Before you design an earthing system, you need to know how well the ground conducts. Soil resistivity measurement gives you that, and the choice of method affects both the effort in the field and the quality of the answer. This guide compares the two most common four-pin methods and explains what a four-terminal earth tester does.
What Resistivity Tells You
Resistivity, measured in ohm-metres, describes the soil itself rather than any particular electrode. Combined with the electrode design, it lets you predict the resistance you will get before you dig. Measured at several depths, it shows whether deeper soil is better or worse than the surface, which decides whether to go deeper or wider.
The Four-Pin Principle
In both methods, a current is passed between two outer electrodes and the resulting voltage is measured between two inner ones. Because the voltage electrodes carry almost no current, contact resistance does not spoil the result. That is why a four-terminal instrument is used for resistivity, and why the DET-2000 and the four-terminal CIE/222M both offer it. See earth resistance testers.
The Wenner Method
Four pins are placed in a line at equal spacing a. Resistivity is approximately 2π × a × R, where R is the measured resistance. To profile depth, the spacing is increased in steps, and the reading roughly reflects soil to a depth comparable to the spacing. It is simple, the formula is easy and the layout is easy to repeat, which is why it is the most common field method.
The Schlumberger Method
Here the inner voltage pins stay fixed near the centre while the outer current pins are moved outward. Because the inner pins are not repositioned at every step, there are fewer moves and it is less sensitive to local variation near the pins. It suits deep investigations where long spacings make repeated repositioning of all four pins impractical, but the calculation is more involved.
| Wenner | Schlumberger | |
|---|---|---|
| Pin layout | Equal spacing, all four pins move | Inner pins fixed, outer pins move |
| Field effort | More pin moves | Fewer pin moves at large spacing |
| Calculation | Simple | More involved |
| Typical use | Earthing design and most surveys | Deeper investigation |
Practical tips
Using the Results
Plot resistivity against spacing. A falling curve means deeper soil is better, which favours long rods. A rising curve means deeper soil is worse, which favours a wide, shallow arrangement. Practical consequences for difficult ground are in earth resistance testing in dry and sandy soil, and the electrode test method is in how to test earth resistance.
Doing a soil survey? Talk to CIE about a four-terminal earth tester.