Multi-electrode arrays (not shown).
Lee array: resembles the Wenner array but has an additional central electrode.
The voltage differences from the centre to the two ‘normal’ voltage
electrodes give a measure of ground inhomogeneity. The two values can be
summed for application of the Wenner formula.
Offset Wenner: similar to the Lee array but with all five electrodes the
same distance apart. Measurements made using the four right-hand and the
four left-hand electrodes separately as standard Wenner arrays are averaged
to give apparent resistivity and differenced to provide a measure of ground
variability.
Focused arrays: multi-electrode arrays have been designed which supposedly
focus current into the ground and give deep penetration without
large expansion. Arguably, this is an attempt to do the impossible, and the
arrays should be used only under the guidance of an experienced interpreter.