the previously acquired data.

2D fold - optimization is done from previous 2D data available sections with different possible

foldage is generated from the existing data at processing center and compared

2D Fold= Number of Channels * Group interval / (2* Shot interval)

3D fold - optimization is done from previous 2D data foldage in the area for same signal to noise

ratio.

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Fold (3D) / Fold (2D)= 4 {(Bx * By tan)/GV} fd

where, G = CMP interval of 2D data,

fd = dominant frequency

The 3D fold needs to be optimized both in in-line and cross-line direction. Ideally the two should

be same. Hence, foldage in the two directions is optimized keeping in view the subsurface

mapping objective and optimal use of inputs.

The stacking fold is the number of traces that are summed during data processing to create the

single image trace positioned at the center of that bin.

In a 3D context stacking fold is the product of inline fold (fold in the direction of the receiver-line)

and cross-line fold (fold in the direction perpendicular to the receiver line)

To build a high quality 3D image, it is critical to create the proper stacking fold across the image

space and also ensure that the fold has a wide range of offsets and azimuths.

Offsets:

This is the distance between shot and receiver and encompasses three aspects, viz. minimum

offset, maximum offset and its distribution. Near offsets are needed for data inversion, far offsets

are needed for velocity analysis, multiple suppression and AVO analysis and the middle offsets

are needed as link between the near and far.

Maximum Near Offset

The near offset for 2D should be less than or equal to one group interval.

The maximum near offset for 3D should be less than 1.0 to 1.2 times the depth of the shallowest

horizon to be mapped.

Far Offset

The far offset should be small enough so that the shallowest reflection reaches just below the first

break and avoid wide angle reflection distortion. It should be large enough for good velocity

analysis for effective multiple suppression. I t should cover a good range at target level for good

AVO analysis

Minimum and Maximum offsets:

Differential move out (if multiples exist)

∆T = X²/ 2TοV²

where, V = stacking velocity ≈ Vrms and X = offset

Velocity analysis (for far trace) X = V √ (2Tο/f)

NMO stretch criteria. X (10%) = V Tο √ (0.21)

The value of far offset is limited on the higher side by the NMO stretch criteria. And it is limited on

the lower side by the differential move-out (multiple attenuation) criteria and velocity analysis

criteria whichever is higher. i.e. Xm, Xv < Xfar < Xnmo

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Rule of Thumb: Xmax should be approximately the same as the primary target depth, usually

expressed as Xmax =Target Depth.

Migration Apron:

It is the area/distance by which the image area is to be extended to get full-migrated

coverage.

Migration apron is normally chosen as the larger of:

The lateral migration movement of each dip in the expected geology,

The distance required to capture diffraction energy coming upwards at a scattering angle of

30°, or The radius of the first Fresnel zone.

Migration Apron = Z * tan

where Z=depth to the target and =dip of the target reflector

The appropriate value of migration apron should be decided from the above calculations

based on the sub-surface complexity and imaging requirements.

In the emerging scenario, the requirement of Pre-Stack Time Migration (PSTM) / Pre-Stack Depth

Migration (PSDM) is becoming almost a routine process; the migration aperture calculations

should take into account the above processes. The calculations require generation and analysis

of unit impulse response and should be done in consultation with the processors and the Client /

Basin Manager.

A.5 Recording Parameters

A.5.1 Record Length:

The record length must be sufficient enough to capture target horizons, migration apron and

diffraction tails.

The record length must be equal to

Tmax= Td+2L

where Td is the time of the deepest selection and L is the length of the longest filter in Time

A.5.2 Sampling Interval

Sample rate in time determines the temporal resolution. It should able to sample at least 4

samples in the time period of the highest anticipated frequency.

A.5.3 High Cut filter

The cut off frequency of a cut frequency depends on the sampling interval. High cut filter is used

to attenuate frequencies above the Nyquist frequency which depends upon the sampling

interval) to avoid their aliasing.

The High cut filter setting is generally kept at 0.5 to 0.7 of Nyquist frequency with the required

slope in dB/octave.

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Appendix-B

EXPERIMENTAL WORKS FOR LAND SURVEYS

B.1 EXPLOSIVES

The following experimental works are required prior to regular seismic work to fine-tune the

acquisition parameters.

Up-hole survey

Optimization of energizing conditions

Noise survey to find out characteristics of dominant noise/ground roll in area.

Geophone Array designing

B.1.2 UP-HOLE SURVEYS

Objective:

To optimize the charge depth for noise survey.

To optimize the charge depth along proposed seismic lines.

To compute the velocity of weathered/sub-weathered layer and thickness of weathered

layer

Preparation of near surface model for optimizing the depth of shot holes

Methodology:

1. Shot hole should be drilled to a depth below the sub-weathered layer.

2. Litho cutting samples to be collected from each shot hole at every 2m-depth interval. The

cutting are to be analyzed and lithology to be identified.

3. Regular shot interval of 2m to be used from deepest interval upto the surface

4. At least four channels with single geophone to be planted at offsets of 1m, 3m, 5m and15m

from the shot hole.

5. A uniform type of seismic source viz. detonators to be used for shooting all the levels. Number

of detonators can be optimized for deepest level so as to record sufficient amplitude at the

surface. It can be varied with depth such that the number of detonators is kept constant in

the zone where optimum depth is expected

6. Uphole surveys should be conducted at an interval of 1-2 km interval/grid covering line

crossings for 2D surveys and at an interval of 1 sq.km. For areas with fast lateral near surface

velocity variation, the interval should be 1 km or even less

7. The planned Upholes should be conducted and interpreted well before shooting the seismic

line/swath

Analysis of data:

1. The recorded slant times are converted into vertical corrected times

2. The time-depth (T-D) plots are generated for all the recorded offsets

3. The velocity and depth of weathered/sub-weathered layers are then computed

4. The amplitude of the first break in the expected zone is studied. Any increase in amplitude

indicates better medium and a decrease indicates a poor medium

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5. Optimum charge depth is derived using the layer velocity, thickness and lithology.

6. All Upholes are to be correlated along the line and a near surface model is prepared in

order to draw optimum shot hole depth based on the lithology, weathering and sub-

weathering velocity.

7. Once the planned Upholes are completed in the area, depth contour map of weathered

and sub-weathered layers to be prepared.

Deliverable:

The optimum charge depth

Near surface model

B.1.3 OPTIMIZATION OF CHARGE

Objective:

Charge size optimization

Charge depth optimization

Methodology:

1. The result of the Uphole survey carried out in the area is utilized to fix the optimum charge

depth for charge size optimization

2. Full spread length as designed in the pre-planning stage is taken

3. Near trace offset is taken slightly less than the optimum obtained from noise survey

4. Unfiltered shot gather plot for all shots

5. Frequency spectra of near, middle and far traces for all shots

6. Data analysis and interpretation:

Charge size optimization

Compare all the shot plots with different charge size recorded at optimum depth and study

the frequency and energy content within the zone of interest. Higher frequency content vis a

vis sufficient energy reaching the far offsets is essential

Compare the amplitude spectra for all the charge size at various offsets. Study the

bandwidth at a common level; say –3 dB and the peak frequency. A wider bandwidth and

higher peak frequency gives better resolution and is therefore preferable

Select the minimum charge size that gives the optimum result

Charge depth optimization

Compare all the shot plots with different charge depths. Study the frequency and energy

content and event mappability within the zone of interest. Higher frequency content with

minimum noise cone is preferable

Compare the amplitude spectra for all the charge depths at various offsets. Study the

bandwidth at a common level; say –3, -6 and -12 dB and the peak frequency. A wider

bandwidth and higher peak frequency gives better resolution and hence is preferable

Select the optimum charge depth and relate to the Uphole result

Deliverable:

The optimum charge size for the area

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The optimum Charge depth at the location.

B.1.4 NOISE SURVEY

Objective:

To study the noise characteristics of the area

To optimize the near trace offset

Methodology:

Noise profile can be conducted in two ways

1. Walk-away spread method, i.e. the shot point location is fixed and the spread is moved

successively away. This method is operationally more time consuming

2. Walk-away shot point method also known as transpose wave-test method, i.e. the spread is

fixed and the shot point is moved. This method introduces some variability due to moving

shots but is operationally less time consuming and is more conventionally used for noise

survey

3. In areas of varying tectonic set-up and surface conditions, noise survey should be

conducted at more than one location. In case the variation is not significant, it can be

conducted at some central location representing the area

4. At each location, noise profile should be shot in both dip and strike direction

5. The offset coverage on the ground should be slightly more than the expected far offset

computed in the pre-planning stage

6. The walk-away shot point method is conducted using a set of shot holes with shot interval

equal to the spread length plus one group interval

7. The number of shots required depends on the maximum offset to be tested

8. The noise profile should be designed based on channel capacity of the party so as to

minimize the number of shots needed to cover the offset.

Analysis of data:

Field monitor analysis is carried out on hard plots or within GUI environment of a seismic data

processing software.

1. Generate noise section for the entire length of the profile by juxtaposing the field monitors

2. Mark the noise events with different colours.

3. Calculate the frequency, velocity and wavelength and tabulate.

4. Mark the offset at which the shallowest reflector of interest is just outside the noise cone.

5. Pick traces corresponding to multiples of channel spacing covering slightly more than the

expected near offset. Compute the amplitude spectrum of traces for full time window.

6. F-K spectrum in two windows, one shallow and one deep. Compute the noise wave

characteristics, i.e. velocity, frequency and wavelength along with their amplitude strength

Interpretation:

1. The coherent noise is essentially characterized by low velocity and low frequency.

2. From the computed values, the range of wavelength of the noise trains are obtained which

gives the first hand idea about the geophone array to be used to attenuate them

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3. The filtered output gives the idea of the noise getting attenuated by low cut filter (LCF)

4. It is preferable to not to use any LCF and all the noise wavelengths present in the unfiltered

section needs to be attenuated using field arrays. However if inevitable, LCF may be used

during acquisition.

5. Study of the amplitude spectrum of the traces indicates the prominence of low frequency

events at smaller offsets and gradual decay towards higher offset. The offset beyond which

these amplitudes are minimum and do not vary appreciably gives the optimum near trace

offset parameter

Conclusions:

The noise wavelength range to be attenuated by geophone array.

Near trace offset to be used.

Affect

B.1.5 GEOPHONE ARRAY DESIGNING

Objective:

To design the optimum geophone array for attenuating the coherent noise

Methodology:

1. The result of the Uphole survey carried out in the area of noise survey is utilized to fix the

optimum charge depth for array optimization

2. The spread is designed by folding the required active channels in 6 or 8 numbers of mini-

spreads. The number of folds depends on the number of arrays to be tested. The group

interval is taken as optimized in pre-planning stage

3. Each mini-spread consists of different array. Normally one spread is laid with 10/12

geophones bunched for comparison of other arrays

4. The other arrays in the spreads are designed based on noise wavelengths obtained from the

noise survey. A geophone array is characterized by number of elements n and element

spacing D

5. The total offset on the ground should cover the computed far offset.

6. The array designing is conducted using a set of shot holes with shot interval equal to the mini-

spread length plus one group interval

7. The number of shots required depends on the maximum offset to be tested. The fold-back

layout should be designed based on available channel capacity so as to minimize the

number of shots needed to record upto the required maximum offset.

Data processing:

1. Simulated unfiltered shot gather plot for each geophone array by juxtaposing the

corresponding shots

2. Simulated shot gather plot for each geophone array with LCF of 8 Hz and 12 Hz

3. Amplitude spectra of near, middle and far traces for each array

4. Two F-K spectra for each array plot, one inside the noise cone and one outside the noise

cone

Data analysis and interpretation:

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1. Each array record is studied by correlating the events within zone of interest

2. Mark the noise wave trends in each plot. This gives idea about the filtering effectiveness of

the arrays

3. Compare the unfiltered plots with LCF applied plots to see the effective efficiency of the

geophone array in association with the LCF

4. Compare the attenuation characteristics of the arrays with their response curves.

5. Compare the amplitude spectra for all the arrays at near, middle and far offsets. Study the

bandwidth at a common level; say –3 dB and the peak frequency. A wider bandwidth and

higher peak frequency gives better resolution and hence is preferable.

6. Compare the signal and noise level of different arrays ‘inside the noise cone’ and “outside

the noise cone” from the F-K spectra respectively.

7. The array providing better signal level and less noise level is selected as the optimum

geophone array

Conclusion:

The optimum geophone array length, number of elements and element spacing.

For a given area of operation and the type of geometry adopted, judicious choice is to be

made between bunching of geophone element at the picket and geophone array,

keeping in view the strength of the ground roll and its interference with the signal at the

objective level.

B.2 VIBROSEIS

The sequences of experimentation for optimizing vibrator parameters for Vibroseis surveys with

monosweep are as under:

B.2.1 LOW END FREQUENCY TEST

Procedure

Sweep Type adopted Linear

No. of Vibrators 4

Drive Force 70%

High End Frequency 80 Hz

Low End Frequency 6 to 16 Hz at 2 Hz interval

Analysis

To look for the continuity of the events at objective levels

To understand the behaviour of ground roll.

B.2.2 HIGH END FREQUENCY TEST

Procedure


No. of Vibrators 4

Drive Force 70%

Low End Frequency As decided above

High End Frequency 60 to 100 Hz at 10 Hz interval

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Analysis

To look for the resolution of the events at objective levels

B.2.3 SWEEP LENGTH TEST

Procedure


No. of Vibrators 4

Drive Force 70%


High End Frequency As decided above

Sweep Length 6 to 24 sec at 2 sec interval

Analysis

To study the signal to noise ratio and continuity of deepest reflector and amplitude buildup of

peak signals.

B.2.4 COMPOSITES / NUMBER OF STACKS

Procedure


No. of Vibrators 4

Drive Force 70%



Sweep Length As decided above

Composites/ No. of stacks 4 to 16 in steps of 2

Analysis

To study continuity of deepest reflector of interest

B.2.5 SWEEP TAPER LENGTH TEST

Procedure


No. of Vibrators 4

Drive Force 70%




Composites/ No. of stacks As decided above

Sweep Taper length 50 to 300 msec at 50 msec interval

Analysis

To study correllogram records for side lobe characters. The side lobes should appear with

minimum amplitude.

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B.2.6 VIBRATOR DRIVE TEST

Procedure


No. of Vibrators 4





Drive Force 50 to 80% in steps of 10%

Analysis

To study the amplitude buildup of weak signal and continuity at the objective level.

B.2.7 SWEEP TYPE TEST

i) Non-linear ii) User Defined

Procedure

No. of Vibrators 4

Drive Force As decided above





Sweep Type Non-Linear with various weight factors

Analysis

To study the resolution at the objective level more tests as given below may be required for

ascertaining and enhancing the effectiveness of various Vibroseis acquisition parameters.

B.2.8 NOISE TEST

B.2.9 RECEIVER ARRAY TEST

B.2.10 UP SWEEP TEST

B.2.11 VIBRO PATTERN TEST

All the above tests are to be carried out as per the standard industry practice.

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Appendix-C

GENERAL SEISMIC DATA PROCESSING STEPS: (In Field)

PROCESS PURPOSE WHEN APPLIED REMARKS

Gain

Recovery

Multiply data by binary gain

codes from gain ranging.

Second, if needed.

Reversible

Locating correct gain

trace.

Editing Remove bad records,

misfired shots, open

channels, noisy traces.

Third, and at other

times during

processing if needed.

Best done in field

during acquisition.

Must scrutinize plots of all

of raw data.

Summing

(Vertical

Stack)

Reduce source and random

noise by acquiring multiple

impacts, shots or sweeps at

same location.

After editing. Often

done during

acquisition, irreversible.

Noisy or unbalanced

shots. False triggers. Strong

60 Hz noise will sum to

harmonics. Large move

up arrays attenuate steep

dips and blur statics.

Correlation Compress vibrator sweeps

into small wavelets.

After summing to save

computer time. Best

after despiking and

editing. Often done

during acquisition,

irreversible.

Incorrect sweeps,

harmonics, spikes produce

ghosts. Acts as a band

pass filler. Very expensive.

Gain

Function

Remove effect of geometric

spreading, amplifying deep

events relative to shallow.

Last step in data

reduction above.

Can destroy true

amplitude information.

Use a reversible function

or save unequalized

dataset.

CMP Sort Arrange traces by common

mid-point.

After data reduction

but before velocity

analysis or NMO

correction.

Incorrect stacking

diagram, crooked seismic

lines.

Elevation

(Datum)

Static

Time correction for elevation

differences.

Correct to at least a

CMP - variable datum

before NMO or

velocity analysis. May

correct to final datum

after stack.

Assumed velocities above

datum, long offsets.

Uphole

Static

Time correction for lateral

velocity variation in

weathering layer.

Before NMO or

velocity analysis.

Assumed depth of

weathered layer; long

offsets.

Velocity

Analysis

Estimate VNMO ,VST After determined time

corrections and

Assumes zero dip, slow

lateral velocity changes,

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sorting, before final

NMO and stacking or

any migration.

strong reflectors at

velocity changes, no

multiples.

NMO

Correction

Correct time on offset traces

to zero offset time.

After sorting and

statics, before

stacking, part of

velocity analysis.

Assumes zero dip, slow

lateral velocity changes,

no multiples, short offsets.

Residual

Static

Correct any remaining time

shifts to straighten out NMO-

corrected events.

After NMO, before

stacking.

Eliminates delay

information useful for

transmission tomography.

Assumes only slow lateral

velocity changes.

Mute Zero out arrivals that are not

primary P-wave reflections.

Before stacking and /

or migration.

Overly sharp clips cause

artifacts in further

processing.

Band pass

Filter

Attenuate noise outside of

reflection frequency band.

Best before stack,

NMO or velocity

analysis; can be after

stack.

Often much noise in signal

frequency band, or weak

signals are filtered out.

Alters true and relative

amplitudes.

Notch Filter Attenuate noise in narrow

frequency hand, such as 50

Hz AC power.

Best before stack,

NMO or velocity

analysis.

Too narrow a notch will

cause artifacts. Destroys

true amplitude and

phase.

Decon Compress source wavelet

shape and duration (spiking),

improve resolution, and

attenuate reverberations

(predictive).

Best before stack,

NMO or velocity

analysis; can be after

stack.

Can unwittingly remove

evidence of real

reflectors; will change

time amplitude and

phase.

2-D (F-K)

Filter

Spatial bandpass filter,

attenuates or enhances

arrivals based on dip, move-

out, or apparent velocity.

Any time alter data

reduction, depending

on type of events. May

require interpolation

for steep events.

Alters amplitudes and can

cause artifacts. High dips

may get aliased.

Stack Zero-offset section,

attenuate random and

much coherent noise.

After sorting, velocity

analysis, muting.

Attenuates dipping

structures, accentuates

lateral coherence.

Depends on inferred

velocities. Mis locates

dipping structures.

Trace

Equalize

(AGC)

Amplify weak events or

traces relative to strong.

Often best used just for

display purposes.

Anytime, usually just

before or after stack.

Lose amplitude

information. Can end up

enhancing noise.

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Post Stack

Migrate

Correctly position dipping

events horizontally.

After stacking and

usually equalization.

Depends on average and

/ or interval velocities.

Cannot improve on steep

or crossed dipping events

that do not stack well.

Depth

Conversion

Correctly position events

vertically.

After stack and usually

migration.

Depth error proportional

to average velocity error.

Pre-stack

Migrate

Correctly position steeply -

dipping and crossing

reflectors. Invert for earth

properties. NMO correction

and stacking are a simplified

migration that assumes zero

dip.

Partial migration (dip

moveout or DMO) can

be done before NMO

and stack. Full

prestack migration

done after data

reduction and often

after filtering,

equalization, and

deconvolution; no

stacking. Usually

applied only to good

data from well-

characterized areas.

Heavily dependent on

velocity estimates and

susceptible to gross errors

when lateral velocity

variations are not correctly

accounted for

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Appendix-D

GLOSSARY

3-C 3-D A 3-D survey which is acquired using a standard source and 3-C geophones.

3-component geophone (3C): A geophone with three orthogonal sensors. The phone must be

planted with known orientation, usually one component inline, one cross-line, and one vertical.

3-D symmetric sampling Symmetric sampling applied in 3-D surveys.

4-C receiver station A receiver station with a 3-C geophone plus a hydrophone.

9-C 3-D A 3-D survey which is acquired using three sources: a standard source, an in-line shear

source, and a cross-line shear source. Each wavefield generated by each source is then

recorded with 3-C geophones.

Achievable resolution A lower resolution than potential resolution caused by noise (multiples,

ground roll, and ambient noise) and by irregular or coarse sampling.

Acquisition imprint: Imprint of 3-D source and receiver geometry onto 3-D data and data

attributes.

Active receiver (station) A receiver (station) belonging to the group of receivers (stations) that

are recording data.

Actual resolution A lower resolution than achievable resolution caused by various sub optimal

processing steps (errors in velocity model, phase errors, etc.).

Air blast The pressure wave that travels through the air from the source to the geophone.

Airgun A marine energy source that creates seismic wavefields by releasing compressed air.

Air-gun array A collection of air guns optimized to generate a sharp source wavelet.

Alias-free sampling Sampling that introduces no improper frequency or wavelength information

into 3-D data.

Ambient noise Noise produced by the environment (engines, people, wind) in contrast to source

generated noise.

Amplitude striping A geometry imprint typical for streamer surveys, may be caused by feathering

but also by width of multisource multistreamer geometry

Amplitude versus azimuth (AVA) Variation in reflection amplitude as a function of source-to-

receiver azimuth.

Amplitude variations with offset (AVO) Variations of reflection amplitude as a function of offset

distance. Behavior depends on Poisson’s ratio of rocks at the reflecting interface.

Anisotropy Variations in rock properties as a function of direction of analysis.

Antiparallel acquisition Sailing adjacent boat passes in opposite directions.

Aperture A range of illumination angles used in migration.

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Apron The width of the fringe area that needs to be added to a 3-D survey to allow proper

migration of any dipping event. Although this is a distance rather than an angle, it has been

commonly referred to as the (migration) aperture. Other synonyms are migration apron and

halo.

Areal geometry Acquisition geometry consisting of a dense (sparse) areal grid of receiver

stations and a sparse (dense) areal grid of sources.

Array (pattern) A geometrical arrangement of sources and/or receivers used to suppress noise of

certain wavelengths.

Array length Number of elements times the distance between the elements.

Array response The amplitude response of an array as a function of wavelength and direction.

Aspect ratio The ratio of the narrow dimension of a rectangle divided by the wide dimension. In

3-D design, the ratio of the cross-line dimension divided by the in-line dimension.

Azimuth Angular direction in degrees relative to north.

Basic sampling interval The sampling interval required for alias-free sampling of the whole

continuous wavefield (including ground roll).

Basic signal sampling interval The sampling interval required for alias-free sampling of the

desired part of the continuous wave field.

Basic subset The 3-D subset of an acquisition geometry consisting of traces those have smoothly

varying spatial attributes. Possible subsets could be cross spread, common-offset gather, CMP

gather, etc.

Bin An area used to gather traces with midpoints that fall inside that area. Bins can be any

shape but are usually square or rectangular.

Bin fractionation An implementation of the orthogonal geometry that intentionally creates

subgroups of midpoints within a natural bin. Uses shifts in station positions between adjacent

acquisition lines to create the effect.

Bin interval The distance between adjacent bins.

Bin rotation Reprocessing 3-D data to create in-line and cross-line orientations that are different

than those involved in the original data. Normally used with bin fractionation or when combining

(interpreting) two or more 3-D surveys where processing used different bin grid angles.

Bin size The area of a bin. Normally determined by source and receiver station spacings.

Binate Literally, take every second sample. Often used as reducing the number of traces by

taking every nth sample, n not necessarily being 2. Because this is a resampling operation

leading to larger sampling intervals, a spatial alias-filter should be applied before bination. See

also decimate.

Boat pass A single crossing of the survey area by a seismic vessel in multisource, multistreamer

acquisition.

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Box The area of a 3-D survey bounded by two adjacent receiver lines and two adjacent source

lines. A box contains all the statistics that describe the middle of the 3-D survey. See unit cell.

Brick or brick-wall geometry An orthogonal geometry in which the source lines are staggered

between receiver lines. Used to reduce the Xmin of a geometry for better shallow coverage.

Button A tightly grouped arrangement of receivers in a button patch geometry.

Button patch geometry A 3-D geometry patented by Arco in which the receivers are laid out in

buttons and sources are positioned around the buttons. Rather than a dense areal grid as in the

areal geometry, this geometry has a checkerboard pattern of dense receivers and empty

spaces.

Cable The wire-connecting receiver groups to the line units.

Center-spread acquisition Acquisition with as many receivers to the right of the source station as

to the left. May also apply to receiver stations where each receiver station has an equal number

of sources on either side. Center-spread acquisition creates symmetry in common-source

gathers and in common receiver gathers.

Chair display An interpretive display in which the 3-D volume is sliced into two depth sections

and a time section connected in a chair shape.

Charge The amount of dynamite (lb or kg) used for one source point, sometimes consisting of

several shotholes.

Circular design A design that uses circular patches.

Circular patch A patch with an outer edge that approximates a circle.

CMP fold The theoretical fold calculated by binning cmps.

COCA gather Common-offset/common-azimuth gather.

Common conversion point (CCP) In converted-wave (PS) acquisition, the CCP is the equivalent

of the common midpoint. It is the point between the source and receiver where the downgoing

P-wave generates an upgoing S-wave.

Common depth point (CDP) The common reflection point for dipping reflectors and complex

velocity fields.

Common midpoint (CMP) The theoretical reflection point that lies midway between a source

and receiver. Assumes no structural dip and no unusual velocity variations exist. A group of

traces that shares the same midpoint.

Common scatter point (CSP) A way of analyzing prestack data (Bancroft and Geiger, 1994). A

reflecting surface is thought of as a specular surface, with each point generating a set of

diffractions. The processing collapses these diffractions prestack to gather the energy to the

appropriate point in the subsurface.

Common-offset gather One of the basic subsets of parallel geometry. For a perfect subset, the

source/receiver azimuth should be constant in the gather, but usually it is not.

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Common-offset stack A display in which traces with the same offset from different sources have

been stacked. The traces are displayed after the application of NMO. This display is useful for

determining mute, detecting multiples, and initial analysis of AVO.

Continuous fold Coverage in the case of proper sampling; reflection points can be

reconstructed for any position within a properly sampled data set.

Conversion point The point in the subsurface where conversion of P-waves to S-waves occurs.

Normally this point is about two-thirds the offset measured from source, (as compared with 1/2

for P-P reflections).

Converted wave An S-wave generated when a portion of a P-wave is converted into shear-

wave energy at a reflecting surface.

Convolution A mathematical process applied to two or more time series that corresponds to

multiplication of the Fourier transforms of the time series.

Correlation The vibrator sends a programmed wave train, or chirp, into the subsurface. Each

reflection is also a chirp. In the correlation step, the reflected trace is correlated with the chirp to

convert each chirp reflection to a short, compact wavelet.

Critical reflection The angle at which waves are refracted instead of reflected.

Cross-line In a 3-D survey this is the direction orthogonal to the receiver lines. It is usually the

same as the direction of the source lines.

Cross-line offset The component of offset that is in the cross-line direction, perpendicular to the

receiver lines.

Cross-line roll A patch (swath) move perpendicular to the receiver lines.

Cross-spread One of the basic subsets of orthogonal geometryconsists of all traces that have a

source line and a receiver line in common.

Decimate Literally: take every tenth sample. Often refers to reducing the number of traces by

taking every nth sample, n not necessarily being 10. As this is a resampling operation leading to

larger sampling intervals, a spatial alias-filter should be applied before decimation.

Deconvolution (“decon”) A mathematical process that collapses wavelet signatures into sharp

spikes. Commonly used in processing to boost high frequencies. Inverse of convolution.

Density The mass per unit volume of rock. Usually measured in kg/m2 or g/cm3. Density has some

effect on seismic velocity.

Depth migration Seismic migration performed in the depth domain rather than the time domain.

Depth structure map A map of a particular horizon where the vertical dimension is depth.

Diffraction A seismic event generated by a scattering point.

Diffraction traveltime surface The collection of travel times associated with a diffraction event.

Dip moveout (DMO) The change in reflection time due to change in position of reflection point

for CMP traces. The processing step that attempts to move reflections to a common-reflection

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point. When combined with NMO correction, the construction of zero-offset traces from offset

traces.

Dip shooting The use of a geometry oriented in the main dip direction.

Dip/strike decision The decision whether to choose dip or strike direction as the shooting

direction in marine surveys and in swath surveys.

Direct wave A wave traveling in the surface layer directly from source to receiver.

Distributed system A 3-D data acquisition system. The signals from several receiver groups are

collected at a remote line unit and then transmitted to the recording truck.

Double zig-zag geometry A 3-D geometry involving two zig-zag paths for the source lines.

Drag The amount of movement of vibrators between each shake within the source array at one

source station (cf., move-up).

Dual-sensor technique OBC with a geophone (array) and a hydrophone (array) in every

receiver station.

Dwell In nonlinear vibrator sweeps, the dwell is the additional sweep effort applied at higher

frequencies; usually quoted as db/octave.

Edge management Optimization of image area, migration apron, fold taper zone, DMO, and

cost considerations to arrive at an efficient design at survey edges.

Effective spread length The product of number of stations and station interval. Should be used in

all 3-D design computations rather than spread length. A similar definition applies to linear arrays

with equidistant elements.

Effort A general term for the amount of vibrator energy put into the ground. Determined by the

number of vibrators, peak ground force, sweep length, and the number of sweeps.

Exclusion area An area that is not accessible because of natural or manmade hazards or a no-

permit area.

Far offset The farthest offset recorded. Used to refer to farthest offset in a particular patch.

Feathering The deviation of towed streamer from track followed by vessel and source.

Feathering angle The average angle between source track and streamer direction.

Final survey plan The plan after the survey is recorded, with all skids and offsets entered. This plat

is often what must be submitted to the regulatory authorities.

Flexi-Bin geometry An implementation of orthogonal geometry that creates subgroups of

midpoints within a natural bin. Uses line spacings that are non-integers of station spacings to

create the effect.

Fold or fold-of-coverage Usually the number of traces in a bin. Sometimes the number of

overlapping basic subsets of a geometry.

Fold rate The increase in fold (either in-line or crossline) per line interval.

Fold taper zone The area around a 3-D survey in which the fold increases from zero to full-fold.

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Fold-of-illumination Usually the number of raypaths hitting a bin on a reflector

Footprint of geometry A. The imprint of acquisition geometry on 3-D data displays. B. The

illumination pattern on reflector that resembles geometry of source and receiver lines.

Fracture porosity Reservoir porosity created by cracks or fractures in the rock, sometimes

enlarged by subsequent dissolution. Commonly sought as a potential gas reservoir, particularly in

carbonates. Oriented fractures will polarize shear waves into fast (parallel to fractures) and slow

(transverse to fractures) components.

Frequency The number of oscillations per second, expressed in Hertz.

Fresnel zone The first Fresnel zone is the area around a reflection point within which constructive

interference occurs. Often used instead of zone of influence.

Full-swath roll An implementation technique for large surveys in which the whole swath is moved

by the full width of the swath when the cross-line roll is done.

Full-fold area The area of the survey where full fold is achieved, neglecting the effects of DMO or

migration (cf., image area).

Gather A collection of seismic traces.

Geometry imprint May occur in two disturbing ways: periodic, reflecting the periodicity in the

acquisition geometry, and non-periodic, such as striping in marine acquisition caused by

feathering and multisource, multistreamer acquisition.

Geophone A sensor that records the particle velocity created by seismic waves.

Geophone array The geophones laid out at a receiver station to achieve a desired array

response.

Geophone group Each receiver station is usually occupied by several geophones in a group to

improve signal-to-noise ratio. The geophones in a group are laid out to form a geophone array.

Geostatistics A mathematical technique of cross-correlating areally distributed data sets. Can

be used for time-to-depth conversion by correlating well-control and seismic data.

Global positioning system (GPS) A satellite positioning system based on calculating the range to

at least four satellites. Most accurate mode of operation is “differential GPS” which can give x,y

accuracy of 1–2 m, and z accuracy of 5–10m at 1 s rate. Greater accuracy can be achieved by

repeating observations. Tree cover or rough topography can obscure the signal.

Gravity coupling Coupling of geophones to the earth using gravity only (most OBS techniques

use gravity coupling).

Ground force The amount of force exerted by a vibrator (cf., peak force).

Ground roll The surface wave generated by a source. These are high-amplitude, low-velocity

waves. Often, the fastest non-P-wave is also called ground roll whereas it is often a first-arrival

shear wave.

Group interval See receiver interval.

Halo A term sometimes used to mean fold taper zone.

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Horizon A particular reflecting surface or its reflection.

Horizon slice An interpretive display in which the displayed surface follows an interpreted

horizon.

Hydrophone An underwater sensor that measures pressure changes instead of the particle

velocity measured by a geophone.

Illumination area The area on a reflector covered by all traces in a basic subset.

Image area The portion of a 3-D survey that has full-fold data after DMO and migration.

Impedance The product of bulk density and wave velocity. In equation form,

impedance Z = density X velocity.

Reflection coefficient = (Z2 – Z1) / (Z2 + Z1).

In-line The direction parallel to the receiver lines in a 3-D survey.

In-line offset The offset in the direction parallel to the receiver lines.

In-line roll The movement of a swath in the direction parallel to the receiver lines. Typically, inline

rolls are only a few stations and are accomplished electronically.

Interleaved acquisition A technique using overlapping boat passes to compensate for large

streamer distances. The distances are large to avoid streamer entanglement.

Inversion Seismic inversion is a mathematical process that calculates the impedance contrasts

that produce the observed seismic response. The process and the results are non-unique.

Isotropic A condition in which a rock system has the same rock properties in all directions.

Largest minimum offset (LMOS) The largest Xmin of all the bins in a box or in some statistically

complete subset of the 3-D survey. The maximum shortest offset (see Xmin).

Lateral resolution The minimum distance over which two separate reflecting points may be

distinguished. Primarily a function of frequency.

Linear moveout (LMO) A static shift applied to each trace equal to offset/velocity, where

“offset” is the source-to-receiver distance and “velocity” is normally chosen to be the

approximate first-break velocity. The effect of the shift is to move all first breaks close to zero time

(i.e., flatten on the first-breaks).

Line geometry Acquisition geometry in which sources and receivers are arranged along straight

acquisition lines.

Line turn The change of direction of the seismic vessel in preparation for the next boat pass.

Live receiver An active receiver ready to record data.

LMOS Largest minimum offset.

Marsh phone A geophone designed to be used in marshy conditions. It must be planted in the

marsh bottom and can be immersed.

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Maxibin or macrobin The neighborhood of bins used for velocity analysis or for time picking in

statics determination.

Maximum cross-line offset The maximum offset in the cross-line direction.

Maximum in-line offset The maximum offset in the in-line direction.

Maximum recorded offset The largest offset recorded in a swath.

Maximum unaliased frequency The highest frequency that can be recorded in a basic subset

without creating aliased frequencies because the trace spacing is too large.

Midpoint A point halfway between a source station and a receiver station.

Midpoint scatter The common situation in 3-D acquisition where the midpoints of traces that

contributes to a bin are spread out across the bin, rather than concentrated in the center of the

bin.

Midpoint/offset coordinate system A coordinate system based on midpoint coordinates and

offset vectors. A simple transformation allows conversion to the source/receiver coordinate

system.

Migration A process in seismic processing in which reflections are moved to their correct

reflection points in space.

Migration aperture The range of illumination angles used in migration and denotes highest value

of angle/offset available in data.

Migration apron The additional distance that must be added to each side of a 3-D survey to

ensure that the migration process can work.

Migration noise Noise created by the process of migration due to irregular or coarse sampling.

Migration stretch Vertical distortion of a seismic wavelet caused by the movement of reflection

energy to a potential reflection point. The distortion is a function of offset (in prestack migration)

and dip.

Minimal data set A single-fold 3-D data set that is suitable for migration (a basic subset,

excluding the 2-D line).

Minimum resolvable distance The smallest distance between two events that can be resolved.

Move-up The distance that vibrators must move between the last sweep of one source point

and the first sweep of the next source point.

Multicomponent recording An acquisition technique that records two or more components of

the seismic wavefield.

Multiple Seismic energy that has been reflected more than once.

Multiple suppression Any process that reduces preferentially the energy of multiple arrivals.

Multisource, multistreamer acquisition The use of one or more source arrays in combination with

many (4 to 12) streamers.

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Mute function In a common-source gather, energy beyond certain offsets is discarded because

it becomes distorted by refractions and other effects. The offsets that are retained increase with

depth. The mute function is the increase of usable offsets as a function of two-way traveltime.

Narrow azimuth geometry A 3-D geometry that has a small aspect ratio. This geometry means

most of the recorded energy comes from a narrow cone of azimuths oriented parallel to the

long axis of the survey.

Natural bin A bin with dimensions of (1/2 source station spacing) _ (1/2 receiver station spacing).

Near-offset trace A trace recorded with a relatively short source-receiver distance.

Near-trace cube A 3-D data set extracted from a 3-D survey using only the nearest offsets. Used

for quality control.

NMO discrimination Using the amount of normal moveout observed to characterize events by

their velocity.

NMO stretch Vertical distortion of a seismic wavelet caused by NMO correction. The distortion is

a function of offset.

No-permit area An area of a 3-D survey which is excluded because a permit could not be

obtained for surface access.

Nominal fold Full fold using all receivers, all offsets, and natural bins.

Non-orthogonal geometry Any 3-D geometry that does not use a orthogonal grid of lines. Used

to refer to line geometries in which the source lines are not orthogonal to the receiver lines.

Normal moveout (NMO) The variation in reflection time as a function of source-receiver distance

(offset).

OBC Ocean-bottom cable. OBS Ocean-bottom seismometer. Ocean-bottom cable technique

(OBC) Marine acquisition technique using receiver cables laid out on the sea floor. Receivers

may be inside or outside the cable. Usually have dual sensors, but 4-C OBCs are available as

well.

Ocean-bottom seismometer (OBS) Self-contained receiving and recording unit. Used mostly by

academia but also being tried for exploration.

Offset a) The distance between a source group center and a receiver group center for a

particular trace. b) Sometimes used to refer to stations that are moved a short distance

perpendicular to the line, usually because of access difficulties.

Offset distribution May mean two different things: distribution of offsets in a CMP or bin (also

called offset sampling), or the distribution of offsets across the bins. Preferably, both distributions

should cover the whole range of offsets occurring in the geometry, whereas the offset intervals

should be irregular. Regularity in the cmps may lead to aliasing of multiples for low-fold data;

regularity across the bins may lead to visible periodic geometry imprint.

Offset sampling The sampling of offsets within a CMP or bin.

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Orthogonal geometry Acquisition geometry with parallel source lines running perpendicular to

parallel receiver lines.

Pad time The sweep length times the number of sweeps for a vibrator source design.

Parallel geometry Acquisition geometry with parallel source lines running parallel-to-parallel

receiver lines.

Patch In an orthogonal survey, a rectangle of receivers that are spread over several receiver

lines. Several sources may have the same patch. The patch moves around the survey for

different source points.

Patch shooting A method of OBC data acquisition. Several receiver lines are deployed and

stationary; source lines are acquired with source points inside and outside the patch. All lines are

picked up in one roll and moved to the next location.

Peak force The maximum amount of force that a particular vibrator is designed to apply to the

ground.

Peg-leg multiple Multiples caused by horizons that are relatively close together. The short time

delay of the peg-leg event makes the velocity of the peg-leg multiple close to the velocity of

the primary events, and therefore harder to separate and suppress.

Phase It is the argument of a wave function. If the wave is represented as function of (Kx - ωt),

then argument (Kx - ωt) is the phase; also Kx is known as spatial phase whereas ωt is called

temporat phase. Phase is expressed in degrees or radians.

Poisson’s ratio The ratio of transverse strain to longitudinal strain, usually denoted by γ, It is one of

the elastic constants that affects both P- and S-wave velocity.

Porosity Pore volume per unit volume, expressed as a percentage.

Potential resolution Theoretically best possible resolution. It Can be computed using Beylkin’s

formula.

Prestack depth migration A migration process applied in the depth domain (instead of the

traveltime domain) to unstacked traces. This process uses ray tracing to compute the diffraction

travel times and can cope with complex velocity models. It essentially entails evolving a credible

interval velocity model in depth.

Prestack process Any process applied before all traces from a particular CMP or bin are

summed together.

Prestack time migration A migration process applied in the time domain (instead of depth) to

unstacked traces. This process uses the double square-root equation to compute the diffraction

travel times.

Proper sampling A data set is properly sampled if the underlying continuous wavefield can be

faithfully reconstructed from the sampled values.

P-wave This is the type of elastic body wave normally considered in seismic work. The particle

motion is in the direction of wave propagation.

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Quasi-random sampling A method of irregular sampling to reduce migration noise caused by

coarse sampling.

Radial Target-oriented acquisition geometry used for known salt domes; source points are along

concentric circles.

Random geometry Geometry with a random distribution of source and receiver locations.

Ray-trace modeling Modeling that computes raypaths as they pass through each layer.

Receiver The recording device in a seismic survey.

Receiver interval The distance between each group of receivers.

Receiver line The line along which receivers are laid out in a straight-line 3-D survey. Receiver

lines are parallel to the in-line direction.

Receiver line interval The distance between receiver lines measured orthogonal to the receiver

lines.

Receiver station A group of geophones linked by a wire.

Reciprocity theorem The assumption that interchanging the position of the source and the

receiver will lead to the same-recorded trace.

Regular sampling Sampling with a constant sampling rate.

Resolution The ability to discriminate between closely spaced subsurface features.

Running mix A summing of traces in which the number of traces summed is larger than the

number of traces advanced between each calculation.

Salvo The number of source points taken before the patch must be moved, i.e., the number of

source points in a template.

Script file The computer file that tells the recording system the geometry of each template in the

survey.

SEG-P1 format An SEG-approved standard format for recording positioning data.

SEG-Y format An SEG-approved format for recording seismic data. There can be many variations

within the SEG-Y format, so it is often necessary to test for compatibility between different

systems.

Seismic reservoir monitoring Monitoring the production of hydrocarbons using seismic

techniques (repeat surveys or time-lapse surveys).

Semblance A measure of multichannel coherence, usually measured as a function of stacking

velocity. The correct stacking velocity should produce the most coherence and the highest

semblance.

SH wave The horizontal component of motion in a shear wave.

Shear wave A body wave in which the wave motion is transverse to the direction of

propagation.

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Shot A dynamite charge used as a source in a seismic survey. Shot is often used to refer to any

seismic energy source (see source point).

Shot hole The hole drilled to contain an explosive charge. Shallow holes should be below the

weathering layer and deep enough not to blow out. Deep holes reduce ground roll.

Signal-to-noise ratio (S/N) The power of the desired energy (signal) divided by the remaining

energy (noise).

Similarity tests Checking to make sure that all the vibrators in an array are in-phase.

Skids Sometimes used to refer to stations moved a short distance along the line, usually because

of access difficulties.

Slowness The inverse of velocity.

Sonic log A well log of seismic traveltime. The frequencies used in a sonic log are much higher

than those in seismic data.

Source The point of energy release in a 3-D survey. The usual sources are dynamite or vibrators

on land and airguns in water.

Source density The number of sources per unit area usually expressed as sources per km2.

Source interval The distance between adjacent sources in a 3-D survey.

Source line (shot line) The line along which source points or vibrator points are placed, usually at

regular intervals.

Source line interval The distance between source lines, usually measured perpendicular to the

source lines.

Source point A location of a source (shot).

Source/receiver coordinate system A coordinate system based on source and receiver

coordinates. A simple transformation allows conversion to the midpoint/offset coordinate

system.

Source-generated noise That part of the seismic wavefield that needs to be removed in

processing.

Source-receiver pair The receiver array and the source point that produce a given recorded

trace.

Spatial continuity The absence of spatial irregularities, such as edges, missing source points,

missing receivers. Slow variation of spatial attributes of all traces in a data set.

Spatial frequency The wave number.

Spider diagram A diagram used to display azimuth distribution in a 3-D design package. Each

leg of the spider points in the direction from the source to the receiver, and the length of the leg

is proportional to the offset.

Spread An arrangement of receivers associated with a source point. In a cross-spread, the line

of receivers forms the receiver spread and the line of source points forms the source spread.

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Spread length The distance between ends of a spread.

SPS format A standard SEG format proposed by Shell for writing script files that contain

comprehensive information about the geometry of the survey.

Stack array The combination of geophone arrays and regular equidistant offset sampling in a

CMP. In 3-D, the stack array concept can be applied only in parallel or full-fold 3-D geometry

Stack response Response as a function of wave number computed for all offsets that contribute

to the stack in a CMP or bin.

Stack section A time section produced by stacking without application of migration.

Static coupling The static correction for each receiver is based on many source paths into that

receiver. If a direct path can be drawn from any receiver to a midpoint, and from there to all

other receivers (via more midpoints), then the static corrections are said to be coupled and

produce a single solution. In a standard orthogonal geometry there are usually several sets of

connected receivers that are not linked to each other, which lead to several independent

statics solutions unless macro-bins are used.

Statics The time corrections applied to compensate for the slow velocities and elevation

differences of the surface weathering layer(s).

Stationary-receiver system A marine acquisition system with receivers in fixed position during

data recording.

Straight-line geometry Any 3-D geometry that uses straight lines for receivers and sources. Source

lines are often, but not necessarily, orthogonal to receiver lines.

Streamer acquisition A marine acquisition technique using towed seismic cables.

Strike shooting The use of survey geometry oriented in the main strike direction.

Sub-bin In bin-fractionation techniques, a smaller group of traces than the natural bin.

Super bin The neighborhood of bins used in velocity analysis.

Surface area The area enclosed by the outermost sources and receivers in a 3-D survey.

SV wave The vertical component of motion in a shear wave.

Swath a. Width of the area over which the sources are being shot without any cross-line rolls,

often with many in-line rolls in one swath. At the end of a swath there is a cross-line roll to set up

the next swath (see also patch). B. The collection of all receiver lines laid out at one time. C. A

single boat pass, or a group of adjacent boat-passes, all acquired in the same direction.

Swath survey In a swath survey, source lines are parallel to the receiver lines (parallel geometry).

Since parallel receiver lines record simultaneously from one parallel source line, swath lines are

created midway between source and receiver lines (terminology in use for land and OBC

surveys trying to mimic marine multistreamer surveys).

S-wave Shear wave.

Sweep The input from a vibrator. Frequencies are varied (“swept”) in a precise manner over

several seconds.

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Sweep length The time needed to sweep across the entire frequency band of the sweep.

Sweep rate The frequency band of the sweep divided by sweep time. Units are Hz/s (or, more

properly, s_2).

Symmetric sampling A seismic sampling technique that applies the same sampling for sources

and receivers, because the properties of the seismic wavefield are the same in common-

source gathers and in common-receiver gathers.

Takeout The electrical connection in a receiver cable where a group of receivers is attached.

Target depth The depth of the prospective horizon for which the 3-D survey is being designed.

Target size The lateral dimensions of the prospective geological reservoir. In 3-D design, the

smallest of these dimensions needs to be resolved.

Target-oriented acquisition geometry Acquisition geometry optimized for a known geologic

structure, e.g., a concentric circle shoot around salt domes.

Telemetry system A 3-D recording system that uses a radio system to relay the recorded

information from the receiver groups to the recording truck.

Template The collection of active receiver stations plus the associated source points.

Time slice A map of any seismic attribute at the constnt two-way traveltime.

Time structure map A map of a particular reflector in two-way traveltime.

Time-depth function For a given point (particularly for a well), a set of two-way traveltimes and

their equivalent depths (true vertical depths), or the mathematical function which approximates

such a set of time-depth pairs.

Time-lapse survey The repeated acquisition of the same survey area as a tool in seismic reservoir

monitoring; also called a 4-D survey.

Total nominal fold or full fold The fold calculated for a 3-D survey assuming that all possible

offsets are recorded and used.

Transition zone An area around a water-land boundary in which neither land nor marine

acquisition techniques may be used without special adaptations. Examples include surf zone,

large marshes, small lakes, mangrove swamps.

Umbilical The pressure hose linking the compressor on a vessel to an airgun array.

Uncorrelated record A recorded trace from a vibrator survey in which the input waveform of the

vibrator has not yet been removed from the data.

Undershooting Most common in marine streamer acquisition where two boats are used to obtain

coverage below an obstacle. On land, examples of undershooting include imaging under rivers,

towns, etc.

Unit cell The area defined by two adjacent source lines and two adjacent receiver lines in an

orthogonal geometry. (See box.)

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Vari-sweep A technique for enhancing specific frequency bands by sweeping over narrow

frequency ranges and summing later.

Velocity control point A point in a seismic survey where velocity analysis has been done.

Velocity distribution A list of (time, velocity) pairs for a given location.

Velocity model The description of subsurface properties in terms of velocities and velocity

boundaries.

Vertical hydrophone cable The arrangement of approximately 12 hydrophones strung along a

vertical cable attached to the sea-floor and kept vertical by a buoy.

Vertical resolution The minimum vertical separation that can be resolved in a seismic survey,

expressed either in terms of traveltime or distance.

VHC Vertical hydrophone cable.

Vibrator A seismic source in which the weight of a specially designed heavy vehicle is supported

by a central pad and then hydraulically shaken in a precisely prescribed set of varying

frequencies. Often several vibrators are used together.

Vibroseis A seismic method in which a vibrator is used as the energy source.

VSP Vertical seismic profile. A seismic survey which combines a surface source and downhole

receivers.

Walkaway VSP A VSP with downhole receivers and sources at various offsets from the well.

WARP Wide-angle refraction and reflection profiling.

Wavelength The distance between two similar points on successive waveforms or on a wave

train of a single frequency.

Wavelet A seismic pulse.

Wave number The number of wavelengths per unit of distance.

Weathering layer A zone of low-velocity along the surface.

Well tie The correlation between the seismic interpretation of a particular horizon and the

occurrence of that same horizon in a well as interpreted from well logs.

Wide-angle profiling (warp) A technique using a very large range of source offsets

Wide-azimuth geometry A 3-D survey geometry that has a broad range of azimuths recorded by

most of the receivers. Large aspect ratio (close to square) patches give wide azimuth ranges.

WSP Well seismic profile. A better alternative name for VSP because wells are not always vertical

and Walkaway VSP is a contradiction in terms.

Xmax The continuous maximum offset recorded in a particular 3-D design.

Xmin The largest minimum of offsets recorded for most templates in a particular 3-D design. The

magnitude of Xmin directly influences how well shallow reflectors can be imaged.

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Xmute The mute distance for a particular reflector. Any traces beyond this distance do not

contribute to the stack at the reflector depth. Xmute varies with two-way traveltime.

Zero offset When a receiver and source are coincident i.e. there is no horizontal distance

between them.

Zig-zag geometry A 3-D geometry in which the source points follow a zig-zag pattern between

each pair of adjacent receiver lines.

Zipper design A 3-D layout strategy for large surveys which uses overlapping swaths.

Zone of influence The area around a reflection point within which interference occurs. The size of

this area depends on the length of the source wavelet. Not to be confused with the (first) Fresnel

zone.

Zone of interest The range of traveltimes or depths that encompasses the prospective horizons.

Documents

Ovl Qc Manual-2012