High resolution characterization of time-lapse tracer experiments using crosshole GPR full-waveform inversion: Synthetic and field studies

Crosshole ground penetrating radar (GPR) full-waveform inversion (FWI) has a high potential to characterize the critical zone with a decimeter-scale resolution, which allows the consideration of small-scale heterogeneity influencing significantly flow and transport. The GPR FWI provides high resolution images of two electromagnetic soil properties: dielectric permittivity and electrical conductivity. While permittivity can directly be linked to soil water content / porosity, electrical conductivity depends on soil texture, clay content, and the electrical conductivity of the pore water. Time-lapse geophysical methods are often applied in hydrogeophysics to monitor dynamic processes, e.g., tracer migration, and contaminant leaks. This study investigates the possibilities and limitations of crosshole GPR FWI to map and monitor different tracer plumes during time-lapse experiments. Firstly, different synthetic tracer tests are analyzed that influence mainly either the permittivity (e.g., ethanol tracer) or the electrical conductivity (e.g., salt and heat tracer) of the FWI results. Thus, informative high-resolution hydrological and geophysical data allow for the Krauthausen alluvial aquifer in Germany, the set-up of a hydrogeological model considering the system inherent heterogeneity realistically. This synthetic transport model was used to derive synthetic background and time-lapse GPR data for three different tracer experiments. We performed the FWI for the background GPR data and for different time steps independent from each other. Thereby, we investigated different starting model strategies for the FWI and visualize the tracer plume by subtracting time-lapse results from the background results. Secondly, insights from the synthetic studies were used to design salt and heat field tracer tests at the Krauthausen site to enhance the aquifer characterization and the information generated by the different used tracers. First analysis of experimental crosshole time-lapse GPR data detects changes in the signal and indicates clearly the presence of the heat tracer in the measured plan and indicates possibilities of the GPR FWI to monitor these different tracer types with a high resolution.