ABSTRACT We propose an optimized method to compute travel times for seismic inversion problems. It is a hybrid method combining several approaches to deal with travel time computation accuracy in unstructured meshes based on tetrahedral elementary cells. As in the linear travel time interpolation method, the proposed approach computes travel times using seismic ray paths. The method operates in two sequential steps: At a first stage, travel times are computed for all nodes of the mesh using a modified version of the shortest path method. The difference with the standard version is that additional secondary nodes (called tertiary nodes) are added temporarily around seismic sources in order to improve accuracy with a reasonable increase in computational cost. During the second step, the steepest travel time gradient method is used to trace back ray paths for each source–receiver pair. Travel times at each receiver are then recomputed using slowness values at the intersection points between the ray path and the traversed cells. A number of numerical tests with an array of different velocity models, mesh resolutions and mesh topologies have been carried out. These tests showed that an average relative error in the order of 0.1% can be achieved at a computational cost that is suitable for travel time inversion.

中文翻译：

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在 3D 四面体网格中计算地震走时的混合方法

摘要 我们提出了一种计算地震反演问题走时的优化方法。它是一种混合方法，结合了多种方法来处理基于四面体基本单元的非结构化网格中的走时计算精度。与线性旅行时间插值方法一样，所提出的方法使用地震射线路径计算旅行时间。该方法分两个连续步骤运行：在第一阶段，使用最短路径方法的修改版本计算网格所有节点的旅行时间。与标准版本的不同之处在于，在震源周围临时添加了额外的二级节点（称为三级节点），以在合理增加计算成本的情况下提高准确性。在第二步期间，最速走时梯度法用于追踪每个源-接收器对的射线路径。然后使用射线路径和遍历单元之间的交点处的慢度值重新计算每个接收器的行进时间。已经进行了一系列不同速度模型、网格分辨率和网格拓扑的数值测试。这些测试表明，可以以适合旅行时间反演的计算成本实现大约 0.1% 的平均相对误差。