论文标题
3D异质各向异性媒体中的本征:第三部分 - 运动学,有限元实现
Eigenrays in 3D heterogeneous anisotropic media: Part III -- Kinematics, Finite-element implementation
论文作者
论文摘要
遵循第一部分中提出的理论,在该理论中,我们得出了欧拉 - 拉格朗日,非线性,二阶运动学射线追踪方程,用于平滑的异质通用各向异性介质,该部分用于其数值有限元元素。对于通过一组节点离散的两个固定端点之间的给定初始猜测轨迹,我们更新节点处的射线轨迹的位置和方向,以获得最近的固定射线路径。从第I部分中得出的Euler-Lagrange方程开始,我们将弱公式和Galerkin方法应用于将二阶的普通微分方程减少到非线性的,局部,一阶,加权的残留代数方程组中。该解决方案基于具有Hermite多项式插值的有限元方法,用于计算节点之间的射线特性。 Hermite插值是一种自然的选择,因为除了节点位置外,节点的射线方向也是有限元方法的独立自由度。这在各向异性媒体中尤其重要。然后,我们构建要最小化的目标函数,在其中区分由两种可能的固定射线触发的两个函数:最小行进时间和鞍点解决方案,主要是在固定路径包括垂体的情况下。目标功能包括两个与两个基本约束有关的惩罚条款。第一个与沿射线的节点的分布有关,而第二个则将射线方向归一化到单位长度。最小化过程涉及计算旅行时梯度向量和Hessian矩阵。后者还用于分析固定射线的类型。最后,我们在三个规范示例中证明了所提出的方法的效率和准确性。
Following the theory presented in Part I, where we derived the Euler-Lagrange, nonlinear, second-order kinematic ray tracing equation for smooth heterogeneous general anisotropic media, this part is devoted for its numerical finite-element solution. For a given initial-guess trajectory between two fixed endpoints, discretized with a set of nodes, we update the location and direction of the ray trajectories at the nodes, to obtain the nearest stationary ray path. Starting with the Euler-Lagrange equation derived in Part I, we apply the weak formulation and the Galerkin method to reduce this second-order, ordinary differential equation into a nonlinear, local, first-order, weighted residual algebraic equation set. The solution is based on a finite element approach with the Hermite polynomial interpolation, for computing the ray characteristics between the nodes. The Hermite interpolation is a natural choice, since, in addition to the nodal locations, also the ray directions at the nodes are independent degrees of freedom of the finite element method. This is in particular important in anisotropic media. We then construct the target function to be minimized, where we distinguish between two functions triggered by the two possible types of stationary rays: minimum traveltime and saddle-point solutions, mainly in cases where the stationary paths include caustics. The target functions include two penalty terms related to two essential constraints. The first is related to the distribution of the nodes along the ray and the second enforces the normalization of the ray direction to a unit length. The minimization process involves the computation of the traveltime gradient vector and the Hessian matrix. The latter is used also to analyze the type of the stationary ray. Finally, we demonstrate the efficiency and accuracy of the proposed method along three canonical examples.