Remote estimation of the hydraulic properties of a sand using full-waveform integrated hydrogeophysical inversion of time-lapse, off-ground GPR data

We used integrated hydrogeophysical inversion of time-lapse, proximal ground penetrating radar (GPR) data to remotely infer the unsaturated soil hydraulic properties of a laboratory sand during an infiltration event. The inversion procedure involved full-waveform modeling of the radar signal and one-dimensional, vertical flow modeling. We combined the radar model with HYDRUS-1D. The radar system was set up using standard, handheld vector network analyzer technology. Significant effects of water dynamics were observed on the time-lapse radar data. The estimated hydraulic parameters were relatively consistent with direct characterization of undisturbed sand samples. Significant differences were particularly observed for the saturated hydraulic conductivity, which was underestimated by two orders of magnitude. Nevertheless, the use of soil hydraulic parameters derived from reference measurements failed to correctly predict water dynamics, whereas GPR-based predictions captured some of the major features of time domain reflectometry measurements and better agreed with visual observations. These results suggest that the proposed method is promising for noninvasive, effective hydraulic characterization of the shallow subsurface and hence, monitoring of water dynamics at the field scale.