Wavepath Traveltime Tomography

The accurately velocity model building is the key for seismic exploration. Because the direct measurement of the velocity field can only be implemented in limited local region, geophysicists try hard to utilize the velocity information included in the seismic wave for velocity requesting. Traveltime tomography is the technique that using seismic wave traveltimes and its corresponding propagation paths for velocity requesting. However the seismic ray which is used in ray traveltime tomography can not depict the seismic wave propagation path accurately, because it is calculated on the assumption that the seismic wave is infinite-frequency. Although based on wave theory, the diffraction tomography may fail when the velocity model is complex, and the seismic waveform information employed is susceptible to disturbing with multi-factors except of velocity, so the diffraction tomography is difficult to be extensively used in seismic exploration.This thesis systematically investigates the wavepath traveltime tomography which is a traveltime tomography method based on wave theory. The wavepath is band-limited seismic wave propagating path which is calculated based on wave-equation theory, It can illuminate the velocity model more evenly, and its calculation depends upon the smoothness of the reference velocity model not as strongly as ray method. So the wavepath tomography method is more adaptable for the imaging of complex structure. The results of numerical examples indicate that the iterative process of wavepath traveltime tomography is steadily convergent even without smoothing or constraint approaches, and it can offer a more reasonably velocity model compared with ray traveltime tomography; The turning-wavepath tomography method that employs the first arrivals for near-surface velocity inversion is presented, and the corresponding near-surface velocity model building method which is based on prestack depth migration for the application of migration imaging is developed. Both numerical experiment and field data processing demonstrate that the software package for shallow structural velocity model building works well and produces encouraging results.