Geotechnical Engineering

DHARM

SOIL EXPLORATION 749

in electrical potential between known horizontal distances within the electric field created by

the current electrodes.

The Wenner configuration with four equally spaced electrodes is simple and is popu-

larly used. The four electrodes are placed in a straight line at equal distances as shown in

Fig. 18.17.

D D D

Battery Milliammeter

Voltmeter

G.L. Electrode

Fig. 18.17 Wenner configuration for electrical resistivity

A direct voltage, causing a current of 50 to 100 milliamperes typically, is applied be-

tween the outer electrodes and the potential drop is measured between the two inner elec-

trodes by a null-point circuit that requires no flow of current at the instant of measurement.

In a semi-infinite homogeneous isotropic materials the electrical resistivity, ρ, is given

by:

ρ =^2 πD

E

I

. ...(Eq. 18.11)
where, D = distance between electrodes (m),
E = potential drop between the inner electrodes (Volts),
I = current flowing between the outer electrodes (Amperes), and
ρ = mean resistivity (ohm/m).
The calculated value is the apparent resistivity, which is a weighted average of all ma-
terial within the zone created by the electrical field of the electrodes. The depth of material
included in the measurement (depth of penetration) is approximately the same as the spacing
between the electrodes.
It is necessary to make a preliminary trial on known formations, in order to be in a
position to interpret the resistivity data for knowing the nature and distribution of soil
formations. Average values of resistivity ρ for various rocks, minerals and soils are given in
Table 18.7.

Two different field procedures are used to obtain information on subsurface conditions.
One method, known as “electrical profiling”, is well-suited for establishing boundaries between
different underground materials and has practical application in prospecting for sand and
gravel deposits or ore deposits. The second method, called “electrical sounding’, can provide
information on the variation of subsurface conditions with depth and has application in site
investigation for major civil engineering construction. It can also provide information on depth
of water-table.