Application Of Vector Wavenumber Transformation Method In Ambient Noise Tomography

The interference between shear wave and longitudinal wave in focal region will produce surface wave propagating along the surface or interface.Rayleigh wave is mainly divided into Rayleigh wave and Love wave.Because Rayleigh wave is easier to collect,Rayleigh wave exploration is widely used in surface wave exploration.Surface wave energy accounts for about 70%of the seismic wave energy,so Rayleigh wave has a strong anti-jamming ability;and Rayleigh wave also has the characteristics of dispersion,the surface wave dispersion characteristic is that the phase velocity of surface wave changes with the frequency.By extracting the dispersion characteristics of Rayleigh surface wave,we can invert the underground velocity structure,so Rayleigh wave is widely used in shallow exploration.Rayleigh surface wave exploration can be divided into active source and passive source because of different seismic sources.Active source includes instantaneous excitation source such as artificial hammer,explosive and high precision vibrator with adjustable steady-state frequency.Active Source Surface Wave(ASSW)prospecting has a high requirement for site,limited exploration depth,and has a certain impact on the environment.Passive source mainly uses environmental background noise as the source.Because noise signals can be collected at anytime and anywhere,and the acquisition process has no impact on the environment,passive source exploration has attracted more and more attention.At present,the main methods of passive source surface wave exploration are spatial autocorrelation method(SPAC),frequency wavenumber method(F-K)and empirical Green’s function method,but these methods can only extract the dispersion information of fundamental surface wave.In fact,the energy of Rayleigh surface wave is distributed in each mode,and the energy of each mode of different formation structure is different..When the formation velocity structure is not uniform,the higher-order surface wave is more sensitive to the medium parameters.Therefore,it is necessary to add the higher-order model to constrain the inversion of the subsurface velocity structure.Vector Wave Number Transform(VWTM)is more flexible and can be arranged randomly than traditional surface wave exploration methods.Firstly,we validate the method with synthetic noise signal.The results show that the VWTM method can achieve high-order imaging effectively.Then,we apply this method to the shallow exploration of Chaohu shoal,establish the initial model according to the basic model,and use the quasi-Newton method to invert.Comparing the two inversion methods which only use the fundamental dispersion curve and the fundamental and the first order joint inversion,the results show that the more accurate underground velocity structure can be obtained by combining the higher order information,which is in good agreement with the borehole formation data,thus verifying the validity and correctness of the vector wavenumber transform method in practical application.