Monitoring Infiltration Process in Structured Soil Using 4D GPR.

Ground Penetrate Radar (GPR) provides a non-invasive way to reveal the subsurface complexity. Our preliminary studies in the Shale Hill Catchment have demonstrated that GPR is suitable to identify subsurface structure in this landscape. In this study, we used 4D GPR to monitor in-situ water infiltration under controlled condition in the hillslope of the Shale Hill catchment. The 4D GPR survey consisted of consecutive 3D surveys at various times.  A 150 (inline) by 70 cm (crossline) grid was established for 3D survey and GPR survey line interval was 10 cm. A hole with a diameter of 8 cm and a depth of 20 cm existed at the 120 cm inline of crossline 60 cm. Water was injected in the hole and the constant head of 20 cm water was maintained until the end of infiltration experiment. The 400 MHz GPR was first used to scan the grid to obtain initial subsurface state before water infiltration. Then eight additional GPR surveys were conducted subsequently along the grid to track the infiltrated water for 75 minutes. Data interpretations, including spectral decomposition and 3D migration, led to the delineation of wetting front boundary and its change with time. By examining the time-lapsed changes of the wetting front, the rate of wetting font movement and its pattern were determined.  This study provided an enhanced understanding of the spatial and temporal evolution of infiltration process in the structured subsurface in the Shale Hills Catchment, which will improve process-based hydrological modeling and prediction.