In 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.

中文翻译：

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利用逐步扩展的数据带宽改进井间探地雷达全波形反演结果

在过去的十年中，时域跨孔探地雷达全波形反演已应用于多个不同的测试站点，并提高了地下特性的分辨率和重建。全波形反演需要几个认真执行的预处理步骤，以确保成功反演并最大限度地减少陷入局部最小值的风险。因此，一个重要的方面是全波形反演的起始模型。通常，足够的起始模型需要满足半波长准则，这意味着基于起始模型的建模数据需要在整个调查区域的测量数据波长的一半以内。基于光线的方法可以提供这样的起始模型，但是在存在高对比度层的情况下，这种结果并不总是符合这一标准，需要改进和更新。因此，必须进行精确细致的数据处理以及对实验探地雷达数据的充分理解，以避免出现错误的全波形反演结果。在这里，我们引入了一种新方法，它改进了起始模型问题，并能够增强地下介质特性的重建。新方法通过在反演建模和测量的探地雷达数据期间应用具有不同通带范围的带通滤波器来解决由高对比度复杂介质引起的非线性问题。因此，这些带通滤波器被考虑进行一定次数的迭代，并逐渐扩展到选定的最大频率带宽。得到的介电常数全波形反演模型用于更新有效源小波，并在全带宽数据的全波形反演中用作更新的起始模型。与标准全波形反演结果相比，这种全波形反演能够增强介电常数和电导率结果的重建。新方法已在两个综合案例研究和一个实验数据集上应用和测试。另外将现场数据与锥入度测试数据进行比较以进行验证。与标准全波形反演结果相比，这种全波形反演能够增强介电常数和电导率结果的重建。新方法已在两个合成案例研究和一个实验数据集上应用和测试。另外将现场数据与锥入度测试数据进行比较以进行验证。与标准全波形反演结果相比，这种全波形反演能够增强介电常数和电导率结果的重建。新方法已在两个综合案例研究和一个实验数据集上应用和测试。另外将现场数据与锥入度测试数据进行比较以进行验证。