Several electrode array methods are used to measure soil resistivity. The most common methods used in electrical grounding engineering are the Wenner four-pin array, the Schlumberger array, and the driven-rod method.
The Wenner four-pin array (also called the Wenner A-spacing method) is the standard method specified by IEEE Std. 81 for grounding system design. Four equally spaced probes are driven into the earth in a straight line. Current is injected between the outer probes and voltage is measured across the inner probes. Apparent resistivity is calculated as ρ = 2πA(V/I), where A is the probe spacing. This method averages resistivity over a depth of approximately A/3.
The Schlumberger array uses unequal probe spacings with the current probes placed much farther apart than the voltage probes. It provides better depth resolution for deep soil layers and is preferred when investigating stratified soils.
The driven-rod method measures the resistance of a single ground rod and is used to verify installed electrode resistance, not to characterize soil structure. It is not suitable for grounding system design.
E&S Grounding Solutions uses the Wenner four-pin method per IEEE Std. 81 for all grounding design projects. Contact us at +1-310-318-7151 or info@esgrounding.com.