Electrical resistivity surveys involve injecting current into the ground between two electrodes, and measuring the voltage difference between two other electrodes - usually all at the ground surface.  For nearly a century, these surveys have been used to perform one-dimensional profiling (detection of lateral changes in underground electrical properties by moving a fixed electrode array along a survey transect) or sounding (detection of vertical changes by expanding an electrode array about a fixed location). Within the last decade, development of multi-conductor electrode cables and computer-driven, automated switching, as well as innovative processing of large resistivity data sets, have allowed simultaneous performance of profiling and sounding to produce two-dimensional “electrical images” depicting detailed variations in subsurface electrical properties.

Since so many underground features have electrical properties that differ from their surroundings, electrical imaging can be used to detect and delineate an incredible array of targets. These include: clay layers; sand or gravel lenses; the top-of-rock and rock pinnacles or float blocks; saltwater intrusion; leachate plumes; non-aqueous phase liquid (NAPL) hydrocarbons; dam seepage; mineralized zones; water-bearing fractures; buried foundation elements; waste disposal pits or trenches; graves; mines, tunnels, and caves; ice or permafrost thicknesses; and more. For some targets, recording the induced polarization or chargeability improves detection and discrimination.

Each electrical image can depict both the lateral and vertical extents of a target. For multiple transects, the data can be directly processed in three-dimensions to produce sliceable, rotatable “block” images.  Click here to see an example of a three-dimensional electrical image.

Enviroscan performs electrical imaging using both a minimally intrusive galvanic system (with driven electrodes) and a completely nonintrusive capacitively-coupled system.  In addition, we have a multi-electrode downhole cable for use in boreholes.

Typically, electrical images are “one-sided”, i.e. with electrodes only at the ground surface. However, where boreholes are available, images can be collected as “two-sided” (e.g. electrodes in two boreholes) or “three-sided” (electrodes in two boreholes and along the surface between the holes) to increase the image resolution and accuracy.