Near Surface Wave Scattering Study

The seismic surface waves are widely used to infer the properties of the media in different fields ranged from global seismology and geotechnical engineering to ultrasonic Nondestructive test(NDT). However, in current analysis of the surface waves the structure to be inferred are usually assumed isotropic and homogeneous transversely, and which does not vary laterally, but only with depth. This assumption is valid only for small lateral variations in the structure. When large variations (which is often seen in practice) occur, the wavefield becomes more complex, and we have to consider the scattering of the surface waves caused by the 3D heterogeneities. This is paid attention by many researchers in the fields at different scales described above. This paper mainly focus on the scattering of the surface waves in the near surface geophysics.Six chapters are included in this paper. Background and introduction are given in chapter 1. Chapter 2 presents a review on the propagation and the scattering of the seismic waves in complex media. Chapter 3 is the general theory about the scattering of the elastic waves, and the analysis of the scattering characteristics of the body waves is also presented in this chapter.Chapter 4 and 5 are the main work of the paper. Mode coupling method for surface waves propagated in the laterally heterogeneous structure are studied in chapter 4. Reference mode coupling method is used to analyze the characteristics of the scattering surface waves in this chapter. This method also use the modes in reference model as the basis to represent the wavefield. However, the Rayleigh and Love modes propagated independently in the laterally homogeneous reference structure will not independent any more due to the 3D heterogeneity and are said to be coupled. The expression of the wavefield can be given by calculating the coupling coefficients between the modes. Single- and Multi-scattering of the surface waves can be achieved by this method. For different laterally heterogeneous structures, the scattering of the surface waves are investigated in this chapter, both in frequency and time domain.Integral equation method is used to calculate the scattering of the surface waves in chapter 5. Different from the chapter 4, this method can handle the situation of the point sources. The wavefield is represented by the Fredholm integral equation. To solve the integral equation, we discretize the heterogeneity to many cells, and replacing the cubic cell by a cylinder with equal volume to calculate the volume integration since the shape of the cell is not important. The volume integration is formed by a surface integration in the horizontal plane and a depth integration in vertical direction. The surface integration of the Green's elements to a circle can be given analytically after replacing the cubic cell by a cylinder. We firstly get the wave field inside the heterogeneity by put the observe point inside. Then the scattered field at arbitrary point can be given by taking the heterogeneity as the secondary source. We investigate the characteristics of the scattering of the surface waves propagated...