To improve the understanding of flow and transport processes in the critical zone, high-resolution and accurate estimation of the small-scale heterogeneity is essential. Preferential flow paths related to high-porosity layers and clay lenses in gravel aquifers greatly affect flow and transport processes in the subsurface, and their high electrical contrast to their surrounding matrix and limited extent can act as low-velocity electromagnetic waveguides. In the past decade, time-domain full-waveform inversion (FWI) of crosshole ground-penetrating radar (GPR) data has shown to provide 2D decimeter-scale resolution images of relative permittivity and electrical conductivity of the subsurface, which can be related to porosity and soil texture. Most studies using crosshole GPR FWI resolved high-porosity zones that were identified by an amplitude analysis approach. But clay lenses or zones with higher electrical conductivity that act as low-velocity waveguides are hard to distinguish in the measured data and amplitude analysis because of the absence of characteristic wave-propagation features. We have investigated a set of nine crosshole GPR data sets from a test site in Hermalle-sous-Argenteau near the Meuse River in Belgium to characterize the aquifer within a decimeter-scale resolution and to improve the understanding of a previously performed heat tracer experiment. Thereby, we extend the amplitude analysis to identify two different types of low-velocity waveguides either caused by an increased porosity or a higher electrical conductivity (and higher porosity). Combining the GPR amplitude analysis for low-velocity waveguide zones with the standard FWI results provided information on waveguide zones, which modified the starting models and further improved the FWI results. Moreover, an updated effective source wavelet is estimated based on the updated permittivity starting models. In comparison with the traditional FWI results, the updated FWI results present smaller gradient of the medium properties and smaller root-mean-squared error values in the final inversion results. The nine crosshole sections are used to generate a 3D image of the aquifer and allowed a detailed analysis of the porosity distribution along the different sections. Consistent structures of the permittivity and electrical conductivity show the robustness of the updated FWI results. The aquifer structures obtained by the FWI results agree with those results of the heat tracer experiment.

中文翻译：

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Hermalle-sous-Argenteau测试地点的3D含水层表征，使用跨孔探地雷达振幅分析和全波形反演

为了更好地了解临界区的流动和输运过程，高分辨率和小规模异质性的准确估算至关重要。与砾石含水层中的高孔隙率层和黏土透镜有关的优先流动路径极大地影响了地下的流动和传输过程，它们与周围基质的高电对比度和有限的范围可以充当低速电磁波导管。在过去的十年中，交叉孔探地雷达（GPR）数据的时域全波形反演（FWI）已显示出提供了地下相对介电常数和电导率的二维分米尺度分辨率图像，这可能与孔隙率和土壤质地。大多数使用井间GPR FWI的研究都可以解决由振幅分析方法确定的高孔隙度区域。但是，由于缺乏特征波传播特征，在低速波导中充当低速波导的粘土透镜或具有较高电导率的区域很难区分。我们在比利时默兹河附近的Hermalle-sous-Argenteau的一个测试点研究了九个井间GPR数据集，以十亿分之一尺度的分辨率表征含水层，并提高了对先前进行的伴热示踪剂实验的理解。因此，我们扩展了幅度分析以识别两种不同类型的低速波导，它们是由孔隙率增加或电导率（和孔隙率较高）引起的。将低速波导区域的GPR幅度分析与标准FWI结果结合起来，可提供有关波导区域的信息，从而修改了起始模型并进一步改善了FWI结果。此外，基于更新的介电常数起始模型来估计更新的有效源小波。与传统的FWI结果相比，更新的FWI结果在最终反演结果中呈现出较小的介质属性梯度和较小的均方根误差值。九个横孔截面用于生成含水层的3D图像，并允许对沿不同截面的孔隙率分布进行详细分析。介电常数和电导率的一致结构显示了更新的FWI结果的稳健性。