Evaluating hydrodynamic parameters accounting for water retention hysteresis in a large sand column using surface GPR

Abstract

This study presents laboratory experiments using surface Ground Penetrating Radar measurements to exhibit and monitor hysteresis of the water retention function of Fontainebleau sand. A commercial impulsionnal GPR system monitored the volumetric water content changes in a large sand column subject to different hydraulic heads applied at its bottom during drainage/wetting cycles. Coupled hydrodynamic and electromagnetic modeling was used to simulate radargrams whereas hydrodynamic modeling coupled with 1D optical ray tracing was used to estimate the hydrodynamic parameters of the sand from GPR reflection two-way travel times. Statistical and uncertainty analysis were performed on numerical and experimental data. The range of optimized parameters obtained from experimental data were compared to those obtained with classical laboratory methods such as the hanging water column and the constant head permeameter. The range of parameters retrieved using GPR monitoring was consistent for the hydraulic conductivity at saturation and the van Genuchten parameters αd, αw. The difference between the GPR method and classical methods for the retrieved λ and n van Genuchten's parameters are believed to be caused by the dynamic character of the proposed method.