Seismic Tomography Technical Research

By analyzing kinematics (traveltime, ray route) and dynamics (waveform, amplitude, phase, frequency) data of seismic wave phases observed, Seismic Tomography is an important geophysics technology which could be used to inverse important information of earth medium covered with a mass of rays, such as structure, velocity distributing and elasticity parameters. The origin of Seismic Tomography is from medical CT, with being developed in geoscience, Seismic Tomography has gotten significant advances, so it is being accepted by more and more people.In this paper, development history of Seismic Tomography and its principles, model constructing, ray tracing and methods of inversing are introduced. At the same time, similarities and differences of damped least square method and genetic algorithm is contrastively analyzed, and these two inversion algorithms above are applied on region of central-eastern part of Qilianshan. Based on same initial velocity model, after joint inversion containing source parameter and velocity structure by near 10 years'digital seismic recorded S ? Pdata with many phases such as Pg,S g,Pm,S m,P n,S nin the research area, we obtained distribution of velocity at different depths, depth of earthquakes and profile images at different locations, and character of depths about Moho. The following conclusions are obtained by comparing two inversion algorithms as well as the results, with reference of geologic structure information and previous research results.1,There are academic differences between the two different inversion algorithms. The damped least square method is a linear solution,which can give discernibility matrix and covariance matrix of the solution estimate directly. The damped factorλ2not only play a role in promoting the balance, but also affects the convergence speed. The genetic algorithm is a global optimization method, which applies the stochastic switching rules to the optimal solution searching, and suits in various optimized questions.However it is the only way to obtain the covariance matrix with the linearized means and unable to solve the model parameter information directly such as resolution and variance component, et al.2,The results of the two inversion algorithms shows that,The travel time remainder error with using damped least square method is bigger than that with genetic algorithm.Moreover, the counting yield of damped least square method is lower. There is bad relocation effect on the marginal seismic parameter with damped least square method, and abnormity of high and low speed structure is larger. On the whole, it is demonstrated that epicenter distribution is relatively more centralized from two inversion algorithms, the distribution of velocity and the depth distribution of Moho surface have the similar characteristic, and the velocity profile fluctuation is more consistent.3,In the research area, most faults thrill through the high and low speed intermediate belt, and their strike are NWW. The earthquakes more occur on faults or their edges. Through the analyzing,it is considered that the research results of this paper are verifiable with the geologic structure information and other seismic tomography results in the region on many aspects.