High resolution imaging of aquifer properties using full-waveform GPR tomography

Abstract:

Highly resolved characterization of the spatial distribution of aquifer properties is critical for the accurate prediction of groundwater flow and transport. Here, we test the value of using full-waveform inversion of cross-borehole ground penetrating radar (GPR) data for alluvial aquifer characterization. Our study is carried out at the Krauthausen test site, where several field and modelling studies, conducted over the last decades, have yielded a rich set of information that provides excellent opportunities to test and validate novel methods. We apply a full-waveform inversion to analyze GPR data acquired along 15 individual tomographic crosshole 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 along transects of several tens of meters length. Although each crosshole plane was inverted separately, consistent spatial structures in the tomographic images are obtained where planes intersect, which indicates robust inversion results. The GPR results are confirmed by independent direct-push porosity logs, which show a strong correlation with porosity estimates derived from GPR using the Complex Refractive Index Model (CRIM). Compared with traditional ray-based inversion techniques, which are limited in resolution, the full-waveform inversion is found to improve the reconstruction of abrupt changes and fine-scale variations in porosity. Based on the GPR results, additional cone penetration tests (CPT) and direct-push injection logs (DPIL) have recently been measured at selected crosshole plane locations. Preliminary comparison with the GPR results indicates that the spatial variations in GPR permittivity and electrical conductivity match the major changes in lithology and hydraulic conductivity 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.