Numerical Study Of The Rayleigh Wave Excited By A Ponit Source For Laterally Inhomogeneous Media

As the velocity of Rayleigh wave is close to the shear wave velocity and has some kinematic characteristics such as velocity dispersion, it is widely used in several geological survey field, such as natural seismic research, oil seismic exploration. In addition to the velocity, Rayleigh waves exhibit a second property: elliptical polarization. In homogeneous media, Rayleigh wave motion trajectory is an ellipse, the ratio between the short axes and the long axes of the ellipse is called ellipticity, which shows approximately linear inverse relationship with the Poisson's ratio of the medium. In inhomogeneous media,the ellipticity is also a function of frequency, which is similar to velocity dispersion, this property is called ellipticity dispersion?As one of the two main characteristics of Rayleigh wave, the velocity dispersion has already made great progress and success in application. Although the ellipticity polarization dispersion has been successfully applied in field frequency test, the application in the field of engineering exploration is still in the initial stage, due to the lack of complete theoretical support, especially for the most common horizontal heterogeneity detection in engineering geology, about which there is not clear record at present.In this paper, the numerical simulation method is used to study the velocity dispersion and ellipticity polarization dispersion in the lateral inhomogeneous media model. The main contents are as follows:(1) The basic theory research of Rayleigh wave velocity dispersion and ellipticity dispersion. Firstly,from the semi-infinite homogeneous media model, the relationship between Rayleigh wave velocity,ellipticity and elastic parameters in homogeneous medium is studied. Then, based on the horizontal layered medium model, the analytical method of velocity dispersion and the ellipticity dispersion of Rayleigh wave are both discussed in detail. At last, the data processing methods in Rayleigh wave dispersion analysis such as Fourier transform, ?-p transform, wavelet transform, complex trace analysis,and the time-frequency domain polarization analysis and so on are discussed in detail.(2) Numerical simulation of Rayleigh wave velocity dispersion and ellipticity dispersion. For the homogeneous half-space, the harmonic wave signal and Ricker wavelet signal are used as the seismic source to simulate the whole wave field. Combined with the theoretical research results, the propagation characteristics of the wave field are observed and analyzed. The accuracy of both numerical simulation method and the theoretical analysis result are verified. And the lateral inhomogeneous media and the vertical rock interface are selected as lateral inhomogeneous media to study the response characteristics of velocity dispersion and ellipticity dispersion, and comparative analyze the sensitivity and resolution of the two methods.(3)Research on the applicability of lateral inhomogeneous media inversion method. Aiming at the inversion algorithm based on the forward model of layered medium, make the shear wave velocity structure to be the inversion target, the applicability of the GA method used in the inversion of lateral inhomogeneous is analyzed by velocity dispersion inversion and ellipticity dispersion inversion,respectively.(4) Engineering application of ellipticity. The ellipticity analysis method is applied to the detection of mined-out area and the vibration monitoring of high-speed railway. The possible application fields and directions of this method and the possible problems in the application are analyzed.Through the research above-mentioned, the main achievements of the research are obtained as follows:(1) It is shown that the ellipticity is inverse proportional to the Poisson's ratio in a homogeneous medium,and is independent of the frequency. But in the case of lateral inhomogeneous medium,there is a highly nonlinear inverse relationship between the ellipticity of the Rayleigh wave and the Poisson's ratio of the media, which is with strong dispersion characteristics.(2) Rayleigh wave velocity dispersion or ellipticity dispersion can be used to detect lateral inhomogeneous media, and can determine the relative properties of inhomogeneous media and the general position of it. Compared with the velocity dispersion, the ellipticity is more sensitive to the lateral inhomogeneous medium, and has a high lateral resolution.(3) For the local inhomogeneous body, the forward model used in the inversion is a horizontal layered medium, which does not match the real model. The inversion errors of both the velocity dispersion and the ellipticity dispersion are very large, so it is not suitable for inversion of lateral inhomogeneous madia.(4) The ellipticity can directly reflect the relative changes of engineering geotechnical parameters,and can be used for the monitoring of high-speed railway subgrade quality and the detection of engineering geological targets such as goaf area, broken belt and other lateral inhomogeneous media.The research of this paper deepens the understanding of Rayleigh wave velocity dispersion and ellipticity dispersion, and improves the theoretical basis of inhomogeneous medium exploration using Rayleigh wave. The research can also provide a more perfect basic theory for Rayleigh wave in engineering application, and provide great help to promote the development of disciplines, and guide the application of Rayleigh wave in practice.