应用网格走时的射线路径计算方法

子射线法（Raylet method）基于走时场的互易性，可用于多路径射线的计算。针对子射线法在拾取总走时（源点到给定点走时与接收点到给定点走时之和）场极值中存在的困难与稳定性问题，提出利用反插值求取总走时场中走时横向或纵向一阶导数为零的点来构成射线路径。此外，结合多级次计算策略，利用上述算法得到了全部的续至射线。在均匀介质和垂向梯度介质中，通过与解析结果的数值对比，发现计算网格尺度与导数算子长度对上述算法的精度有一定影响；在横向变速介质中，数值试验表明该算法相对于常规运动学射线追踪只有很小的计算偏差，其对横向变速具有很强的适应能力。低速体模型和Marmousi模型试算结果表明，本文算法对复杂模型具有令人满意的计算精度和计算效率。

A ray path calculation method based on grid traveltime

The raylet method based on reciprocity of traveltime field is potentially capable of dealing with multi-paths.However this method cannot readily obtain the extremum of the total traveltime field which is the sum of a field from the source and a field from the receiver as the ray path.So we calculate transverse or longitudinal first-order derivative of the total traveltime field,and pick up its zero points to constitute the ray path by means of the inverse interpolation.Furthermore,besides a first-arrival ray,other later-arrival rays can be also obtained with a multi-stage algorithm.We test influence characteristics of grid size and the length of derivative operator on our computation accuracy in a homogeneous media and a media with a constant gradient in depth compared with exact analytical results.Then we indicate applicability of our method in a desired laterally velocity-varied media in contrast to the conventional kinematic ray tracing.Finally our tests in both a low-velocity multi-path model and Marmousi model prove that our proposed method achieves satisfactory computational efficiency and accuracy.