Ground-Penetrating Radar reflection data sensitivity to van Genuchten parameter variations GPR reflection data sensitivity to van Genuchten parameters

We are interested in Ground Penetrating Radar (GPR) as a geophysical tool useful for determining the depth of the ground water table (GWT) and for monitoring shallow water infiltration in sandy soils. At hydrostatic equilibrium, the water content distribution in an homogeneous ground from the vadose zone down to the water saturated zone depends on the medium water retention function. A classical way to model retention curve data is to use a van Genuchten continuous model. Using Finite Difference Time Domain simulations, we study the sensitivity of the GPR signal reflected by a van Genuchten type transition to the hydrostatic parameters. We show a power type relationship between the reflected signal amplitude and the slope of the retention curve. Furthermore, for simulating GPR reflection data acquired above a transition from unsaturated to saturated soil, geophysicists often approximate the soil water retention curve by a piece-wise linear model. We test the validity of such an approximation depending on the frequency of the radar signal and the abruptness of the retention curve. We illustrate our results with high resolution GPR data (1600 MHz) acquired above a fluctuating water table in a sand column at the laboratory scale.