Chapter-Six
GROUNDWATER GEOPHYSICS
6.1. INTRODUCTION
Water is most important for any type of land development and conservation of
groundwater is economically important. Geophysical methods has been used for a
number of applications particularly groundwater potential zone identification.
Accurate and reliable results has been obtained when these methods are proposed with
thoroughly understanding of geological, geomorphological and hydrogeological
environments, water table conditions and topography in a specific location. There are
four important techniques like gravity, magnetic, seismic, and geoelectrical
prospecting methods are available. Out of this electrical resistivity method plays
solving groundwater problems through its highest resolving power and economical
viability. Electrical resistivity methods are used to investigate the different
lithological formations, bed rock dispositions, the depth to water table or zone of
saturated formations, thickness of weathered zones, detection of fissures, fractures,
fault zones, establishment of their depths, thickness and lateral extent of aquifers,
groundwater flow directions, valley fills, depth to basement in hard rocks, fresh and
salt water intrusions, groundwater prospective zones, for locating ore deposits and
archaeological studies. In this study area electrical resistivity method has been
worked to analyse geoelectrical parameter identification.
6.2. PRINCIPLES OF ELECTRICAL RESISTIVITY SURVEY
Electrical Resistivity Survey is performed by passing a known electric current
into ground by means of two current electrodes and the potential differences between
the other two potential electrodes is measured. The potential variations may be
126
changes due to size, shape and conducting capacity of the material in the subsurface
and from the quantities of potential differences and the current applied the resistance
is calculated.
6.2.1 Electrical Resistivity Survey (ERS)
Generally the prospecting of geophysical methods is fundamental to illustrate the
physical character of the chosen sites. In the prospecting developments in geophysics,
physical characteristics like density, magnetic susceptibility, elasticity, radioactivity
and electrical resistivity (or electrical conductivity) are considered. Any geological
formation is characterized by an electrical resistivity whose main factor is the ease
with which the electrical current passes through it. Electrical resistivity can be termed
in ohm- meter of ohm-feet of ohm-centimeter and in electrical exploration ohm-meter
is recorded as a standard unit. Electrical Resistivity Surveys could find out a good
electrical resistivity difference between the water bearing formations and the
surrounding rocks (Zohdy et. al., (1974).Figure 6.1 Shows that the Schematic
diagram of basic of electrical resistivity configuration method. In this process a
known value of electric current (I) is passed into the ground by two outer metal stakes
(C1 and C2) that are buried in the ground. The potential variation (∆V) is measured
between two inner electrodes termed potential electrodes (ρ1and ρ2). The ratio of
∆V/I provides the resistance (R) and by multiplying R with the geoelectrical factor
(K) of the electrode separation, the resistivity ‘ρ’, and it is inverse of conductivity of
the ground may be described. The potential variation passes because of the externally
pressing current between various electrodes, the apparent resistivity of the elements
established in the given geologic formation which does not match the average
resistivity and it may be lower than the lowest and higher than the highest resistivity
within the subsurface to which it pertains. The apparent potential value of ‘ρ’
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corresponds to the true resistivity, if the ground is homogeneous and isotropic when it
is obtained from the measurements over a layered or heterogeneous ground, then it is
only an apparent resistivity and is signed by ‘ρa’, the quality being used in the
interpretation of electrical methods.
6.3. LITHOLOGICAL FORMATIONS AND THEIR RESISTIVITY
VALUE RANGES
The resistivity of geological subsurface formations differs very broadly not only
from formation to formation but also within one lithological unit and is related to
(i)
Size and shape of the aquifer materials, density, porosity, pore size of the
material
(ii) Quality of water, size, shape, pore space and density of the aquifer horizons
(iii) Distribution of water in the rocks due to the structural and textural
characterstics and
(iv) The temperature of the subsurface of the water environment
Kollert (1969); Vingoe (1972); Sharma (1976); Telford et. al., (1976);
Bhimashankaram and Gaur (1977), Patangay and Murali (1984), Ramachandran
(2000) and Venkateswara Rao et. al., (2004) has
given comprehensive list of
resistivity values of various rock types, minerals and soils. Mooney (1980) has also
given representative resistivity values for different kinds of earth materials (Table
6.1). Sakthimurugan and Balasubramanian (1991) tabulated the range of resistivity
values of common hard rocks and their water bearing decomposed products of the
Indian Peninsula (Table 6.2). The resistivity of highly weathered saturated gneiss of
Archaean age group rock formation ranges from 27 to 120 ohm-m.
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Dry rocks
whether Non-porous or Porous are almost non-conductors otherwise the resistivity
and saturated clayey sediments have low resistivity clay, sand and gravel deposits that
are saturated with groundwater of low ionic strength have high resistivity. Porous
geological formations, which are saturated with groundwater of high ionic strength,
have very less resistivity (Gilkeson and Cartwright, 1983).
Table 6.1 Representative Resistivity values of Regional Soil Resistivities
(After Mooney, 1980)
Soil
Wet areas
500-200 ohm-meters
Dry areas
100-500 ohm-meters
Arid regions
200-1000 ohm-meters (sometimes as
low as it the soil is saline)
Waters
Soil water
1 to 100 ohm-meters
Rain water
30 to 1000 ohm-meters
Sea water
Order of 0.2 ohm-meters
Ice
10 5 to 108 Ohm-meters
Representative Resistivity values for Earth materials
Low resistivity material
Medium resistivity material
High resistivity material
Less than 100 ohm-meters
100 to1000 ohm-meters
Greater than 1000 ohm-meters
Rock types below the water table
Igneous and Metamorphic
100 to 10,000 ohm-meters
Consolidated sediments
10 to 1000 ohm-meters
Unconsolidated sediments
1 to 100 ohm-meters
Ores
10-4 to 1 ohm-meters
Massive sulphides
Non-metallics
Order of 1010 ohm-meters
(Gypsum, Quartz, dry rock salt)
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Table 6.2 Hydrogeological Characteristics of Bulk Resistivity
(After Sakthimurugan and Balasubramanian, 1991)
Bulk
Resistivity
(ohm.m)
<20
50-70
20-30
Aquifer characteristics
Authors
Indicates a chloride ion concentration of
Stewart et.al.,
250 ppm (aquifer fine sand and limestone)
(1983)
Porosity is the principal determinant of
Stewart et.al.,
resistivity
(1983)
Pore fluid conductivity dominates
Stewart et.al.,
(1983)
30-70
Affected by both water quality and lithology
Stewart et.al.,
(1983)
<10
<1
Delineates sediments enriched with salt
Stewart et.al.,
water
(1983)
Clay/sand saturated with salt water sand
Zohdy et.al., (1974)
15-600
gravel saturated with fresh water
<5
Salt water or clay with salt water
Arora and Bose
(1981)
< 19
Clay/ clay mixed with kankar weathered
64-81
sandstone weathered granite
<10
Singhal (1984)
Saline coastal zone sand (sedimentary)
Balasubramanian
10-20
Clay with or without diffused water
et.al., (1985)
20-60
Fresh water zones
(EP-Electrical Profiling; DES-Dipole Electrical Sounding; FEMS – Frequency
Electro Magnetic Sounding;
Polarization;
IP – Induced Polarization;
RVES – Radial Vertical Electrical Sounding;
SP – Spontaneous
IPVES –Induced
Polarization Vertical Electrical Sounding; RIS – Radio Interference Sounding; RP –
Radio wave Profiling; VES – Vertical Electrical Sounding)
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6.4 MATERIALS AND METHOD
Different types of electrical methods are commonly employed to deal with
various geological problems (Bhimashankaram and Gaur (Op. cit), Patangay and
Murali (1984) have catalog listed in Table 6.3 which shows that majority of the
groundwater problems to be solved using the Vertical Electrical Sounding (VES) that
has been conducted in Nanjangud taluk in order to satisfy the following reasons such
as to investigate the nature and extent of the potential aquifers of groundwater and to
carry out the thickness of saturated zones and depth to basement rock formations.
6.4.1 Depth Sounding
Depth sounding was employed to determine the vertical variation in the
Apparent Resistivity due to different geological units (Zohdy et. al., Op.cit). In
Vertical Electrical Sounding (VES) the central electrode was arranged at a fixed point
and at the same time the electrode spacing was successively increased at logarithmic
intervals.
Increase in the electrode spacing gives near-surface variations in the
resistivity whereas great electrode spacing of high sensitive sounding and expansion
of electrode arrangement and depth of the penetration of electric current is increased
and hence information on the vertical succession of different zones, their individual
true resistivities conductive zones and thickness of the aquifer can be obtained. The
main purpose of Vertical Electrical Sounding is to deduce the variation of electrical
resistivity with depth below and to obtain geological knowledge in order to work out
the substructure in addition detail. The basis of theory behind this procedure is that a
fraction of electric current that passed into the ground penetrating below a given,
which increases with current electrode separation.
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6.5. ELECTRODE ARRANGEMENTS
Many different electrode arrangements have been proposed and used for
resistivity exploration. Whiteley (1973) presented a comprehensive review of 25
arrangements with comments on the advantages and disadvantages of each. Some of
the less well known arrangements offer significant benefits for certain problems,
Wenner, Lee modification of Wenner, Schlumberger and Axial Dipole-Dipole
arrangements are widely used because the interpretation tools are well developed and
they will be adequate for application to groundwater and shallow geologic problems.
The Schlumberger arrangement is widely used for quantitative interpretation in
Vertical Electrical Sounding compared with Wenner.
It offers the important
advantage of being less sensitive to unknown lateral homogeneities because the
potential electrode (M & N) remains in fixed position during a large number of
sensitive measurements.
The Schlumberger arrangements have two disadvantages
compared with Wenner. Certain adjustments must sometimes be made to the field
data prior to interpretation, and more sensitive measuring equipment is required. In
this work, Schlumberger arrangement has been used for resistivity sounding with a
speed of current electrodes AB/2 upto 260m. Four electrodes are placed along a
straight line on the earth’s surface (Fig. 6.1 a,b & c) in the same order, AMNB, as in
the Wenner array, but with
AB ≥ 5 MN. For any linear, symmetric array AMNB of
electrodes, the equation in the form
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(AB/2)2 – (MN/2)2
∆V
ρ =
………………….. (6.1)
MN
I
But of MN → 0, then this equation can be written as
E
2
ρs = (AB/2)
…………………………………….. (6.2)
I
∆V
MN
where E = lim,
MN → 0
= electric field
Where ρa apparent resistivity
∆V -
Potential difference
I
-
Current
K
-
Geometric factor
A
-
Space between two electrodes
Conrad Schlumberger defined the resistivity in terms of the electric field E rather
than the potential difference ∆V (as in the Wenner array). It can be seen from the
equation 6.2 that the Schlumberger apparent resistivity “ρs” is a function of a single
distance variable (AB/2). In practice it is possible to measure “ρs” according to the
equation 6.2 but only in approximate manner. The apparent resistivity “ρs” usually is
calculated by using equation 6.1 provided that AB
5 MN (Deppermann, 1954;
Keller and Frischknech, 1966).
6.6. INSTRUMENTATION
Aquameter CRM-20 (Computerized Resistivity Meter) has been used to conduct
the geophysical survey in this study (Fig.6.2) Aquameter is a modern version of earth
resistivity meters, which make use of advanced technology of microprocessor. The
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generator circuit in Aquameter CRM-20 is so designed to maintain the current
flowing through the ground at the present level, irrespective of variations in the soil
and contact condition. The current passes through the C1C2 electrodes into the ground
and the potential generated across V1V2 electrode is sensed by instrument. This
instrument is capable of detecting even a very low signal riding on a noisy
background. The noise elimination takes place at various stages of amplification and
the filtered signal is the processed so as to derive the resistance information. For
excessively noise condition the ‘averaging mode’ is provided externally in addition to
the internal built-in averaging.
This facilitates geophysical prospecting even in
difficult terrain.
Fig 6.2 .Instrument of CRM 20 Aquameter
The microprocessor based circuit always ensures the proper functioning of the
instrument. Any malfunction such as low battery voltage, a poor contact at the
current electrodes or improper current setting etc, is immediately detected and
conveyed to the operator through LCD display. This eliminates the possibility of any
erroneous readings.
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A seven-position switch is provided to select the appropriate current. The
current ranges are 0.2, 0.5, 1.0, 2.0, 5.0, 10, 20 mA. Accuracy of the result will be
higher, the higher the value of current. The aquameter is provided by a rechargeable
high power battery of 12 V. the generator circuit inside the aquameter converts the 12
V D.C into a high voltage supply of 150 V D.C, which is made to alternate at regular
intervals giving 300 pp (peak to peak). This constant current generator circuit sends
out the current through the C1-C2 electrodes and maintains it at the present value
throughout the measurement. By selecting the number of cycles depending on the
ground noise, one can get noise free readings (The available are 1, 4, 16 and 64).
Using the range switch it is possible to select the resistance range 10 ohm, 1000-ohm,
10 K ohm or 1 M ohm depending on the soil resistivity. Normally the lowest range is
very suitable for accurate readings. Self-potential cancellation is done automatically.
6.7. FIELD DATA
For evaluating the groundwater potential zones in the investigated area, the
vertical electrical sounding data of 55 locations (Map.6.1) conducted with the help of
CRM-20 (Computerized Resistivity Meter) has been used. The maximum depth of
investigation was 100 meter. The spread was commonly in steps of half meter interval
up to 10 meter, 20 meter interval up to 30 metres, 5 metres interval up to 60 metres
and 10 metres interval up to 100 metres.
6.8. ANALYSIS OF VES DATA
The main objective of the Vertical Electrical Sounding is to transform the field
data in terms of subsurface geology or hydrogeology, so that suitable maps can be
drawn for various geophysical parameters of interests. To succeed on this main
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objective, it is very basic to analyse the field data and interpret them suitably. The
details required to relate the data can be represented by (1) Qualitative and (2)
Quantitative methods. Zhody et. al., (Op.cit), Patagay et. al., (Op.cit) have explained
the different methods of qualitative and quantitative interpretations and the sequence
of interpretations.
6.8.1 Quantitative Interpretation
Quantitative interpretation of Vertical Electrical Sounding data is detailed the
thickness of the various geological formations having different resistivities and the
interpretation may be analytical or empirical or semi empirical.
6.8.2 Analytical Method
Analytical methods are fundamentally the calculation of theoretical sounding
curves that match the field curves. The Apparent Resistivity Data of VES covering
the entire basin has been plotted on log-log graph sheets and matching done with the
master curves of Orellona and Mooney (1966), particularly prepared Wenner for
configuration. The mathematical basis of their method could be found in Griffiths
et.al. (1966).
6.8.3. Types of Curve
The simplest sounding curves can be different types related to the geological
and hydrogeological environments and the high electrodes spread. These curves are
of two types like (1) Ascending type (2) Descending type. The ascending type of
curves are established where the ground has a two-layer structure, the topsoil or
weathered layer and hard compact basement (highly resistivity). The descending types
136
of curves are established where a top layer is overlying a thick clay or saline water
aquifer.
‘A’-type curves are obtained in the hard rocks with conductive topsoils. In
this case, the resistivity of the layers will be continuously increasing (ρ1<ρ2<ρ3).
Sounding curves have maximum peak and occupied by low resistivity values
(ρ1<ρ2>ρ3). are termed ‘K’ type curves and such curves result from different
environment. In the category to three layered ground structure four types of sounding
curves are possible. The listed ρ1,ρ2 and ρ3 are electric resistivities of the three
following other layers, sounding curves with central minimum (ρ1>ρ2<ρ3) is called
‘H’ type curves. Type of sounding curves are not normally in hard rock formations
and it consists of dry top soil of high resistivity as the first layer, water saturated
weathered layer of minimum resistivity as second layer and compact hard with very
high resistivity as the third layer. A sounding curve with continuously decreasing
resistivity (ρ1>ρ2>ρ3) is said to be ‘Q’ type curve and this type of curves are
commonly obtained in coastal region because of the saline water. (Map.6.2). Based
on the above results interpretation of the Nanjangud taluk, the following types of
curves are noticed.
6.8.4. Qualitative interpretation
A qualitatative method is useful to learning the common information about the
geological structure and the changes in the geological section of the region. The
technique of interpretation is very helpful after the quantitative interpretation of the
same is undertaken. Interpretations are worked out by preparing iso-resistivity maps,
thickness of different layers, longitudinal conductance maps, transverse resistance and
aquifer anisotropy maps to demonstrate the subsurface information and their features.
137
6.9. COMPUTER INVERSION TECHNIQUES (CIT)
As there is a chance of error in interpretation and judgments of manual curve
matching procedures, several numerical computer programs for automated fit of
resistivity data have been developed. The computer inversion program IPI2WIN has
been used to compute the geoelectrical parameters of the study area.
The input data given for execution include
(i)
The field measurement as spacings and apparent resistivities
(ii)
The type of electrode arrangement used (Wenner, Schlumberger or Dipole).
(iii) The number of layers and
(iv)
Assumed layer of resistivities (ρ) and thickness (h).
This is an iterative procedure of utilizing a set of guessed model parameters to
compute a sequence of apparent resistivity values. The computed ‘ρa’ values are
compared at each iteration with the observed field ‘ρa’ values and subsequent attempts
are mode in refining the layer parameters. When the error between the observed and
computed value is less, the resultant model is produced as an output. Instead of an
arbitrary initial guess, the parameters derived through curve matching have been
utilized for inversion and the best fit results were obtained. It has been found that this
strategy produced a very good automated fit and reduced the number of iterations, and
compute processing time.
Fig. 6.3. Shows the automated fit of matched curves
obtained through this computed inversion program for three locations. The resistivity
and thickness of different layers obtained through manual curve matching and
compute inversion have been compared for all the vertical electrical sounding stations
138
of the study area. The results of these two procedures are showing similar layer
details and are tabulated (Table. 6.3) for further interpretation.
6.10. INTERPRETATION OF VES DATA
The geophysical parameters of the aquifers of the Nanjangud taluk (curve
type, resistivity of the layers and the thickness of the first two layers) has been
tabulated for 55 villages (Table 6.4). From this data the following points are emerge.
The four types of resistivity cures for the 3-layered geo-electrical sections (A,K,H and
Q) do exist in this Nanjangud taluk.
Curve types
Number of Sites
A
08
K
22
H
21
Q
04
Total
55
Locations with ‘K’ type curves predominate in the Nanjangud taluk
followed by ‘H’, ‘A’ and ‘Q’type. Location with ‘A’ and ‘H’ curves are in general
suitable for shallow wells, where as location K type could be exploited by deep bore
wells or dug cum borewells. Fig 6.3 gives the observed field curves as well as
computed field model curves using the IPI2WIN
program along with the
geoelectrical model. These diagrams show clearly that the IPI2WIN model exactly
coincides with the field data and could clearly be used for the interpretation of
resistivity data of hard rock areas. Water saturated horizons at AB/2 equal to 20, 40,
60, 80 and 100 meters has been separately prepared and shown as five maps.(Maps
139
6.3 and 6.7). It was very clearly shown that areas with 20-60 Ohm-meters resistivity
is ideal for potable water well sites and areas with > 60 Ohm-meters resistivity are
mostly dry in the hard rock areas and Karnataka (Sharma, 1982; Prasad, 1984;
Balasubramanian, 1986; Venugopal, 1988; Chandrashekar, 1988; Indira, 1988;
Siddaraju, 1996; Nagaraju, 1996; and Mastan Rao, 1998).
Map
AB/2 mts
6.3
20
6.4
40
6.5
60
6.6
80
6.7
100
From the maps as well as the table given above it could be observed that the
area of water saturated horizon increases with depth upto 40 m AB/2 and then
decreases slowly depth wise and drastically by the time AB/2 reaches 100 m
indicating that dug, dug-cum bore and moderate depth bore wells will be more
successful than shallow dug-wells or deep bore wells in this area (Table 6.4). Maps
6.8 and 6.11 give the iso-resistivity contours of 20 and 60-Ohm meters respectively
for the first and second layers. This has been particularly chosen due to the fact that
potable groundwater exists in the resistivity areas falling between 20-60 ohm meters,
water quality deteriorates when resistivity is < 20 Ohm meters and the formations will
be dry if resistivity escalates beyond 40 meters. Thus both these maps have shown
three horizons namely non-potable groundwater (< 20 Ohm meters) potable
groundwater (20-60 Ohm meters) and dry rocks (>60 Ohm meters).
140
The thickness of the first layer h1, h2 and h3 is given in Maps 6.9, 6.12 and
6.15. A large area of the Nanjangud taluk with less than 20 meters thickness and
considerably small areas with thickness greater than 20 meters is shown by Map 6.9
thus, groundwater exploitation has to be done with shallow wells. In contrast to this
Map 6.11 the area of the falling is less than 20 meters thickness is lowest, 20-60
meters is larger and greater than 60 meters is in between the first two. This clearly
shows that the shallow wells are of no use when deeper aquifers are to be exploited.
Super position of Map 6.8 and 6.9 results is Map 6.10, which gives the groundwater
potential zones of the first layer. The entire area could be divided into six resistivity
thickness horizons as follows.
Thickness
Resistivity
Groundwater
(M)
(Ohm meters)
Development by / Quality
< 20
< 20
Dug wells, Non-potable water
< 20
20-60
Dug wells, potable water
< 20
> 60
Not feasible
> 20
< 20
Shallow bore wells, Non-potable water
> 20
20-60
Shallow bore wells, potable water
>20
> 60
Not feasible
The third column in the above table gives the type of wells for groundwater
development and the quality of water, which will be encountered in the area. Larger
areas of the Nanjangud Taluk could be served by dug wells development with potable
water area.
For deeper groundwater exploitation and identification of groundwater
potential zones Maps 6.11 and 6.12 are superimposed over each other and Map 6.13
141
gives the groundwater potential zones. And the third layer groundwater potential
zones maps are decipher on Maps 6.14, 6.15 and 6.16.
Thickness
Resistivity
Groundwater
(m)
(Ohm meters)
Development by / Quality
< 20
< 20
Dug wells Non-potable water
< 20
20-60
Dug wells potable water
< 20
> 60
Not feasible
20-60
< 20
Shallow bore wells Non- potable water
20-60
20-60
Shallow bore wells potable water
20-60
> 60
Not feasible
> 60
< 20
Deep bore wells, Non potable water
> 60
20-60
Deep bore wells, potable water
> 60
> 60
Not feasible
The above table shows 9 horizons with the types of wells for development
(dug, shallow borewells, deep borewells) quality of water (potable, non potable) and
not feasible areas. It is interesting to note that a large area could be served by shallow
bore wells followed by deep bore wells both yielding potable water.
6.10.1 Geoelectrical Parameters
In general, geologic section varies from a Geoelectrical section when the
boundaries of geologic layers are not similar with the boundaries between layers,
properties of various resistivities causing electric boundaries of individual layers of
various resistivities may or may not be similar with boundaries of individual layers of
various geologic ages or various lithological characteristics. For example, the type of
142
salinity of groundwater in a given type of rock is different in depth and number of
Geoelectrical layers that may be recognized within a lithological homogenous rock
and also in the heterogeneous situation layers of various lithologies or ages or both
may have the same resistivity and thus from a single Geoelectrical layer.
A Geoelectrical layer is characterized by two essential parameters like
resistivity ‘ρ’ and thickness ‘h’. The other geoelectrical parameters could be obtained
from these variables:
1. Total longitudinal unit conductance (S)
2. Total transverse unit resistance (T) and
3. Aquifer anisotropy ( λ)
1. The total longitudinal unit conductance (S) is calculated by using the formula.
For ‘n’ layers, the total longitudinal conductance is
S=
n
hi
Σ
i=1 ρi
h1
=
h2
+
ρ1
hn;
+………
ρ2
……….. (6.3)
ρn
Where h1and ρ1 are thickness (m) and Resistivities of the ith layer upto nth layer.
Different is ‘S’ from one VES to other has been used in qualitative sense to
show changes in the total thickness of low resistivity materials (Zohdy, 1969; Henriet,
1975; Worthington, 1977; Galin, 1979). High ‘S’ values are typical of low aquifer
Resistivities. Empirical connection between longitudinal conductance of weathered
layer and groundwater yields has been noticed especially for winter months during
resistivity investigations for groundwater in metamorphic areas in the Dhanbad
district in India (Verma et. al., 1980).
143
2. The total transverse unit resistance (T) of a location can be observed by using the
equation.
The total transverse unit resistance is
n
T= Σ
hi ρi = h1ρ1 + h2 ρ2 + ……………hnρn;
i=1
……
(6.4)
Where h1 and ρ1 are the thickness (m) and resistivity (ohm-meters) of the ith
Layer up to nth layer.
The total transverse resistance has normally been interpreted either by a
qualitative or quantitative manner. The quantitative interpretation technique effort to
develop a comparison between T and mean borehole test yield (Worthington Op. Cit)
or between T and aquifer transmissivity values (Scarascia, 1976 and Kosinski, 1977).
The characteristic has helped to point out the varying thickness of high
resistivity materials and for variations in their transverse resistance (Zohdy et. al., Op.
cit). Increasing T values are supported by an increase in the thickness of the high
resistivity materials (Balasubramanian, 1986). It has been reported (Matzner, 1983)
that increasing T and S are similar with high transmissivity of aquifers.(Map.6.18)
3. The coefficient of anisotropy (λ) of a formation could be observed by using the
formation.
The coefficient of anisotropy is
√TS
λ=
………………(6.5)
H
Where,
T – Is the total transverse resistance (ohm-m2)
S – Is the total longitudinal conductance (mhos) and
H – Is the total thickness of the formation (m)
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The result presented as coefficient is commonly higher than 1.00 but does not
often exceed 2.00 (Zohdy et. al., Op. cit). It could be measure of carryout the extent
of anisotropies in an area of interest. It accounts to 1.02-1.10 for alluvium, 1.05-1.15
for sandstones and shales 1.40-2.25 for slates and 2.0-0.8 for graphite schist
(Balasubramanian, Op. cit). As the hardness and compaction of rocks increase, the
coefficient of anisotropy also increases (Keller et. al., 1966). Those parameters have
been described and shown in Table 6.4
6.11 SIGNIFICANCES
Geophysical techniques have emerged as important tools in prospecting for
groundwater. In spite of the manifestation of surficial features of any terrain. Actual
potable groundwater in areas of hard rock has Resistivities ranges from 30 to 150
ohm-metres. Whereas in the sedimentary formations, resistivity values may reach as
low as 1 –ohm-m.
In general hard rocks groundwater could be tapped in the
weathered, fissured and fractured zones are commonly found at comparatively very
shallow depth, such zone and pockets have less resistivity values and were correlated
to the high compact and fresh rocks and could be simply located by resistivity
surveys. Water holds in joints and fissures of the under composed rocks may also be
found may be depending on the low resistivity values. Balakrishna et. al., (Op. cit),
Krishnaraju (1983), Prasad (1984) Sharma and Sastri (1986) have worked and
published about related studies in hard rock terrains. Hydrogeological evaluation of
the groundwater potential of the Akure metropolis,
South Wastern Nigeria
demarcated by Olorunfemi et al., (1999). Adedeji (2001) suggested the application of
integrated geophysical methods in groundwater exploration and protection. Electrical
resistivity surveys for groundwater in the upper Gunjanaru catchment, Cuddapah
145
district, Andhra Pradesh an integrated approach by Janardhana Raju et.al.,(1996).
Vertical sounding curves in the Nanjangud taluk are shown in the Map 6.2 and the
figure curve types are classified into four categories, out of this most of the curve fall
under the ‘A’ type followed by ‘H’ type than ‘K’ type and ‘Q’ type. It could be seen
from this figure that ‘A’ curve type indicates that most part of the study area falls
under rock terrain and massive nature.
Convenient occurring of the longitudinal
conductance (Map. 6.17) and it can be represented that high longitudinal conductance
values are characteristic of deeper basement topography very close to river basin.
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