Research And Application Of Initial Model Inversion Algorithm Of Seismic Tomography

Computer technology has been widely applied in various fields.It is used to seismic tomography as a means to solve problems when detecting the heterogeneous structure of the earth's interior.Seismic travel time tomography is an imaging method that uses a large number of seismic travel time data to inverse the three-dimensional velocity structure of the study area.The initial model,as an important factor,has a great influence on the convergence speed of three-dimensional velocity structure inversion and the quality of imaging results.A new inversion algorithm of initial model is proposed to obtain a more reasonable and reliable initial model in this paper,applied to the actual seismic tomography to obtain a high-resolution three-dimensional crustal velocity structure.The main research works are as follows:(1)This paper has carried out theoretical research and algorithm implementation of the initial model inversion algorithm.The algorithm avoids the interference of human factors as much as possible.It uses the Snell's theorem and the MPI parallel technology to track the ray paths and calculate the theoretical travel time,which effectively improves the calculation speed of the algorithm.At the same time,considering the influence of the depth change of the subsurface discontinuity on the travel time,this study carries out joint inversion by dividing the weights of the velocity and the depth of the discontinuity to obtain a reliable initial model.(2)In this paper,a large number of evenly distributed theoretical seismic data are generated in the theoretical initial model and used for inversion to modify the model before inversion.The inversion results are compared with the theoretical initial model to verify the correctness and effectiveness of the initial model inversion algorithm.(3)This paper compares the convergence and imaging quality of the threedimensional velocity structures obtained by different initial models which come from different initial model inversion methods.By comparing the three-dimensional imaging results and convergence process corresponding to the initial models,the reliability and advantages of the initial model inversion algorithm in this study are highlighted,and the dependence of three-dimensional imaging results on the initial model is further demonstrated.(4)This paper uses a large number of real travel time data of seismic waves and combines with the more objective initial model calculated by this research algorithm to carry out three-dimensional tomography of the Longmenshan fault zone.The reliability of the imaging results and the practicability of the initial model inversion algorithm are proved by analyzing the results of resolution test and recovery test.Through the above works,both the verification of one-dimensional layered velocity model inversion process and the comparison of three-dimensional imaging results have proved that the proposed algorithm has great feasibility and advantages.In addition,the practical application of the algorithm in three-dimensional velocity structure inversion is analyzed,which further proves the practicability of the algorithm.