Mapping sand layers in clayey till using crosshole ground-penetrating radar

Abstract

Fluid transport through clayey tills governs the quantity and quality of groundwater resources in the Northern Hemisphere. This transport is often controlled by a 3D network of macropores (biopores, fractures, and sand lenses) within the clayey till. At present, a nondestructive technique that can map and characterize the sand-lens network does not exist, and full excavation or extensive drilling is therefore the only solution. Acquisition and modeling of crosshole ground-penetrating radar (GPR) may provide the answer to this problem. We collected 1D and 2D crosshole GPR data at a field site in Denmark from four 8 m deep boreholes with horizontal distances varying between 2.64 and 5.05 m. We find that the depth, thickness, and tilt of a coherent sand layer within the clayey till (approximately 0.4–0.6 m thick), as well as the underlying sand formation, can be mapped accurately using GPR data. We efficiently identify the sand as a highly resistive section with high electromagnetic (EM) wave velocities, whereas the clayey till is conductive with lower EM wave velocities. We find that the exact location of the sand occurrences is better delineated by the increase in amplitude than the increase in EM wave velocity. We believe that crosshole GPR may contribute significantly to groundwater protection and contaminant remediation initiatives.