Wenner method: Measuring soil resistivity

The Wenner method uses four electrodes (rods or pins) placed in a straight line in the soil at equal spacing (a). The two outer electrodes (C1 and C2) inject current, while the two inner electrodes (P1 and P2) measure the voltage drop. This configuration measures the average resistivity of the soil volume between and beneath the electrodes to a depth of approximately 1.5 × a.

Where:

• ρ = soil resistivity in ohm-meter (Ω·m)

• a = distance between electrodes (in meters)

• V = voltage measured between the inner electrodes

• I = current applied through the outer electrodes

This method provides the average resistivity of the soil volume beneath and around the electrodes, typically to a depth of ~1.5 × a.

• Ensure good contact between electrodes and soil – avoid loose or dry surface soil.
• Take measurements at multiple distances (a) to detect soil layering.
• Avoid interference from nearby conductors, buried utilities, or active electrical systems.
• Use a tester with automatic calculation, like the EM4058 by Megabras, to minimize errors.