Crosshole GPR full-waveform inversion and waveguide amplitude analysis: Recent developments and new challenges

Over the last years, crosshole GPR full-waveform inversion has proved to be a powerful tool to map and characterize aquifers with a decimeter-scale resolution. Especially the detection of small-scale high contrast layers that can be related to zones of high porosity and zones of preferential flow improved our understanding of the propagation of the electromagnetic waves related to these features. Here, we give an overview of the potential and challenges of applying the full-waveform inversion to experimental data and discuss the obtained results for crosshole GPR data acquired at different test sites. Thereby, we also demonstrate the theoretical developments and illustrate the necessary steps that are required to achieve reliable full-waveform inversion results, which are not only indicated by a good fit of the measured and modelled traces, but also by the absence of a remaining gradient for the final models. One requirement and important step is to obtain good starting models. Whereas ray-based approaches sometimes cannot provide sufficient good starting models, the waveguide amplitude analysis can help to improve these starting models.