Electrical resistivity method

This method is based on Ohm’s law. Electrical resistivity is the resistance offered by a unit cube of a medium when traversed by a unit current at right angles to one face. For material with cross sectional area A and length L:

ρ = (R.A)/L (4.5.1)

Where ρ= resistivity in Ohm-metre (Ωm)

R = Electrical resistance offered by the cube in Ohms (Ω) A = cross sectional area (m²)

L = length (m).

In measuring electrical resistivity of different subsurface layers, a known current is sent through a pair of electrodes thrust into the soil, the resulting potential difference being measured by another pair of electrodes. The objective of such measurements is to locate layers of low resistance which may be associated with groundwater, as the water within the pores of rock renders them more conductive to flow of electric current. The electrical resistivity of the strata is controlled by two components: the solid material and pore fillings. If the pores are filled with water, the rock will have lower resistivity; even lower when the water is mineralised.

In Figure 4.5.1 A and B are two electrodes that are used to send current into the ground and are referred as current electrodes, whereas M and N are used to measure the potential difference and thus referred to as potential electrodes. The potential difference varies with position and geometry of the four electrodes for the given strata segment. The most common electrode configuration is that of Wenner and Schlumberger. In the Wenner configuration all the four electrodes are equally spaced, and are disposed symmetrically; in the case of Schlumberger array the distance between current electrodes is at least 5 times the distance between the potential electrodes.

The different layers of the earth being non-homogeneous cause distortion or refraction of current flow lines and thus of the electric potential field. The measured resistivity is generally called an apparent

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resistivityρ_a. The apparent resistivity may represent the weighted average of true resistivity (10⁰ to 10⁵Ohm-m) of formations and that of other underlying features (Karanth, 2001), and also depends upon several variables such as electrode spacing, geometry of the electrode array and anisotropic properties. Resistivity surveys are generally carried out in two different ways 1) resistivity profiling and 2) vertical resistivity soundings (VES). A resistivity profiling survey focuses on the lateral changes in the resistivity characteristics of strata, while in sounding the vertical variations of the electrical parameters of rocks are evaluated. For profiling, the Wenner electrode configuration is most suitable and the apparent resistivity value at a site is obtained by shifting the whole electrode array along a profile. Profiling is particularly useful to locate the vertical contacts of different geological structures such as faults, dykes, a stream channel or the fresh water/sea water interface. Vertical electrical soundings are useful in defining the electrical characteristics in depth section, and particularly useful in delineating low resistive layers indicating the possible presence of groundwater. The depth dependent variation in apparent resistivity is particularly useful for determining the depth of a water bearing formation, its thickness, delineation of different lithological strata, the depth of bedrock and the fresh water/salt water interface. Using VES with Wenner or Schlumberger configuration the thickness of different layers may be calculated.

The newly developed 2D (Barker, 1978 and 1992) resistivity imaging, using 20 or more electrodes (Fig. 4.5.2a) combines sounding and profiling to investigate complicated geological structures with lateral and vertical resistivity changes. The advantages of 2D measurements are their vertical and lateral resolution along the profile, computer based data acquisition and small field crew required. The information regarding an aquifer, its exact depth etc. is obtained through the concept of pseudo sections, which divide the subsurface into a number of rectangular blocks that provide apparent resistivity of the pseudo section that agrees with actual field measurements (Fig. 4.5.2b).

Fig. 4.5.3a shows the application in a basaltic area to identify shallow and deep aquifers. The method is also useful for delineating intrusive lineaments such as dykes or quartz veins. This can be illustrated by an example shown in Fig. 4.5.3b obtained by an ERT (Electrical Resistivity Tomography) survey that was carried out at the Kothur quartz vein (~25 m thick) in the Maheshwaram watershed located in a Figure 4.5.1. Principle of the resistivity method using two current electrodes and two potential

electrodes. Strata with different resistivity affect the current distribution by refraction of the field in the earth (Kovelesky et al. 2004)

granitic terrain near Hyderabad, India. The resistivity image reveals the weathering effect at the contact zone, which was confirmed by successful drilling of a bore well.

With proper survey design 2D as well as 3D pictures of the subsurface can be obtained. Significant variations in the topography can also be taken into account in a final modeling of the electrical resistivity data providing parameters such as depth of the water bearing strata, stratification, and the fresh water/salt water interface which no other geophysical method can provide. However, there are limitations to electrical methods especially when the depth range exceeds 500 m and further hetero -geneity of different strata reduces the resolution. The presence of highly conductive layers at shallow depths (e.g. reactive water) masks the less conductive layers thus making the method less effective.

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Figure 4.5.2a Photograph of multi-electrode system being used in the field for 2D resistivity imaging

Figure 4.5.2b. A field layout for 2D resistivity imaging

Similarly, if the upper layers are too resistive, the method becomes less effective for delineating conductive layers below.

Deep resistivity measurement is a well-established tool for delineating deeper aquifers in sedimentary terrains, identified by zones of low resistivity. Based on such data, drilling at six sites in the drought prone Barmer district, Rajasthan, was successful in identifying aquifers resistant to drought with yields ranging from 2.5 to 12.5 l s^-1of potable water (Singh et al, 1990).