Experimental Estimation of the Penetration Depth of the GPR Groundwave

The Ground Penetrating Radar (GPR) groundwave is a direct wave that travels between the transmitting and receiving antennas in the shallow subsurface. Several researchers have shown that both variable-offset and common-offset groundwave data can be used to estimate soil moisture, indicating that GPR is a promising tool for field-scale water content estimation. However, the exact penetration depth of the GPR groundwave is unknown, and this uncertainty limits the efficacy of the groundwave technique for large-scale soil water content monitoring. Modeling studies of the GRP groundwave have indicated that the penetration depth is a function of GPR frequency and soil water content. This research experimentally investigates the penetration depth of the GPR groundwave as a function of frequency under controlled conditions within a large tank filled with a fairly homogeneous sandy soil. In this experiment, water was added to part of the sand, and this sand was thoroughly mixed to produce a homogenous soil with a very high water content. The remainder of the sand was oven-dried to produce a homogeneous soil with a very low water content. The wet sand was placed in the bottom 20-cm of a large tank, and variable-offset and common-offset GPR data were collected over the wet sand using 100-, 250-, 500-, and 1000-MHz antennas. Then, a 3-cm layer of the dry sand was placed on top of the wet sand, and the GPR surveys were repeated. Additional layers of dry sand were placed in the tank in 3-cm lifts, and GPR data were collected after each sand layer was added until the GPR data showed no changes in electromagnetic velocity with additional layers of dry sand. The depth of dry sand where the electromagnetic velocity is no longer influenced by the underlying wet sand layer is assumed to represent the penetration depth of the GPR groundwave in this soil. Data analysis of this experiment is ongoing, but preliminary results confirm the theoretical expectation that penetration depth is a function of GPR frequency.