Soil resistivity

Soil resistivity is a measure of how much the soil resists the flow of electricity. It is a critical factor in design of systems that rely on passing current through the Earth's surface. Knowledge of the soil resitivity and how it varies with depth in the soil is necessary to design the grounding system in an electrical substation. It is needed for design of grounding (earthing) electrodes for High-voltage direct current transmission systems. In single wire earth return power transmission systems, the earth itself is used as the path of conduction from the end customers (the power consumers) back to the transmission facility.

The soil resistivity value is subject to great variation, due to moisture, temperature and chemical content. Typical values are:

Usual values: from 10 up to 1000 (Ωm)
Exceptional values: from 1 up to 10000 (Ωm)

The SI unit of resistivity is the Ohm-meter (Ωm); in the United States the Ohm-centimeter (Ωcm) is often used instead.

A wide range typical soil resistivity values can be found in literature.

1 Measurement
2 Soil resistivity variability
3 Corrosion
4 References

Measurement

Because soil quality may vary greatly with depth and over a wide lateral area, estimation of soil resistivity based on soil classification provide only a rough approximation. Actual resistivity measurements are required to fully qualify the resistivity and its affects on the overall transmission system.

Several methods of resistivity measurement are frequently employed:

Wenner method

The Wenner four-pin method, as shown in figure above, is the most commonly used technique for soil resistivity measurements.^[1]^[2]^[3]^[4] Using the Wenner method, the apparent soil resistivity value is:

$\rho_E=\frac{4\cdot {\pi}\cdot a\cdot R_W}{1%2B\frac{2\cdot a}{\sqrt{a^2%2B4\cdot b^2}}-\frac{a}{\sqrt{a^2%2Bb^2}}}\,$ ^[5]

where

ρ_E = measured apparent soil resistivity (Ωm)

a = electrode spacing (m)

b = depth of the electrodes (m)

R_W = Wenner resistance measured as “V/I” in Figure (Ω) If a is small compared to b, as is the case of probes penetrating the ground only for a short distance (as normally happens), the previous equation can be reduced to:

$\rho_E=2\cdot \pi\cdot a\cdot R_W\,$ ^[5]

Schlumberger method

In the Schlumberger method^[1]^[3]^[4] the distance between the voltages probe is a and the distances from voltages probe and currents probe are c (see figure above).

Using the Schlumberger method, if b is small compared to a and c, and c>2a, the apparent soil resistivity value is:

$\rho_E={\pi}\frac{c\cdot (c%2Ba)}{a}R_S\,$

where

ρ_E = measured apparent soil resistivity (Ωm)

a = electrode spacing (m)

c = depth of the electrodes (m)

R_S = Wenner resistance measured as “V/I” in Figure (Ω)

Conversion between measures using Schlumberger and Wenner

The conversion between values measured using the Schlumberger and Wenner methods is possible only in an approximate way.^[4] In any cases, for both Wenner and Schlumberger methods the electrode spacing between the currents probe corresponds to the depth of soil investigation and the measured apparent soil resistivity is referred to a soil volume as in figure aside.

The current tends to flow near the surface for small probe spacing, whereas more current penetrates deeper soils for large spacing. Thus, it is usually a reasonable approximation to assume that the resistivity measured for a given current probe spacing represents the apparent resistivity of the soil to a depth of when soil layer resistivity contrasts are not excessive.

If the apparent soil resistivity measured with Schlumberger method ρ_E (with the corresponding electrode spacing a_S and c) is given, assuming that the soil resistivity are referred to a volume as in the figure aside with a=L/3 follows:

$R_W=\frac{\rho_E}{2\cdot \pi\cdot a_W}\,$

with

$a_W=\frac{a_S%2B2c}{3}\,$

where:

R_W = equivalent Wenner resistance (Ω)

a_W = equivalent electrode spacing with Wenner method (m)

a_S = electrode spacing between voltages probe with Schlumberger method (m)

c = electrode spacing between voltages and currents probe with Schlumberger method (m)

If the measured Schlumberger resistance is given, before to the calculate the and values, the apparent soil resistivity has to be calculate as follows:

$\rho_E={\pi}\frac{c\cdot (c%2Ba_S)}{a_S}R_S\,$

The Wenner method is more laborious than the Schlumberger method because it requires many time, long measure cables, large free space, and for big electrode spacing one person per electrode is necessary to complete the survey in a reasonable time. However, the Wenner four-pin method is the most efficient in terms of the ratio of received voltage per unit of transmitted current.

The soil resistivity measurements have not to be affected by existing grounded electrodes. Buried conductive objects in contact with the soil can invalidate readings made by the methods described if they are close enough to alter the test current flow pattern. This is particularly true for large or long objects.