Dispersion Characteristics And Inversion Of High-frequency Surface Waves In Horizontal Layered Models With Velocity Not Increasing With Depth

High-frequency surface wave method is based on the dispersion feature of surface wave.It possesses stable and efficient inversion algorithms to invert phase-velocity dispersion curves of surface waves.At present,it is widely used to solve different kinds of near-surface geophysical and geotechnical problems.For normal earth models,S-wave velocities of different layers will increase with the depth.High-frequency surface wave methods estimate the Shear-wave(S-wave)velocity of each layer well for such kinds of models.The near surface,however,is complex.The theory of high-frequency surface wave methods remains insufficient for low-velocity-layer and high-velocity-layer models.This dissertation made a systematic research on imaging,forward modelling and inversion for horizontal layered models with velocity not increasing with depth.This dissertation proposed a data-processing step to enhance resolution of the ?-p transformation,f-k transformation,phase shift,and frequency decomposition and slant stacking method.It enables these four algorithm to image dispersion energy with the same resolution as high-resolution linear Radon transformation.It is demonstrated that resolution of the five methods is equivalent in substance for noise-free synthetic data.By computing theoretical dispersion curves and their eigen-displacements,and comparing them with dispersion energy images of synthetic data,this dissertation distinguished dispersion curves of surface waves and channel waves in models with velocity not increasing with depth,and studied their dispersion characteristics,which laid a foundation for inverting surface-wave dispersion curves due to these models.This dissertation noted that the dispersion curves calculated by the generalized reflection and transmission coefficient method in which only the dispersion function of the surface layer is considered,match the simulated dispersion energy completely,and can be confirmed to be surface wave.Therefore,this method is applicable for the forward modelling of horizontal layered models with velocity not increasing with depth.Based on this method,this dissertation calculated sensitivities of phase-velocity dispersion curves due to variations in S-wave velocities in horizontal layered models with velocity not increasing with depth.The results show that the sensitivity of the low-velocity-layer is high;sensitivities of the high-velocity-layer and layers beneath the irregular layer(the low-velocity-layer or the high-velocity-layer)are low.Consequently,it is expected to acquire accurate S-wave velocity of the low-velocity-layer.Confidence of the inversion results of the high-velocity-layer and layers beneath the irregular layer,however,is not high.A priori knowledge on the irregular layer is preferred during the inversion.By setting the initial model appropriately,the damping least-square algorithm effectively handles the inverse problem of continuous dispersion curves(due to a weak or thin low-velocity-layer model).Differential evolution algorithm is able to conquer the difficulties that brought by the discontinuous dispersion curves(due to a strong or thick low-velocity-layer model).