CHAPTER III

METHODOLOGY - ELECTRICAL RESITIVITY METHOD

3.1 Introduction

Electrical methods of geophysical prospecting are the most important methods and

they yield significant insights into the resistivity distribution for the subsurface structural

studies and groundwater investigations. The electrical resistivity method is one that has

been widely used because of the theoretical, operational and interpretational advantages.

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, the related software etc. Although most of the geophysical

methods find application in subsurface structural studies and groundwater prospecting,

the choice of a particular method for delineating potential aquifer zone in a given area

depends solely on electrical resistivity method. The theory and practice of this method is

well documented by Bhattacharya and Patra,(1968), Parasnis, (1997) Telford et al,

(1976), Keller. and Frishchnecht(1966). It has emphasized a broad treatment on the

interpretation of resistivity data and its application to groundwater investigations (Zohdy,

1974).Both profiling and vertical electrical sounding (VES) techniques are made use of

with appropriate electrode arrays for understanding lateral as well as vertical distribution

of electrical resistivity and have been successfully applied to subsurface studies as well as

groundwater studies by Flathe (1955), Zohdy (1969),Yungal (1996) and Zohdy et al,

(1974).

3.2 Resistivity method

The most important electrical property of subsurface structure is due to the

electrical resistivity changes, otherwise known as specific electrical resistance and

apparent resistivity. When electrical current is passed into the ground, the magnitude and

distribution of current lines in the subsurface are mostly dependent on effective electrical

resistivity of the subsurface of the study area.

Electrical conduction in subsurface structures can be electronic or ionic due to

electrolytes. However, groundwater available in pores, joints, fissures etc. is conductive

because of the presence of aquifer and that gives rise to finite conductivity in subsurface

formation. Thus the resistivity of a given geological formation is dependent on the nature

and amount of water contained in it and hence resistivity method can effectively be used

to study the groundwater conditions and subsurface structure of a given area.

In general, electrical resistivity method is employed to map contacts separating

formations of different electrical resistivity. The methods, which are used for

investigating lateral and vertical variations of resistivity, are known as profiling and

vertical electrical sounding (VES) respectively (Verma et al, 1980)

3.2.1 Apparent resistivity

In practice, one can measure the true resistivity of formation by using any

electrode array in a homogeneous and isotropic medium. However, in nature, one

encounters heterogeneity and anisotropy more often, and therefore, the resistivity

measured under such conditions is not the true resistivity of the medium, but is the

apparent resistivity.

The apparent resistivity is a formal concept and should not be considered to be

some sort of average resistivity encountered in heterogeneous surface (Parasnis 1997).

Unlike the true resistivity, the apparent resistivity is not a constant physical property and

the measured resistivity is dependent on factors such as (a) there resistivity contrast of the

layers (b) the geometric factors of the electrode configuration (c) the thickness of geo-

electrical layers and (d) the position of the electrodes with respect to lateral

inhomogeneties.

The concept of apparent resistivity is the foundation of the resistivity method.

Though it is not directly related to the true resistivity of the layers, it can however be used

to deduce the true resistivity of subsurface layered structure by making a series of

measurements of the apparent resistivity for different electrode separations. The apparent

resistivity, when plotted against half electrode separations, yields vertical electrical

soundings (VES).

3.2.2 Direct current resistivity method

The direct current (DC) is a traditional way of measuring the resistivity of the

subsurface. By this technique, artificial source of electric current is injected into the

ground through galvanic contact and point electrodes creating stationary current flowing

through the earth as shown in Figure 3.1(a) (Parasnis, 1997; Telford et al, 1990). The

effective resistivity of the subsurface for a given electrode geometry depends upon the

measured potentials. The electric field (E) of the continuous current flows through a

homogenous and isotropic medium and can thus be regarded as the gradient of a scalar

potential. This is a far reaching assumption because it excludes any variation of the field

with time. The type and geometry of the anomaly must be known for the apparent

resistivity as a diagnostic property. These basic techniques are very much useful in the

Figure 3.1 (a) Current flow between the two current electrodes A and B (b) Wenner

Arrangement(c) Schlemberger Arrangement

complex geology, where the subsurface is often highly heterogeneous and has localized

anomalous features. For these applications, more appropriate model of the subsurface is a

two dimensional (2D) model, where the resistivity changes on the vertical as well as

horizontal direction along the survey line.

The construction of the two dimensional images of the subsurface from the

resistivity data is known as 2D electrical resistivity pseudocross section. This technique

reflects different degree of depth realism. The electrical resistivity study involves

measuring a series of constant separation traverse with increase of electrode spacing with

each successive traverse. The increase of electrode spacing increases the depth of

penetration, so that apparent resistivity measured at various depths is used to construct a

vertical contoured section, displaying the variation of resistivity both laterally and

vertically over the section. The 2D electrical resistivity pseudo section study was used in

different locations in order to characterize the geology of the subsurface lithological,

structural and hydrological condition (Queralt et al, 1991; Griffiths and Barker, 1993;

Dahlin,1996; Loke and Barker, 1996; Dahlin and Bernstone, 1997; Dahlin and Loke,

1998; Sullivan and La Brecque, 1998; Josep Jordana et al, 1999; Dahlin, 2000; Michael

Van Schoor, 2002; Zhou and Dahlin, 2003).

3.3 Electrical properties of geological formations

The electrical resistivity of a geological formation is a physical characteristic that

determines the flow of electric current in the formation. Bulk resistivity from surface to

subsurface is controlled by electrolytic conduction by way of pores, fractures, faults and

shear zones. It is dependent on the texture of the rock, nature of mineralization and

conductivity of electrolyte viz, water with dissolved salts contained within the area

Parkomenko et al, (1967). Resistivity changes not only from formation to formation but

even within a particular formation (Sharma, 1997). Besides the resistivity of

unconsolidated formation, it is also governed by other factors like grain size, porosity and

more importantly on clay content. Resistivity is known to increase with grain size and it is

maximum when the rains are coarse (Sharma and Bhashat Rao,1962) and very fine sand

(0.06mm to 0.2mm) tends to have lower resistivity. For any geological formation to be

conductive, it is not necessary that it should contain only conductive minerals, that in

several cases, the presence of clay also decreases the resistivity of the formation

considerably, depending upon the amount and nature of its occurrence. Weathering of

hard rocks gives rise to clayey material which may fill the fractures in hard rocks making

them to be less resistive. In unconsolidated formations, clay materials are commonly

dispersed throughout as coatings on grains or disseminated masses or as thin layers or

lenses. Thus, in the case of saturated rocks, lowering of resistivity can either be due to the

presence of clay or due to salinity. Thus, it is usually possible to distinguish clay or

salinity on the basis of resistivity alone without additional information, viz. geological or

hydro geological or borehole information. The factors which influence the resistivity of a

particular formation are important for groundwater investigations, and subsurface

structural rocks should be taken into consideration while interpreting the electrical

resistivity data.

3.4 Electrode arrangement

3.4.1 Wenner arrangement

A simple method of determining the resistivity of the ground using four electrodes

was explained by Wenner (1916) as in Figure 3.1(b). When four electrodes are placed in a

line, and a known current is passed through the two extreme electrodes, the potential

difference measured between the two inner electrodes gives a measure of the resistivity of

the ground. The value of resistivity ρ is determined by the formula:

ρ= (V/I) х 2 Π a

where, V is the potential in volts measured between the two inner electrodes, and I

is the current in amperes passed into the ground, ‘a’ is the distance between the

successive electrodes and 2 Π, a constant. Since V/I = R, the resistance (in ohms), the

formula may be expressed as

ρ = 2 Π a R, (in ohm-m)

In the above electrode arrangement, the two current electrodes are usually

designated as C1 and C2, while the two inner electrodes, which pick up the potentials in

the ground, are designated as P1 and P2, the potential electrodes or the measuring

electrodes as shown in Figure 3.1(b).

3.4.2 Schlumberger arrangement

In the Schlumberger arrangement, the current electrodes are denoted by A and B

while the potential electrodes are denoted by M and N. The interval between M and N

may be denoted by b, while the interval AB/2 is denoted by a. It is proposed that the

current electrodes AB may be placed as outer electrodes, and the two potential electrodes

M and N as inner electrodes as shown in Figure 3.1(C). This arrangement has the

advantage that in depth soundings, long current carrying cables may be avoided. If the

ground consists of two or more layers having different resistivity, and the electrode

separation (a) is less than the thickness of the first layer, the measured resistivity value

(ρ1) will pertain to the first layer; on increasing the separation to, say (2a), the second

layer having a different resistivity will make itself felt in the apparent resistivity

measured. Therefore, by successively increasing the electrode separation about a central

point, deeper layers of the earth may be involved in the measurement. The apparent

resistivity values obtained by expanding the electrode intervals are plotted against the

respective electrode separations,(a) in the case of Wenner arrangement. In depth

soundings by the Schlumberger arrangement, the electrodes M and N are kept fixed but

the electrodes A and B are moved farther away on either side, i.e., increasing the interval

a, i.e. (AB/2) in successive steps and obtaining the resistivity values for a series of such

increases for one setting of MN with interval of B. With this arrangement, deep

exploration can be made in two or more settings, increasing also the spacing of the

potential electrodes and then moving out the current electrodes farther. When the readings

are completed for a particular spacing of the potential electrodes MN, the spacing of the

latter is increased; whenever such a shift of MN is made, a duplicate reading is obtained

while the current electrodes are still in the old setting. Thereafter, only the current

electrodes are shifted for the next set of observations. The resistivity curves obtained in

depth soundings readily indicate whether two or more layers are involved. Under

favorable conditions, one can determine the thickness and true resistivity of the layers

involved in the measurement.

As already stated, if the ground is homogeneous and isotropic, the values of ρ will

be the true resistivity. Such ideal conditions seldom exist in the ground, and the value of

ρ is usually affected by the variations in the lateral and vertical dimensions, particularly

layered medium. Therefore, the value of resistivity measured refers to an apparent

resistivity ρ of the material in a block of ground whose depth is governed by the electrode

separation measurement. The various techniques for the measurement of resistivity of the

ground will be dealt with in the sequel.

3.4.3 Dipole-dipole arrangement

Resistivity surveys involving depths of more than 1 km, as in oil exploration for

locating basement highs etc., system of electrode arrangements known as Dipole

Sounding Arrangements, has been proposed and extensively used, particularly by Soviet

geophysicists. The Dipole arrangements are usually characterized by the large distance

separating the pair of current electrodes (current dipole) from the pair of potential

electrodes (measuring dipole). Further, the spacing between the potential electrodes is

much smaller than that between the current electrodes. As in the Schlumberger

arrangement the current dipoles are designated as A and B, and the potential dipoles as M

and N. The distance from the center of AB to that of MN may be designated as six

electrode configurations, termed Axial, Parallel, Perpendicular Radial, Azimuthal and

Equatorial spreads. Of these, the Axial, Equatorial and Azimuthal spreads are more

commonly used( Bhattacharya and Patra, 1966).

All the various electrode arrangements mentioned here should give theoretically

the same values of resistivity in a medium, which is homogeneous and isotropic. But in

practical field measurements, there will be differences due mainly to the variations in the

lateral and vertical dimensions of the formations. The geometrical factor G of the

electrode arrangements used will also influence the values of the apparent resistivity that

is measured.

Arrange all the three electrode arrangement, the Wenner and Schlumberger are

widely used in the present study for profiling and sounding respectively. The relative

merits of the Wenner and the Schlumberger arrangements have been discussed by

Bhattacharya and Patra, 1966 and also by Bhimasankaram et al, 1967. They consider that

the Schlumberger arrangement is more advantageous. It would appear that for soundings

Figure 3.2Electrical resitivity meter (DDR3) and field work on electrical resistivity

involving a depth of 100meters, both the arrangements are equally good. However, depth

soundings by Schlumberger arrangement require comparatively larger amount of power

to be sent into the ground than that required for Wenner arrangement.

3.5 Instrumentation

Electrical resistivity methods are very much successful in delineating lateral and

vertical variation of subsurface geology. Detailed Geophysical investigations with

electrical resistivity were carried out with DC resistivity meter IGIS DDR3. It is an

indigenous Digital resistivity meter with several innovative features built into its design

and is known for its high quality data acquisition capability as well as for its field

worthiness. DDR3 resistivity meter is a very compact, elegant and highly reliable

equipment, which is used for resistivity investigations as shown in Figure (3.2). DDR3

resistivity meter consists of two units (i) Current unit (C unit), and (ii) Potential unit (P

unit). This unit has the facility to measure the potential difference across the potential

electrode as also the resistance values provide direct display over digital panel metre.

While the current units serve the purpose of sending the required output of constant

current, the potential unit provides an accurate measurement and display.