01 Report

ra02e 2005-01-17

12

CHAPTER 3: GEOPHYSICAL STUDY

E- GEOTECHNICAL FACTUAL REPORT 24 August 2007. ISSUED FOR BID Strategic Storage of Crude Oil at Visakhapatnam

Project 2158005;

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

36

SUMMARY AND CONCLUSION

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 fourlayer 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 Line name Start Point End Point E N E N Line E1-W1 1599 846 1273 700 Line E2-W2 1623 789 1327 657 Line E3-W3 1644 739 1317 594 Line S1-N1 1645 743 1562 929 Line S2-N2 1571 688 1491 868 Line S3-N3 1488 657 1404 842 Line S4-N4 1426 589 1332 785 Line S5-N5 1342 540 1248 754 Total Coverage Table 1b: End Coordinates of Resistivity Profiles Coordinates Line name Start Point End Point E N E N Line W1- E1 1296 709 1599 846 Line W2- E2 1319 656 1623 789 Line W3- E3 1330 600 1644 739 Line S1-N1 1645 743 1562 929 Line S2-N2 1571 688 1491 868 Line S3-N3 1488 657 1425 796 Line S4-N4 1426 589 1332 785 Line S5-N5 1342 540 1248 754 Total Coverage

Proposed length (m)

Actual length (m)

357.00 357.00 357.00 115.00 115.00 115.00 115.00 115.00

370.00 345.00 380.00 235.00 230.00 235.00 235.00 235.00 2265.00

Proposed length (m) 357.00 357.00 357.00 115.00 115.00 115.00 115.00 115.00

Actual length (m) 355.00 355.00 355.00 235.00 235.00 175.00 235.00 235.00 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 500-1500 with completely weathered rock altered into residual soil (Velocity range between 1400 to 1500 m/sec may possibly be the saturated zone) Weathered rock (lower velocity indicates higher 1500-3000 degree of weathering, whereas the higher velocity indicates lower degree of weathering) 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 1500

2500

3500

4500

5500

6500

0.1

1.0

10.0

100.0

1000.0

m/sec Q-Value

0.01

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 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 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 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 2300 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 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 anomalous zone has been observed in the basement rock along this profile. The interpretation of this profile is given in tabular form in Table 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 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 481m/sec and varying thickness between 0.0 to 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. Seismic section along S3-N3: This profile was carried out westward of S2-N2 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 449m/sec and varying thickness between 1.3 to 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 anomalous zone has been observed in the basement rock along this

profile. The interpretation of this profile is given in tabular form in Table 5f and the Seismic section is in Fig. 5f. Seismic section along S4-N4: This profile was carried out westward of S3-N3 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 485m/sec and varying thickness between 0.3 to 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

having seismic velocity of the order of 2500 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 anomalous zone has been observed in the basement rock along this profile. The interpretation of this profile is given in tabular form in Table 5g and the Seismic section is in Fig. 5g. Seismic section along S5-N5: This profile was carried out westward of S4-N4 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 485m/sec and varying thickness between 0.7 to 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

having seismic velocity of the order of 2500 m/sec. This is followed by massive khondalite having seismic velocity of the order of 3600 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 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 loglog 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 multielectrode 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. LINE W2-E2: Resistivity line W2-E2 runs roughly West-East close to the valley on southern side. This line is 355 m long from higher elevation to lower side. The inverted model is given in Figure-9b. The inverted resistivity section of this profile is interpreted in terms of three-layered model, having low resistivity of the order of 188 Ohm-m to 450 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 800 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. LINE W3-E3: Resistivity line W3-E3 runs roughly West-East close to the valley on southern side. This line is 355 m long from higher elevation to lower side. The inverted model is given in Figure-9c.

The inverted resistivity section of this profile is interpreted in terms of three-layered model, having low resistivity of the order of 250 Ohm-m to 550 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 1000 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. 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. The inverted resistivity section of this profile is interpreted in terms of three-layered model, having low resistivity of the order of 174 Ohm-m to 500 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 750 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. 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. The inverted resistivity section of this profile is interpreted in terms of three-layered model, having low resistivity of the order of 177 Ohm-m

to 500 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 750 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. LINE S3-N3: This profile was carried out westward of S2-N2 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 170 m long. The inverted model is given in Figure-9f. 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 500 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 800 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. LINE S4-N4: This profile was carried out westward of S3-N3 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-9g. The inverted resistivity section of this profile is interpreted in terms of three-layered model, having low resistivity of the order of 100 Ohm-m to 600 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 800 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. LINE S5-N5: This profile was carried out westward of S4-N4 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-9h. The inverted resistivity section of this profile is interpreted in terms of three-layered model, having low resistivity of the order of 250 Ohm-m to 600 Ohm-m indicating residual soil. This is followed by a layer having resistivity of the order of 700 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. 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

Velocity (m/sec)

569

2600

4040

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

Weathered

Massive Khondalite

Soil

rock mass

Overburden

Layer 4 5774

Remarks

Thin saturated zone is encountered between layer 1 &2 Table 5b: E2-W2

Line No. Layer 1

Layer 2

Layer 3

Layer 4

Velocity (m/sec)

409

2300

3500

4950

Resistivity (Ohm-m)

188-450

800-2000

>2000

-

Thickness of layer (m)

0.0-3.3

3.3 – 11.7

3.2 – 21.6

Continued

Geology

Residual

Weathered

Jointed

Massive

Soil

rock mass

Rock mass Khondalite

Overburden Remarks

Thin saturated zone is encountered between layer 1 &2 Table 5c: E3-W3

Line No. Layer 1

Layer 2

Layer 3

Layer 4

Velocity (m/sec)

444

2350

3500

-

Resistivity (Ohm-m)

250-550

1000-2000

>2000

-

Thickness of layer (m)

1.8-5.4

1.8 – 26.0

Continued

Geology

Residual

Weathered

Jointed

Soil

rock mass

Khondalite

Overburden Remarks

Thin saturated zone is encountered between layer 1 &2 Table 5d: S1-N1

Line No. Layer 1

Layer 2

Layer 3

Layer 4

Velocity (m/sec)

400

2250

3500

4633

Resistivity (Ohm-m)

174-500

750-2000

>2000

-

Thickness of layer (m)

1.3 – 5.8

1.5 – 15.3

1.7 – 26.0

Continued

Geology

Residual

Weathered

Jointed

Massive

Soil

rock mass

Khondalite

Khondalite

Overburden Remarks

Thin saturated zone is encountered between layer 1 &2 Table 5e: S2-N2

Line No. Layer 1

Layer 2

Layer 3

Layer 4

Velocity (m/sec)

481

2250

3500

4793

Resistivity (Ohm-m)

177-500

750-2000

>2000

-

Thickness of layer (m)

0.0 – 5.3

0.5 – 16

1.3 – 40.0

Continued

Geology

Residual

Jointed

Jointed

Massive

Soil

rock mass

Khondalite

Khondalite

Overburden Remarks

Thin saturated zone is encountered between layer 1 &2 Table 5f: S3-N3

Line No. Layer 1

Layer 2

Layer 3

Layer 4

Velocity (m/sec)

449

2497

4223

-

Resistivity (Ohm-m)

200-500

800-2000

>2000

-

Thickness of layer (m)

1.3-5.3

2.6-37.3

Continued

Geology

Residual

Jointed

Massive Khondalite

Soil

rock mass

Overburden Remarks

No saturated zone is encountered. Table 5g: S4-N4

Line No. Velocity (m/sec)

Layer 1

Layer 2

Layer 3

Layer 4

485

2500

3500

-

Resistivity (Ohm-m)

100-600

800-2000

>2000

Thickness of layer (m)

1.3-5.3

2.6-37.3

Continued

Geology

Residual

Jointed

Weathered

Soil

rock mass

Khondalite

-

Overburden Remarks

Thin saturated zone is encountered between layer 1 &2 Table 5h: S5-N5

Line No. Layer 1

Layer 2

Layer 3

Layer 4

Velocity (m/sec)

518

2500

3600

-

Resistivity (Ohm-m)

250-600

700-2000

>2000

-

Thickness of layer (m)

2.0-5.3

3.4-21.3

Continued

Geology

Residual

Weathered

Massive

Soil

rock mass

Khondalite

Overburden Remarks

Thin saturated zone is encountered between layer 1 &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