The detection of cavities with geophysical methods is a challenging task for which a general approach has not yet been found. We have found that viscoelastic SH full-waveform inversion (FWI), focusing primarily on reflection events, is able to accurately locate the position of cavities, areas of decompacted sediments, and, more generally, seismic low-velocity anomalies down to 30 m depth. The key for a successful FWI application is the enhancement of the reflected wavefield relative to the surface wavefield. For this purpose, we apply automatic gain control normalization in the objective function. By focusing the inversion on the reflected wavefield, we determine that one can differentiate between air-filled cavities with zero shear-wave (S-wave) velocity and low-velocity zones. Additionally, we test the FWI approach on a field data set, with a known collapsed tunnel system inside a 32 m high monumental antique grave mound. The results show that the location and extent, as well as the density and S-wave velocity of the collapsed tunnel system, can be determined with sufficient accuracy by applying a 2D FWI approach to intersecting profiles, despite the 3D nature of the problem.

中文翻译：

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SH波全波形反演的腔体检测—一种以反射为中心的方法

用地球物理方法检测腔体是一项具有挑战性的任务，目前尚未找到通用的方法。我们发现，粘弹性SH全波形反演（FWI）主要关注反射事件，能够准确定位腔体的位置，解压的沉积物区域，并且更广泛地讲，可探测低至30 m深度的地震低速异常。成功的FWI应用的关键是相对于表面波场增强反射波场。为此，我们在目标函数中应用自动增益控制归一化。通过将反演集中在反射波场上，我们确定可以区分剪切波（S波）速度为零的充气腔和低速带。此外，我们在现场数据集上测试FWI方法，在一个32 m高的巨大古董墓丘中有一个已知的坍塌隧道系统。结果表明，尽管问题具有3D性质，但通过将2D FWI方法应用于相交剖面，可以足够准确地确定坍塌隧道系统的位置和范围以及密度和S波速度。