The ambient vibration analysis is a non-invasive and low-cost technique used in site characterization studies to reconstruct the subsurface velocity structure. Depending on the goal of the research, the investigated depth ranges from tens to hundreds of meters. In this work, we aimed at investigating the deeper contrasts within the crust and in particular down to the sedimentary-rock basement transition located at thousands of meters of depth. To achieve this goal, three seismic arrays with minimum and maximum interstation distances of 7.9 m and 26.8 km were deployed around the village of Schafisheim. Schafisheim is located in the Swiss Molasse Basin, a sedimentary basin stretching from Lake Constance to Lake Geneva with a thickness ranging from 800 to 900 m in the north to 5 km in the south. To compute the multimodal dispersion curves for Rayleigh and Love waves and the Rayleigh wave ellipticity angles, the data were processed using two single-station and three array processing techniques. A preliminary analysis of the inversion results pointed out a good agreement with the fundamental modes of Rayleigh and Love waves used in the inversion and a quite strong disagreement with the higher modes. The impossibility to explain at the same time most of the dispersion curves was interpreted as the co-existence, within the investigated area, of portions of the subsurface with different geophysical properties. The hypothesis was confirmed by the Horizontal-to-Vertical spectral analysis (H/V) which indicated the presence of two distinguished areas. The observation allowed a new interpretation and the identification of the Rayleigh and Love wave fundamental modes and of the S-wave velocity profiles to be reconstructed for each investigated zone. It results in two S-wave velocity profiles with similar velocities down to 15 km deferring only in their shallow portions due to the occurrence of a low velocity zone at a depth of 50–150 m at the centre of the investigated area.

中文翻译：

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地下发生横向变化时大规模阵列的环境振动分析：瑞士的一个研究案例

环境振动分析是一种非侵入性和低成本技术，用于场地特征研究以重建地下速度结构。根据研究的目标，调查的深度从几十米到几百米不等。在这项工作中，我们旨在研究地壳内更深的对比，特别是深入到位于数千米深处的沉积岩基底过渡。为了实现这一目标，在 Schafisheim 村周围部署了三个最小和最大站间距离为 7.9 m 和 26.8 km 的地震阵列。沙菲斯海姆位于瑞士莫拉塞盆地，这是一个从康斯坦茨湖延伸至日内瓦湖的沉积盆地，北部厚度为 800 至 900 m，南部厚度为 5 km。为了计算瑞利波和洛夫波的多模色散曲线以及瑞利波椭圆度角，使用两种单站和三阵列处理技术处理数据。对反演结果的初步分析表明，与反演中使用的瑞利波和洛夫波的基本模式有很好的一致性，但与更高的模式有很大的不同。无法同时解释大多数色散曲线被解释为在调查区域内，具有不同地球物理特性的地下部分共存。水平到垂直光谱分析 (H/V) 证实了该假设，该分析表明存在两个不同的区域。观察允许对每个研究区域重建瑞利波和洛夫波基模以及 S 波速度剖面的新解释和识别。由于在研究区域中心的 50-150 m 深度处出现低速带，它导致两个具有相似速度的 S 波速度剖面，低至 15 公里，仅在它们的浅部延迟。