Precision ground penetrating radar measurements to monitor hydrologic processes occurring in unsaturated subsurface material.

摘要

The temporal scales at which water moves through the unsaturated subsurface, from its input as precipitation, to its output as stream discharge, has been, and continues to be, a fundamental focus of hydrological research. Previous laboratory and small scale field studies have invasively explored how the vadose zone responds to precipitation, however, such methods potentially change the way water migrates through unsaturated material. In contrast, non-invasive geophysical techniques, such as surface based ground penetrating radar (GPR), may be used to assess the hydrologic processes occurring in the subsurface. Thus it is necessary to know the precision of the GPR observations.; Sixteen high quality independent common mid-point (CMP) soundings using a surface based GPR system indicate that observations of radar velocities and interval thicknesses may optimally be precise at the 95% confidence level to +/-0.001 m/ns and +/-0.2 m, respectively. In addition a correction procedure to minimize drift in the GPR system is developed.; The typical moisture conditions in the subsurface after a heavy precipitation event are shown to cause dispersion in the GPR data, due to the region near the ground surface with increased water content acting as an asymmetric waveguide. The resulting radargram exhibits significant dispersion and traditional traveltime analysis is challenging. However, the data are interpreted using dispersion analysis, and the accuracy of the analysis is demonstrated by the agreement between the thickness of the waveguide and the amount of rain distributed ideally in a porous material.; Finally, multiple CMP soundings collected at specific times before, during but immediately prior to an intense rainstorm, and after an extended precipitation period, to infer hydrologic processes occurring in the subsurface in response to the precipitation. Surface based GPR observations indicate that water was rapidly delivered to the deeper subsurface 1.5 hours after the intense rainstorm.