Study Of Fresnel Zone Traveltime Tomographic Velocity Modeling Method

With the sustaining development of world economy, increasingly high demand of oil and gas was called for by society. The main target of seismic exploration has been transformed from tectonic reservoirs to lithologic reservoirs and unconventional reservoirs. Pre-stake depth migration is considered to be the most effective method to uncover these oil and gas sources. The successful implement of pre-stack depth migration is determined by the accuracy of seismic velocity analysis. Being one method with high efficiency and precision, tomographic velocity modeling has undergone rapid development within decades. This article study the Fresnel Zone travel time tomography, including the determination of First Fresnel Zone, the establishment of sensitivity kernel of Fresnel Zone tomography and the solution of tomographic equations.The determination of First Fresnel Zone is the fundamental problem of our methods. Firstly we start from the mathematic implication of First Fresnel Zone to decide its boundary. Next, we introduce three kinds of traveltime calculation method and discuss the influence of traditional traveltime tomography which brought by the introduction of Fresnel Zone.As is known to all, the sensitivity kernel establishes the linear relationship between velocity and time perturbation and it’s the crucial factor for successful application of tomography. Taking the advantage of frequency domain we derive the nine points difference scheme of scale wave equation and deduce the sensitivity kernel of Fresnel Zone tomography by using Born&Rytov approximation. This linear relationship describes the contribution of velocity perturbation to traveltime perturbation more correctly and makes the propagation of wave in a more realistic way.In this article, we also take the wavefront healing into consideration and discuss the influence of wave healing on the precision of tomography. As a kind of compromise between ray and wave equation tomography, Fresnel Zone tomography possesses the advantage of stability and high accuracy.We should take the inadequate of observation angle, the optimal match between the modeling traveltime and observation traveltime, the constraint of prior information into consideration. We use the least square QR factorization by confine of regularization and logging and geology prior information to solve the linear equation. We repeat the iterative flow until the precision reach our expectation. We apply the Fresnel Zone tomography to model inversion, the result prove the effectiveness and correctness of our method.Finally, we apply our method to calculate the static correction of a desert real data. The result of actual seismic data proves that Fresnel Zone tomography is a feasible and adaptive tomographic method.