Imaging Tracer Experiments Using Time-Lapse Crosshole GPR Full-Waveform Inversion

The monitoring of flow and transport processes in aquifers is of crucial importance for the understanding and management of groundwater resources. In recent years, crosshole GPR full-waveform inversion (FWI) has demonstrated a high potential for the characterization of aquifers at a decimeter-scale, which is essential for the enhancement of flow and transport models. To date, the majority of studies were conducted in a steady state, and the establishment of correlations with flow and transport processes has been proven to be challenging. In this study, we show strategies for time-lapse GPR FWI, which enable direct monitoring of flow and transport processes in aquifers. Since GPR FWI can obtain the permittivity and electrical conductivity of the medium, it can analyze different effects on the properties caused by different tracers. We performed different tracer experiments at the Krauthausen test site in Germany using salt, heat or both in combination. Saltwater tracer showed an increase in electrical conductivity exclusively, while the combined salt-hot water enhanced both electrical conductivity and permittivity. Depending on the tracer, different acquisition and inversion strategies are proposed. The studies demonstrated successfully the potential of the FWI to obtain high-resolution images and to monitor flow and transport processes in aquifers, irrespective of the tracer type. A specific workflow for the time-lapse GPR FWI was developed, depending on the tracer type, and guidelines are provided for the acquisition, processing and inversion of time-lapse GPR data.