水平层状VTI介质高效射线追踪计算方法

The calculation of traveltime and raypath in 1D layered VTI medium with enough accuracy and high efficiency is the foundation for seismic prestack time migration and microseismic velocity inversion. Two commonly used raytracing algorithms for 1D layered VTI medium are the shooting method and the shortest-path raytracing method. The shooting method is simple in principle and easy to implement, but a small change in the initial emergence angle can cause a large offset variation in the raypath at the longer offset. The principle of the shortest-path raytracing method is intuitive, but it requires calculating the traveltime along the ray group velocity direction (group angle) in anisotropic medium. It costs a lot of time to find the group velocity when the group angle is given. We combine the advantages of the above two methods and propose a high-efficient raytracing method for 1D layered VTI medium. A bunch of rays propagates from the source simultaneously and a dynamic grid model is set up based on the ray intersection with the velocity interfaces. The grid nodes around the receivers become denser adaptively to satisfy the traveltime accuracy requirement. The traveltime and raypath of the receivers are obtained by the cubic spline interpolation and the adaptive node method. Our proposed method calculates the group velocity and group angle by a given phase angle while no weak anisotropy assumption is employed. It can calculate the rays for the direct and reflected waves of qP, qSV and qSH in layered strong anisotropic VTI medium. Compared with the traditional shortest-path raytracing method, the numerical examples demonstrate that our method is more efficient and accurate, suitable for seismic prestack time migration and real-time microseismic monitoring.