A convenient method for measuring the soil resistivity is the four-point method given
in Figure 6.32. Four auxiliary electrodes (spikes) are driven into the ground at the same
depth d (d ,1 m) in a straight line with equal spacings s.
A current I is passed between the outer electrodes, and the voltage drop U is
measured between the inner electrodes. The ration U/I gives a resistance value R
and the average soil resistivity to a depth of s metres is then