GPR full-waveform inversion of a variably saturated soil-aquifer system

Abstract

Characterizing the critical zone is highly important to improve the mapping of small-scale spatial heterogeneity of soil properties and the understanding of flow and transport processes. Thereby, crosshole GPR full-waveform inversion of experimental data can significantly improve imaging results of permittivity and electrical conductivity compared to standard ray-based inversion approaches. Recently, the full-waveform inversion was used to characterize the saturated domain of several different aquifers based on highly resolved permittivity maps at the decimeter-scale. Until now it was not possible to invert the entire depth range of the variably saturated soil-aquifer system including the unsaturated and saturated part of the subsurface because of the high medium contrast between the unsaturated and saturated zone and differences in borehole fillings. Here, we propose a crosshole GPR full-waveform inversion that simultaneously inverts for both the saturated and unsaturated part of the subsurface. In the proposed method, we estimate four effective source wavelets rather than one for transmitter and receiver antennae combinations being present in both saturated, both unsaturated, or saturated-unsaturated and unsaturated-saturated, respectively. The GPR full-waveform inversion that investigated the soil-aquifer system is adapted accordingly, where different source wavelets are used during the inversion depending on the transmitter and receiver being present in the saturated and/or unsaturated zone. The new full-waveform inversion was applied to image a gravel aquifer and higher resolution images were obtained compared to standard ray-based inversion. Moreover, the measured and modeled data showed a good correspondence in both shape and amplitude for antennae in the saturated domain. For transmitters and receivers in the unsaturated zone, the shape of the modeled data is also in good correspondence with the measured data, whereas in the amplitude still a misfit can be noticed. Therefore, five different starting model tests were performed to improve the results in the unsaturated domain and to define the optimal solution for the full-waveform inversion. These tests indicated that either more a priori information is needed or more data is necessary to reliable invert the unsaturated zone. Nevertheless, we have demonstrated that the new GPR full-waveform inversion for variably saturated soil-aquifer systems for different borehole fillings can be used to image the entire depth range from the critical zone and can be applied to a wide field of applications.