Research On Tomographic Inversion Of Near-surface Velocity In Complex Areas

In the central and western regions of our country,because of the terrain,thick weathering layer heaving,fault distribution and accompanied by a strong folding complex near surface factors,generally lower than the signal-to-noise of seismic data,so the near surface velocity structure accurately becomes the key to improve the quality of seismic imaging.First arrival time tomography inversion is a good choice to get near surface velocity.However,the complex and variable speed of the model makes the distribution of ray path very uneven,which will greatly increase the ill posed nature of the solution process,thus affecting the resolution and reliability of tomography.Therefore,further research on higher precision,more robust and efficient travel time tomography inversion is of great significance.First of all,in order to obtain a kind of computational efficiency and accuracy are relatively high and can be applied to the complex model of ray tracing method,the system research of the ray tracing method,which includes straight ray tracing,the ray equation based on solving,based on wavefront construction,based on the Gauss beam,based on finite difference,based on the shortest path ray tracing method based on interpolation and linear travel time.After analyzing their respective advantages and disadvantages through numerical simulation,a shortest path ray tracing method for complex velocity structures under fluctuating terrain is put forward and verified by model trial calculation.This ray tracing method combines the advantages of linear traveltime interpolation and high accuracy,and the shortest path is easy to globally converge.It takes account of both computational efficiency and computational accuracy.The algorithm is stable and reliable,and does not depend on the complexity of the model.Then,the paper introduces the basic principle of inversion,that is,how to use the travel time residual radiation passes through the grid length and slowness to establish the inversion equations and the ART algorithm,solving the inverse equations of the SIRT algorithm and LSQR algorithm of 3 kinds of inversion algorithms,and put forward the ups and downs of the near surface under the inversion scheme and process,verify the chromatography the establishment of inversion scheme level and fluctuation theory model.Finally,the paper in order to analyze the factors influencing the complex near surface inversion accuracy,including inversion method,initial iteration model,covering the number of iterations,the velocity field smoothing algorithm and offset several influence factors,established a theoretical model of a typical complex near surface,and the different influence factors of the model tomography inversion,by contrast analysis of the inversion results of them,the influence of various factors on the inversion results of the discussion,and proposed to the inversion processing in the actual production of advice.In particular,a new tomographic inversion method—iterative tomographic inversion between different offset ranges is proposed.Numerical simulation shows that this method is better than the traditional tomography inversion in retrieving shallow and medium depth formation velocity near the surface.