GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 401 ON THE REVERBERATION EFFECT IN MULTI-CHANNEL ANALYSIS. SURFACE WAVES POLARIZATION AS QUICK TOOL FOR SUBSOIL CHARACTERIZATION I. Barone 1 , J. Boaga 1 , G.P. Deidda 2 , G. Cassiani 1 1 Dipartimento di Geoscienze, Università degli Studi di Padova, Padova, Italy 2 Dipartimento di Ingegneria civile, ambientale e architettura, Università degli Studi di Cagliari, Cagliari, Italy It is known that resonance phenomena linked to buried impedance contrast can be adopted for subsoil characterization (Lunedei and Malischewsky 2015). Since the 70’s resonance effects were observed and analyzed, both in case of telluric events and microtremors studies (Nogoshi and Igarashi 1970). From the multi-component analysis of horizontal and vertical components of soil motion, we can retrieve information about the presence of an impedance contrast, the depth and the rigidity of the layers involved (e.g. HVSR technique). These measurements have strong implications on seismic microzonation studies, since the presence of an impedance contrast may lead to strong amplifications in case of earthquake events (Bonnefoy-Claudet et al., 2004), while the local resonance frequency of the subsoil has great importance for the a-seismic design. Surface waves polarization phenomena were also observed in case of strong impedance contrast in the investigated subsoil, leading to the osculation modes phenomenon (Malischewsky et al., 2008, Hobiger et al., 2009, Boaga et al., 2013). Nowadays, the standard approaches for the determination of the soil resonance frequency and of the shear-wave velocities are single-station noise multi-component recordings (HVSR technique) and multi-channel analysis of surface waves (MASW; Park et al., 1999), the latter generally making use of single-component receivers. We here observe, in both synthetic and real datasets, a reverberation effect in controlled-source vertical receiver arrays, due to the presence of a buried impedance contrast. This reverberation is linked to the polarization of Rayleigh wave motion, and is clearly visible both in time domain (shot gathers before any processing) and in frequency domain (clear peak in the amplitude spectrum). We here show the strong dependency of the vertical reverberation frequency on the shear-wave velocity contrast, and on the shear-wave velocity, depth and Poisson’s ratio of the top layer, and its relation to the site resonance frequency. In conclusion, when seismic reverberations are observed on vertical-component data, a very quick characterization of the first subsoil can be deducted, without the need of multi-component analysis, multi-component sources or long acquisition time. References Lunedei E. andMalischewsky P.; 2015: A Review and Some New Issues on the Theory of the H/V Technique for Ambient Vibrations . In A. Ansal (ed.), Perspectives on European Earthquake Engineering and Seismology. Geotechnical, Geological and Earthquake Engineering, 39 , doi:10.1007/978-3-319-16964-4_15. Nogoshi M. and Igarashi T.; 1970: On the propagation characteristics of microtremor . Journal of the Seismological Society of Japan, 23 (4), pp. 264–280. Park C. B., Miller R. D. and Xia J.; 1999: Multichannel analysis of surface waves . Geophysics, 64 , pp. 800–808, doi: 10.1190/1.1444590. Boaga J., Cassiani G., Strobbia C. and Vignoli G.; 2013: Mode misidentification in Rayleigh waves: ellipticity as a cause and a cure . Geophysics, 78 (4), pp. 1–12, doi: 10.1190/GEO2012-0194.1. Malischewsky P. G., Scherbaum F., Lomnitz C., Tran Thanh T., Wuttke F. and Shamir G.; 2008: The domain of existence of prograde Rayleigh-wave particle motion for simple models . Wave Motion, 45 (4), pp. 556–564, doi: 10 .1016/j. wavemoti.2007.11.004. Bonnefoy-Claudet S., Koeler A., Cornou C., Wathelet M., and Bard P. Y.; 2008: Effects of Love waves on microtremor H/V ratio. Bulletin of Seismological Society of America, 98 , pp. 288–300. Hobiger M., Le Bihan N., Cornou C. and Bard P. Y.; 2009: Rayleigh wave ellipticity estimation from ambient seismic noise using single and multiple vector-sensor techniques . Proceedings of 17th European Signal Processing Conference (EUSIPCO 2009).