A Field Guide to Soil Resistivity Testing

Soil resistivity is the single most influential variable in any earthing design, yet it is the one most often guessed at. A measured profile turns a hopeful sketch into an engineered system, and the good news is that the field procedure is well understood and repeatable.

The Wenner four-pin method spaces four electrodes equally and injects a known current through the outer pair while measuring voltage across the inner pair. It is fast, intuitive and the default for shallow surveys. The Schlumberger arrangement keeps the potential electrodes close together and widens the current electrodes, which improves signal at greater depths and reduces the labour of moving every pin at each step.

Whichever method you choose, remember that resistivity changes with depth, moisture and temperature. Take readings at several spacings to build a layered model, and repeat surveys across seasons where possible — a site measured after heavy rain can read several times lower than the same ground in a dry summer.

Finally, translate the numbers deliberately. Feed your layered model into the electrode design rather than applying a single average value. A two-layer soil with conductive material at depth may favour deep-driven rods, while uniformly high resistivity may call for a wider grid or chemical backfill. The measurement is only useful if it shapes the design.