Impact of ERT data inversion uncertainty on the assessment of local hydraulic properties from tracer test experiments

The use of geophysical methods as tools for the characterization of subsurface transport processes has increased considerably in recent years. Time-lapse electrical resistivity imaging (ERT), in particular, has been demonstrated to be a powerful tool for solute transport characterization since a full picture of the spatio-temporal evolution of the process can be obtained. ERT can provide spatially and temporally highly resolved information on subsurface parameters, which are closely linked to both structural and transport properties. However, the quantitative interpretation of solute tracer experiments is made difficult by the uncertainty related to the data inversion as well as to the a priori unknown hydraulic properties (e.g. porosity, hydraulic conductivity, storativity, etc.) in heterogeneous natural formations. A modeling approach based on the Lagrangian formulation of transport and the ensemble Kalman Filter (EnKF) data assimilation technique has been proved to be useful for the assessment of the concentration evolution and the retrieval of the local aquifer heterogeneity when “true” synthetic concentration data are assimilated. This modeling framework has been developed under the assumption that the solute spreads as a passive tracer, so that for high values of the Peclet number the spatial moments of the evolving plume are dominated by the porosity and the spatial distribution of the hydraulic conductivity. In this presentation we investigate the influence of the uncertainty of ERT data inversion on the model's retrieval capability. The data consist of 3-D images of tracer test experiments derived by cross-hole ERT for a synthetically generated heterogeneous aquifer. The assimilation of ERT data in terms of either spatially distributed concentration or resistivity allows the correction of the system state (in terms of concentration) as well as the retrieval of the spatial distribution of saturated hydraulic conductivity.