A shallow geothermal experiment in a sandy aquifer monitored using electric resistivity tomography

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole observations of the temperature field at a few locations. Bulk electric resistivity variations can bring important information on temperature changes in aquifers. We have used surface electric resistivity tomography to monitor spatially temperature variations in a sandy aquifer during a thermal injection test. Heated water (48 degrees C) was injected for 70 hours at the rate of 87 l/h in a 10.5 degrees C aquifer. Temperature changes derived from time-lapse electric images were in agreement with laboratory water electric conductivity-temperature measurements. In parallel, a coupled hydrogeologic saturated flow and heat transport model was calibrated on geophysical data for the conceptual model, and on hydrogeologic and temperature data for the parameters. The resistivity images showed an upper flow of heated water along the well above the injection screens and led to a new conceptualization of the hydrogeologic source term. The comparison between the temperature models derived from resistivity images and from the simulations was satisfactory. Quantitatively, resistivity changes allowed estimating temperature changes within the aquifer, and qualitatively, the heated plume evolution was successfully monitored. This work demonstrates the ability of electric resistivity tomography to study heat and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems.