The Research On LTI-SIRT Tomography Static Correction Method And Application Effect Analysis

The seismic data processing and interpretation for the near-surface structure of complicated region is a major task of seismic exploration on land .Seismic tomography is a seismic data processing methods which reconstructs statum and inverse velocity. The use of the first break tomography to build near-surface velocity field can eliminate distortion of the time-distance curves. In this paper, following jobs have been done:1. The experiment shows that the existing ray tracing algorithm can not adapt to complicated velocity model as well as velocity model with large change in velocity gradient.At the same time, there are also some problems in the inversion algorithm ,such as low stability, the strong dependence of the initial model and so on. By comparison, this paper uses LTI(Linear Traveltime Interpolation), and SIRT(Simultaneous Iterative Reconstruction Technique) to realize the forward and inversion of the velocity field.2. In the inversion process,the selection of the largest offset has great effect on the inversion because the error of the deep statum will affect the accuracy of the near-surface velocity inversion. But if the largest offset is too small,it will cause the rays are not enough to cover near surface which will also affect the accuracy of inversion.Largest offset can be determined by the depth of the ray traveling in initial model and the required invesion depth3. In order to verify the stability and convergence of the tomographic inversion algorithm,theoretical model experiment is done,in which the depth error and randon error are introduced. The conclusions of the experiment are following:â‘ tomography algorithm inversion trend is correct.â‘¡pick up in the accurate measurement and Inversion results is better with accurate measurement and pick-ups.â‘¢Error can lead to worse convergence of the velocity model.â‘£In model inversion, the influence of the overall deviations is bigger than that of random error.4. In the realization of LTI and SIRT algorithm, first of all,three kinds of initial model are built,and the seismic data of a two-dimensional survey line is selected. By comparison,the inverstion result of the model is better. Second,the reasons for the effect of velocity inversion are identified; Interpolation and smoothing technique are used to improved the accuracy.5. Traditional softwares usually usd fixed depth to extract the isovelocity surface, this may lead to the wrong amount of static correction. In order to solve this problem, we improved the methods of extraction of isovelocity surface to improve the inversion accuracy . At the same time, in order to eliminate some of the velocity mutation of the isovelocity surface we use median filter and mean filter method to smooth the surface velocity6. There is improvement, but also a place to show signs of deterioration in static Correction profiles.The result in this outcome is expecting further study.In this paper, theoretical models and actual data inversion results show that the method can reconstruct near-surface distribution of the velocity field stably, effectively and rapidly . To perform well in this anti-structure, it is necessary to rely on errors reduction of field observation system ,use of first examination of information , the high adaptivity of the algorithm, and the appropriate choice of parameters.