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 electrode(s) are placed within the zone of interest and a grid of receiving electrodes 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. The figure below shows an example of the setup for the RPM method:
Conceptual model for an RPM survey.
Potentials predictably decrease as a function of increasing distance from the transmitting source and are influenced by the type of geologic media in the area. Our method of potential mapping differs from typical mise a la masse processing in the manner in which we remove the primary field. This innovation allows us to merge multiple data sets recorded from different transmitter locations into a cumulative distribution of potential measurements.
The following figure shows an example RPM dataset collected by HGI for Tucson International Airport. In this application the RPM method consists of injecting electric current directly into the paleo-channels at an approximate depth of 120 feet and observing the potential field at 10 ft intervals along a surface grid. The processed data show electric field distortions corresponding to the location of the paleo-channels (in red). Multiple confirmation boreholes were drilled to evaluate the success of HGI’s predictions. Confirmation drilling resulted in a success rate of over 80 percent for hitting gravel versus non-gravel units.
Three-Dimensional example of RPM dataset collected at Tucson International Airport showing an interpreted gravel unit that was subsequently drilled at 80% confidence rate.
As with any geophysical method, the success of characterization using the RPM method depends on the physical property contrasts and geologic conditions that exist within the site area. As such, we always suggest that some amount of testing be completed within a smaller area, before production coverage is ordered.
Applications of Residual Potential Mapping
The method was used to map historic coal mine workings (tunnels, voids, etc.) in areas where city development has encroached on former mine sites. The unpredictable nature of these voids precludes development unless they can be accurately mapped and subsequently remediated. RPM can effectively map the tunnels in areas where the voids are now below the water table and are therefore solution filled. Our Residual Potential Mapping (RPM) technique was applied to nearly 25 acres of undeveloped land. In this case, water-filled voids approximately 60 feet below ground surface were energized with an electric current that creates an electric field mapped at the surface. HGI recommended 5 confirmation sites to verify our results and all predictions proved successful. The figure below shows the RPM dataset from this survey.
RPM Case study results for delineation of underground coal mine workings.
RPM works equally well on shallow depth projects including leak detection. In a sense, a leak within a lined pond which contains a solution, is geophysically very similar to a water bearing fracture and offers a contrast to surrounding areas. Using RPM, we transmit beneath the retaining pond in question and measure resulting potential within the pond itself. Any leaks will allow current to flow into the pond causing a potential response that can be mapped.
Advantages of using RPM:
- It is non-intrusive – the majority of the measurements are made from the surface
- Geophysical data acquisition equipment (electrodes, wire, cables) are temporary
- RPM can be deployed within active industrial sites with heavy cultural interference such as, roads, railroads, parking lots, concrete aprons
- The method has demonstrated almost a 90% success rate for prediction of target feature as evaluated by confirmation borings