复杂各向异性介质初至波射线追踪

First arrival ray tracing in complex anisotropic medium

准确求取各向异性初至波走时与射线路径对偏移成像及层析反演具有重要意义,目前基于图论的最短路径方法虽然能同时得到走时与射线路径且无条件稳定,但仅适用于弱各向异性介质。利用牛顿下山法求解群相关系方程,由群角得到较为精确的相角,进而求得较为精确的群速度和走时,使得最短路径射线追踪方法可以适用于复杂各向异性介质。将此射线追踪方法应用于均匀各向异性介质及层状介质模型,并将试算结果分别与理论走时和弹性波动方程有限差分结果进行比较,验证了方法的准确性;通过复杂各向异性介质模型试算,验证了方法对复杂介质的适用性。

The accurate calculation of anisotropic first arrival traveltime and raypath is critical for migration imaging and to- mographic inversion. The shortest-path method based on graph theory can get the traveltime as well as the raypath with high stability. Thus, we proposed a new shortest-path ray tracing algorithm for anisotropic mediurrL To compute the traveltime in a grid, Newton downhill method is used to solve the equation relating group angle with phase angle. By this way, a more accu- rate phase angle can be achieved from a group angle, and then a more accurate group velocity and traveltime can be obtained. Correspondingly,the shortest-path method can be applied to strong anisotropic mediurr~ Finally, this ray tracing algorithm was applied to homogeneous anisotropic medium, a layered model and a complex model, and its effectiveness was proved through comparing the computed traveltimes with those using the finite difference implementation of elastic wave equation.