59 CHAPTER – 4 GEOPHYSICAL INVESTIGATIONS 4.1 Introduction Electrical resistivity methods of geophysical prospecting are well established and the most important method for groundwater investigations. The electrical resistivity method is one that has been widely used because of the theoretical, operational and interpretational ease. The advantages of electrical methods also include control over depth of investigation, portability of the equipment, availability of wide range of simple and elegant interpretation techniques, and the related software etc. Direct current (D.C.) resistivity (electrical resistivity) techniques measure earth resistivity by driving a D.C. signal into the ground and measuring the resulting potentials (voltages) created in the earth. From the data obtained, the electrical properties of the earth (the geoelectric section) can be derived. In turn, from those electrical properties we can infer the geological characteristics of the earth. In geophysical and geotechnical literature, the terms “electrical resistivity” and “D.C. resistivity” are used synonymously. Several geological parameters which affect earth resistivity (and its reciprocal, conductivity) include clay content, soil or formation porosity and degree of water saturation. The theory and practice of this method for groundwater investigations is well documented by Bhattacharya and Patra (1968) and Parasinis (1973). The interpretation of resistivity data and its application to groundwater studies has been given in detail by Zohdy (1965 and 1975). D.C resistivity techniques may be used in the profiling mode (Wenner surveys) to map lateral changes and identify near-vertical features or they may be used in the sounding mode (Schlumberger array) to determine depths to geoelectric horizons (Ex. depth to water table). Both profiling and vertical electrical sounding (VES) has been successfully applied to various geological formations by (Flathe 1955; Ogilvy 1967; Ogilvy et al. 1980; Zohdy et al. 1974). Common applications of the D.C resistivity method include delineation of aggregate deposits for quarry operations, estimating depth
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CHAPTER – 4
GEOPHYSICAL INVESTIGATIONS
4.1 Introduction
Electrical resistivity methods of geophysical prospecting are well established and
the most important method for groundwater investigations. The electrical resistivity
method is one that has been widely used because of the theoretical, operational and
interpretational ease. The advantages of electrical methods also include control over
depth of investigation, portability of the equipment, availability of wide range of simple
and elegant interpretation techniques, and the related software etc. Direct current (D.C.)
resistivity (electrical resistivity) techniques measure earth resistivity by driving a D.C.
signal into the ground and measuring the resulting potentials (voltages) created in the
earth. From the data obtained, the electrical properties of the earth (the geoelectric
section) can be derived. In turn, from those electrical properties we can infer the
geological characteristics of the earth.
In geophysical and geotechnical literature, the terms “electrical resistivity” and
“D.C. resistivity” are used synonymously. Several geological parameters which affect
earth resistivity (and its reciprocal, conductivity) include clay content, soil or formation
porosity and degree of water saturation.
The theory and practice of this method for groundwater investigations is well
documented by Bhattacharya and Patra (1968) and Parasinis (1973). The interpretation of
resistivity data and its application to groundwater studies has been given in detail by
Zohdy (1965 and 1975). D.C resistivity techniques may be used in the profiling mode
(Wenner surveys) to map lateral changes and identify near-vertical features or they may
be used in the sounding mode (Schlumberger array) to determine depths to geoelectric
horizons (Ex. depth to water table). Both profiling and vertical electrical sounding (VES)
has been successfully applied to various geological formations by (Flathe 1955; Ogilvy
1967; Ogilvy et al. 1980; Zohdy et al. 1974). Common applications of the D.C resistivity
method include delineation of aggregate deposits for quarry operations, estimating depth
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to water table and bedrock or to other geoelectric boundaries, mapping and/or detecting
other geologic features (Verma et al. 1980).
4.2 Electrical properties of geological formation
The electrical resistivity of a geological formation is physical characteristic,
determines the flow of electric current in the formation. Resistivity varies with texture of
the rock nature of mineralization and conductivity of electrolyte contained within the
rock (Parkhomenko et al. 1967). Resistivity not only changes from formation to
formation but even within a particular formation (Sharma 1997). Resistivity increases
with grain size and tends to maximum when the grains are coarse (Sharma and Rao
1962), also when the rock is fine grained and compact. The resistivity drastically reduces
with increase in clay content and which are commonly dispersed throughout as coatings
on grains or disseminated masses or as thin layers or lenses. In saturated rocks, low
resistivity can be due to increased clay content or salinity. Hence, the resistivity surveys
are the best suited for delineation of clay or saline zone.
Further, combining resistivity data with insitu total dissolved solids (TDS) or
electrical conductivity measurements in wells can help identify shallow contaminated
zones. A combination of Hydrogeological, geophysical and geochemical investigations
can be very effective in the detection of contaminant migration (Sankaran et al. 2005).
Detection of contamination due to mine seepage, oil field leakage and hazardous waste
disposal were discussed by Warner 1969; Kelly 1976; Urish 1983; Mazac et al. 1987;
Ebraheem et al. 1990 and 1996 and Barker et al. 1981. Similarly, in the present study also
attempt has been made to trace the extent of pollution due to industries and anthropogenic
activities within the study area based on the resistivity methods.
4.3 Electrical Resistivity Method
In resistivity method of electrical prospecting, an electric field is artificially
created in the ground by means of either galvanic batteries (DC) or low frequency AC
generators. The energizing current is sent in to the ground by means of two grounded
electrodes, called the current electrodes designated as „A‟ and „B‟ placed at two selected
points. The potential in the area is measured by another two more grounded electrodes
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called the potential electrodes designated as „M‟ and „N‟. Electrical resistivity is defined
as the resistance offered by a unit cube of material for the flow of current through its
normal surface. If „L‟ is the length of the conductor and „A‟ is its cross-sectional area,
then the resistance (R) is defined as
R=L/A
In MKS system the unit of resistivity is Ohm-meter(W-m). The reciprocal of
resistivity is called conductivity and denoted by σ, the unit of conductivity is mho/meter.
4.3.1 Apparent resistivity
For a homogeneous and isotropic conducting medium ρ is independent of the
position of electrodes on the surface and electrode configuration while measuring the
potential difference between any two points in a four-electrode array comprising a pair of
current and potential electrodes. Hence, it is designated as true resistivity of the medium
(Bhattacharya and Patra 1968 and Sharma 1997). For heterogeneous medium, the
resistivity is called the apparent resistivity. The apparent resistivity of geologic formation
is equal to the true resistivity of fictitious homogeneous and isotropic medium in which,
for a given electrode configuration and current strength, I, the measured potential
difference ∆V is equal to that for the given heterogeneous and anisotropic medium. The
apparent resistivity depends upon the geometry and resistivity of the elements
constituting the given geologic medium.
a = K (∆V/I)
Where K is the geometrical factor having the dimension of length (m). Resistivity
of rock formations varies over a wide range; depending on mineral constituents of rock,
density, porosity, pore size and shape, water content, quality of water and temperature.
There is no fixed limit for resistivity of various rocks; igneous and metamorphic rocks
yield values in the range of 102 to 108 Ω-m; sedimentary and unconsolidated rocks vary
between 1 to 104 Ω-m.
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4.3.2 Resistivity measurements
Generally, for measuring the resistivities of the subsurface formations, four
electrodes namely two current electrodes A and B and two potential electrodes M and N
are required. There are different electrode arrangements for measuring the potential
difference, which are uniquely used for different purposes in exploration techniques
(Keller and Frishknecht, 1966). The most popular among them are Wenner (1915) and
Schlumberger (1920).
4.3.3 Schlumberger array
The Schlumberger array, consist of four co-linear point electrodes to measure the
potential gradient at the midpoint. In this array, the current electrodes and potential
electrodes are spaced in the ratio of 1:5 and the geometrical factor K for this array is
given by
K = {(AB/2)2-(MN/2)
2}/MN
(i. e.) K = (s2 - b
2)/2b
Apparent resistivity a is calculated as a = K (∆V/I)
Where, s = half spacing of current electrodes and b = half spacing of potential electrodes.
A M O N B
____________ ___ ___ ___________
s 2b s
Where s 5b
The above sketch is the schematic representation of Schlumberger electrode
configuration, when AM = MN = NB = s, results the Wenner configuration.
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4.4 Vertical electrical sounding (VES)
Resistivity sounding is the study of resistivity variation with depth for fixed center
i.e. vertical investigations of subsurface geological layers. It is also called as vertical
electrical sounding (VES). This method gives the information about depth and thickness
of various subsurface layers and their potential for groundwater exploitation. Since the
fraction of total current flows at a depth varies with the current electrodes separations, the
field procedure is to use a fixed center with an expanding spread. The Wenner and
Schlumberger arrays are particularly suited to this technique, where in Schlumberger
array has some specific advantages. There are always some naturally developing potential
(self-potential, SP) in the ground, which have to be eliminated and nullified. Thus, in
such electrode configuration, the potential difference for a selected value of AB/2 is
measured and in turn, the festivities are obtained. The resistivities are plotted against
AB/2 on a double log graph. A log-log plot of the apparent resistivity versus current
electrode spacing (AB/2) is commonly referred to as the “sounding curve”. Resistivity
data is generally interpreted using the “modeling” process. A hypothetical model of the
earth and its resistivity structure (geoelectric section) is generated. The theoretical
electrical resistivity response over that model is then calculated and compared with the
observed field response. The differences between the observed and the calculated are
then adjusted to create a response, which very closely fits the observed data. When this
iterative process is automated, it is referred to as “iterative inversion” or “optimization”.
The product from a D.C resistivity survey or VES is generally a “geoelectric”
cross section showing thickness and resistivities of all the geoelectric units or layers. If
borehole data or a conceptual geologic model is available, then a geologic identity can be
assigned to the geoelectric units. A two dimensional geoelectric section may be made up
of a series of one-dimensional soundings joined together, which yield the required
subsurface information.
4.5 Multi-electrode resistivity imaging (MERI)
The improvement of resistivity methods using multi-electrode arrays has led to an
important growth of electrical imaging for subsurface surveys (Griffith et al. 1990;
Griffith and Barker 1993). The multi-electrode resistivity technique is now fairly well
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established with respect to theory, practical application and interpretation techniques