Electrical Resistivity Monitoring of Heat Tracer to Characterize Lab-Scale Hydraulic Conductivity Distributions

Knowledge of the spatial variations of hydraulic conductivity (K) is crucial to almost every hydrogeological investigation. The representative scale of K estimates from traditional slug and pumping tests are, however, inadequate to accurately predict hydrogeological processes. There is increasing interest in the application of electrical resistivity tomography (ERT) to quantify spatially continuous K variations. ERT estimation of high-resolution K distributions, however, requires continuous injection of saline tracer (ST) into an aquifer over an extended period, which is feasible but impractical. Here, we present electrical resistivity thermography (ERTh) to evaluate the potential application of time-lapse ER monitoring of heat tracer (HT) to characterize high-resolution K architectures. Unlike ST, long term HT experiments are comparatively easier to manage and repeatable with minimal environmental impact. We estimate K variations via petrophysical coupling of flow and heat transport with joint time-lapse ER and discrete multi-level temperature breakthrough curves. We illustrate the strategy with a 2-D lab-scale sandbox experiment. To construct the heterogeneous field, three lenses with high-K properties with each consisting of gravel, coarse sand, and a mixture of coarse and fine sand, were created within a background of comparatively low-K fine sand. The experiment involved continuous injection and extraction of heat, respectively, at the left and right boundaries of the lab-scale aquifer. We simultaneously performed time lapse ER monitoring of the heat transport and temperature measurements at four discrete multi-levels near the heat extraction well. Results of the coupled inversions demonstrate that ER monitoring of heat tracer provides a unique opportunity to characterize high-resolution spatially continuous K variations, which seems more practical for field applications in contrast to that of the traditional ST.