New findings in high-frequency surface wave method

Multi channel Analysis of Surface Waves(MASW)analyzes high-frequency Rayleigh waves to determine near-surface shear(S)-wave velocities.This method is getting increasingly attention in the near-surface geophysics and geotechnique community in the past 20 years because of its non-invasive,non-destructive,efficient,and low-cost advantages.They are viewed by nearsurface geophysics community as one of most promise techniques in the future.We introduce some research results about propagation and applications of high-frequency surface waves proposed by near-surface geophysical research group at China University of Geosciences(Wuhan)in recent years.Non-geometric wave exists uniquely in near-surface materials,especially inunconsolidated sediments.It is valuable for aquick and accurate estimation of S-wave velocity of the surface layer.Our study shows that non-geometric waves are leaky waves and they are dispersive.Leaky surface wave could cause misidentification when treating the leaky-wave energy as fundamental or higher modes Rayleigh wave.Such misidentification will result in wrong inversion results.By obtaining Rayleigh-wave Green′s function after separating fundamental-and higher-mode Rayleigh waves,we verify the feasibility of virtual source method in Rayleigh-wave survey,which could tremendously decreases the cost of field works.Compared to Rayleigh waves,a fewer parameters are involved in Multichannel Analysis of Love Waves(MALW),which makes Love-wave dispersion curves simpler than Rayleigh waves.As a result,inversion of Love waves is more stable and the degree of non-uniqueness is reduced.Images of Love-wave energy are usually sharper and of higher resolution than those from Rayleigh waves.This make picking Love-wave phase velocities much easier and more accurate.Analysis on relationship between surface-wave wavelength and penetrating depth by using Jacobian matrix shows that:as for fundamental mode with the same wavelength,Rayleigh wave can see 1.3~1.4times deeper than Love waves,however,their penetrating depths are similar for higher modes.We also make some attempts on time-domain Love-wave waveform inversion.We divide the subsurface model into different sizes of blocks according to resolution of Love waves.We remove the source effect by deconvolution,and achieve an appropriate subsurface S-wave velocity model via updating Swave velocity of each block to fit observed waveforms.This method does not need horizontallayered-model assumption,and can be applied to any kind of 2D media.