Of course, the best place to measure the geo-electric soil characteristics is at the location of interest itself. So what if you want to determine the soil resistivity at the location where a substation (with an earthing grid) is already present? What is the influence of the conductors on the measurement? To answer this question, some examples are simulated in software.
Substation and location of measurement
An example earthing grid with 5×5 m meshes and a total size of 50×50 m is modelled, using a double layer soil model with a top layer of 600Ω.m and 2 m thickness, and a lower layer of 200Ω.m, refer to the picture below. Wenner measurements have been simulated at three locations:
1. right in the middle of the earthing grid, where a horizontal conductor is directly below the measurement electrodes.
2. inside the area of the earthing grid, but starting in the middle of a mesh.
3. five meters outside of the earthing grid.
As mentioned in this article, the electrodes have been placed at a maximum of 10% of the electrode spacing and a maximum of 0.2m deep, whichever is applicable.
Simulation results
The apparent soil resistivity is determined for all three measurement locations and compared to an ideal measurement, refer to the figure below. Based on this graph you can see that all measurements are influenced by the presence of the earthing grid.
Looking more closely you can see however that the soil resistivity value for the top layer is not really affected. The curves more or less have the same origin. Then, the location in the graph where the curve starts to bend, which typically corresponds to the thickness of the top layer, is affected. This means that the soil layer thickness probably is affected by the earthing conductors in the earth. And then there is the final parameter in this example, being the apparent soil resistivity of the lower layer. The value for this turns out to be different in all cases, as the curves all go in different directions as the depth increases.
An analysis of these values results in the following:
| Top layer | Top layer | Lower layer | |
| Resisitivity (Ω.m) | Thickness (m) | Resisitivity (Ω.m) | |
| Ideal | 600 | 2.00 | 200 |
| Measurement 1 | 649 | 1.02 | 4 |
| Measurement 2 | 594 | 1.72 | 48 |
| Measurement 3 | 605 | 2.13 | 172 |
Based on this you can say for this example that you should be able to collect some useful information about the top layer resistivity, with an accuracy of about 10%. If you know where the conductors are placed (e.g. with an underground cable detector) the error may be lower. The layer thickness will probably be estimated to be less thick than it actually is, some 15 to 50% in this example. An estimation of the resistivity of the lower layer is not really possible.
Moving the measurement to several meters outside of the area where the earthing grid is located is really beneficial for the outcome accuracy. A slight overestimation of the values may be applicable for the top layer, and you may expect an underestimation of the lower layer, which seems logical.
Be careful though
The greatest risk is that there are unidentified earthing provisions that may influence the measurements. You may think that you have sufficient distance to conducting elements, but other objects may be also be present and they may disturb the measurements.
Conclusions
Based on the calculations it seems to be that you are able to collect some useful information on the geo-electrical soil characteristics at the location of interest. In general you should be as far away as possible form any objects that can disturb the measurements.
This example is only valid under the given conditions and using the listed parameters. However, it provides some useful insight on the effect of conducting elements in the vicinity of a measurement location.
Learn more about this in our e-learning on soil characteristics, measurements and potentials.