QUANTITATIVE APPRAISAL OF ELECTRICAL RESISTIVITY TOMOGRAPHY FOR MONITORING SALINE TRACER TESTS IN HETEROGENOUS SEDIMENTS

Electrical resistivity tomography (ERT) is examined as a method to provide spatially continuous images of saline tracer concentrations during transport through an unconfined aquifer. Field data were collected at the Massachusetts Military Reservation, Cape Cod, Massachusetts. High-resolution 3-D images of the movement of an electrically conductive sodium-chloride tracer are inferred from the geophysical data through time. Sixty 3-D ERT snap shots were collected between four corner-point wells for 20 days following the 9-hour injection. Concentrations with an estimated range of 0 to 600 mg/L NaCl were measured at a 15-point multilevel sampler centrally located within the ERT array, at the production well, and at two wells external to the central array.

The tomograms indicate movement of the saline tracer over time and space that is consistent with measured concentration data. The magnitude of the tracer concentration, however, is underestimated due to scaling effects, geophysical regularization, and an averaging process. To use the geophysical data quantitatively, calibration of the resistivity tomograms with respect to known concentrations is required. By accounting for the expected geophysical response, the ERT data serve as an appropriate surrogate for concentration maps that are otherwise impossible to obtain. Such maps can serve as data to help delineate aquifer heterogeneity.