What matters most
A Wenner test places four electrodes in a straight line at equal spacing. The instrument drives current through the outer electrodes and measures voltage between the inner electrodes. Resistance is calculated from voltage and current, then converted to apparent soil resistivity using the array geometry and applicable correction considerations.
Key takeaways
- The four electrodes must be aligned and equally spaced for the basic Wenner arrangement.
- Outer pins carry test current; inner pins sense potential.
- A larger spacing influences a larger and generally deeper ground volume, not one exact depth.
- Raw resistance, spacing, direction, condition, and calculated resistivity belong in the record.
Lay out the array before driving electrodes
The field crew establishes a station, selects a workable direction, measures equal intervals, and checks utilities, fences, pavement, rails, foundations, grounding conductors, and other features that may affect safety or the current path.
Electrode insertion, lead routing, and instrument setup follow the project procedure and applicable method. The line is documented so a reviewer can understand what ground volume and site features influenced the reading.
Measure resistance and calculate apparent rho
The instrument applies a test signal through the current electrodes and measures the response between the potential electrodes. A stable reading is recorded with spacing, units, and any instrument quality indicator used by the procedure.
For the basic ideal Wenner geometry, the geometric relationship converts resistance to apparent resistivity. Field conditions, electrode depth, large spacings, and nonideal geometry may require the correction or interpretation established by the governing method and project engineer.
Repeat across spacings, directions, and stations
Changing spacing changes the influenced volume. A sequence can reveal layering or a change in subsurface behavior that one spacing would hide. Repeat or perpendicular lines can help assess directionality, interference, or questionable contact.
The finished dataset should preserve every valid reading and explain omitted or rejected points. Averaging unlike stations without showing variability can erase the feature the survey was intended to find.
Electrode spacing is not an exact test depth. Describe the spacing, influenced volume, site setting, and model assumptions separately.
This page provides general educational information, not a grounding-system design, cathodic-protection design, installed-system acceptance test, or parcel-wide resistivity conclusion. A Wenner result is apparent soil electrical resistivity for the documented array, spacing, location, orientation, and field condition. The responsible grounding or corrosion engineer must evaluate the complete dataset, earth model, asset geometry, governing documents, and uncertainty.
Frequently asked
Questions about this topic
How many pins are used?+
Four electrodes are used in the basic Wenner array: two current electrodes and two potential electrodes.
Why use several spacings?+
Different spacings influence different ground volumes. A sweep provides information about layering and depth-related changes that one spacing cannot show.
Can the line bend around an obstacle?+
The basic Wenner geometry is a straight, equally spaced line. Obstacles may require a relocated station, a different direction, or another approach approved for the project.
Primary references and further reading
- ASTM International — ASTM G57-20 Official record for the Wenner four-electrode field and four-electrode soil-box method.
- IEEE Standards Association — IEEE 81-2025 Current IEEE guide for measuring earth resistivity, grounding-system impedance, and earth-surface potentials.
- Soil Tests Laboratory — Wenner Four-Pin Testing Calicore Engineering’s testing division and field-service information.