ESTIMATING SPATIAL MOMENTS WITH ELECTRICAL RESISTIVITY TOMOGRAPHY: EFFECT OF SPATIALLY VARIABLE RESOLUTION DURING A SALINE TRACER TEST

Crosswell electrical resistivity tomography (ERT) was used to monitor the migration of a saline tracer in a two-well tracer test conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts. After injecting 2200 mg/L of sodium chloride for 9 hours, ERT data sets were collected from 4 wells every 6 hours for 20 days. Each ERT data set was inverted to produce a sequence of 3D maps that track the plume. Using modified moment analysis of the resistivity tomograms, the mass, center of mass, and spatial variance of the imaged tracer plume were estimated. Although the tomograms provide valuable insights into field-scale tracer migration behavior and aquifer heterogeneity, standard tomographic inversion and application of Archie’s Law to convert electrical conductivities to solute concentration results in underestimation of tracer mass. Such underestimation is attributed to 1) reduced measurement sensitivity to electrical resistivity values with distance from the electrodes and, 2) differential smoothing (regularization) from tomographic inversion. The center of mass calculated from the ERT inversions coincides with that estimated by migration of the tracer plume using 3D advective-dispersive simulation. The 3D plumes imaged using ERT exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well.