Abstract
SUMMARYThe difficulty and the high cost to assess the subsurface properties led to the development of several geophysical techniques. Generally, the focus of a site study is the reconstruction of the S-wave velocity profile down to few tens to hundreds of metres (e.g. 30–300 m), but not the investigation of deeper structures, such as the transition to the crystalline basement. However, such deeper structures are of interest when seismic hazard products have to relate to a reference rock-velocity profile, for example in regional seismic hazard assessment and microzonation studies. To estimate the S-wave velocity profiles down to several kilometres, we study the potential of Rayleigh and Love waves at low (down to 0.1 Hz) and high (up to 20 Hz) frequencies using two seismic arrays of increasing size. The small array, with a maximum inter-station distance of 900 m and a recording time of 3 hr, was aimed at constraining the shallow subsurface down to about 350–400 m, while the big one, with a maximum inter-station distance of more than 29 km and 23 hr of recording had the goal to constrain the deeper structure. The arrays were deployed in northern Switzerland (east of the village of Herdern) within the Swiss Molasse basin, a sedimentary basin north of the Alps stretching from Lake Constance to Lake Geneva; its thickness increases from 800 to 900 m in the northeast to more than 5 km in the southwest. The seismic data recorded by the two arrays were analysed using the techniques developed for the analysis of small-aperture arrays. The results were inverted for the S-wave velocity profile in two steps: first, the Rayleigh and Love wave phase dispersion curves were inverted together. Secondly, the previous dispersion curves were jointly inverted with the measured Rayleigh wave ellipticity angle. The resulting S-wave velocity profiles are similar and show agreement with the available geological and geophysical data, confirming the potential of surface waves to investigate deep structures. Moreover, our analysis proves the feasibility of site characterization techniques to large arrays and the possibility to estimate the P- and S-wave velocity profiles down to 5 km, deeper than the contrast between Molasse basin and crystalline rock at around 2.1 km.
Highlights
The Swiss Molasse basin is the northern foreland of the Alps located between the Alpine chain to the south and the Jura Mountains to the northwest (Fig. 1a)
For the analysed work, taking into account the picked dispersion curves (Fig. 8) and the results of the inversion (Fig. 11), 3CFK is the method providing the most continuous and clearest curves for both arrays. This method is less sensitive to the subsurface variation, to the source distribution and to the selected array geometry; Wavefield decomposition (WaveDec) and Modified SPatial AutoCorrelation (MSPAC) are more sensitive to the presence of lateral variations when wide areas are investigated
Based on the good overlap of the RayDec ellipticity curve computed for the small array with the 3CFK ellipticity curves and its similarity with the results of WaveDec, we performed a joint inversion of this curve, as ellipticity angle, together with the Rayleigh and Love wave dispersion curves
Summary
The Swiss Molasse basin is the northern foreland of the Alps located between the Alpine chain to the south and the Jura Mountains to the northwest (Fig. 1a). We extend the application of such techniques to large scale arrays to investigate the deeper structures focusing on the transition between the sedimentary rock cover and the crystalline basement at several thousand of metres of depth To achieve this goal, two arrays of three-component seismic sensors were deployed in northern Switzerland, east of the village of Herdern (Fig. 1a). The area was chosen for three reasons: (1) geological and geophysical information at depth were collected by SEAG in the 1980s with the Herdern 1 borehole (Roth et al 2010), (2) a 3-D geological model of the Swiss Molasse Basin was developed within the GeoMol project (Allenbach et al 2017) and (3) no passive seismic site characterization measurement was previously performed in the area. We present the deployed seismic arrays, the analysis performed using single-station and array processing techniques, an overview of the picked Rayleigh and Love wave dispersion curves and the ellipticities. The inverted P- and S-wave velocity profiles are compared with the available geological and geophysical information
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