Suitability of GPR for characterizing variably saturated sediments during transient flow

Ground-penetrating radar (GPR) is potentially rich in information about both subsurface structure and water saturation, though non-unique interpretations of GPR data are possible since the dielectric constant, often inferred from GPR travel times, is a function of both porosity and water content. Further, water is often non-uniformly distributed in the vadose zone, its distribution being influenced by the spatial structure of hydraulic parameters and transient conditions at the ground surface. Synthetic time-lapsed GPR surveys are shown useful in this work for improving overall characterization of the Vadose zone and for giving valuable information about flow processes. Variably saturated flow and GPR are simulated simultaneously for a highly heterogeneous vadose zone (outcrop) model in order to obtain synthetic GPR images before and during infiltration experiments. Aside from delineating the advancing water front, GPR crosshole surveys obtained during redistribution of water (after the infiltration front passes) provide additional information. Transients in water content, and therefore in electrical parameters, are seen to be related to soil type, and even more so to the spatial structure of fluid permeability. Instead of requiring a direct relationship between the dielectric constant of soil and its permeability, the goal of this work is to take advantage of the sensitivity of GPR to changes in water content and ultimately relate estimated changes in water content to fluid permeability through inverse modeling.