Residual Potential Mapping
Residual potential mapping (RPM) is partially based on the concept of excitation of mass or mise a la masse, and consists of electrically energizing a target body and measuring the resulting distribution of electric potentials at the surface. Transmitting electrodes (TX) are placed within the zone of interest (within a borehole, spring, etc.) and a grid of receiving electrodes (RX) is placed on the surface. Optimal results for RPM are obtained when the target response is of considerable ionic strength and the background response is relatively resistive.
HGI uses residual potential mapping as a supporting method with other resistivity surveys. For example, a 3D resistivity campaign using a set of closely spaced parallel lines can be augmented with electrodes in a groundwater well.
The figure below shows an example of the setup for the RPM method.
Resistivity mapping of the subsurface can be conducted with several different methods, including two dimensional profiling, three dimensional imaging, and time lapse resistivity. Lately, HGI has been using the RPM data as a supporting method with other resistivity methods. For example, a 3D resistivity campaign using a set of closely spaced parallel lines was used to map a sulfate plume. Nearby groundwater wells were augmented with an electrode in each of the nine wells and four seeps to act as both TX and RX electrodes. The extra groundwater wells increased sensitivity of the resistivity method deeper than from surface electrodes alone. The plot below shows the voltage potential distribution at the surface while transmitting current on well ZL-315. The high voltage is centered at the well. The RPM data in the middle plot were created by removing a primary field comprised of a resistivity of 340 ohm-m. The residual resistance after subtracting the primary field shows areas that are lower and higher in resistivity than near the well. The well appears to be hydraulically connected to the stream at the north end of the site and to the far fault creating the natural drainage. The last image shows a slice through a 3D model that was created by using all surface and well electrode data. The southern portion of the site is conductive and is the location of reclamation activities. A conductive feature extends from the reclaimed area to the stream, which provides evidence for contaminant pathways.