Rayleigh Surface Wave Forward Modeling And Inversion Research

Rayleigh surface wave exploration is a non-destructive and cost-effective exploration method that utilizes the dispersion characteristics of Rayleigh surface waves to infer the shear wave velocity of the shallow subsurface.Surface wave inversion primarily involves three aspects:forward modeling of dispersion curves,energy imaging of dispersion,and inversion of dispersion curves.In order to gain a deeper understanding of the imaging capabilities of Rayleigh surface waves for subsurface shear wave velocity structures,this study conducted an investigation and research on these three aspects.Regarding the forward modeling of dispersion curves,the Knopoff fast algorithm was utilized to simulate dispersion curves.The characteristics of dispersion curves for different models and the extent to which model parameters affect the dispersion curves were analyzed.Regarding energy imaging of dispersion,a comparison was made among the imaging results of phase-shift method,f-k method,and τ-p method in terms of dispersion energy maps,as well as the accuracy of dispersion curves extracted by these three methods.Among them,the phase-shift method is effective in imaging the fundamental mode energy of Rayleigh waves,while the τ-p method is more advantageous for extracting higher-order dispersion curves.Furthermore,in order to gain a deeper understanding of the propagation characteristics of Rayleigh waves,the highorder staggered grid finite difference method was employed to numerically simulate Rayleigh waves.By obtaining the dispersion energy maps of simulated seismic records,the phenomena of "mode kissing" and "energy discontinuity" were identified,which have significant implications for practical exploration.In terms of inversion of dispersion curves,this study employed the damped least squares algorithm to invert the phase velocity dispersion curves of Rayleigh waves.Singular value decomposition(SVD)technique was introduced during the inversion process.By establishing different models to validate this algorithm,the results demonstrate that it possesses high accuracy and fast computational speed.To address the issue of limited depth penetration in active-source surface wave exploration,this study employed background noise data for Rayleigh wave inversion.The phase-weighted stacking technique was utilized to obtain cross-correlation functions with a higher signal-to-noise ratio.Additionally,low-frequency Rayleigh wave phase velocities were extracted from the empirical Green’s functions derived from background noise.Subsequently,the shear wave velocities of deeper subsurface media were evaluated based on these extracted velocities.Finally,this study utilized actual data collected on a campus to conduct processing and inversion,resulting in a quasi-2D shear wave velocity profile of the shallow subsurface.This validation of the method demonstrates its correctness and reliability.