Characterization of low-velocity waveguides in crosshole GPR data using amplitude analysis and full-waveform inversion

High contrast zones within aquifers like preferential flow paths or impermeable clay lenses can be related to low-velocity waveguides resulting in complicated electromagnetic wave propagation behavior in borehole GPR data. Recently, an amplitude analysis was introduced that was able to identify continuous low-velocity waveguides between boreholes already in the measured data. Here, we extend this approach by investigating synthetic data for continuous and discontinuous waveguides in more detail. We found that discontinuous waveguides show characteristic wave propagation behavior causing late arrival high amplitude spreading in the data. The amplitude analysis was able to detect the waveguide and the boundaries at the borehole where the waveguide is present, and the intensity of the spreading can be used to quantitatively estimate the extend of the waveguide into the domain. Applying the amplitude analysis and full-waveform inversion to crosshole GPR data from the Boise Hydrogeophysical Research Site, we were able to detect two discontinuous waveguide caused by changes in porosity (validated by porosity logs), that were confirmed by the full-waveform inversion results that provided a detailed image of the waveguides between the wells.