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Seismic Reflection
Speakers:
-Atok Yuliantono -Intan Dewi Meutia SariMuslihudin
-Rizky Gustiansyah -Intan Widya
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OUTLINE
1 Introduction
2
Acquisition
3 Processing
4 Interpretation
5 Case Study
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Introduction
Seismic Reflection is a method of
exploration geophysics that uses the
principles of seismology to estimatethe properties of the Earth's subsurface
from reflected seismic waves.
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Introduction
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Basic Concept
Fermats Principle
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Basic Concept
Huygens Principle
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Basic Concept
Snells Law
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Medium earth consists of several layers of rock, whichis between the layers of rock with another rock layers
can be different density and wave speed response.
According to Snell's law, can seismic waves change
direction when passing through the boundary between
the layers because of refraction and reflection.
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Acqusition
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Planning
Select and describe primary and secondary targets.
Estimate potential production and profits.
Budget acquisition costs.
Specify and document program objectives and priorities.
Establish data quality standards. Set reasonable schedules and deadlines.
Locate desired lines of survey on maps (survey design).
Select specific methods and equipment to be used.
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Permitting
Determine who all these owners are
Gain permission for seismic work for them, and
Communicate to the field crew any restrictions imposed by the
owners.
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Acquisition Requirements
Surveying/navigation system: precise locations of source and
receiver positions must be known.
Energy sources: all about appropriate amplitudes & frequency
spectra.
Receivers
Cables
Recording system
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Energy source
Desirable characteristics of seismic sources include:
Signal high amplitude, broad frequency bandwidth produced
Safety hazard in use, storage and maintenance can be
managed without excessive precautions
Cost total cost of equipment
Operation relatively simple, efficient, and fast operation
generally preferred
Environment minimal physical and biological damage to the
surroundings.
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Energy Sources
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Energy Sources
Explosive
Most often loaded at the bottom of a drilled holes to avoid
the low velocity zone.
Those holes are drilled in a geometrical patter or array to
enhances the signal and attenuates surface waves at the
source.
The charge is usually dynamite or ammonium nitrate
fertilizer mixed with diesel fuel.
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Energy Source
Explosive
Principal advantage:
o produce high energy and a broadband signal.
o A direct measure of time through low-velocity zone
can be obtained when the explosives are shot in
drilled holes.
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Energy Sources
Explosives
Disadvantages:
o Much energy lost in blow.
o Produce high amplitude horizontal noise
o
Expensiveo Strict safety regulations are imposed and tight security
is required.
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Energy Sources
Vibrator
A vehicle that uses hydraulic energy to produce a signal.
Usually using 2 to 4 vibrator trucks are positioned at source
points within source array.
Vibrators allow the selection of signals frequency content.
Available frequencies range from 5 Hz to 511 Hz.
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Seismic Receivers
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Seismic Array
A group of two or more elements (source or receivers)
arranged in a geometrical patter.
The function is to do spatial filtering.
An array response depends upon wavelength or wavenumber
of seismic energy produced or received.
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Seismic Array
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Acquisition Method
2-D Acquisition Method
3-D Acquisition Method
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2-D Acquisition Method (Land)
Line configuration (depends on target depth)
Off end spread
Pulling the spread
Pushing the spread
Split spread Symmetrical
Asymmetrical
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2-D Acquisition Method (Marine)
Using off end spread with pulling the
spread movement.
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2-D Acquisition Method (Marine)
In 2005, In 2005, Ocean Bottom
Nodes/Seismic (OBN / OBS) - an
extension of the OBC method that uses
battery-powered cableless receivers
placed in deep water.
This method is called Ocean Bottom cablesystem.
Usually used in shallow marine & transition
zone (
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General Acquisition Parameter
Line parameters
Number and orientation of lines
Line spacing
Line length
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General Acquisition Parameter
Source parameters
For explosives
Size (e.g., pounds of dynamite)
Number of holes
Hole depth
Pattern
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General Acquisition Parameter
Source parameters
For vibrators
Number and layout of source positions per source point
Number of units
Sweep type
Number of sweeps
Sweep length
Initial and final frequencies
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General Acquisition Parameter
Source parameter
For airguns
Number and sizes of guns
Array designs
Number of arrays
Depth at which array is towed
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General Acquisition Parameter
Spread parameters
Spread types
Off end
Source pulling or pushing spread
Split spread Gap
Symmetric or assymetric
Number of groups
Group interval Maximum and minimum offsets
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General Acquisition Parameter
Fold
Each spread provides spread of subsurface coverage.
Moving the spread spread length between shots thus
provides continuous coverage of the subsurface below the
line.
Common depth point (CDP) and CMP concept.
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3-D Acquisition Method
Because of the shortcoming of 2-D, such as:
1. Distortion of the image of geologic structure
2. Inadequate subsurface sampling to define small-scale
geologic features
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3-D Acquisition Method
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3-D Acquisition Method
Procedure (Based on Figure 4.41):
1. Eight receiver lines are laid but only six are active at a time.
This total length of the six lines is called a swath.
2. Patch is the receiver groups used for the active source.
3. The patch and source are moved up along the active
swath.
4. When the first swath is completed, one or more receiver
lines are moved laterally (rolled) such that there is overlap
in surface and /or subsurface coverage.
5. This continues until all sources have been shot and the
entire survey area covered.
3 D A i i i M h d
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3-D Acquisition Method
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Processing
S i i D t P i
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Seismic Data Processing
Is to process the raw seismic data whichreceived from field seismic acquisition to extract
a good quality and quantity final product as an
input to interpretation step on the line of seismic
exploration.
Fl Ch t
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Flow Chart
DEMULTIPLEX
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DEMULTIPLEX
REARRANGE DATA FROM FIELD TO PROCESSING
ORDER.
CONVERT FROM FIELD FORMAT (MANY, VARIABLE) TO
INTERNAL FORMAT.
PROVIDE FIRST LOOK AT THE RAW DATA.
G t
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Geometry
G t S ifi ti
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Geometry Specification
Source Geometry
Geometry Specification
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Geometry Specification
Receiver Geometry
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Data Editing
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Data Editing
BAD RECORDS
BAD TRACES
- ISOLATED, RANDOM
- NOISY GEOPHONE GROUPS
- MISSING GROUPS (ENDS OF LINES)
NOISY TIME ZONES
- SPIKES
- NOISE BURSTS
Editing Option
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Editing Option
Killing Traces
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Killing Traces
before after
Top Mute
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Top Mute
before after
Datum Correction
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Datum Correction
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Gain (Amplitude Variations)
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Gain (Amplitude Variations)
Geophone Output (Ungained Recorder Trace)
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Geophone Output (Ungained Recorder Trace)
Common Gain Problems
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Common Gain Problems
Shot Records
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Shot Records
Objectives of Gain
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Objectives of Gain
(AGC or TRACEWISE BALANCE)
- Best continuity
- Events visable at all times
- Bright spots visible
- Amplitudes proportional to reflectioncoefficients
Signal and Noise
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Signal and Noise
NOISE IS WHAT DO NOT WANT
NOISE IS WHAT IS NOT IN THE MODEL
SIGNAL IS WHAT WE DO WANT
SIGNAL IS DESCRIBED BY THE MODEL
Split- Spread Field Record
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Split Spread Field Record
Types of Noise
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Types of Noise
Causes of Poor Signal to Noise
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Causes of Poor Signal to Noise
NON-OPTIMAL FIELD PROCEDURES
STRONG COHERENT NOISE
SCATTERING OR ABSORPTION
NEAR-SURFACE PROBLEMS
IMPROPER PROCESSING (STACK)
Filtering
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Filtering
Types of Filter
Bandpass Filters
Deconvolution
Wave shaping
F-K Filters
Bandpass Filter
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a dpass e
Spectrum Amplitude after Bandpass Filter
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p p p
One Dimensional Filter
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Two Dimensional Filter (FK Filter)
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( )
FK Filter
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before after
Deconvolution
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NMO
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NMO : Natural curvature of reflection events onField records & CMP gathers
NMO Corrections : Time shifts in the computer to
change the curvature (to flat)
NORMAL MOVEOUT
The variation in reflection arrival time with offset distance between
source and receiver.
MOVEOUT AT OFFSET X:
Surface
Reflector
S R
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WITHOUT NMO CORRECTION WITH NMO CORRECTION
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Picking Velocity
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Residual Static
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elevation static correction put the shot pointand geophone at the same datum level so that
the influence of different elevation can be
eliminated.
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CMP GATHERS BEFORE AFTER RESIDUAL STATICS
Stacking
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Stacking is the sum of traces in one gather that aims to
enhance the signal to noise ratio (S / N).
STACK BEFORE & AFTER RESIDUAL STATICS
Migration
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Migration is to move the position of the visible reflectors on
seismic data recorded into the actual position according to the
position below the surface.
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Interpretation
Flow ChartData Collecting &
Verification- Regional Geology
- Bouguer Map
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Loading Seismic and Wells Data
Loading Supporting Data(Checkshot, Formation Market, etc.)
Seismic Well Tie
Seismic Interpretation /Fault Reconstruction
Picking Horizon
Structural Map, Isopach
& Isochron
Lead & Prospect
Risk Analysis
Iteration
Y/N
Reflection Pattern
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Reflection Pattern
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The purpose of interpretation is to obtain depth map (structural
map) of The surveyed area. We can divide interpretation into two parts
The interpretation ofstructure using the geometry of the beds
The interpretation oflithology using seismic signatures and
seismic attributes.
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Structural interpretation is relatively straightforward and is
largely visual.
the internal geometry of layered strata is revealed
sediment packages can be identified
erosion surfaces can be identified channelling can be identified
We must remember the various scale distortions that may exist
in a seismic record.
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Offshore sparker survey
timescale lines 40ms apart.
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It is possible to estimate the lithology (sediment type) from a
seismic record, although this is less precise than determiningthe structure.
The key is the seismic signature of the material. This is the
internal appearance of a bed, arising from the composite effect
of numerous small reflectors within it.
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A key issue concerns the sound source, since this influences
the signature as well as does the sediment type. The signatures obtained in marine surveys in particular are
very sensitive to the sound source in use.
Thus, in a given material, a boomer may produce a different
signature from a sparker. This is due to the differing frequencyspectra and resolving power of the two sources.
This is less of a problem in terrestrial surveys since the higher
frequencies (=details) are usually lost.
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92
False layering produced by non-lithological features
These signatures are both
from identical lithologies
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Signatures are broadly characteristic of the parent materials
(with the above proviso). This leads to the idea of a seismicfacies.
A seismic facies is a unit of sediment that has a consistent
seismic appearance. It is often assumed that this implies a
consistent lithology. The full geophone record can be analysed statistically as a
time series to obtain eg its frequency content, average
amplitude, autocorrelation etc.
These are known as seismic attributes and can becharacteristic of particular layers.
93
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Acoustic Impedance & Reflection Coefficient
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Acoustic Impedance & Reflection Coefficient
VV
VV
AIAI
AIAI
122
1122
12
12
RC
AI1
AI 2
AI 3
AI 4
RC1
RC2
RC3
AI = V
Model Seismic Responses - Input
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Courtesy of ExxonMobil
10%
Porosi ty
Gas
Oil
Br ine
20%
Porosi ty
30%
Porosi ty
Model Seismic Responses - Output
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Courtesy of ExxonMobil
10% Po rosi ty
Offset OffsetOffset
30% Po rosi ty20% Poro si ty
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Synrift Example
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y p
Could you tell me where is Fluvial Environment ?
Could you tell me where is Deltaic Environment ?
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Could You Tell Me Where is Fluvial Environment (Braided Stream, Fan DelCould You Tell Me Where is Deltaic Environment ?
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Marine? Marine?
DELTAIC
FLUVIAL
TRANSITION
Lacustrine?
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Courtesy of ExxonMobil
Seismic DHI
s are anomalous seismic responses related to the presenceof hydrocarbons
Acoustic impedance of a porous rock decreases as hydrocarbon replacesbrine in pore spaces of the rock, causing a seismic anomaly (DHI)
There are a number of DHI signatures; we will look at a few common
ones: Amplitude anomaly Fluid contact reflection Fit to structural contours
DHI=DirectHydrocarbonIndicator
DHIs: Amplitude Anomalies
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Courtesy of ExxonMobil
High AmplitudeLow
Change in amplitudealong the reflector
Anomalous amplitudes
DHIs: Fluid Contacts
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L12 Data Analysis Courtesy of ExxonMobil
Hydrocarbons are
lighter than waterand tend to form flat
events at the gas/oil
contact and the
oil/water contact.
Thicker Reservoir
Fluid contactevent
Fluid contactevent
Thinner Reservoir
DHIs: Fit to Structure
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L12 Data Analysis Courtesy of ExxonMobil
Since hydrocarbons are
lighter than water, the
fluid contacts and
associated anomalous
seismic events are
generally flat in depth
and therefore conform
to structure, i.e., mimic
a contour line
Intro to Exercise
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Goal: To map the extent of the A1 gas-filled reservoir
Courtesy of ExxonMobilFigure 1Inline 840
A1 Gas
Sand
W E
Changes in Amplitude Indicate Fluid
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L12 Data Analysis Courtesy of ExxonMobil
Inline 840 Figure 1
Gas SandWater Sand
Traces areclipped
Fluids within the A1 Sand
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L12 Data Analysis Courtesy of ExxonMobil
Inline 840 Figure 1
Extent of Gas
References
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Brown, A.R. 2004. Interpretation of Three Dimensional
Seismic Data. AAPG Memoir 42 SEG Investigations inGeophysics. Tulsa
Munadi, S. 2000.Aspek Fisis Seismologi Eksplorasi. Program
Studi Geofisika UI. Depok.
Munadi, S., D. Rubyanto dan B. Triharjanto. 1995. ResolusiSeismik. Lembaran Publikasi Lemigas No.2. Jakarta.
Russell, B. H. 1991, Introduction to Seismic Inversion Methods,
S.N. Domenico. Editor Course Notes Series. Volume 2 3rd
edition. USA.
References
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Sismanto. 1996. Modul 1: Akuisisi Data Seismik.Laboratorium Geofisika UGM. Yogyakarta.
Sismanto. 2006. Dasar Dasar Akuisisi dan Pemrosesan
Data Seismik. Laboratorium Geofisika UGM. Yogyakarta.
Sukmono, S. dan A. Abdullah. 2001. KarakteristikReservoar Seismik. Lab. Geofisika Reservoar Teknik
Geofisika ITB. Bandung.
Umam, M. S. 2004. Seismic Interpretation in Petroleum
Exploration and Production. Course by Chevron. Pekanbaru.