An Ultra-Precise System for Electrical Resistivity Tomography Measurements

One of the most widely used geophysical methods for monitoring flow within the shallow subsurface is Electrical Resistivity Tomography (ERT). The latest generation of ERT systems can monitor data to a precision of around 1%, allowing in situ changes to be interpreted within a few percent. In turn, subsurface temperature changes can be monitored to within a few degrees Centigrade, low levels of tracers ¿ only slightly higher than background variations ¿ can be tracked, and fracture propagation on a site of several thousand square meters can be watched. This project will make improvements to hardware and data collection procedures that will raise ERT precision by an order of magnitude. In Phase I, modifications were made to a number of interdependent hardware and software systems, resulting in reductions in both random and systematic noise sources. Random noise was reduced by implementing longer data averaging periods and by improving stacking algorithms. Systematic noise was reduced by improving hardware design, including the isolation of transmitter and receiver paths in the multiplexer and the use of temperature-stabilized higher-precision components to improve calibration. Electrode and cable effects also were found to be larger sources of noise than anticipated. Phase II will investigate the use of alternative materials for the electrode and cable, in order to mitigate the noise from these sources. Alternative data collection strategies will be investigated. Improvements to hardware and software will focus on calibration. Field trials will be held in difficult environments, such as the Idaho National Laboratory, Box Canyon fractured rock study area, and Vadose Zone Research Park. Commercial Applications and other Benefits as described by the awardee: An improved ERT technology should increase measurement precision, increase resolution, and enhance effectiveness in existing monitoring applications without significantly increasing costs. These improvements should broaden the range of applications for the method, particularly in monitoring flow in the shallow subsurface. The DOE complex alone has been estimated to contain 200 million cubic meters of contaminated soils, including those at the Hanford Reservation, the Savannah River Site, the Idaho National Laboratory, and the Nevada Test site