The simplest array is one in which one of the current electrodes and one of the potential electrodes are placed so far away that they can be considered at infinity. This configuration with its formula for apparent resistivity is shown below.
This array can actually be achieved for surveys of small overall dimension when it is possible to put the distant electrodes some practical distance away. For a survey in an area of a few square meters infinity can be on the order of a hundred meters. Pole-pole sounding data is plotted as apparent resistivity vs. “a”.