Using ground penetrating radar to investigate the water table depth in weathered granites : Sardon case study, Spain

Precise and non-invasive measurement of groundwater depth is essential to support management of groundwater resources. In that respect, GPR is a promising tool for high resolution, large scale characterization and monitoring of hydrological systems. We applied GPR in a semi-arid catchment (Sardon, Salamanca, Spain) in order to investigate the water table depth in weathered granites. We used a pulse radar with a single 200 MHz bowtie antenna combined with a differential GPS and a survey wheel for accurate positioning. Measurements were performed following a series of transects crossing perpendicularly the bed of the Sardon streams, which were dry during the survey period (September 2009). In order to transform the GPR data from time to depth we estimated the soil dielectric constant using frequency domain reflectometry (FDR) or water level depth information from several observation wells. Electrical resistivity tomography (ERT) was applied along the GPR profiles and compared to the GPR results. GPR signals were also simulated using forward modeling (GprMax2D) of several hypothetic configurations of the subsurface. Those techniques helped us to better understand and interpret the GPR data. In general, the shallow water table was sparsely detected in the GPR profiles ranging from ∼ 1 to ∼ 3 meters the entire catchment. The results showed a good agreement of ERT and GPR profiles. The comparison of measured and simulated GPR data showed multiple reflections in presence of the saturated fractured granite