Field studies of the GPR ground wave method for estimating soil water content during irrigation and drainage

Knowledge of ground wave penetration depth and methods for facilitating ground wave velocity analysis are important practical aspects to consider when measuring soil water content with surface ground penetrating radar. A field study was conducted to optimize the wide angle reflection and refraction and fixed offset methods of measuring the ground wave velocity and to find the effective ground wave sampling depth under irrigation and drainage conditions. In this study, a PulseEkko 1000 GPR system with 450 MHz antennas was used at a sandy loam soil site. Water contents measured with time domain reflectometry (TDR) were used to determine the sampling depth of GPR based water content estimates. Cumulative irrigation and drainage calculated with GPR were found to be more closely related with cumulative irrigation and drainage measured with shorter TDR probes, suggesting that the ground wave sampling depth was in the 0.2–0.5 m range. During the infiltration phase the depth of the ground wave penetration was found to be in the 0–0.56 m range, assuming a sharp boundary between wet and dry sand. Comparison of water contents measured with the WARR and FO methods revealed that an antenna separation distance of 1.5–2.0 m for the FO method was required to obtain similar results between the two methods.