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Transcript of 01 Report
E- GEOTECHNICAL FACTUAL REPORT
24 August 2007. ISSUED FOR BID Strategic Storage of Crude Oil at Visakhapatnam Project 2158005;
ra02
e 20
05-0
1-17
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
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 (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
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 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.
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 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.
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
Soil
Overburden
Weathered
rock mass
Jointed
Rock mass
Massive
Khondalite
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
Soil
Overburden
Weathered
rock mass
Jointed
Khondalite
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
Soil
Overburden
Weathered
rock mass
Jointed
Khondalite
Massive
Khondalite
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
Soil
Overburden
Jointed
rock mass
Jointed
Khondalite
Massive
Khondalite
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
Soil
Overburden
Jointed
rock mass
Massive Khondalite
Remarks No saturated zone is encountered.
Table 5g:
S4-N4 Line No.
Layer 1 Layer 2 Layer 3 Layer 4
Velocity (m/sec) 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
Soil
Overburden
Jointed
rock mass
Weathered
Khondalite
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
Soil
Overburden
Weathered
rock mass
Massive
Khondalite
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