Research On High-frequency Surface Wave Imaging Technique For The Shallow Subsurface

The S-wave velocity is one of the most critical seismic parameters of the shallow subsurface.Extracting the S-wave velocity information from the high-frequency surface waves has always been a research hotspot in the field of the shallow subsurface geophysics.To deeply understand the propagation law of high-frequency surface waves and the imaging ability to medium,the following research contents are covered in this paper:(1)The field data of Rayleigh waves is processed by the classic Multichannel Analysis of Surface Waves(MASW),and the processing flow for generating 1D S-wave velocity and pseudo 2D S-wave velocity profile is introduced in detail.Finally,according to the regional geological profile of the survey area,the weathering interface of the stratum is identified from the pseudo 2D S-wave velocity profile and the profile of its gradient.Meanwhile,the limitations of MASW are pointed out.(2)To deeply understand the propagation law of Rayleigh waves and Love waves in elastic medium,the simulation of propagation for surface waves in two-layer model are realized by highorder staggered-grid finite-difference method.The dispersion energy of the multichannel records for Rayleigh waves and Love waves are generated by using the tau-p transformation.By analyzing the dispersive energy,it is found that the energy distribution of each mode of Love waves is uniform and there is no "mode kissing" phenomenon compared with Rayleigh waves.In conclusion,the multi-mode dispersion curve is easy to extract from the record of Love waves.The linear mapping is used in the nonlinear inversion algorithm of the Generalized Pattern Search(GPS)for the fourlayer geological model with a weak interlayer,and the ability of Rayleigh waves and Love waves to evaluate the S-wave velocity is further analyzed.The results show that the addition of high-mode Rayleigh waves can significantly improve the accuracy of the S-wave velocity evaluation results and the robustness of the inversion system.The addition of high-mode Love waves would make the inversion system oversensitive to the anomalous layer above the half-space,which is the parameters are overestimated.The accuracy of the half-space S-wave velocity evaluation result is significantly improved.The results of the field data show that the penetration depth of the high-mode Love waves is deeper.(3)The low-frequency(less than 10 Hz)surface waves in the active source seismic record has not developed into a stable propagating wave due to the too short offset.Therefore,it is difficult to accurately extract the phase velocity of the low-frequency surface waves from the active records and evaluate the deeper(greater than 30 m)S-wave velocity.Aiming at this problem,we attempt to measure the phase velocity of the low-frequency surface waves from the ambient noises by frequency-time analysis and realize the evaluation of the S-wave velocity of the deeper medium in this paper.(4)The surface waves dispersion analysis based on the horizontal layered medium assumption is only applicable to a medium with a slow lateral change,and it is difficult to achieve 2D or 3D imaging of a medium with a sharp lateral change.Aiming at this problem,we study the Full-Waveform Inversion(FWI)of direct matching waveforms,including the numerical test of gradient-based and preconditioned algorithms.A series of numerical test results show that the preconditioned technique based on the inverse scattering theory can greatly improve the convergence rate of the misfit,enhance the seismic illumination intensity of deep medium,and improve the imaging accuracy.Finally,the preconditioned conjugate gradient algorithm is applied to realize the FWI of the measured Love wave data,and the influence of the consistency of the seismic wavelet on the imaging accuracy is discussed in this paper.(5)The medium has been widely confirmed to have anisotropy and viscoelasticity,and it is assumed that the medium is an isotropic elastic medium of many surface waves exploration techniques.For this purpose,the propagation laws of Rayleigh waves and Love waves in KelvinVoiget(a viscous model that can describe the characteristics of formation attenuation)medium are researched.The dispersion laws of viscoelastic Rayleigh waves and Love waves in the layered model are analyzed and compared with the elastic Rayleigh waves and Love waves in the same layered model.The results show that the viscoelasticity of the medium affects the amplitude of the waveform and greatly attenuates the high frequency components in the signal.The viscoelasticity of the medium narrows the dominant frequency band of the seismic wave energy,and the peak frequency of the seismic trace moves toward the lower frequency.In addition,the viscoelastic properties of the medium suppress the development of high-order surface waves,especially highorder surface waves at high frequencies.