High resolution subsurface characterization of the Krauthausen aquifer using GPR full-waveform inversion (Invited)

Abstract:

High resolution subsurface characterization is critical for the accurate prediction of flow and transport. Over the last decades, several field and modelling studies of the alluvial aquifer at the Krauthausen testsite, Germany, have yielded a rich set of information that provides excellent opportunities to test and validate novel methods. Here, we use full-waveform inversion (FWI) of crosshole ground penetrating radar (GPR) data acquired along several cross-borehole tomographic planes. By stitching together the tomographic images for adjacent crosshole planes, we are able to image the spatial distribution of subsurface electrical properties (dielectric permittivity, electrical conductivity) at the decimeter scale over several tens of meters. Our results show that the dielectric permittivity obtained from GPR correlates strongly with independently measured porosity logs and with grain size data available along boreholes. Compared with traditional ray-based inversion techniques, which are limited in resolution, the FWI results improve the reconstruction of abrupt changes and fine-scale variations in porosity. Densely spaced cone penetration test (CPT) porosity logs, measured several years ago and located close to the GPR transect show a strong correlation with the porosity estimates derived from GPR using the Complex Refractive Index Model (CRIM). Based on the GPR results, additional cone penetration tests (CPT) and direct-push injection logs (DPIL) have been recently measured at selected crosshole plane locations which show detailed local variations in lithology and hydraulic conductivity. Preliminary comparison with the GPR FWI results indicates that the spatial variations in GPR derived permittivity and electrical conductivity match the major lithology and hydraulic conductivity changes obtained from CPT and DPIL. In conclusion, our study suggests that full-waveform inversion of crosshole GPR data can be used to map lithological changes in alluvial aquifers at the field scale with a spatial resolution at the decimeter scale. In particular, the combination of GPR with complementary direct-push investigation tools appears to be a promising strategy for alluvial aquifer characterization.