Microtremor survey method based on inversion of the SPAC coefficient of multi-mode Rayleigh waves and its application

To make full use of the information found in the Rayleigh waves which propagate along the earth surface, a microtremor survey method was proposed and studied in this paper, based on the inversion of the SPAC coefficient of multi-mode Rayleigh waves observed in the microtremor signal. This technique provides a useful and reliable option for urban geological investigation by mapping the S-wave velocity structure of the near surface. First, the energy ratios of the multimode Rayleigh waves were computed using the medium response of each mode; Then, the SPAC coefficients of the fundamental and higher modes Rayleigh waves were calculated by forward modeling, with the influence of the finite arrays on the SPAC coefficient in practical work being taken into account; Finally, fast simulated annealing algorithm was used to invert the SPAC coefficient to obtain the subsurface shear wave velocity structure. Synthetic microtremor datasets from three typical geological models were studied using the mode algorithm of Herrmann, and compared with the inverted SPAC coefficients from field data. This technique was successfully used in the underground investigation project in downtown Shanghai, and the inversion results from the theoretical data and field data matched well. Our study shows that the microtremor survey method can provide more accurate and high resolution mapping of the near surface, especially for the geological structures with weak or hard interlayers.