High-resolution investigation of the capillary transition zone and its influence on GPR signatures

Abstract

The boundary between the unsaturated and saturated zones is commonly not characterized by a sharp interface, but rather by a gradual increase of water content. The shape of this capillary transition zone (CTZ) depends on the pore-size distribution of the soil and has a strong influence on the signature of GPR reflections. This can be observed when mapping groundwater tables with GPR: high frequencies do not cause distinct reflections and the groundwater table derived from GPR reflections appears higher than the groundwater table that is measured in observation wells, which are utilized by hydrologists. We present a technique that uses waveguides in shallow boreholes to deduce velocity-depth distributions. Numerical simulations and experiments show that it is possible to measure dielectric permittivity and accordingly water content distributions around the borehole with a high vertical resolution of a few centimeters. We apply the technique at test sites with shallow groundwater tables and fit water retention functions to the data. The resulting permittivity distributions are used as input models for FDTD simulations and the wavelets reflected at the CTZ are analyzed. Both amplitude and traveltime show strong dependence on the shape of the CTZ and the frequency of the source wavelet. Another observation is that the spectrum of the incident wave is shifted to lower frequencies when reflected at the CTZ. These findings are in accordance with observations in the field. The results of our investigation can be used to i) choose appropriate antennas for groundwater table mapping in different geologic environments and ii) to correct the traveltimes of GPR reflections so that the depths correspond to groundwater tables measured in observation wells.