Joint Tomography With Seismic First-break And Reflection Travel-time

Seismic wave velocity is important during every procedure of seismic prospecting, including collection, processing and explanation of seismic data. Therefore, how to precisely get propagation velocity of seismic wave underground is one of the essential problems in seismic prospecting all the time.Seismic travel-time tomography is one of inversion method which can inverse important information of underground medium such as structure, velocity distribution and elastic parameter by reconstructing algorithm with models using seismic travel-time information collected. Seismic travel-time tomography is nonlinear inversion problem. Comprehensive velocity information can't be obtained just through single wave inversion. So inversion with as more seismic information as possible has always been an important way sought by geophysicists. First-break has high stability and obvious advantages in velocity analysis of cross-hole or near surface and complex surface static move-out because it is easy to distinguish and trace while reflection wave can reflect deeper and finer structure and parameter distribution underground because of its strong energy with abundant underground information. Consequently, joint tomography with first-break and reflection travel-time is needed to provide more stable and precise velocity analysis for high-precision seismic prospecting. The main purpose of this paper is to construct mid-depth velocity model by the method of joint tomography with first-break and reflection travel-time, which can provide velocity information for data processing and explanation of high-precision seismic prospecting.Travel-time tomography is needed to resolve forward problerm and inversion problerm. In this paper, linear travel-time interpolation (LTI) ray tracing method is used in forward simulation, and LSQR method is adopted in reversion progress. We do reasearch about the calculation precision, calculation efficiency, function expansion in the forward simulation and mixed set feature of travel-time tomography in the inversion progress.1. In traditional LTI method, ray from different direction isn't completely considered during tracing forward, which leads to traced ray path dissatisfy minimum travel-time in medium with strong transverse velocity variation. Calculating travel-time of each grid node by all-round cycle method during tracing forward is adopted in this paper. Model test shows that the improved method is more suitable for complex model.2. LTI method is based on linear assumption. Its precision depends on grid division. But calculation time become longer while the grid become smaller. In order to ensure both the precision and efficiency, new idea of inserting secondary nodes in grid boundary is put forward in this paper. It is proved by model test that the new method has higher precision and is less time consuming in the case of same grid division.3. LTI method traces first-break based on minimum travel-time theory. Multi-stage LTI method is proposed in this paper combining the improved LTI method with the multi-stage calculation technology, which can trace any multiple or single transmission wave as well as converted wave and reflection wave in two dimensional complex lamellar medium. The stability and precision of this method is checked by model test and error analysis. It provides a new way for multiple wave simulation.4. In order to solve the mixed set problem of Jacobi matrix during reversion caused by non-uniform ray covering in travel-time tomography, multi-scale gradual inversion method is raised, which weight and regularize model parameter in Jacobi matrix and then get the solution by LSQR iteration. It proves to be valid through multiple group model test and result analysis.