Parameter estimation for soil hydraulic properties using zero-offset borehole radar:: Analytical method

Inverse methods to obtain soil hydraulic parameters are becoming increasingly popular, due to their more rapid, complete, and robust estimations of hydraulic parameters compared with traditional direct methods. We present a method to infer hydraulic parameters based on first arrival travel time measurements made with zero-offset borehole ground penetrating radar (BGPR). Borehole ground penetrating radar offers many advantages for field-scale monitoring of transient processes including the ability to measure rapidly, over relatively large soil volumes, with high temporal resolution and to great depths. The BGPR measurements are used to infer the position of the wetting front during infiltration. The analysis makes use of critical refraction at the edge of the wetting front, which gives rise to a linear increase in BGPR travel time with time as the wetting front passes beneath the antennae. The slope of this response is used directly to calculate the hydraulic conductivity. We demonstrate that unique determination of the van Genuchten alpha and n parameter is not possible with BGPR data alone; at least one pressure head measurement in the dry range (early time) is required. We employ a nonlinear least squares parameter estimation code to obtain the optimal alpha and n parameters for synthetic data. The method could potentially be applied to areas of artificial recharge in an infiltration basin, natural recharge in an ephemeral stream, or agricultural settings where the surface is flooded with irrigated water.