Abstract
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.
Highlights
The ambient seismic noise consists of a mixture of body (P- and S-waves) and surface (Love and Rayleigh waves) waves propagating through the subsurface in different directions and with different phase velocities
The corresponding mode for the Rayleigh waves (R0) consists of the curve picked for the big array (0.07–1.33 Hz), the mode picked for the intermediate array at lower frequency and the curve picked for the small array up to 14.6 Hz (Fig. 7b)
The ellipticity angle derived from Wavefield Decomposition (WaveDec) for the fundamental mode shows a retrograde particle motion for the dispersion curves picked using the big and the small arrays, in agreement with our interpretation of fundamental mode
Summary
The ambient seismic noise consists of a mixture of body (P- and S-waves) and surface (Love and Rayleigh waves) waves propagating through the subsurface in different directions and with different phase velocities. An important feature of the surface waves is their dispersive behaviour, or in other words, the dependence of their phase velocity with frequency This feature allows the investigation of shallow and deep layers: the first are swept by the high-frequency content of the wavefield (e.g. generated by factories and traffic), while the second are sampled only by the long wavelengths (e.g. caused by oceans and winds). In site characterization and engineering studies, in general, the subsurface is investigated down to several hundreds of meters (e.g. 100–400 m). Several single-station and array processing techniques were developed for site characterization studies. They assume a homogeneous layered subsurface below the study area, investigate the geophysical properties and provide useful information for the P- and S-wave velocity profiles estimation. Single-station methods investigate the subsurface using the seismic recordings of three-
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