Convergence of shear wave velocity structure models obtained from ellipticity analysis of surface waves of ambient vibrations1

Shear wave velocity structure of the surficial layer of the earth is one of the requirements in many of site investigation programs. Conventional methods, especially in sites with thick alluvial layers, have limitations for this purpose. In this study, single-station measurement of micro-tremors and surface-wave ellipticity inversion were used to identification of soil structure. The study site is located in the south of Tehran and has a considerable thickness of soft alluvial layers. The ellipticity ratio was obtained using Geopsy software employing the time-frequency analysis method. Due to the uniformity of the subsurface layers of the site, the mean ellipticity ratio curve of the four seismic measurements stations was used to inversion and extraction of the shear wave velocity structure. Due to the uncertainty in the inversion process and to investigate the convergence and stability of the solutions, five different initial models are considered and the inversion process has been repeated 3 times in each model. There is a very little error in inversion results, where they show that the seismic bedrock exists at a depth of about 100 meters. Also, different models have good convergence indicating the reliability of the method used to derive the shear wave velocity structure.