Characterizing a low-velocity waveguide using crosshole GPR full-waveform inversion

For accurate prediction of flow and contaminant transport a detailed quantification of the local spatial hydraulic conductivity is necessary. In particular, decimeter-scale high contrast layers caused by increased porosity or clay content are important because they can have a dominant effect on solute transport. Such embedded layers when characterized by high dielectric permittivity can act as low-velocity electromagnetic waveguides and be readily identified and characterized using high-frequency crosshole GPR. We show by means of a GPR field example from a hydrological test site in Switzerland that the full-waveform inversion, which exploits the full information content of the data, is able to image a sub-wavelength thickness dipping low-velocity wave-guiding layer. Further, we show an approach to identify low-velocity waveguides from the measured data by analyzing the amplitude and energy behavior within the data. For transmitters present within the waveguide, high amplitude elongated wave-trains are detected for receivers straddling the waveguide depth range, with significantly larger amplitudes than on receivers outside the low-velocity layer, whereas transmitters outside the waveguide show an energy minimum for receivers within the waveguide.