What matters most
Increasing Wenner electrode spacing generally increases the lateral and vertical volume that influences the measurement. There is no universal rule that makes the spacing equal to one exact investigation depth. The useful spacing sequence comes from the grounding or corrosion objective, anticipated layering, facility size, and available straight-line distance.
Key takeaways
- Spacing is array geometry, not an exact depth label.
- Use a sequence wide enough to reveal important trends.
- The largest spacing needs substantial clear linear distance.
- Plan alternate lines when access or buried infrastructure prevents the intended sweep.
Current spreads through a volume of earth
Current from the outer electrodes does not travel along a narrow ray. It spreads through the ground, and the potential electrodes sense the resulting field. Every reading is influenced by a broad three-dimensional volume.
Larger spacings extend that field farther. Surface material still contributes, while deeper material generally becomes more influential. This is why a spacing sweep can show vertical trends without directly sampling one isolated layer.
Choose spacings for the receiving model
The grounding engineer may need close spacings for shallow behavior and larger spacings for the volume surrounding a large grid. A buried-utility corrosion study may prioritize route coverage and a specified spacing series instead.
Use the project layout, target structures, expected layers, available property, and method limits to set the sequence. A geometric or project-defined progression is more defensible than improvising after the crew arrives.
Confirm the site can physically support the array
The full four-electrode line, lead routing, safe clearances, and access must fit. Fences, roads, buildings, slopes, wetlands, crops, energized equipment, grounding conductors, and property boundaries can remove an otherwise convenient direction.
Send a plan showing proposed stations and the longest requested spacing. Calicore can flag likely constraints, backup directions, and whether the project needs additional property access or a revised station plan.
For long-spacing work, clear distance and interference control are often the limiting factors—not instrument lead length.
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
Does a 100-foot spacing test exactly 100 feet deep?+
No. The reading is influenced by a broad ground volume. Depth sensitivity depends on the array and subsurface model; report the spacing separately from any interpreted depth.
How much straight-line space is needed?+
It depends on the largest equal spacing, complete four-pin geometry, safe clearances, lead routing, and backup directions. Provide a site plan for layout review.
Can I use only the largest spacing?+
A single large spacing may miss shallow or layered behavior. The appropriate sequence depends on the project model and specification.
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.