Electrical Resistivity Tomography (ERT)
Subsurface Imaging/Mapping is used to help identify seen or unforeseen subsurface features to better target detailed investigation or help reduce the risk of unexpected ground. Electrical Resistivity Tomography (ERT) and Seismic Refraction Tomography (SRT) are both used to image below the ground to much greater depths than other systems like Ground Penetrating Radar (GPR).
Dual systems (electrical and seismic) are often deployed together to improve the chances of “discovery”. ERT measures variations in subsurface resistivity. These variations can assess groundwater, changes in soil type, saturation, buried features and voids. The technique produces a 2D profile below the ground showing the apparent ground resistivity as coloured contours. Coupling this dataset to a geology model can provide valuable insight into understanding variability below your feet.
The depth of investigations are typically linked to the length of the survey line, 20-25% length to depth. RDCL most commonly uses a 72-electrode system capable of a 360m array, which could image 75-80 m below ground.
Engineering clients often assume a geophysics 2D profile is a direct image or cross-section of geology. Many times seismic techniques are mis-understood to what they deliver and sometimes a different approach is needed. Although ERT doesn’t indicate direct engineering properties, it can often build up a better understanding of the ground, which is crucial for geotechnical or hydrological engineers to design better and improve outcomes.