Characterizing Spatially Limited High-Porosity Layers in Aquifers Using Crosshole GPR Full-Waveform and Waveguide Amplitude Analysis

Abstract:

Small-scale high-contrast layers related to high porosity zones or impermeable clay lenses can have a significant effect on water flow and transport processes within aquifers. Such layers can act as electromagnetic waveguides causing late arrival high amplitude elongated wave trains in the GPR data. Recently, we introduced a waveguide amplitude analysis approach that is able to identify continuous low-velocity waveguides between two boreholes including their upper and lower boundaries. Here, we analyze synthetic waveguide models for different thickness, dip, extent, permittivity, and conductivity parameters. These results showed that 1) high-amplitude elongated wave trains in the data are most probably caused by a change in porosity rather than a change in clay content, 2) spatially limited high-porosity layers have characteristic wave behavior forming late-arrival high-amplitude spreading in the data, 3) for a high porosity lens that is not connected to any of the boreholes, the amplitude analysis can be used to identify the lens, but not to estimate the lens boundaries. The waveguide amplitude analysis was extended to detect spatially limited high porosity layers and was applied to an experimental crosshole GPR dataset. Similar events in the data were observed as caused by the modelled spatially limited waveguides and two wave-guiding structures could be identified. Using the ray-based inversion results as starting model for the full-waveform inversion was not able to explain all the data due to the unreliable ray-based inversion results close to the water table caused by the low ray-coverage. Information gained from the amplitude analysis was necessary to improve the starting model for the full-waveform inversion. The final results of the full-waveform inversion imaged two high porosity layers with limited lateral extension that were caused by high porosity sand units embedded in lower porosity gravel and were confirmed by neutron-neutron logging data.