Estimation of Shear Wave Velocity from MASW and H/V Joint Inversion in a Noisy Area of Sao Paulo City, Brazil

Abstract:

<span'';">The parameters shear wave velocity and soft sediment thickness are very important for the site investigation. Shear wave velocities are usually obtained from the dispersion curve analysis of surface wave (MASW), which are obtained by active or passive seismic array. Dispersion curve provides a reliable velocity estimate at frequencies above the fundamental resonant frequency of the site (Hobiger et al, 2009), however dispersion curve analysis does not provide information about the deeper layers,. The information of deeper layer can be included from H/V spectral ratio analysis, which provides a very good estimate of fundamental frequency of site. A joint inversion analysis of dispersion and H/V curves have been made, the dispersion curve has been obtained by active source (MASW) while H/V ratios are obtained from seismic ambient noise recording through single 3 component broadband seismometer. H/V curve inversion are linked with Rayleigh wave ellipticty curve, however Bonnefoy-Claudet (2008) have shown that both Rayleigh and Love waves contribute to the H/V spectrum, so it is necessary to accurately extract Rayleigh wave ellipticity from single station recording. We have utilized two techniques to retrieve Rayliegh wave ellipticty from our noise recording. One is random decrement technique, RayDec (Hobiger et al, 2009) and second, time frequency analysis (Neries project, 2010). This Rayleigh wave ellipticty from H/V recording and dispersion curve from MASW have been jointly inverted. The inversion has been carried out using neighborhood algorithm introduced to geophysics by Sambridge (1999). Shear wave velocity profile have been compared with borehole result (at same location), which showed a very promising correlation, especially within zone of high velocity contrast and depth of bedrock.

References 

1. Bonnefoy-Claudet, (2008), Effects of Love waves on microtremor H/V ratio, Bull. Seismol.Soc 2. Hobiger, (2009), Single station determination of Rayleigh wave ellipticity by using the random decrement technique (RayDec), Geophys. Res. 3. NERIES group. (2010). Using Ellipticity Information for Site Characterisation. Sixth Framework Program.. 4. Sambridge, M. (1999). Geophysical inversion with a Neighbourhood algorithm, I, Searching a parameter space.Geophys. J. Int