Ground Penetrating Radar Measurements in a Controlled Vadose Zone: Influence of the Water Content

Ground penetrating radar (GPR) is a nondestructive method, which, as with other geophysical methods, has been successfully used to estimate the water content or hydraulic properties of soils. We performed GPR measurements to calibrate and compare water content estimates with actual water contents in a sand box. A vadose zone was simulated by injecting water in a sand box. We obtained four GPR data sets: for dry sand, for sand with water tables at 72- and 48-cm depths, and for sand after drainage. Using the reflections (or diffractions) from the bottom of the sand box (or objects buried in the sand), mean relative dielectric permittivities were determined at several depths in the sand box. These relative dielectric permittivities were used to calculate “real” mean relative dielectric permittivities of a sand box made up of three layers (dry sand, unsaturated sand, and fully saturated sand), knowing that a layer can be subdivided into more layers depending on the depth of the reflections (or diffractions) recorded. We used three relationships between relative dielectric permittivity and the water content to estimate the mean water content for each layer. From these water contents and the known volume of sand considered, we estimated the amount of water in the sand box for each water table. Subtracting the volume obtained for dry sand from the volume obtained for the different water tables gave estimates of the variations in water quantities in the sand box; these were compared with the quantities injected in the sand box. Despite uncertainties in the determination of the mean relative dielectric permittivities, the calculated variations in water quantities were very similar to those injected in the sand box.