起伏海底地形时间域海洋电磁三维自适应正演模拟

本文基于自适应非结构有限元算法实现海洋电磁起伏海底地形三维正演模拟.通过采用隐式后推欧拉时间离散技术,保证在较大的时间步长条件下获得正确结果.为获得多时间道海洋电磁正演模拟的有效网格,我们采用基于法向电流连续的后验误差估计的自适应方法和网格融合技术;同时为了控制网格数量和保证正演模拟稳定性,我们还在网格融合过程中应用了随机网格挑选技术.对于方程组求解我们使用MUMPUS直接求解器.当时间步长不变时,只需对系数矩阵进行一次分解,大大提高计算速度.将本文计算结果与半空间模型解析解进行对比,验证了本文算法精度.针对海洋电磁半拖曳式和双船拖曳式工作方式,我们通过典型模型的模拟计算,研究海底地形影响及海底高阻层识别特征.

3D adaptive forward modeling for time-domain marine CSEM over topographic seafloor

We realize 3D modeling for time-domain marine controlled-source electromagnetic (CSEM) method using adaptive unstructured finite-element method. For the time discretization, we use the implicit backward Euler technique to ensure a stable and accurate modeling even for large time intervals. To obtain an effective mesh for accurate multichannel MCSEM modeling, we use adaptive method based on the continuity of normal current for the posterior errors estimation and mesh integration technique. To control the number of grids while maintaining stability for the forward modeling, we run a random choice in the process of mesh integration. We use MUMPUS to solve the system of linear equations that needs only to decompose the matrix once and thus enhances the modeling speed. Checking against the semi-analytical solution for a half-space under the ocean verify the accuracy of our algorithm. Via numerical experiments for both semi-towed and dual-ship towed MCSEM systems, we can easily investigate the EM characteristic of topographic ocean floor and the resistive target underneath.