Introduction
The Wenner four-pin method is the most widely recognised method for soil resistivity testing, using four pins at equal distance from each other along a straight line. This method has been primarily used in geological surveying since it was first developed in 1915 by Dr. Frank Wenner.
Beyond Wenner’s initial work there have been several developments since, identifying proven alternative methods for measuring soil resistivity.
This post has been put together noting the key factors and necessary information for three of the lesser known methods of soil resistivity testing.
Equatorial Dipole-Dipole Soil Resistivity
To a depth of 0.5m, with a dipole length of 1m, the depth curve sensitivity is considerably higher than the standard 4 pin Wenner method.
At a depth greater than 0.5m, the sensitivity-depth-curve decreases exponentially.
The required maximum depth reading shall not exceed 2.5 metres (due to loss of sensitivity).
A length of 0.5m with a dipole length of 0.3m provides the most sensitive-depth-curve.
Evidently, Equatorial Dipole-Dipole soil resistivity testing is most efficient at shallower depths with shorter dipole lengths.
With this method, you can penetrate further into the ground than the dipole-dipole standard array (so long as the length of the survey line is the same).
Although the equatorial array penetrates further into the ground, doing so will generate a loss in signal as the depth is expanded, which greatly limits its use.
This method is incredibly useful for situations where multiple readings of top-surface soil is required to a maximum depth of 2.5 metres.
Summary
Ideal for shallow depths requiring highly sensitive applications out in the field.
Wenner Schlumberger Soil Resistivity
A combination of the Wenner (most widely used method for soil resistivity and earthing purposes) and Schlumberger methods (high voltage signal, less sensitive.)
A large difference between this method and the more commonly know ‘Wenner’ method is that using Wenner, the pins are all equally distanced apart whereas in Schlumberger, the pins are not equally distanced and primarily, only two electrodes are moved to take a new reading whereas using Wenner, all four electrodes need to be moved to take a new reading.
This method of soil resistivity testing is best suited to increased depth requirments due to the high voltage signal.
Less applicable for shallower, more sensitive operations.
More practical to use when the task is to plot soil resistivity at several different depths.
Best suited for ground water and aggregate mineral terrain.
This method also takes less time to deploy than the standard Wenner array when changing pin spacing (i.e. moving two outer pins as opposed to all four pins).
Summary
Ideal for greater depths, with reduction in time spent altering pin spacings when compared to standard 4 Pin Wenner technique.
Dipole-Dipole Soil Resistivity
A dipole is a pair of oppositely charged electrodes that are so close together that the electrical field forms a single electrical field rather than a field from 2 different electric poles.
To conduct a survey using the dipole-dipole method, you place a large number of electrode stakes out with equal spacing between the stakes. i.e. 100 electrodes spaced 1 metre apart – This would generate a 99 metre long profile of the surveyed area to a depth of 1 metre.
To collect the high number of depth samples from the pin array, specialist equipment with a multiple core harness is required.
The apparent resistivity data is plotted at the midpoint between the 2 dipoles and at a depth half the distance between the 2 dipoles.
Typically 99 readings are measured and stored in approximately 15 minutes (this excludes the installation of the pin array and connection of the cable harness).
Summary
Ideal for high sensitivity measurement at increased depths. However, specialist equipment is necessary.
