The Wenner method drives four electrodes in a line, equally spaced, injects current through the two outer ones and measures the voltage between the two inner ones. The resulting resistance gives the apparent soil resistivity, which represents the ground down to a depth close to the electrode spacing. It is repeated at several spacings and at several points of the site.
Short answer. The Wenner method measures soil resistivity with four electrodes driven in a straight line at equal spacing. Current is passed between the two outer ones and the voltage is measured between the two inner ones. Voltage divided by current gives a resistance, and from it comes the apparent resistivity of the soil down to a depth close to the electrode spacing. IEEE Std 80-2013 describes it as the most commonly used method and refers to IEEE Std 81 for the details.
What it is measured for
Soil resistivity governs the grid resistance and the step and touch voltages. Clause 13.1 states that investigating the resistivity of the site is essential to know the general composition of the soil and how homogeneous it is. Estimates by soil type give only a rough approximation, so actual tests are imperative (clause 13.3).
The principle of the method
Four electrodes are driven in a straight line, each separated from the next by the same distance a and driven to a depth b. Current is injected between the two outer ones and the voltage is measured between the two inner ones. With the resulting resistance R, the apparent resistivity is:
ρ_a = 2π · a · R
when the electrode depth b is small compared with the spacing a (equation 50 of the guide). Here ρ_a is in Ω·m, a in meters and R in ohms. If the electrodes are not shallow, the guide gives a fuller expression that takes b into account (equation 49).
Current flows close to the surface when the spacing is small and reaches deeper layers when it is large. It is therefore accepted as a reasonable approximation that the resistivity measured with spacing a represents the apparent resistivity of the soil down to depth a, as long as the contrasts between layers are not excessive. Repeating the measurement at increasing spacings gives a curve of apparent resistivity against spacing, which is the basis for interpreting the soil structure.
Why it is the most used method
The guide lists the advantages of the four-electrode method:
- It gives the resistivity of deep layers without driving the electrodes down to those depths.
- It does not require heavy equipment.
- The results are not greatly affected by the resistance of the test electrodes or by the holes they leave when driven.
The guide also mentions a variant, the Palmer method, which gives greater sensitivity for large spacings, and the driven-rod method, based on the three-electrode or fall-of-potential method. The latter shows how deep rods can be driven into the ground, which can avoid redesigning the grid. Its drawback is that a rod driven very deep tends to lose contact with the soil through vibration and through the larger-diameter couplings, which gives resistances higher than the real ones.
What errors appear
| Source of error | What it causes |
|---|---|
| Nearby buried metallic objects | They alter the path of the test current, especially if they are large or long: pipes, rails, cables or the conductors of an installed grid |
| A single measurement point | It does not detect lateral or deep changes; the guide calls for measuring at several places in the yard |
| Too few spacings | It does not show how resistivity varies with depth |
| No record of conditions at the time | Resistivity changes with moisture and temperature; the records must include them |
| Misreading the apparent value | Treating the apparent value of one spacing as the resistivity of a layer |
The guide states that the measurement records should include the soil temperature and moisture at the time of the test, and all available data on buried conductive objects in the area.
How the results are used
Interpreting the apparent resistivities is, in the words of the guide, perhaps the most difficult part of the measurement program. The goal is a soil model that closely approximates the real soil. If the variation of apparent resistivity is moderate, a uniform soil model may serve; if it is large, that model is unlikely to give accurate results and a two-layer model is advisable. The guide also mentions that there are seasonal variations due to climate.
Where it is done
The measurement is made before the design, at the substation site. It requires calibrated measuring equipment, auxiliary electrodes, cables and, for large spacings, free space in a straight line. The detailed procedure, the choice of spacings and the processing of the data depend on each site and are part of our service.
For a grid that is already built, ground resistance is measured with a different method, fall of potential; see ground resistance testing. The instruments that make these measurements are described in Metrel MI 3152 and MI 3155 earth resistance testers and in Metrel MI 3290 earth analyser. To turn your resistivities into a grid design, you can use our free grounding resistance tool, which fits a two-layer model to your Wenner readings and warns when the variation between measurements is large. The later verification, with the potential map, is covered in potential mapping and integrity of the installed grid.
We measure your grounding system
Our team measures the resistivity of your site, validates the data, interprets the soil model and delivers the result ready for the grid design. Field measurements are done in Colombia, or elsewhere on request. See our testing and commissioning service or write to us through contact.
Related guides
- IEEE 80: Substation Grounding Safety
- Soil Resistivity: Factors and Chemical Treatment
- Two-Layer Soil Model from Field Measurements
- Ground Resistance Test: Fall-of-Potential and IEEE 81
- IEEE 81: How a Grounding System Is Measured
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 13.1 (soil structure investigation), 13.3 (resistivity measurements; equations 49 and 50) and 13.4 (interpretation of measurements).
- IEEE Std 81, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.
Frequently asked questions
What does "apparent resistivity" mean?
It is the value calculated from the resistance measured for a given spacing. If the soil is not uniform, it is not the resistivity of one particular layer but a blend of what the current passes through. One value is obtained per spacing, and how the soil changes with depth is deduced from the variation between them.
Why is a measurement at a single point not enough?
Because a site with uniform resistivity over its whole area and to great depth is rare. IEEE 80 calls for tests at several points and, if there are large variations or values so high that they suggest a safety problem, for more readings.
Can you measure inside a site where a grid is already installed?
It is difficult: buried conductors distort the readings. The guide notes that only shallow readings near the center of a very large grid escape the effect, and that approximate readings are sometimes taken outside the grid, with the electrodes placed so as to minimize its influence.