Study On Seismic Wave Field Under Complex Surface Conditions Of Special Physiognomy In West China

Modeling and understanding the effect of seismic wave propagation involving the complex near surface in West China has been one of the important issues in seismic exploration. In fact, the earth's free surface is rugged topography at complex free surface condition areas. So based on the conditions the exact solution can not get. But this is usually the location at which seismic observations are made, topography may have a significant influence on record data. In fact at complex free surface areas, such as some classical topography model in West China, the research of the construction of recorded data is more slowly than the practice of exploration.Generally the complex free surface has included rugged topography and variation of velocity in lateral near free surface rock layer or soil layer. Professor Sun Jianguo has given a generally summary which included arbitrary combination three as down lines is complex free surface. And the three down lines are : (1) the earth's free surface topography;(2) the complex geological construction near free surface;(3) the complex media and rock physical parameters distribution near free surface. For exact geological solutions from the observation seismic data, we must do the modeling of seismic wave field.The use of synthetic seismograms is one of most useful methods in estimating the response of media at a given receiver. Various numerical approaches have been proposed to solve the seismic wave equations. I present synthetics of seismic wave propagation near free surface topography. Based on existing direct and imaging methods of a 10th order staggered finite-difference scheme, an improved algorithm for staggered finite-difference is proposed to implement rugged topographic free boundary conditions. This method assumes that the free surface can be implemented with horizontal and vertical free surface segments and their corners. Based on the same principle, the method is adapted to elastic wave equations, visco-elastic wave equations and isotropic elastic wave equations.Main research contents in the article include:(1) The construction of mathematical physical model;(2) Synthetics of seismic wave propagation near free surface topography;(3) The influence of free surface topography through the seismic wave modeling;(4) The influence of the type of media through the seismic wave modeling;(5) The influence of the distribution of rock physical parameters near free surface through the seismic wave modeling;(6) The influence of some seismic parameters by the wave modeling. And by the work of this thesis, I get some conclusions in two fields about the development of modeling computation technology and some features of seismic wave propagation.In computation technology, I use high-order staggered grid finite difference method to solve the seismic wave equations. This is a ripe method of seismic modeling. And my key work is to develop the combination method general imaging method with zero velocity method to deal with the topography. Besides the elastic media seismic wave equations, the method is adapted to many other earth models, such as isotropic media, visco-elastic media and so on. And it can be used in 3D computations.By the modeling of seismic wave propagation, some features are drawn:(1) In all effect factors of complex near surface, the topography is the most one, and the second one is the distribution of rock physical parameters near free surface;(2) The ratio of signal/noise of cave model's response is more better than of apophasis model;(3) Besides the distortion in x-axis of symmetry by the topography, it also does the distortion in z-axis of symmetry by the asymmetric topography; (4) The topography may be choose some frequencies to absorb;(5) The dispersion by topography is not obvious, so it isn't to expect filtering the influence by topography.