# ELECTRICAL RESISTIVITY TEST OF SOIL – GEOPHYSICAL METHOD OF SOIL EXPLORATION

This method relies on the difference in electrical resistance of different types of soil (and rock). The flow of current through soil is mainly due to electrolytic action and therefore depends on the concentration of dissolved salts in the pores. The mineral particles of soil are bad conductors of current. Therefore, the resistivity of the soil decreases as both the water content and salt concentration increase.

For example, clear sand densely above water level will exhibit a low degree of saturation and high resistivity due to the apparent absence of dissolved salts. Saturated soils of high void ratio, on the other hand, will exhibit low resistivity due to the relative abundance of pore water and free ions in that water.

There are several methods by which field resistivity is measured. The most popular of the methods is the Weiner method.

## Weiner’s method

In the Weiner arrangement there are four equally spaced (a) Drive the electrode about 20 cm into the ground as shown in the following figure.

In this method a dc current of known magnitude (I) is passed between two external (current) electrodes, creating an electric field within the soil, the pattern of which can be determined by the resistivity and boundary conditions of the soil within the field. Potential drop through the inner electrode’I‘ Current flow lines are measured for the surface. apparent resistivity’R‘, is calculated using the following equation Where,

a in centimeters,

I in volts,

I in amperes, and

R Ohm-cm. In

The apparent resistivity represents the weighted average of the actual resistivity to the depth a In a large amount of clay, the soil closer to the surface is more heavily loaded than the soil at greater depth. The presence of a level of low resistivity forces the current to flow closer to the surface resulting in a higher voltage drop and hence a higher value of apparent resistivity. The opposite is true if the low resistivity level is below the high resistivity level.

The method known as electrocution is used when variation of resistivity with depth is required. From this the type and depth of the levels can be estimated. A series of readings are taken, the (equal) spacing of the electrodes being increased for each successive reading. However, the center of the four electrodes remains at a fixed point. As the distance increases, the apparent resistivity is affected by greater soil depth. If the resistivity increases with increasing electrode spacing, it can be concluded that an inherent level of higher resistivity is starting to affect the reading. If the resistivity decreases with increased separation, on the other hand, the reduced resistivity begins to affect the reading.

The apparent resistivity is, preferably, plotted against the spacing on log paper. The characteristic curves for the two layer structure are shown in the following figure. 