01 Report

E- GEOTECHNICAL FACTUAL REPORT

24 August 2007. ISSUED FOR BID Strategic Storage of Crude Oil at Visakhapatnam Project 2158005;

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e 20

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12 CHAPTER 3: GEOPHYSICAL STUDY

INDIAN STRATEGIC PETROLEUM

RESERVES LTD. (ISPRL)

Report on

GEOPHYSICAL INVESTIGATION WORK FOR

STRATEGIC STORAGE OF CRUDE OIL IN

UNDERGROUND ROCK CAVERN PROJECT AT

VISHAKHAPATNAM

August 2007

RITES (A Govt. of India Enterprise)

RITES Bhawan No. 1, Sector 29, Gurgaon-122001

CONTENTS

ABSTRACT 1 1.0 INTRODUCTION 2 2.0 PROJECT AREA 2 3.0 SITE CONDITIONS AND ACCESSIBILITY 3 4.0 GEOLOGICAL SETTING 3 5.0 SCOPE OF WORK 4 6.0 SEISMIC REFRACTION SURVEY 6

6.1 Basic Principle of Seismic Refraction Method 6

6.2 Methodology. 8

6.3 Interpretation of Seismic Refraction Method. 10

6.4 Straitigraphy as per Seismic Refraction Results. 10

6.5 Limitations of Seismic Refraction Method. 18

7.0 RESISTIVITY SURVEY 19 7.1 Basic Principle of Electrical Resistivity Survey. 19

7.2 Traditional Resistivity Surveys. 20

7.3 Relationship between Geology and Resistivity. 23 8.0 2-D ELECTRICAL IMAGING SURVEY 25

8.1 Introduction. 25

8.2 Field Survey Method, Instrumentation and Measurement 26

Procedures.

8.3 Pseudosection Data Plotting Method. 27

8.4 Dipole-Dipole Array. 28

8.5 Inversion Method. 29

8.6 Data Processing and Interpretation. 31

8.6.1 Data Processing. 31

8.6.2 Interpretation of Resistivity Imaging. 31

9.0 CORRELATION 35 10.0 SUMMARY AND CONCLUSION 36

ABSTRACT In July 2007 seismic refraction surveys and resistivity profiling were performed

by RITES Ltd. on Yerada Hills and Lova garden at Vishakhapatnam for

ISPRL. The aim of the investigations was to obtain the quantitative knowledge

of the rock condition for geotechnical study being conducted for detailed

project report for the proposed additional and extended cavern for

underground oil storage tank. The planned contribution of geophysical

investigations was to measure seismic velocities, true resistivity of

overburden, weathered and hard rock interface with its thickness and to

identify any possible anomalous zone in basement rock. The interpretation of

the seismic and resistivity data has been carried out with special attention to

find out any low velocity or resistivity zone in the basement rock along the

lines.

The seismic refraction survey was carried out along eight (8) profiles covering

a total length of 2265m and the resistivity survey was also carried out along

these seismic profiles covering a total length of 2180 m.

The seismic and resistivity data was of good quality and provided a detail

insight about the overburden and basement rock condition. In general four-

layer model was established based on geophysical data. Compressional wave

velocities were measured to know the thickness of different layer and the

conditions of basement layer and the resistivity profiling were carried to know

the lateral variation of resistivity of the subsurface material.

1.0 INTRODUCTION

Indian Strategic Petroleum Reverses Limited awarded the work

through Engineers India Limited, New Delhi for Geophysical

Investigation works for the Strategic Storage of Crude Oil in

underground rock cavern project at Vishakhapatnam to RITES vide

letter no. ISPRL/EIL/SGT-VIZAG dated 22.06.2007. Surface

geophysical investigations including Seismic Refraction and Electrical

Resistivity Imaging survey have been conducted to ascertain the depth

of weathered layer, depth of bedrock and delineate the structural

discontinuities in the area where the underground rock cavern to be

proposed.

The purpose and objective of the survey are:

1. To establish the nature & thickness of overburden

2. To obtain the bedrock profile & interfaces of different geological

strata.

3. To identify zones such as faults, fractures and extent of weathering

zones in basement rock,

4. To assess the geological setting of the area including ground water

levels.

2.0 PROJECT AREA

The project site is only 1 km south of the Visakhapatnam harbour

entrance channel and immediately west of the so called “ dolphins

Nose” with approximate position being North 17˚ 41’ and East 83˚ 17’.

The proposed area is an E-W striking hill range that terminates along

the shoreline close to the crude oil jetty. The ground elevation varies

from +10m to + 125m. The surrounding hill sides are relatively steep,

reaching up to an elevation of approx. +150m. The inner part of the

“site valley” is only approx. 50m wide and the elevation of the natural

bottom is located at approx. +20m. The outer part of the “site valley” is

approx. 100m wide and has a natural inclination of approx. 1:10, from

approx. +10m at the entrance to an elevation of up to approx. +25m at

the middle of the valley. The present investigations are to be carried

out in the valley area.

3.0 SITE CONDITIONS AND ACCESSISBILITY:

The alignments of the proposed seismic lines are located on the top of

the Yerada Hills. The area was covered with dense bushes and jungle.

Hence the seismic lines were not accessible without clearing of

bushes. Access to the line locations was made through dense bushes

and lines were cleared for planting of geophones, shots and laying

down geophone and source cables. The preparation of seismic lines

was extremely difficult and time consuming though bushes. It took lot of

time for the field crew to start acquisition work. It took almost one week

to complete line preparation. The lines were only available to the end

points of the lines. To do far shots additional line cutting was arranged

on either sides of the lines. In most of the cases the far off clearance

restricted far shots within limited distances and in some cases far shots

could not be made to the required distances.

4.0 GEOLOGICAL SETTING

The area under investigation represents the Eastern Ghats of India

generally covered with Granulite grade rocks, which are further

classified as Khondalite group, Charnockite group and Leptynites.

Khondalite group of rocks occupies the major part of the area and is

dominantly made up of Garnet Sillimanite gneiss with minor bands of

quartzite, and calc granulites.

Charnockite group of rock generally consists of hypersthene bearing

gneisses with mafic granulites.

The Khondalites represent the major hill ranges while the garnet biotite

Gneisses (Leptynites) occurs as low lying mounds.

5.0 SCOPE OF WORK

The purpose of geophysical investigations using seismic refraction

profiling and resistivity profiling was to define the subsurface conditions

of the rocks upto a depth of 60 m. The scope of work includes 8 Nos. of

seismic refraction profiles and 8 Nos. of resistivity profiles covering

total lengths of 2000 m each, almost in grid pattern for better

appreciation of bedrock configuration. However, as per the actual

ground coverage by seismic and resistivity profiles were 2265 m &

2180 m respectively. The end coordinates of seismic and resistivity

profiles are given in the Table 1a & 1b and profile locations are given in

figs. 1a & 1b. Total coverage made along the above traverse by

seismic profiling is 2265 m and by resistivity profiling is 2180 m.

Table 1a: End Coordinates of Seismic Profiles

Coordinates Start Point End Point

Line name

E N E N

Proposed length (m)

Actual length (m)

Line E1-W1 1599 846 1273 700 357.00 370.00 Line E2-W2 1623 789 1327 657 357.00 345.00 Line E3-W3 1644 739 1317 594 357.00 380.00 Line S1-N1 1645 743 1562 929 115.00 235.00 Line S2-N2 1571 688 1491 868 115.00 230.00 Line S3-N3 1488 657 1404 842 115.00 235.00 Line S4-N4 1426 589 1332 785 115.00 235.00 Line S5-N5 1342 540 1248 754 115.00 235.00 Total Coverage 2265.00 Table 1b: End Coordinates of Resistivity Profiles

Coordinates Start Point End Point

Line name

E N E N

Proposed length (m)

Actual length (m)

Line W1- E1 1296 709 1599 846 357.00 355.00 Line W2- E2 1319 656 1623 789 357.00 355.00 Line W3- E3 1330 600 1644 739 357.00 355.00 Line S1-N1 1645 743 1562 929 115.00 235.00 Line S2-N2 1571 688 1491 868 115.00 235.00 Line S3-N3 1488 657 1425 796 115.00 175.00 Line S4-N4 1426 589 1332 785 115.00 235.00 Line S5-N5 1342 540 1248 754 115.00 235.00 Total Coverage 2180.00

6.0 SEISMIC REFRACTION SURVEY: 6.1 Basic Principle of Seismic Refraction method

Seismic refraction survey is one of the important tools in the family of

exploration geophysics. Seismic investigations utilize the fact that

elastic waves (also called seismic waves) travel with different velocities

in different rocks. By generating seismic waves at a point and

observing the time of arrival of these waves at a number of other points

on the surface of the earth, it is possible to determine the velocity

distribution and locate the subsurface interfaces where the waves are

reflected or refracted. The underlying theory of seismic refraction

survey is that whenever a seismic wave impinges on the boundary

separating two media, energy is partly reflected and partly transmitted.

Hence, by choosing the refracted arrivals alone, we can relate the

delay in the arrival times of refracted seismic waves at different

locations to a lateral or transverse variation in the velocity of different

subsurface layers.

Figure-2a: Diagram showing theory of seismic refraction survey

Normally, seismic wave velocities increase with depth, and hence

travel-time plot of arrival of seismic waves in an array of sensors

(geophones) spread linearly will show the presence of various layers

based on the chainage in slope of different segment of the first arrival

time plot. Seismic waves generated by a hammer blow or shot travels

through material medium and is recorded by an array of sensors

spread along the profile line. We record the first arrival time in different

sensors from which the velocity of the two layers involved in the

refraction as well as the depth to this refracting layer are determined.

Seismic (P-Wave) velocity of materials relates to the strength

properties and the degree of weathering and joint sets available in-situ.

This defines rocks in various sub-categories such as Hard, Weathered

and Soft in terms of range of P-wave velocities in them. Therefore, a

comprehensive knowledge of the seismic velocities in different medium

is basis of the interpretation of seismic survey data. In fact, the entire

stratigraphy of the area as deciphered from the seismic refraction

survey is a velocity imaging of the area. Later this variation in velocity

is correlated to the local geology by using standard table of seismic

wave velocities in different geologic medium in dry and wet conditions,

as shown in figure-2b or with a prior information of rock types (local

geology) or based on laboratory investigations of the core samples.

Figure 2b: Seismic wave velocity in different geological materials

6.2 Methodology:

The seismic refraction survey covering a total length of 2265.00 m

were conducted on the proposed site. The survey lines were marked

on the ground in grid pattern as shown in fig. 1a. Seismic refraction

survey has been carried out in the Project area to determine:

• Overburden and bed rock configuration

• Thickness of the overburden

• Compressional wave velocity for soil and bed rock

The equipment used in the SRS method was 48-Channel Geometrics

made digital seismograph, which are manufactured by Geometrics Inc.

USA. This is a high-resolution digital seismograph with facility of data

stacking, frequency filtering and various other digital signal processing

capabilities available on-line for optional selection of data acquisition

parameters. 10 Hz vertical geophones were used as sensors. The

geophones were hooked on to the acquisition unit through specially

provided multiple take-out cables. 65 Kg SPT hammer as weight drop

and 5 kg. Sledgehammer was used as seismic source. Signal at each

shot point was stacked 10-20 times to improve signal to noise ratio.

The field set-up of the present seismic refraction survey made use of

the following parameters:

• Data acquisition unit : Geometrics Strata Visor NZ

• No. of channels : 48

• Source type used : Sledge hammer (7.5 kg)

• Trigger mode : Trigger Switch

• Channel spacing : 5m

• Sampling interval : 250 msec.

• No. of stacks : 10-20 (cumulative)

• Recording format : SEG 2

• Operating software : SeisImager

• Display type : Monochrome in wiggle or other

formats

• Data processing : Computer controlled software.

Seven sets of shots were gathered for each seismic spread, three in

the forward, three in the reverse, one center shot in between the profile

at various positions. The far offset shots were recorded at a distance of

30 to 50 m either side of the spread.

6.3 Interpretation of seismic refraction survey Results The seismic data was of good quality. Analysis of the seismic data was

carried out by establishing an initial model using time intercept time

method. Which was further refined by inversion technique with the help

of PLOTREFA software. The seismic sections thus obtained were

interpreted in terms of geological cross sections along each seismic

line. The interpretation of the seismic data is presented in the form of

seismic profiles along each line. Fig 5a to 5h.

6.4 Stratigraphy as per the Seismic Survey Results Seismic wave velocity in soil and rock is dependent on the soil type

and its condition. For rocks degree of weathering, jointing, fracturing

etc., are important. Based on the observed velocities in the surveyed

area the stratigraphy can be stated as follows:

Seismic sections give the information about the subsurface stratigraphy

in terms of their seismic velocities, which are directly related to the

quality and the strength of the medium. As a representative exercise in

this report, following seismic velocity classification is used for indexing

subsurface strata with different layer properties as given in table 2.

Following bar chart in Figure 4 suggests the range of seismic velocity

for various types of soil and rock under unsaturated and saturated

conditions.

Table 2: Classification of Subsurface Strata in Terms of Seismic wave

velocity.

Subsurface Strata Seismic Velocities (m/sec)

Overburden material comprises of loose top soil

with completely weathered rock altered into

residual soil (Velocity range between 1400 to

1500 m/sec may possibly be the saturated zone)

500-1500

Weathered rock (lower velocity indicates higher

degree of weathering, whereas the higher velocity

indicates lower degree of weathering)

1500-3000

Jointed Rock Mass 3000-3500

Hard/Massive Rock (Khondalite) >3500

These layers might not reflect a change in the geologic medium or a

change in subsurface rock type, nevertheless, they represent a

significant change in the engineering properties of the rock mass.

Seismic velocities in the rock mass can be correlated to other

engineering properties by specific empirical relationship. By using

these empirical relations, Q-value of the strata encountered can be

assessed. In order to have a preliminary assessment of the

tunneling/cavern media, a reference is made to the expected Q-value

of the rock type. This is based on an empirical relationship (Barton et

al, 1993) used extensively in civil engineering practice as shown in

table-3.

Table 3: Empirical relationship between Seismic Wave Velocity Vp and

Q-Value. Vp in

m/sec

1500 2500 3500 4500 5500 6500

Q-Value 0.01 0.1 1.0 10.0 100.0 1000.0

This parameter is a vital input for design consideration for any

subsurface excavation and is widely used as a correlation tool with

seismic refraction survey.

The site stratigraphy as deciphered from the seismic refraction survey

are summarized in Table 4 given below

Table 4: Stratigraphy of the site as per the Seismic Refraction survey

Seismic section along E1-W1:

This profile was carried out northern side of the valley. The area under

investigation is mainly comprises of three layer model. On the top

overburden comprising of residual soil having seismic velocity of the

order of 569 m/sec and varying thickness between 0.6 to 1.9 m along

the profile. This is followed by a layer comprising of saturated soil

having seismic velocity of the order of 1437 m/sec. This is followed by

a layer of Jointed rock mass strata having seismic velocity of the order

of 2600 m/sec. This is followed by massive khondalite having seismic

velocity of the order of 4040 m/sec which further increase with depth.

Depth of weathering profile in bedrock and the depth of basement is

shown in the corresponding seismic section. No anomalous zone has

been observed in the basement rock along this profile. The

interpretation of this profile is given in tabular form in Table 5a and the

Seismic section is in Fig. 5a.

Seismic section along E2-W2: This profile was carried out on southern side of the valley. The area

under investigation is mainly comprises of three layer model. On the

top overburden comprising of residual soil having seismic velocity of

the order of 409 m/sec and varying thickness between 0.0 to 3.3 m

along the profile. This is followed by a layer comprising of saturated soil


a layer of moderately weathered strata having seismic velocity of the

order of 2300 m/sec. This is followed by Jointed khondalite having

Seismic Velocity (m/sec) Interpreted Lithology

400 – 600 Residual Soil Overburden

1350-1500 Saturated overburden

1500 – 3000 Weathered rock mass

3000-3500 Jointed rock mass

≥3000 Massive Khondalite

seismic velocity of the order of 3500 m/sec which further increase with

depth representing massive khondalite having seismic velocity 4950

m/sec. Depth of weathering profile in bedrock and the depth of

basement is shown in the corresponding seismic section. No

anomalous zone has been observed in the basement rock along this

profile. The interpretation of this profile is given in tabular form in Table

5b and the Seismic section is in Fig. 5b.

Seismic section along E3-W3: This profile was carried out on southern side of the valley further

southward of E2-W2. The area under investigation is broadly

comprises three to four layer model. On the top overburden comprising

of residual soil having seismic velocity of the order of 444 m/sec and

varying thickness between 1.8 to 5.4 m along the profile. This is

followed by a layer comprising of saturated soil having seismic velocity

of the order of 1400 m/sec. This is followed by a layer of moderately

weathered strata having seismic velocity of the order of 2350 m/sec.

This is followed by jointed khondalite having seismic velocity of the

order of 3500 m/sec. Depth of weathering profile in bedrock and the

depth of basement is shown in the corresponding seismic section. No



5c and the Seismic section is in Fig. 5c

Seismic section along S1-N1: This profile was carried out on the eastern side across the valley. The

area under investigation is broadly comprises three to four layer model.

On the top overburden comprising of residual soil having seismic

velocity of the order of 400m/sec and varying thickness between 1.3 to

5.8 m along the profile. This is followed by a layer comprising of

saturated soil having seismic velocity of the order of 1400 m/sec. This

is followed by a layer of moderately weathered strata having seismic

velocity of the order of 2250 m/sec. This is followed by Jointed

khondalite having seismic velocity of the order of 3500 m/sec which

further increase with depth representing massive khondalite having

seismic velocity 4633 m/sec. Depth of weathering profile in bedrock

and the depth of basement is shown in the corresponding seismic

section. No anomalous zone has been observed in the basement rock

along this profile. The interpretation of this profile is given in tabular

form in Table 5d and the Seismic section is in Fig. 5d.

Seismic section along S2-N2: This profile was carried out westward of S1-N1 across the valley. The




5.3 m along the profile. This is followed by a layer comprising of

saturated soil having seismic velocity of the order of 1350 m/sec. This

is followed by a layer of moderately weathered strata having seismic

velocity of the order of 2250 m/sec. This is followed by massive

khondalite having seismic velocity of the order of 3500 m/sec which

further increases with depth. Depth of weathering profile in bedrock

and the depth of basement is shown in the corresponding seismic

section. No anomalous zone has been observed in the basement rock

along this profile. The interpretation of this profile is given in tabular

form in Table 5e and the Seismic section is in Fig. 5e.





5.3 m along the profile. This is followed by a layer of moderately

weathered strata having seismic velocity of the order of 2497 m/sec.

This is followed by massive khondalite having seismic velocity of the

order of 4223 m/sec. Depth of weathering profile in bedrock and the

depth of basement is shown in the corresponding seismic section. No



5f and the Seismic section is in Fig. 5f.





1.2 m along the profile. This is followed by saturated overburden

having seismic velocity of the order of 1400 m/sec. This is underlain by

a highly weathered layer having seismic velocity of the order of 1900

m/sec. This is followed by a layer of moderately weathered strata


Jointed khondalite having seismic velocity of the order of 3500 m/sec.




interpretation of this profile is given in tabular form in Table 5g and the

Seismic section is in Fig. 5g.





1.0 m along the profile. This is followed by saturated overburden

having seismic velocity of the order of 1300 m/sec. This is underlain by

a highly weathered layer having seismic velocity of the order of 1900

m/sec. This is followed by a layer of moderately weathered strata


massive khondalite having seismic velocity of the order of 3600 m/sec.




interpretation of this profile is given in tabular form in Table 5h and the

Seismic section is in Fig. 5h.

6.5 Limitations of Refraction Method In the seismic sections various refracting layers are identified based on

the change in seismic velocity of the strata. Surface relief should be

properly surveyed at each source and receiver location and should be

properly fed at the data processing stage for correct interpretation. The

errors in surface relief used at the processing stage will cause multifold

error in the subsurface position. This is particularly important while

surveying in a hilly terrain. For 5 meter geophone spacing used in data

collection, it is highly likely that layers lesser than 1m thicknesses might

not be identified.

In case of hidden zone or blind zone the depth of the subsurface

interfaces would either be over estimated or underestimated. In such

cases depth of subsurface interfaces would be corroborated with

borehole data.

The errors in subsurface relief at source and receiver locations might

restrict the accuracy of the depths to various horizons within 10%,

but with digital data recording and computerizes data processing

combined with errors in surface relief within 0.1 meter would pegged

down the accuracy within 5%.

7.0 RESISTIVITY SURVEYS 7.1 Basic Principle of Resistivity Survey

The purpose of electrical surveys is to determine the subsurface

resistivity distribution by making measurements on the ground surface.

From these measurements, the true resistivity of the subsurface can be

estimated. The ground resistivity is related to various geological

parameters such as the mineral and fluid content, porosity and degree

of water saturation in the rock. Electrical resistivity surveys have been

used for many decades in hydrogeological, mining and geotechnical

investigations. More recently, it has been used for environmental

surveys.

The resistivity measurements are normally made by injecting current

into the ground through two current electrodes (C1 and C2 as in fig. 6),

and measuring the resulting voltage difference at two potential

electrodes (P1 and P2). From the current (I) and voltage (V) values, an

apparent resistivity (pa) value is calculated.

ρa = k V / I

Where k is the geometric factor which depends on the arrangement of

the four electrodes. Figure 3 shows the common arrays used in

resistivity surveys together with their geometric factors.

Resistivity meters normally give a resistance value, R = V/I, so in

practice the apparent resistivity value is calculated by

ρa = k R

The calculated resistivity value is not the true resistivity of the

subsurface, but an “apparent” value, which is the resistivity of a

homogeneous ground, which will give the same resistance value for

the same electrode arrangement. The relationship between the

“apparent” resistivity and the “true” resistivity is a complex relationship.

To determine the true subsurface resistivity, an inversion of the

measured apparent resistivity values using a computer program must

be carried out.

7.2 Traditional Resistivity Surveys The resistivity method has its origin in the 1920’s due to the work of the

Schlumberger brothers. For approximately the next 60 years, for

quantitative interpretation, conventional sounding surveys (Koefoed

1979) were normally used. In this method, the centre point of the

electrode array remains fixed, but the spacing between the electrodes

is increased to obtain more information about the deeper sections of

the subsurface. Conventional method for conducting resistivity survey

is shown in figure-2.

Figure-6: A conventional four-electrode array.

Figure-7: Common arrays used in resistivity surveys and their geometric factors.

The measured apparent resistivity values are normally plotted on a log-

log graph paper. To interpret the data from such a survey, it is normally

assumed that the subsurface consists of horizontal layers. In this case,

the subsurface resistivity changes only with depth, but does not change

in the horizontal direction. A one-dimensional model of the subsurface

is used to interpret the measurements. Despite this limitation, this

method has given useful results for geological situations (such the

water-table) where the one-dimensional model is approximately true.

Another classical survey technique is the profiling method. In this case,

the spacing between the electrodes remains fixed, but the entire array

is moved along a straight line. This gives some information about

lateral changes in the subsurface resistivity, but it cannot detect vertical

changes in the resistivity. Interpretation of data from profiling surveys is

mainly qualitative.

The most severe limitation of the resistivity sounding method is that

horizontal (or lateral) changes in the subsurface resistivity are

commonly found. The ideal situation is rarely found in practice. Lateral

changes in the subsurface resistivity will cause changes in the

apparent resistivity values, which might be, and frequently are,

misinterpreted as changes with depth in the subsurface resistivity. In

many engineering and environmental studies, the subsurface geology

is very complex where the resistivity can change rapidly over short

distances. The resistivity sounding method might not be sufficiently

accurate for such situations.

Despite its obvious limitations, there are two main reasons why 1-D

resistivity sounding surveys are common. The first reason was the lack

of proper field equipment to carry out the more data intensive 2-D and

3-D surveys. The second reason was the lack of practical computer

interpretation tools to handle the more complex 2-D and 3-D models.

However, 2-D and even 3-D electrical surveys are now practical

commercial techniques with the relatively recent development of multi-

electrode resistivity surveying instruments (Griffiths et al. 1990) and

fast computer inversion software (Loke 1994).

Figure-8: Three different models used in the interpretation of resistivity measurements.

7.3 Relationship between Geology and Resistivity Before going for the interpretation of 2-D resistivity surveys, we will

briefly look at the resistivity values of some common rocks, soils and

other materials. Resistivity surveys give a picture of the subsurface

resistivity distribution. To convert the resistivity picture into a geological

picture, some knowledge of typical resistivity values for different types

of subsurface materials and the geology of the area surveyed is

important.

Table 6 gives the resistivity values of common rocks, soil materials and

chemicals (Keller and Frischknecht 1966, Daniels and Alberty 1966).

Igneous and metamorphic rocks typically have high resistivity values.

The resistivity of these rocks is greatly dependent on the degree of

fracturing, and the percentage of the fractures filled with ground water.

Sedimentary rocks, which usually are more porous and have higher

water content, normally have lower resistivity values. Wet soils and

fresh ground water have even lower resistivity values. Clayey soil

normally has a lower resistivity value than sandy soil. However, note

the overlap in the resistivity values of the different classes of rocks and

soils. This is because the resistivity of a particular rock or soil sample

depends on a number of factors such as the porosity, the degree of

water saturation and the concentration of dissolved salts.

The resistivity of ground water varies from 10 to 100 ohm-m.

depending on the concentration of dissolved salts. Note the low

resistivity (about 0.2 ohm-m) of sea water due to the relatively high salt

content. This makes the resistivity method an ideal technique for

mapping the saline and fresh water interface in coastal areas.

The resistivity values of several industrial contaminants are also given

in Table 6. Metals, such as iron, have extremely low resistivity values.

Chemicals, which are strong electrolytes, such as potassium chloride

and sodium chloride, can greatly reduce the resistivity of ground water

to less than 1 ohm-m even at fairly low concentrations. The effect of

weak electrolytes, such as acetic acid, is comparatively smaller.

Hydrocarbons, such as xylene, typically have very high resistivity

values.

Resistivity values have a much larger range compared to other

physical quantities mapped by other geophysical methods. The

resistivity of rocks and soils in a survey area can vary by several orders

of magnitude. In comparison, density values used by gravity surveys

usually change by less than a factor of 2, and seismic velocities usually

do not change by more than a factor of 10. This makes the resistivity

and other electrical or electromagnetic based methods very versatile

geophysical techniques.

Table-6. Resistivities of some common rocks, minerals and chemicals.

Material Conductivity Resistivity (Ohm.m)

Igneous and Metamorphic Rocks

Granite 5x103-106

Basalt 103-106

Slate 6x102-4x107

Marble 102-2.5x108

Quartzite 102-2x108

Sedimentary Rocks

Sandstone 8-4x103

Shale 20 – 2x103

Limestone 50 – 4x102

Soils and waters

Clay 1 - 100

Alluvium 10 -800

Groundwater (fresh) 10 -100

Sea water 0.2

Chemicals

Iron 9.074x10-8

0.01 M Potassium chloride 0.708

0.01 M Sodium chloride 0.843

0.01 M acetic acid 6.13

Xylene 6.998x1016

8.0 2-D ELECTRICAL IMAGING SURVEYS 8.1 Introduction

The greatest limitation of the resistivity sounding method is that it does

not take into account horizontal changes in the subsurface resistivity. A

more accurate model of the subsurface is a two-dimensional (2-D)

model where the resistivity changes in the vertical direction, as well as

in the horizontal direction along the survey line. In this case, it is

assumed that resistivity does not change in the direction that is

perpendicular to the survey line. In many situations, particularly for

surveys over elongated geological bodies, this is a reasonable

assumption. In theory, a 3-D resistivity survey and interpretation model

should be even more accurate. However, at the present time, 2-D

surveys are the most practical economic compromise between

obtaining very accurate results and keeping the survey costs down.

Typical 1-D resistivity sounding surveys usually involve about 10 to 20

readings, while 2-D imaging surveys involve about 100 to 1000

measurements.

In many geological situations, 2-D electrical imaging surveys give

useful results that are complementary to the information obtained by

other geophysical method.

8.2 Field Survey Method - Instrumentation and Measurement Procedure

One of the new developments in recent years is the use of 2-D

electrical imaging/tomography surveys to map areas with moderately

complex geology (Griffiths and Barker 1993). Such surveys are usually

carried out using a large number of electrodes, 72 or more, connected

to a multi-core cable. A laptop microcomputer together with an

electronic switching unit is used to automatically select the relevant

four electrodes for each measurement.

An SYSCAL Imaging system (72-electrodes) from IRIS Instruments

(France) was used for automatic data collection with 72 electrodes

spaced at 10m intervals. Dipole-Dipole array was used for data

acquisition. Before starting data collection by the instrument a

sequence was made using ELECTRE-II software for dipole-dipole

array using 72 electrodes and desired number of datum points to reach

desired depth of investigation which was loaded into the system.

Data acquisition takes place through a mice computer, which is

connected, to the imaging system. This equipment is capable of

running self-checks for connectivity of electrodes and generates

warnings on bad contacts. Bad contacts were resolved by pouring salt

water around the electrode from a water cane.

Normally a constant spacing between adjacent electrodes is used. The

multi-core cable is attached to an electronic switching unit, which is

connected to a laptop computer. The sequence of measurements to

take, the type of array to use and other survey parameters (such the

current to use) is normally entered into a text file which can be read by

a computer program in a laptop computer. After reading the control file,

the computer program then automatically selects the appropriate

electrodes for each measurement. In a typical survey, most of the

fieldwork is in laying out the cable and electrodes. After that, the

measurements are taken automatically and stored in the computer.

8.3 Pseudosection Data Plotting Method

To plot the data from a 2-D imaging survey, the pseudosection

contouring method is normally used. In this case, the horizontal

location of the point is placed at the mid-point of the set of electrodes

used to make that measurement. The vertical location of the plotting

point is placed at a distance, which is proportional to the separation

between the electrodes.

Another method is to place the vertical position of the plotting point at

the median depth of investigation (Edwards 1977), or pseudodepth, of

the electrode array used. The pseudosection plot obtained by

contouring the apparent resistivity values is a convenient means to

display the data. The pseudosection gives a very approximate picture

of the true subsurface resistivity distribution. However the

pseudosection gives a distorted picture of the subsurface because the

shape of the contours depend on the type of array used as well as the

true subsurface resistivity. The pseudosection is useful as a means to

present the measured apparent resistivity values in a pictorial form,

and as an initial guide for further quantitative interpretation. One

common mistake made is to try to use the pseudosection as a final

picture of the true subsurface resistivity.

8.4 Dipole-Dipole Array

Dipole-Dipole array has been considered for this survey due to the fact

that it is very sensitive to horizontal changes in resistivity, this means

that it is good in mapping vertical structures. This array has been

widely used in resistivity/I.P. surveys because of the low E.M. coupling

between the current and potential circuits. The spacing between the

current electrodes pair, C2-C1, is given as “a” which is the same as the

distance between the potential electrodes pair P1-P2. This array has

another factor marked as “n”. This is the ratio of the distance between

the C1 and P1 electrodes to the C2-C1 (or P1-P2) dipole separation

“a”. For surveys with this array, the “a” spacing is initially kept fixed and

the “n” factor is increased from 1 to 2 to 3 until up to about 6 in order to

increase the depth of investigation. The sensitivity function plot in

Figure 8c shows that the largest sensitivity values are located between

the C2- C1 dipole pair, as well as between the P1-P2 pair. This means

that this array is most sensitive to resistivity changes between the

electrodes in each dipole pair. Note that the sensitivity contour pattern

is almost vertical. The median depth of investigation of this array also

depends on the “n” factor, as well as the “a” factor. In general, this

array has a shallower depth of investigation compared to the Wenner

array. However, for 2-D surveys, this array has better horizontal data

coverage than the Wenner.

This means that for the same current, the voltage measured by the

resistivity meter drops by about 200 times when “n” is increased from 1

to 6. One method to overcome this problem is to increase the “a”

spacing between the C1-C2 (and P1-P2) dipole pair to reduce the drop

in the potential when the overall length of the array is increased to

increase the depth of investigation.

8.5 Inversion Method

All inversion methods essentially try to find model for the subsurface

whose response agrees with the measured data. In the cell-based

method used by the RES2DINV and RES3DINV programs, the model

parameters are the resistivity values of the model blocks, while the

data is the measured apparent resistivity values. It is well known that

for the same data set, there is a wide range of models whose

calculated apparent resistivity values agree with the measured values

to the same degree. Besides trying to minimize the difference between

the measured and calculated apparent resistivity values, the inversion

method also attempts to reduce other quantities that will produce

certain desired characteristics in the resulting model. The additional

constrains also help to stabilize the inversion process. The RES2DINV

(and RES3DINV) program uses an iterative method whereby starting

from an initial model, the program tries to find an improved model

whose calculated apparent resistivity values are closer to the

measured values. One well-known iterative inversion method is the

smoothness-constrained method that has the following mathematical

form.

(JTJ + uF)d = JTg - uFr C.1

where F = a smoothing matrix

J = the Jacobian matrix of partial derivatives

r = a vector containing the logarithm of the model resistivity values

u = the damping factor

d = model perturbation vector

g = the discrepancy vector

The discrepancy vector, g, contains the difference between the

calculated and measured apparent resistivity values. The magnitude of

this vector is frequently given as a RMS (root-mean-squared) value.

This is the quantity that the inversion method seeks to reduce in an

attempt to find a better model after each iteration. The model

perturbation vector, d, is the change in the model resistivity values

calculated using the above equation which normally results in an

“improved” model. The above equation tries to minimize a combination

of two quantities, the difference between the calculated and measured

apparent resistivity values as well as the roughness (i.e. the reciprocal

of the model smoothness) of the model resistivity values. The damping

factor, u, controls the weight given to the model smoothness in the

inversion process. The larger the damping factor, the smoother will be

the model but the apparent resistivity RMS error will probably be larger.

The basic smoothness-constrained method as given in equation C.1

can be modified in several ways that might give better results in some

cases. The elements of the smoothing matrix F can be modified such

that vertical (or horizontal) changes in the model resistivity values are

emphasized in the resulting model. In the above equation, all data

points are given the same weight. In some cases, especially for very

noisy data with a small number of bad datum points with unusually high

or low apparent resistivity values, the effect of the bad points on the

inversion results can be reduced by using a data weighting matrix.

Equation C.1 also tries to minimize the square of the spatial changes,

or roughness, of the model resistivity values. This tends to produce a

model with a smooth variation of resistivity values. This approach is

acceptable if the actual subsurface resistivity varies in a smooth and

gradational manner. In some cases, the subsurface geology consists of

a number of regions that are internally almost homogeneous but with

sharp boundaries between different regions. For such cases, an

inversion formulation that minimizes the absolute changes in the model

resistivity values can sometimes give significantly better results.

8.6 Data Processing and Interpretation 8.6.1 Data Processing

After storing the field data in the system, a PROSYS software

was used to down load the field data to the computer and after

doing basis processing and loading elevation data the file was

saved in RES2DINV format for further processing and

interpretation by RES2DINV program, where data processing is

derived from finite difference forward modeling and inversion

was made using finite element method. Unit electrode spacing

of 5 m was used in dipole-dipole array for this survey. Hence all

the profile results obtained show unit electrode spacing as 5 m.

The profile results are presented in colored contour plots of

resistivity (Figure 9a to 9h).

8.6.2 Interpretation of Resistivity Imaging The field data obtained were critically examined, processed and

the final interpretation has been made which are represented in

the form of geoelectric sections and interpreted in terms of

geological sections. The results of these sections bring out the

following inferences concerning the subsurface conditions.

Interpretation of each profile is done separately as described

below.

LINE W1-E1: Resistivity line W1-E1 runs roughly West-East close to the valley on

northern side. This line is 355 m long from higher elevation to lower

side. The inverted model is given in Figure-9a.

The inverted resistivity section of this profile is interpreted in terms of

three layered model, having low resistivity of the order of 200 Ohm-m

to 350 Ohm-m indicating residual soil. This is followed by a layer

having resistivity of the order of 500 ohm-m to 2000 ohm-m, which is

interpreted as moderately weathered/jointed rock. This is further

followed by comparatively higher resistivity strata having resistivity

more than 2000 ohm-m, which is interpreted as massive khondalite

forming the basement. No anomalous zone having lower resistivity has

been encountered in the basement along this profile.


southern side. This line is 355 m long from higher elevation to lower

side. The inverted model is given in Figure-9b.


three-layered model, having low resistivity of the order of 188 Ohm-m









southern side. This line is 355 m long from higher elevation to lower

side. The inverted model is given in Figure-9c.










LINE S1-N1: Resistivity line S1-N1 runs roughly South-west across the valley on the

eastern side of the area to be investigated. This line is 235 m long. The

inverted model is given in Figure-9d.










LINE S2-N2: This profile was carried out westward of S1-N1 across the valley. This

profile runs roughly South-West across the valley on the eastern side

of the area to be investigated. This line is 235 m long. The inverted

model is given in Figure-9e.













model is given in Figure-9f.













model is given in Figure-9g.












of the area to be investigated. (This line is 235 m long. The inverted

model is given in Figure-9h.










9.0 CORRELATION

Interpretation of Seismic and Resistivity Sections were correlated has

been found a reasonable correlation among themselves for basement

rock. In general depth to the basement rock is fairly matched with

seismic and resistivity interpretation. Only one borehole CHR could be

available for correlation of the seismic and resistivity data. The

borehole CHR is crossing the Seismic line E1- W1 at 2.5m along profile

and at 135m along seismic line S1- N1. The borehole data reveals that

thickness of the overburden comprising highly weathered rock with less

than 20% RQD has been attributed upto 5.0m depth. From 5.0m to

16.0m depth, a zone of weathered/ jointed rock has been further

interpreted, where the RQD values varies from 25% to 65%. Beyond

the depth of 16m the CR and RQD has improved substantially, The CR

and RQD beyond 16m depth varies from 83% to 100% and 73% to

100%. This indicates the rock mass below 16m is massive nature.

From the seismic study, thickness of the overburden comprising of

residual soil and highly weathered rock is interpreted upto 3.50m

having seismic velocity of the order of 569 m/sec to 1437m/sec at E1 –

W1 and 3.2m thickness having seismic velocity of the order of

400m/sec to 1350m/sec at S1 – N1. The resistivity of this zone varies

from 175 Ωm to 500Ωm. This layer is further underlain by weathered

rock mass having seismic velocity of the order of 2250m/sec to

2600m/sec. Resistivity of this zone varies from 500Ωm to 2000Ωm.

This layer extends upto the depth of 7.0m from the surface, where as

the borelog data indicates that this zone extends upto 16.0m based on

the RQD values. Through the seismic and resistivity study, the depth to

basement is found to be at 7.0m.

Ground water table as recorded in borehole CHR is at about 7.0m,

which has been further correlated with seismic and resistivity values for

the saturated zones. As per the seismic interpretation the weathered

rock starts from depth of around 2.0m, which has seismic velocity of

the order of 2600m/sec. Generally the water saturated zone in alluvium

shows seismic velocity of the order of 1450 m/sec where as in the case

of rock which has seismic velocity more than the velocity of water. In

such conditions water table cannot be inferred from seismic section.

However the resistivity section indicates water table at a depth of about

5.0m in E1 – W1 section with low resistivity of the order of 400 Ωm to

500 Ωm in weathered rock. Probably this is in correlation with the

observed water table at CHR borehole. On the basis of this it is

interpreted that the similar resistivity range in other resistivity profiles

also indicates the saturated zone.

10.0 SUMMARY AND CONCLUSION

The data of seismic and resistivity profiling has revealed the required

information.

The study area is mainly characterized by three layers with varying

thickness. Overburden on the top comprises of dry residual soil. This

top layer has a thin saturated zone with varying thickness along

profiles. This layer has seismic velocity of the order of 400 m/sec to

569 m/sec in dry zone and 1400 m/sec in saturated zone. This

overburden layer has resistivity value of the order of 100 Ohm-m to 600

Ohm-m.

Second layer is moderately weathered having seismic velocity of the

order of 2250 m/sec to 2600 m/sec and resistivity of the order of 500

Ohm-m to 2000 Ohm-m. The upper portion of second layer is highly

weathered which has been inferred along few lines and has seismic

velocity of the order of 1900 m/sec.

Third layer is jointed/massive khondalite forming the basement. This

has seismic velocity of the order of 3000 m/sec to 5700 m/sec and

resistivity more than 2000 Ohm-m, which increases with depth.

The interpretation of seismic and resistivity profiles is summarized in

Tables 5a to 5h as under:

Table 5a:

E1-W1 Line No.

Layer 1 Layer 2 Layer 3 Layer 4

Velocity (m/sec) 569 2600 4040 5774

Resistivity (Ohm-m) 200-350 500-2000 >2000 -

Thickness of layer (m) 0.6 – 1.9 0.0 – 13.0 1.5 – 16.0 Continued

Geology Residual

Soil

Overburden

Weathered

rock mass

Massive Khondalite

Remarks Thin saturated zone is encountered between layer 1

& 2

Table 5b:

E2-W2 Line No.


Velocity (m/sec) 409 2300 3500 4950


Thickness of layer (m) 0.0-3.3 3.3 – 11.7 3.2 – 21.6 Continued

Geology Residual

Soil

Overburden

Weathered

rock mass

Jointed

Rock mass

Massive

Khondalite


& 2

Table 5c:

E3-W3 Line No.


Velocity (m/sec) 444 2350 3500 -


Thickness of layer (m) 1.8-5.4 1.8 – 26.0 Continued

Geology Residual

Soil

Overburden

Weathered

rock mass

Jointed

Khondalite


& 2

Table 5d:

S1-N1 Line No.


Velocity (m/sec) 400 2250 3500 4633


Thickness of layer (m) 1.3 – 5.8 1.5 – 15.3 1.7 – 26.0 Continued

Geology Residual

Soil

Overburden

Weathered

rock mass

Jointed

Khondalite

Massive

Khondalite


& 2

Table 5e:

S2-N2 Line No.


Velocity (m/sec) 481 2250 3500 4793


Thickness of layer (m) 0.0 – 5.3 0.5 – 16 1.3 – 40.0 Continued

Geology Residual

Soil

Overburden

Jointed

rock mass

Jointed

Khondalite

Massive

Khondalite


& 2

Table 5f:

S3-N3 Line No.


Velocity (m/sec) 449 2497 4223 -


Thickness of layer (m) 1.3-5.3 2.6-37.3 Continued

Geology Residual

Soil

Overburden

Jointed

rock mass

Massive Khondalite

Remarks No saturated zone is encountered.

Table 5g:

S4-N4 Line No.


Velocity (m/sec) 485 2500 3500 -



Geology Residual

Soil

Overburden

Jointed

rock mass

Weathered

Khondalite


& 2

Table 5h:

S5-N5 Line No.


Velocity (m/sec) 518 2500 3600 -



Geology Residual

Soil

Overburden

Weathered

rock mass

Massive

Khondalite


& 2

From the interpretation of seismic and resistivity data no anomalous

zone has been encountered in the basement rock along the survey

profiles.

It is strongly recommended that the seismic survey and resistivity

profiling results need to be correlated with the borehole logs and other

geological informations to give precise geological interpretation.

Figure 5a: SEISMIC SECTION ALONG LINE#E1-W1

Figure 5b: SEISMIC SECTION ALONG LINE#E2-W2

Figure 5c: SEISMIC SECTION ALONG LINE#E3-W3

Figure 5d: SEISMIC SECTION ALONG LINE#S1-N1

Figure 5E: SEISMIC SECTION ALONG LINE#S2-N2

Figure 5F: SEISMIC SECTION ALONG LINE#S3-N3

Figure 5G: SEISMIC SECTION ALONG LINE#S4-N4

Figure 5H: SEISMIC SECTION ALONG LINE#S5-N5

Figure 9a: RESISTIVITY SECTION ALONG LINE#W1-E1

Figure 9b: RESISTIVITY SECTION ALONG LINE#W2-E2

Figure 9c: RESISTIVITY SECTION ALONG LINE#W3-E3

Figure 9d: RESISTIVITY SECTION ALONG LINE#S1-N1

Figure 9e: RESISTIVITY SECTION ALONG LINE#S2-N2

Figure 9f: RESISTIVITY SECTION ALONG LINE#S3-N3

Figure 9g: RESISTIVITY SECTION ALONG LINE#S4-N4

Figure 9H: RESISTIVITY SECTION ALONG LINE#S5-N5

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