Estimation of shallow subsurface S-wave velocity structure in urban area based on inversion of apparent dispersion curve

Due to its efficiency, convenience, non-destructive nature and strong anti-interference capability, the microtremor survey method (MSM) has found wide applications in urban geological surveys. The spatial autocorrelation method is diffusely applied to extract the dispersion curves from microtremor signals, which needs to satisfy the assumption that the energy of the fundamental Rayleigh wave is dominant. However, for layered media containing a layer with a significant low- or high-velocity contrast, this assumption is distinctly incorrect for certain frequency ranges. We present a processing methodology comprising the extraction and inversion of the apparent dispersion curves based on extended spatial autocorrelation method and fast simulated-annealing algorithm. We analyse synthetic microtremor signals for three selected geological models, and then compare the S-wave velocity structures estimated from their inversions with the actual models. Subsequently, a filed data example is given to detect the shallow stratigraphic structures in Guangzhou city, China, in which the new MSM was used. The estimated two-dimensional S-wave velocity model provided an accurate description of the thickness and depth of the strata in the study area, based on a priori information. Moreover, the S-wave velocity structures estimated from the MSM and the results from the drilling match very well, indicating that MSM is a reliable geophysical technique in urban geological surveys. Combined with available borehole information, MSM can be a very robust and effective method for detecting the shallow three-dimensional velocity structures in an urban area.