Electrical resistivity tomography (ERT) is widely used in environmental applications to study processes associated with subsurface contaminants and contaminant remediation. This contamination often originates from industrial areas rife with buried metallic infrastructure such as transfer pipes, well casings, and storage tanks. Such infrastructure interferes with ERT results, generally limiting their utility when ERT might otherwise prove useful for subsurface investigation and monitoring. 

PNNL has developed a method of imaging the subsurface electrical conductivity distribution that removes the effects of the subsurface metallic structures, enabling more accurate assessment and execution of ERT imaging within industrial areas. ERT works by injecting current into the subsurface across a pair of electrodes and measuring the corresponding electrical potential response across another pair of electrodes. Many such measurements are strategically taken across an array of electrodes to produce an ERT data set. These data are then processed to produce an image of the subsurface conductivity structure. PNNL’s E4D software applies a numerical correction to simulate the presence of metallic structures at known locations and dimensions. The method then removes the influence of these structures on the results to show a more accurate image. The process is flexible and customizable to a particular problem, making it suitable for both advanced research applications and more common imaging.

In addition to modeling the influence of metallic structures on the subsurface potential distribution, PNNL’s method allows those structures to be used as current injection and/or potential measurement electrodes, thereby expanding ERT capabilities. The method can also model structures that are discontinuous in space, but electrically connected (for example, two or more buried tanks connected by above-ground transfer piping). Because PNNL’s method enables ERT to focus on estimating soil conductivity, engineers have the opportunity to 1) accurately assess whether imaging in areas with dense metallic infrastructure is feasible, 2) determine what electrode configurations and survey geometries are required to effectively image in the presence of metallic infrastructure, and 3) significantly improve modeling capabilities in areas with conductive infrastructure.

Applications

PNNL’s subsurface infrastructure modeling method can be used to image subsurface electrical conductivity in the presence of buried pipes, tanks, and well casings and other industrial areas with buried metallic infrastructure. It has also been used to assess and develop electrical measurements for long-term autonomous monitoring of pipe integrity (e.g., leak detection monitoring) and deep wellbore integrity.