As part of our services for energy and infrastructure, TerraDat regularly carry out soil resistivity measurements for earthing installation design and corrosion engineering. These resistivity measurements are made by passing a DC electrical current through the ground using a pair of electrodes and measuring the resulting potential gradient within the subsurface using a second electrode pair. There are a number of different electrode configurations and by gradually increasing the spacing between the current/potential electrodes the depth of investigation is increased.
Traditionally, these tests are carried out using a 4-pin survey technique, which provides a vertical electrical sounding profile at a central location point. However this survey approach has some inherent limitations which include:
An assumption of lateral homogeneity of the ground within the test area which may be up to 150m away from the central test location point for deeper readings.
The results are usually reported in terms of apparent resistivity values i.e. average values from surface.
No allowance for topography which can result in errors in the calculated depth and lateral positioning of the readings.
Given the limitations of the traditional 4-pin methodology, TerraDat resistivity tests are carried out using a multi-electrode resistivity tomography survey approach. Not only does this provide true (as well as apparent) resistivity values at the selected depths, the modelled true resistivity section provides detailed cross-sectional information that will allow the identification of, and compensate for, lateral changes such as geological boundaries and hydrological variations within the subsurface.