Study On Inversion Methods Of Near-surface Parameters Based On Surface-wave Information

Surface wave is a category of special mechanical waves that mainly exist near the interface between two layers with high-impedance contrast.In most cases,it is referred to Rayleigh-wave and Love-wave,both of which propagate along the earth surface,in onshore seismic exploration.Geophysical exploration methods based on the theory of surface-wave have been playing an increasingly vital role in solving the problems in engineering geology and environment up to date.This category of methods,which is composed of three core parts,is primarily used for near-surface shear wave velocity characterization by the inversion of phase velocity characteristics of surface waves.The three parts include the forward of dispersion curves,dispersion imaging,and the inversion of dispersion curves.In order to succeed in applying the theory and method of surface-wave to oil seismic exploration,this paper has conducted a systematically research on aforementioned three aspects.In the aspect of dispersion forward,the paper systematically reviews three surface-wave dispersion computation methods—the transfer matrix method of Thomson-Haskell,the delta matrix method and the reduced delta matrix method.On that basis,a recursive algorithm to analytically compute the group-velocity dispersion curves is proposed by using the reduced delta matrix method and theorem of implicit function.The paper builds a few typical layered horizontal models,computes the phase-velocity dispersion curves and the group-velocity ones of Rayleigh-wave and Love-wave,and verifies the feasibility of this algorithm.In addition,the paper compares and analyses the curves of eigen-displacement of Rayleigh-wave and Love-wave,and also points out that the eigen-displacement can help to interpret the phenomenon of mode jumping appearing in the dispersion images in the case of a low velocity layer embedded.As far as dispersion imaging is concerned,improvements have been made on the phase-shift method and fk-music method,respectively,the cross-correlation phase-shift method and fv-music method have been proposed.The validity of the two new methods has been confirmed by processing surface-wave data from engineering and oil seismic exploration.Compared to the conventional phase-shift method,the cross-correlation phase-shift method can obviously suppress the background noise and other disturbances,which improves the quality of dispersion images.In the case of fv-music,it belongs to a kind of high-resolution dispersion imaging methods.After comparing with the conventional high-resolution linear Radon transform,it is demonstrated that fv-music can obtain phase velocity images with a higher resolution when the length of receiving array is shorter.At present,the dispersion characteristics of surface-wave in an elastic medium have been well studied.However,the researches in a viscoelastic medium are still relative few.In this paper,I present a unified solution to the Love-wave dispersion equation in the elastic medium,viscoelastic medium with constant Q and Kelvin-Voigt viscoelastic medium based on zeros solving technique of analytic function.The movement laws and differences of Love-wave dispersion characteristic roots in two cases in the plane of complex phase velocity are summarized: layer velocity increases with depth and with low velocity layer embedded.The study also shows that the attenuation of fundamental mode Love waves in the viscoelastic layer with constant Q increases approximately linearly with frequency;while it increases approximately exponentially with frequency in the Kelvin-Voigt viscoelastic layer.In the aspect of inversion of dispersion curves,this paper realizes a simultaneous linear inversion strategy of velocities and thicknesses of the layers according to the characteristics of surface-wave encountered in the oil seismic exploration and itself.This inversion strategy has made a comprehensive application of the control of parameters in log-domain,the smoothness regularization,and a recursive algorithm to analytically compute the elements of Jocobian matrix.Through these techniques,the inversion strategy can stably invert more than 20 layers and show some kind of “automatic detection” in the inversion results.By comparing with the inversion results published earlier in other papers,the feasibility and advantages of this inversion strategy have been validated.Lastly,inversions of multiple-mode dispersion curves have been conducted on the real seismic data from eastern and western areas in our country,and the 2D near-surface shear velocity profiles also have been acquired.