Soil Resistivity Testing Method -The Schlumberger Array

The Schlumberger array is a geotechnical investigation method which determines the electrical resistivity of the soil. This is similar to the Wenner probe test, but it uses multiple current electrodes rather than just two. This allows for a more detailed and accurate measurement of soil resistivity.

Soil resistivity testing carried out using different methods, this is another of the three popular approaches to perform soil resistivity testing: 

To complete a Schlumberger array test, you will need the following equipment:

• Multiple current electrodes (usually copper rods or pipes)
• Two potential electrodes (usually copper rods or pipes)
• A resistivity meter or multi-meter
• A hammer or other tool for driving the electrodes into the ground

Process for conducting a Schlumberger array test:

1. Determine the desired length of the probe. This will depend on the depth of soil you want to test and the spacing between the electrodes.
2. Set up the electrodes. The current electrodes should be spaced a set distance apart, typically 20 cm, 50 cm, or 100 cm. The potential electrodes should be placed at a fixed distance from the center of the current electrodes, typically about twice the distance between the current electrodes.
3. Drive the electrodes into the ground. The electrodes should be driven to the desired depth.
4. Connect the resistivity meter or multi-meter to the electrodes. The meter should be connected to the current electrodes and the potential electrodes.
5. Take a reading. The resistivity meter will measure the resistance between the current electrodes and the potential electrodes.
6. Repeat the process at multiple locations to get a representative sample of the soil.
7. Calculate the average resistivity of the soil using the readings from each location.

Arrangement for the Schlumberger Array.

Figure 2 shows the arrangement for the Schlumberger Array. When contact resistance, which is the resistance of the top layer of the ground, is very high, therefore this can lead to an error in the reading of the machine during soil resistivity testing.

Additionally every machine has maximum loop impedance, in some cases where the soil resistivity of the top layer is very high, that will lead to a loop impedance more considerable than the maximum loop impedance of the machine.

In this case, apply the Schlumberger array to the reciprocity theorem. This soil resistivity test method, known as the Inverse Schlumberger Array (ISA). Also, using this method it provides a safer working environment for the resistivity tester under high current supply. Additionally, it reduces the more massive cable needed during the soil resistivity test.

Soil Resistivity Calculation

Calculate the soil’s resistivity using equation 2:

Where:

 L is the distance the centre from the outer probe

 l distance to the centre from the inner probe

Alternative Soil Resistivity Testing Methods:

• Wenner method
• Driven Rod

Click here to find out how to avoid the common mistakes of Soil Resistivity Testing.