Parameter estimation for unsaturated soil hydraulic properties using full-waveform hydrogeophysical inversion of time-lapse GPR data

Summary: Knowledge of soil water content distribution and dynamics at the field scale is essential for sustainable and optimal agricultural and environmental management of land and water resources. To constrain full-waveform inversion of time-lapse, proximal ground penetrating radar signals we used a hydrodynamic model to simultaneously identify shallow soil hydraulic properties and continuous vertical electric profiles. The radar model involves a full-waveform solution of Maxwell's equations for wave propagation in three-dimensional multilayered media. Hydrodynamic modeling was based on a onedimensional solution of Richard's equation and was solved numerically using HYDRUS-1D code. The uniqueness of the inverse solution was analyzed using numerical experiments for different textured soils to estimate three soil hydraulic parameters (α, n, and K_s) of Mualem-van Genuchten (MVG) model. The top boundary conditions was time-variable and we assumed initial condition to be a priori unknown. A laboratory experiment was performed to test the proposed method for a transient infiltration event in a homogeneous sandy soil. Radar data were acquired in the frequency domain using a standard vector network analyzer combined with an off-ground monostatic horn antenna. The results suggest that the proposed method is promising for characterizing the shallow subsurface hydraulic properties at the field scale with a high spatial resolution.