Ambient Vibration Analysis on Large Scale Arrays When Lateral Variations Occur in the Subsurface: A Study Case in Switze
- PDF / 18,152,439 Bytes
- 23 Pages / 547.087 x 737.008 pts Page_size
- 42 Downloads / 156 Views
Pure and Applied Geophysics
Ambient Vibration Analysis on Large Scale Arrays When Lateral Variations Occur in the Subsurface: A Study Case in Switzerland DARIO CHIEPPA,1 MANUEL HOBIGER,1 PAOLO BERGAMO,1 and DONAT FA¨H1 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-toVertical 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. Keywords: Sedimentary basin, crustal structure, Love waves, Rayleigh waves, seismic array, seismic noise, surface waves.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00024-020-02516-x) contains supplementary material, which is available to authorized users. 1 Swiss Seismological Service (SED), ETH Zurich, Zu¨rich, Switzerland. E-mail: [email protected]; manuel.hobiger@ sed.ethz.ch; [email protected]; [email protected]
1. Introduction 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. Surface waves are widely used in site effect evaluation (e.g. Wathelet et al. 2008; Picozzi. et al. 2009; Bergamo et al. 20
Data Loading...
