Abstract
ABSTRACTIn the last decade, time‐domain crosshole ground‐penetrating radar full‐waveform inversion has been applied to several different test sites and has improved the resolution and reconstruction of subsurface properties. The full‐waveform inversion requires several diligent executed pre‐processing steps to guarantee a successful inversion and to minimize the risk of being trapped in a local minimum. Thereby, one important aspect is the starting models of the full‐waveform inversion. Generally, adequate starting models need to fulfil the half‐wavelength criterion, which means that the modelled data based on the starting models need to be within half of the wavelength of the measured data in the entire investigation area. Ray‐based approaches can provide such starting models, but in the presence of high contrast layers, such results do not always fulfil this criterion and need to be improved and updated. Therefore, precise and detailed data processing and a good understanding of experimental ground‐penetrating radar data are necessary to avoid erroneous full‐waveform inversion results. Here, we introduce a new approach, which improves the starting model problem and is able to enhance the reconstruction of the subsurface medium properties. The new approach tames the non‐linearity issue caused by high contrast complex media, by applying bandpass filters with different passband ranges during the inversion to the modelled and measured ground‐penetrating radar data. Thereby, these bandpass filters are considered for a certain number of iterations and are progressively expanded to the selected maximum frequency bandwidth. The resulting permittivity full‐waveform inversion model is applied to update the effective source wavelet and is used as an updated starting model in the full‐waveform inversion with the full bandwidth data. This full‐waveform inversion is able to enhance the reconstruction of the permittivity and electrical conductivity results in contrast to the standard full‐waveform inversion results. The new approach has been applied and tested on two synthetic case studies and an experimental data set. The field data were additionally compared with cone penetration test data for validation.
Highlights
Detailed and high-resolution characterization of variable saturated soil-aquifer systems is critical and full of challenges, but highly important to improve the understandings of flow and transport processes of subsurface water
The realistic synthetic crosshole ground-penetrating radar (GPR) data are modelled with the same time-domain 2D finite-difference time-domain (FDTD) approach as used for the full-waveform inversion (FWI)
We have presented a new crosshole ground-penetrating radar (GPR) full-waveform inversion (FWI) approach using progressively expanded bandwidths of the measured and modelled data (PEBDD) with frequency bandpass filters to improve the reconstruction of the subsurface properties
Summary
Detailed and high-resolution characterization of variable saturated soil-aquifer systems is critical and full of challenges, but highly important to improve the understandings of flow and transport processes of subsurface water. To enhance our understandings of such processes, high-resolution images and accurately described subsurface properties, especially for small-scale heterogeneities of aquifers, are highly important (e.g., Klotzsche et al, 2019b). Geophysical methods have been developed and applied to obtain images of the near subsurface and to improve the characterization of hydrogeological properties (Binley et al, 2015), such as seismics (e.g., Doetsch et al, 2010), electrical resistivity tomography (ERT; e.g., Coscia et al, 2011) and ground-penetrating radar (GPR; e.g., Klotzsche et al, 2018). The good consistency illustrates crosshole GPR is able to close the gap between small-scale investigations with highresolution (e.g., coring) and large-scale zones mapping (e.g., flowmeter tests)
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