Soil Resistivity Calculator

What is Soil Resistivity?

Soil resistivity is a measure of how much the soil resists the flow of electricity. It is a critical factor in electrical grounding grid design, lightning protection systems, and cathodic protection design for buried pipelines and structures. Lower soil resistivity allows electrical currents to dissipate more easily into the earth. For related electrical tools, check our Voltage Drop Calculator and Resistance Converter.

The Wenner Four-Pin Method

The Wenner method is the most common test used to measure apparent soil resistivity. It involves driving four electrodes (pins) into the ground in a straight line at equal spacing $S$. An electric current $I$ is injected through the outer two pins, and the voltage drop $V$ is measured across the inner two pins.

The apparent resistivity ($\rho$) is calculated using the formula:

$$\rho = 2\pi \cdot S \cdot \frac{V}{I}$$

Where:

• $\rho$ = Apparent soil resistivity (Ω·m)
• $S$ = Electrode spacing (m)
• $V$ = Measured voltage (V)
• $I$ = Injected current (A)

Electrode Spacing and Depth

In a Wenner array, the effective measurement depth is approximately equal to the electrode spacing $S$. By increasing the spacing between the electrodes, engineers can measure resistivity at progressively deeper soil layers, allowing them to construct a resistivity profile with depth.

Typical Soil Resistivity Ranges

Resistivity varies widely depending on moisture content, temperature, salt concentrations, and soil type:

• Wet Organic Clay: $5$ to $30\text{ }\Omega\cdot\text{m}$ (highly conductive)
• Loam / Topsoil: $30$ to $100\text{ }\Omega\cdot\text{m}$
• Moist Sand / Gravelly Clay: $100$ to $300\text{ }\Omega\cdot\text{m}$
• Dry Sand / Gravel: $300$ to $5000\text{ }\Omega\cdot\text{m}$
• Bedrock / Solid Granite: $10000$ to $100000+\text{ }\Omega\cdot\text{m}$ (highly resistive)