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.
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
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.
Curve C. For1 The resistivity of layer 1 is less than the resistivity of layer 2; Curve C. For2, Layer 1 has a higher resistivity than Layer 2. The curves become asymptotic to the lines representing the actual resistances R1 and R2 of the respective layers. The estimated layer thickness can be obtained by comparing the observed curves of electrode spacing versus resistivity with a set of standard curves. The process known as electrical profiling is used in the investigation of lateral variation of soil types. A series of readings are taken, with the four electrodes subsequently moved as a unit for each successive reading; The electrode spacing remains constant for each reading of the series. On a natural scale, the apparent resistivity is plotted against the center position of the four electrodes; Such a plot can be used to locate soil conditions with high or low resistivity. The resistivity in a given region can be plotted. The electrical method of exploration has not been found to be as reliable as the seismic method because the apparent resistivity of a particular soil or rock can vary over a wide range of values. Representative values of resistivity are given in the following table.
|Representative values of resistivity. Value 10. are expressed in units of3 ohm cm(after Peck et al, 1974)|
|clay and saturated silt||0 – 10|
|sandy soil and wet silty sand||10 – 25|
|clay sand and saturated sand||25 – 50|
|Sand||50 – 150|
|pebbles||150 – 500|
|weathered rock||100 – 200|
|sound rock||150 – 4000|