Soil Resistivity Surveys: Wenner vs Schlumberger

An earthing study is only as good as the ground data feeding it. Model the wrong soil and every number downstream — electrode resistance, earth potential rise, touch and step voltages — is wrong with it. The soil resistivity survey is where that data comes from, and it's worth understanding before you commission one.

Why Soil Resistivity Matters

Soil resistivity (measured in ohm-metres) sets how readily current spreads from an earth electrode into the ground. It directly drives the electrode resistance and therefore the earth potential rise during a fault. Real ground is layered — a dry, high-resistivity topsoil over a wetter, lower-resistivity layer, or the reverse — and those layers change the answer, so the survey has to reveal them.

The Wenner Method (Four-Probe)

The workhorse. Four probes are driven into the ground in a straight line at equal spacing a. Test current is injected through the two outer probes and the voltage measured across the two inner probes. The apparent resistivity is ρ = 2πa·R, where R is the measured resistance. Increasing the spacing a pushes the current deeper, so a set of readings at growing spacings builds a picture of resistivity with depth.

The Schlumberger Method

A variation where the inner potential probes stay close together while the outer current probes are moved progressively outward. It's faster for deep soundings — you move only the outer probes — and holds a better signal at large spacings, at the cost of a slightly more involved calculation. For deep or large sites it's often the more practical choice; for routine work the Wenner array's simplicity wins.

From Readings to a Soil Model

The raw output is apparent resistivity plotted against probe spacing. That curve is then inverted into a layered soil model — typically two or three layers with a resistivity and thickness each. It's this model, not the raw readings, that goes into the earthing software (SES MultiFields / the CDEGS suite) to compute electrode resistance and EPR.

What a Good Survey Delivers

• Readings taken to a maximum spacing at least as large as the electrode system you're modelling — too short and you miss the deep layer that dominates a large earth grid.
• More than one traverse, ideally in different directions, to catch lateral variation.
• Recorded probe spacings, instrument, date and recent weather (soil moisture strongly affects results — a survey after a drought reads very differently from one after rain).
• Notes on buried services and fences that could distort readings, kept clear of the array.

What This Means for Your Study

If you're commissioning an earthing study, get the soil survey specified properly up front — it's the cheapest way to avoid a re-visit. We can advise on the survey scope before it's done and interpret the data into a layered model afterwards. For background on what the model then produces, see what is earth potential rise.