Cross-well Seismic First Arrival Wave Travel-time Tomography Method Research

Crosswell seismic technology is an effective method to reflect the internal structure of the earth,and the crosswell seismic tomography is an important part of the crosswell seismic.With the application of crosswell seismic technology in oil and gas exploration more and more widely.At present,the main method of crosshole seismic tomography is the first arrival traveltime tomography based on ray theory.Ray tracing technique is the main method to calculate the sensitive kernel function and travel time.The first arrival has the advantages of easy identification,high stability,carrying a large number of underground information,so the first arrival traveltime tomography can provide more efficient and more accurate velocity for velocity analysis,and lay a solid foundation for the subsequent processing of seismic data.Thesis mainly studied the primary wave travel-time based on the theory of the ray tomography,its main divided into two parts,the forward ray tracing and inversion imaging.Tomography inversion of the article adopts damping least-square LSQR algorithm of the high precision and fast convergence rate,deducing the iterative formula of the algorithm and introducing the algorithm for selecting the damping factor in standard.The method has been tested for three different models,three main factors of tomography are discussed in detail: ray coverage,grid size and inversion algorithm.The results showed that:1)travel-time linear interpolation ray tracing algorithm is easy to programming and the inversion results of high precision.2)The damping LSQR algorithm is of high precision,the iteration speed,strong practicability and can obtain ideal inversion result.3)through the theoretical model of the test get satisfactory results,further illustrates the forward(LTI)algorithm and the inversion damping LSQR algorithm is the correctness and reliability,provides correct and efficient theoretical support for the practical work of the seismic exploration.4)On the theory,the closer for ray and the smaller the grid size,the higher the inversion precision,but considering the actual cost,to arrange the number of shot points and geophone,to ensure that the cost and improve the inversion accuracy.