The Wenner four-probe method is commonly used for soil resistivity testing, where the standard formula for apparent resistivity (ρ = 2πAR) assumes point electrodes at the earth’s surface. In practice, bare rod stakes are driven to a certain depth, and the point-source assumptions of this formula start to break down because current injection and voltage sensing occur along the entire rod length.
An engineer may opt to compensate for this error by using the apparent resistivity equation in IEEE 81 that compensates for probe depth, but current standards do not provide a lot of guidance on the assumptions or limitations of this formula. Can it safely be used for any probe depth?
This presentation/paper will provide a comprehensive derivation of the IEEE 81 probe depth compensation formula to provide the reader an understanding of its effect when used for computing a soil resistivity model. We investigate the error introduced when applying both the uncompensated and compensated formulas to realistic probe depths and spacings by simulating tests with numerical computations. Results show that, for bare driven rods, the IEEE depth-compensation equation can overestimate apparent resistivity for small spacings, while the uncompensated equation underestimates it. An improved rule-of-thumb is proposed which aims to address these under/over compensations.