soil resistivity:

soil resistivity:

Soil resistivity, denoted as ρ, refers to the electrical resistance of a cubic block of soil with dimensions of 1 meter in length, width, and height.

Measurement of ρ
In general, there are two main methods for measuring soil resistivity:
1. Single-rod method
2. Four-point method (which includes the Wenner and Schlumberger techniques).

Four-Point Test
The basic principle of the four-point method involves:
• Applying current through the two outer electrodes.
• Measuring the voltage between the two inner electrodes using a high-impedance voltmeter.
After measuring the voltage (V) and current (I), the resistance (R) can be calculated using Ohm’s Law:
R=VIR = \frac{V}{I}R=IV

Types of Four-Point Methods
1. Wenner Method (Equal Spacing)
o Electrodes are equally spaced apart.
2. Schlumberger Method (Unequal Spacing)

Electrode spacing varies to allow deeper soil resistivity profiling.

Measurement of Soil Resistivity – Wenner Method

In the Wenner method, four electrodes are driven into the ground in a straight line, each spaced equally at a distance a apart. The depth of each electrode in the soil is the same and denoted as b.

After connecting the leads, the resistance R is measured directly using the earth tester (megger or ground resistance tester). The soil resistivity (ρ) is then calculated using the following formula:

ρ=2πaR(1+2aπ(ln⁡a2+4b2a−2ba2+4b2))\rho = 2 \pi a R \left(1 + \frac{2a}{\pi} \left(\ln \frac{\sqrt{a^2 + 4b^2}}{a} - \frac{2b}{\sqrt{a^2 + 4b^2}}\right)\right)ρ=2πaR(1+π2a(lnaa2+4b2−a2+4b22b))

For practical cases where the electrode depth is small compared to the spacing (b ≪ a), the formula simplifies to:

From a theoretical perspective, the driven rods should ideally have a point contact with the soil. However, in practice, it is recommended that the depth b be taken as less than 10% of the spacing a. In this case, b can be neglected compared to a, and the simplified formula can be used:

ρ=2πaR

In this formula, ρ represents the soil resistivity at a depth equal to the electrode spacing a.

👉 The main advantage of the Wenner method is that the parameter a directly indicates the depth at which the soil resistivity test is performed.

Soil Resistivity Measurement – Schlumberger Method

Four-Point Test – Schlumberger Method

As mentioned earlier, the basis of four-point testing is to inject current into the soil through the two outer electrodes and measure the potential difference between the two inner electrodes using a high-impedance voltmeter. The soil resistance is then calculated by applying Ohm’s Law (R = V/I).

In this method:

The two inner electrodes are placed at a distance A from each other.
The two outer electrodes are positioned at a distance of 2d from each other.
The penetration depth of all electrodes into the soil is equal and denoted as p.

Important Notes:

The condition 2.5 × A < d must always be satisfied.
The distance d should be within the range of 2 to 30 meters.
Here, d represents the depth at which the soil resistivity test is effectively performed.

Short link: