基于非结构有限元的时间域海洋电磁三维反演

海洋电磁法是一种有效的钻前储层评价手段，可识别出海底构造是否储油气，从而减少干井率降低勘探成本.近年来，频率域三维海洋电磁反演得到了快速发展，但受到空气波的影响，其在浅海环境中无法取得很好的效果.为解决这一问题，本文研究时间域海洋电磁数据三维正反演.正演模拟和计算中，我们选择基于非结构网格的矢量有限元方法.该算法中空间离散采用非结构四面体网格，可很好地拟合复杂海底地形条件和地下复杂结构；而对于时间离散，我们采用无条件稳定的后推欧拉方法，以确保任意时间步长数值计算的稳定性.反演计算中，灵敏度信息采用伴随正演隐式进行计算；同时，依据时间域反演方法的特点采用L-BFGS方法计算模型修正量.通过利用合成数据反演结果证明了本文提出的三维时间域反演方法可用于复杂海底环境，特别是在浅海环境下的有效性.

3D inversions of time-domain marine EM data based on unstructured finite-element method

Marine electromagnetic (EM) method is an effective tool for pre-drilling reservoir evaluation that can distinguish the existence of oil at ocean bottom and thus reduces the exploration cost and dry-wells. In recent years, 3D marine EM inversion in the frequency-domain is developing rapidly, but due to the influence of airwave no good results can be obtained in shallow water. To solve this problem, we carry out the 3D marine EM modeling and inversions in time-domain. For the forward modeling, we choose the unstructured vector finite-element method. This method discretizes the space domain using tetrahedral grids and thus can well model the complex seafloor and underground structures. For the time discretization, we adopt the unconditionally stable backward Euler scheme to guarantee the stability of modeling for all time channels. In the inversion process, we use the adjoint forward modeling to calculate the sensitivity, while we use the L-BFGS method to obtain the parameter updates. We use the synthetic data to validate our 3D inversion algorithm for the complex geology under the ocean, especially in the shallow water environment.