3D characterization of an aquifer using full-waveform inversion and amplitude analysis

For accurate prediction of flow and contaminant transport in aquifers, a high resolution method is necessary, that is able to detect small-scale high-contrast layers. Such layers can act as low-velocity waveguides in the GPR signal and can be related to a zone of preferential flow or impermeable clay lenses. Here, we characterize a saturated gravel aquifer in 3D by applying 2D full-waveform inversion and an amplitude analysis approach that explores the information content present in the measured GPR data. The full-waveform inversion results of the permittivity and conductivity show decimeter-scale high resolution images and similar results at the borehole crossing and at the intersection of the diagonal planes. In all six planes, a high permittivity layer between 5m-6m depth was resolved, which acted due to the high contrast to the surrounding as a low-velocity waveguide indicating a zone of higher porosity. The amplitude analysis of the measured data showed significant wave propagation for transmitter located in and outside this zone. By using this information, the method was able to detect the waveguide layers and their boundaries in the measured data, which were confirmed by the full-waveform inversion results. Permeability logs indicate a zone of preferential flow between 5m-6m depth, which shows a good agreement with the high permittivity/porosity zone detected by the full-waveform inversion.