Soil Resistivity Testing 10 Common Mistakes
This is a series of short posts on Soil Resistivity Testing and the common mistakes encountered, with practical advice on how to avoid Soil Resistivity Testing 10 Common Mistakes.
A Soil Resistivity Testing weak signal can result from a low-power source, a low-voltage source, or a high contact resistance of one or both of the current injection electrodes.
The problem is most often experienced when driving electrodes in high resistivity surface soils or when the electrode spacing becomes large (the signal strength is inversely proportional to the electrode spacing, for the Wenner 4-pin method, and inversely proportional to the square of the electrode spacing, for the Schlumberger 4-pin method, all other things being equal).
Use of a powerful, high-voltage source is an obvious first step to eliminating this problem.
Even with a good source, however, contact resistance can easily become a problem in high resistivity soils at the larger electrode spacings.
The solution in this case is to drive the current-injection electrodes as deep as possible and wet the soil around these electrodes with saltwater2: this should be done only for the larger electrode spacings.
If need be, multiple rods can be driven into the ground and connected together to constitute a larger, lower impedance electrode. On solid rock or in rock with a shallow soil layer over it, the electrodes can be driven into the ground almost horizontally or laid horizontally on the rock and covered with conductive material, such as salt water moistened earth.
If the rock is highly localised, then the electrode position can be altered (and noted) to avoid the rock; interpretation from competent technical resources using software such as the RESAP module of the CDEGS software package will account for this.