Direct inversion of multimode Rayleigh-wave spatial autocorrelation coefficients from high-frequency ambient noise

ABSTRACT In general, the traditional spatial autocorrelation (SPAC) method uses the fundamental-mode surface-wave dispersion curve for inversion. Ignoring the higher-mode surface-wave dispersion curves affects the accuracy of the SPAC method in detecting the S-wave velocity structures, especially for the inversion of deeper structures. To fully and accurately use the multimode nature of surface waves, we investigate a direct inversion method of subsurface S-wave velocity structures from the SPAC coefficients of multimode surface waves extracted from the seismic ambient noise. In this new method, the SPAC coefficient is directly inverted to obtain the S-wave velocity. The key step is to obtain the theoretical SPAC coefficients of a layered model by taking into account the energy ratio of each mode of surface waves in the studied frequency band and the influence of the finite station pairs and by weighting the Bessel functions corresponding to the phase velocities of each mode with the energy ratio as the weighting factor. The direct inversion method for the SPAC coefficient significantly improves the efficiency of data processing by omitting the extraction of the dispersion curves. Meanwhile, joint inversion of the fundamental and higher-mode SPAC coefficients of surface waves ensures the reliability of inversion results, increases the exploration depth, strengthens the stability of inversion, and significantly reduces the inversion misfit.