Estimating soil conductivity for GPR surveys

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Courtesy of Geoscanners AB

The results of a ground penetrating radar survey depends on many factors. In order to get useful data and good penetration, one of the conditions is that the conductivity of the ground is not too high. The conductivity of the material together with the operating frequency and the dielectric constant allow us to calculate the loss factor that is used to roughly estimate the theoretical penetration.

There are many ways for estimating the bulk relative dielectric constant, one of them with the radar itself. By using some reflection from an object at a known depth and a velocity analysis tool like the one in the GPRSoft PRO software package we can accurately find out the layer's relative dielectric permittivity or RDP.

It is however, a little bit more difficult to estimate the conductivity of the soil if one doesn't have a resistivity meter or a similar measuring instrument. Since this is quite an important parameter I'll try to explain here a simple, yet effective method to estimate the conductivity of the soil under survey.

A little bit of theory first

A while ago, almost one hundred years soon, Mr. F. Wenner suggested that a linear array of four equally spaced electrodes would make all the problems they had at that time with soil-electrode contact go away. Ever since most of the resistivity measurements are carried out using the four electrodes principle.

If we insert four electrodes into the ground, as shown in Figure 1., and apply the current between the electrodes A and B then an electrical potential will appear between the electrodes M and N.

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