Application Of The Tomo-inversion-based Statics Techniques

Near-surface variations of elevation, lithology, velocity and structure etc. often lead to complicated static time shifts to subsurface reflections and thus cause severe problems to stacked reflections in terms of poor imaging and false structures. Aiming at this near-surface-caused problem, this paper uses seismic tomography to first invert near-surface velocity model and then calculates statics; and satisfying results were obtained. This paper first introduces the basic concepts and causes of near-surface static time shifts, the advert effects on seismic exploration and the statics estimation techniques such as field statics technique (near-surface refraction surveying plus up-hole surveying), refraction-based statics techniques, tomo-statics techniques. Among these techniques, the field statics technique is only suitable for simple near-surface conditions, not for the ones with abrupt changes of velocity or structure. Though the refraction-based statics techniques are widely used, they have two basic assumptions, a stable refractor across the survey area and known near-surface velocities; therefore, the scope of their application is also very limited. Using first arrival times, the tomo-statics techniques invert weathering velocities with gradual depth-velocity variations, thus avoid the layered-velocity assumption and can be applied to surveys with various complicated near-surface velocity models.Focusing on tomo-inversion-based statics technique, this paper introduces two aspects of the tomo-statics techniques, ray-tracing techniques and inversion techniques. For the ray-tracing techniques, four techniques are introduced: analytical method, shooting method, ray-bending method and the shortest path method. The first two of the four are initial-value-oriented ray tracing and the last two are boundary-oriented ray tracing. Each of the four has their own limit, for instance, the analytical method is only suitable for a few special velocity distributions; the shooting method could lead to blind sector and/or non-least travel time ray paths; in case of complex velocity distribution and long distance tracing, the ray-bending method is less efficient than the shooting method; one of the advantages of the shortest path method is that it can always find least travel time ray path however complex the velocity distribution is; the other is that it has high computational efficiency, for it performs only addition, subtraction, multiplication, division and radication. For the inversion techniques: FBP(Filter-Back Projection Techniques), ART(Algebraic Reconstruction Technique), SIRT(Simultaneous Iterative Reconstruction Technique) and LSQR(Least Squares QR-factorization) are introduced. BPT has a too low resolution, thus can only be used for coarse velocity scanning; though ART has a little higher computational efficiency, it diverges sometimes; SIRT is robust, but its computational cost is high; for normal-condition equations LSQR is equivalent to CGLS (conjugate gradient least squares ); but for ill-condition equations, LSQR is more robust than CGLS.Real data application and basic procedure of tomo-statics estimation are also introduced. Ray-tracing was used for modeling and LSQR was used to solve the large sparse inversion matrix in inverting near-surface velocity model. The top of the high-velocity layer was picked from the obtained velocity model and static correction values were calculated and applied to real data. Finally, the theory of tomography is presented, and the formulas for calculating statics are given and compared with those for field statics calculation. It is theoretically expounded that the future statics estimation techniques will develop towards tomography.