Determination Of Near-surface Velocity Structure Based On 3D Seismic Tomography With Non-uniform Grids

Static correction subjected for solving the warping of first break of shot gather and the inequality of first arrival of offset gather is requisite for reducing the bad effects caused by complicated near-surface condition. The existence of static correction problem produces the deterioration of seismic records and depresses the quality of stack profile and migration profile. The approaches for static correction is predominantly consist of elevation static correction, refraction static correction, and tomography static correction. The suitability of static correction is closely related to the accuracy of near surface velocity model. The non-uniform grid is better than uniform grid in velocity molding of near-surface.We have discussed several issues that are concerning near-surface tomography. Those contains the approaches, for illustrating velocity models, of discretization and of mesh generation, the ray tracing algorithm which based on ray vector, and the inversion strategy, Algebra Reconstruction Technique(ART), and its application.The approach for illustration of geological structure models by non-uniform grid is superior compared with velocity function method which has low accuracy, and with the conventional mesh model which holds its drawbacks-large data redundancy. Non-uniform grid approach provides series of mesh groups with different data density to construct velocity model of near-surface. The near-surface model has a hierarchical feature from background velocity to micro-structure velocity distribution.We have described a well performed ray tracing algorithm which based on non-uniform mesh modeling and ray vector. As a forwarding approach, it results the azimuth angle of ray and travel time of first pick. We have developed a software in C++ and have obtained satisfactory results.Utilizing ART for near-surface tomography has long been an effective technique. Adjusting the velocity of a group of voxels, a ray path has gone though, for delimited the travel time residual. It is appealing for near-surface tomography by ART because we had obtained the high quality stack profile after static-correction, which results from near-surface velocity model inversed by ART, compared with the profile after elevation static-correction in a seismic prospecting area of Hunan.