If only one of the current electrodes is placed at infinity the configuration and the apparent resistivity are as shown:
Roa = 2*PI*[b(a+b)/a]*V/I
This array is used frequently in resistivity surveying and the spacing are usually described, and taken, in integer multiples of the voltage electrode spacing b = na. The standard nomenclature is to call the potential electrode spacing “a” so the configuration and apparent resistivity becomes:
Pole-dipole sounding data is plotted as apparent resistivity vs. “a”.