Application Study Of Rayleigh Wave Dispersion Imaging In Transverse Inhomogeneous Media

The classical Multichannel analysis of surface waves(MASW)method in seismic surface wave exploration is based on the midpoint hypothesis of the horizontally stratified medium that the dispersion curves represent the dispersion information of the stratum below the midpoint of the received arrangement and the results obtained from the inversion of the MASW method based on this hypothesis are the 1D average results of the subsurface medium.Based on this hypothesis,the MASW method’s inversion is the 1D average result of the subsurface medium,which only applies to transverse homogeneous medium.In transverse non-homogeneous media,this method cannot directly realize 2D imaging of the medium,and can only obtain pseudo-2D shear wave velocity profiles.In order to overcome the limitations of MASW method,this paper explored the Multiscale Window Analysis of Surface Waves(MWASW)method and investigated its applicability,lateral resolution,and dependence on the initial model in shallow horizontal heterogeneous media.The paper also applied the MWASW method to archaeological geophysics.The main research contents of this paper are as follows:(1)Analyzed the dispersion error of the MASW method based on the assumption of horizontal layered media with midpoint.A tilted model,a trench model,and a barrier model were established to analyze the error of the midpoint assumption with the same layer thickness of the horizontal layered model.The numerical simulation results show that the dispersion curve error is small between geological models with lateral gradient and the horizontal layered model.However,the dispersion curve error significantly increases in laterally non-uniform media models,which indicated that the applicability of the MASW method may be limited in lateral inhomogeneous media.(2)Based on the beamforming analysis technique,a fast forward method for Rayleigh wave dispersion curves in laterally heterogeneous media is developed,which directly compute the energy spectrum of surface wave dispersion from the model.The beamforming analysis fast forward method was used to compute the energy spectra of three models,including earth levy model,double earth trench model,and step model,and compared with the dispersion energy spectra extracted from seismic records using the MASW method.The results showed that the former had less interference and stronger focusing,and the maximum phase velocity difference between the two dispersion curves was only 11m/s.This verified the effectiveness of the beamforming analysis fast forward method.(3)On the basis of the subsurface media information contained in Windows of different scales(i.e.,different receiving array lengths)in surface wave seismic records,The MWASW method is tested on the ground-barrier,double-graben,and stepwise models following the multi-scale dispersion analysis strategy,and compared it with MASW method.The results showed that the former method is more clear and accurate in revealing anomalous bodies and characterizing stratigraphic interfaces,with weak smoothing effect.It can obtain more real 2D horizontal shear wave velocity structure,which verified the strong applicability of MWASW method in the lateral heterogeneous models.(4)Aiming at three laterally heterogeneous media models,namely ground barrier,double graben and step,the dependence analysis of MWASW method on the initial model is carried out,and the influence of several uniform half-space initial models and layered initial models on the inversion results is analyzed in detail.the results showed that the MWASW method has low dependence on the initial laminar model.The accuracy of the MWASW method inversion results will be improved if some information about the anomalous body is present in the MASW method inversion results,which is used as the initial model.(5)With the background of shallow urban surface complex medium,the MWASW method is used to invert the multi-window dispersion curve of transverse inhomogeneous media.The inversion results obtained high-precision shear wave velocity structure,revealed the location and morphology of tombs and tombs with low velocity characteristics,and verified that the MWASW method has strong applicability and high inversion accuracy in transverse heterogeneous media.