Soil Resistivity Survey Calculator
Reduce a table of four-electrode field readings — Wenner, Schlumberger, or both together — to apparent resistivity. Shows the geometric factor for every row, the sounding curve on a common AB/2 axis, the voltage each reading implies as a field sanity check, and what the spread costs a real electrode.
One reading is not a soil model. A single four-electrode reading returns one apparent resistivity, which in layered ground is neither layer — sweeping the spacing is what turns a number into a model.
The reduction ρa = k·R is exact for a uniform half-space and nothing more. This page does not invert: it will not name a layer resistivity or a layer depth from a curve, and it uses no "depth of investigation is about the spacing" rule of thumb. Fitting layers to a sounding is what a computed model exists for. BS 7430, BS EN 50522, ENA TS 41-24 and IEEE Std 81 are named here as where this work is properly documented, and are not reproduced.
Why Both Arrays, and Why One Axis
Wenner and Schlumberger are not two formulas — they are two ways of standing the same four probes. Superposing the point-source potential ρI/(2πd) of a uniform half-space over the four separations gives ρa = 2πR/(1/AM − 1/AN − 1/BM + 1/BN), and both named arrays fall out of it: 2πa for Wenner, π(L² − l²)/2l for Schlumberger. This tool computes every row through that general factor, so the closed forms are a consequence rather than a second source of truth.
The geometric factor also decides how much signal a survey has: the measured voltage is ρa·I/k, so a geometry with a large factor reads a small voltage — which is why Schlumberger arrays widen MN as AB grows. And it is why every row here is reported against AB/2, the half-separation of the current electrodes: plot Wenner against its own spacing a and the two arrays appear to disagree about the same ground.
More on the method in how a soil resistivity survey works, and take a resistivity into the earth electrode & EPR calculator to size an arrangement.