INTRODUCTION

The electrical resistivity test is a useful technique for investigating subsurface structures by analyzing variations in their resistance to the flow of electric current, thereby aiding in the identification of groundwater.

The primary objectives of this method in groundwater exploration include:

• Identification of groundwater-bearing formations
• Determination of the thickness and lateral extent of aquifers
• Estimation of the depth to the water table
• Delineation of wetlands
• Determination of the depth to bedrock
• Identification of subsurface structures and stratigraphic conditions such as fractures and dykes
• Assessment of the distribution and configuration of the saltwater–freshwater interface

Thus, the electrical resistivity method serves as an important tool for understanding subsurface geological and hydrogeological conditions.

The resistivity values of rocks are governed by several factors, including the chemical composition of minerals, density, porosity, water content, water quality, and temperature. In general, the distance between the current electrodes is proportional to the depth of investigation. Typically, the effective depth of the soil sample is approximately one-third of the distance between the electrodes.

The geoelectrical method operates on the principle of injecting electrical current into the ground. The electrode system consists of two current electrodes (C₁ and C₂) that introduce the electric current into the subsurface, and two potential electrodes (P₁ and P₂) that measure the resulting potential difference after the current passes through the soil and rock layers.

These four electrodes are inserted into the ground at specified distances. Increasing the spacing between the current electrodes allows the electric current to penetrate deeper into the subsurface layers. As the current flows through the ground, it generates an electrical potential. This potential difference at the surface is measured using a multimeter connected to the potential electrodes (P₁and P₂), which are placed at a smaller compared to the current electrodes (C₁–C₂).

When the spacing between the current electrodes is increased, the depth of current penetration also increases. Consequently, the measured potential difference changes in response to the properties of the subsurface materials, providing information about the type of rock and its characteristics at greater depths.

Measurements in resistivity surveys are carried out by injecting electric current into the ground through two current electrodes and measuring the resulting voltage difference across two potential electrodes. In its simplest form, a resistivity meter consists of a current source and a voltage-measuring circuit, connected by cables to a minimum of four electrodes.

The primary data obtained from a resistivity survey include:

• The electric current (I) injected into the ground
• The positions of the current and potential electrodes
• The resulting voltage difference (V) measured between the potential electrodes

These measurements are used to determine the apparent resistivity of the subsurface.

From these measurements, the apparent resistivity (ρₐ) of the subsurface can be calculated using the equation:

Where,
ρₐ = apparent resistivity (ohm-meters)
V = measured potential difference (volts)
I = current (amperes)
k = geometric factor, which depends on the electrode configuration and spacing

The geometric factor k depends on the configuration and spacing of the current and potential electrodes. Since the subsurface is generally heterogeneous, the measured resistivity represents a weighted average of the resistivities of the different rock materials present. Therefore, the value obtained is referred to as the apparent resistivity.

Several electrode configurations are commonly used in resistivity surveys, including the Wenner array, Schlumberger array, tri-electrode array, and dipole–dipole array.

The Wenner and Schlumberger arrays are generally employed for shallow investigations, whereas dipole systems are typically used for deeper studies. However, the Schlumberger electrode configuration is often preferred due to its advantages in both field surveying and data interpretation.