Study On The Structure Of Sichuan Basin By Using High Mode Dispersion Curves Of Ambient Noise

In this paper,we review the research status of ambient noise method and the theoretical basis of extracting empirical Green's function from background noise data.The data processing method of extracting empirical Green's function from background noise data and the process flow is summarized.the dispersion curve is calculated by the generalized reflection and transmission coefficient method,which provides the basis for the forward modeling in the inversion phase;the theory of dispersion curve inversion has a long history,and it is developed from the initial half-wavelength method.Up to now,various bionic algorithms have emerged in an endless stream,and the quasi-Newton method-BFGS correction-trust region method is deduced in detail.Vector wavenumber transform is a newly proposed method for dispersive energy imaging of surface wave signals.A dispersion energy diagram containing high-order dispersion energy is obtained from the background noise cross-correlation function,we Based on the principle of high-resolution Radon transform,the high-resolution vector wavenumber transformation formula is derived and the simulation data is simulated.It shows that the high-resolution vector wavenumber transform can effectively improve the resolution of the dispersion energy map.The data obtained from cross-correlation has a poor signal-to-noise ratio,and compares the effects of smooth denoising,wavelet threshold denoising,empirical mode decomposition denoising,and sparse decomposition denoising on the simulated data.The results show that the sparse decomposition denoising effect is the best,and the noise energy in the dispersion energy map can be effectively reduced.The method is applied to the Sichuan Basin region,and the research results are obtained through the processing of ambient noise data in the Sichuan Basin and its surrounding areas.A dispersion energy map with high-order dispersion energy,strong resolution,better continuity,wide frequency band distribution,and high-order dispersion energy.After obtaining the dispersive energy map,the one-dimensional shear wave velocity model of each sub-region is obtained by using the inversion algorithm.The geological conclusion that the low-speed region exists in the depth of 20km-40 km in the Tibet Plateau is verified.The shear wave velocity is as low as 3.25km/s;Due to the influence of sedimentary layers in the Sichuan Basin,the phase velocity can reach 2.5km/s at 0.3Hz,and the inversion results show that the thickness of the sedimentary layer is between 5km and 10 km.The inversion results in the Yunnan-Guizhou Plateau show there is a low speed zone at a depth of 60-70 km on the west side of the upper mantle.Through analysis and comparison,the reason for the base-order dispersion bifurcation and energy disturbance is explained by the difference in horizontal velocity in the shallow region.