带复杂海底地形的海洋可控源电磁法三维正反演研究

The marine controlled-source electromagnetic (MCSEM) method is a new developed marine geophysical exploration tool for detecting seafloor hydrocarbon reservoirs and mineral resources. Nowadays, MCSEM tends to be practical in the international, however, in China, the instruments as well as the forward modeling and inversion techniques for MCSEM method are developed later, thus it is still in the theoretical and experimental period and many research efforts are needed before put into practices. In this project, the unstructured meshes that allows for accurate description of arbitrary submarine topography and complex structures is incorporated, a posteriori error estimation based on gradient recovery operator is employed to guide the meshes refinement automatically, to realize the three-dimensional adaptive unstructured finite element program for MCSEM modeling incorporating complex seafloor topography. After that, a mesh-partitioning technique is applied to partition the origin problem domain, which normally consists of a huge number of elements into smaller subdomains. Thanks to the partitioning, many computations can be done simultaneously using the Message Passing Interface (MPI) technique, that is in parallel, which may reduce the total execution time of the program substantially. Finally, we use the damped Gauss-Newton method to solve the MCSEM three-dimensional inverse problem, to realize the three-dimensional inversion algorithm of MCSEM data incorporating complex submarine topography. The result of this project can provide a powerful tool of interpretation for three-dimensional MCSEM data incorporating complex submarine topography, also it can promote the capability of MCSEM exploration in complex submarine cases.

海洋可控源电磁法（MCSEM）是探测海底油气资源和矿产资源的一种新兴海洋地球物理勘查技术。目前海洋电磁法在国际上已趋于实用化，但国内由于仪器设备、正反演技术研究起步较晚，正处于理论研究、方法实验向实用化迈进的重要阶段。本项目拟采用能精确逼近任意起伏海底地形和复杂构造的非结构网格对模型剖分，并采用梯度恢复型后验误差估计指导网格自动细化，实现带复杂海底地形的MCSEM三维自适应非结构有限元正演模拟；然后采用区域分解算法将模型区域分解为若干子区域，并利用MPI技术实现MCSEM三维有限元正演的并行化，进一步缩短正演的计算时间；最后采用计算效率较高的阻尼高斯-牛顿法求解MCSEM三维反问题，实现带复杂海底地形的MCSEM三维反演算法。本项目不仅能为复杂海底地形下MCSEM三维资料解释提供一套切实可行的工具，也有助于提升复杂海底地形下MCSEM勘探的能力。

目前海洋电磁法在国际上已趋于实用化，但国内由于仪器设备、正反演技术研究起步较晚，正处于理论研究、方法实验向实用化迈进的重要阶段。鉴于传统基于结构网格的正反演中网格的设置和细化主要靠人工经验和手动调节，不利于对起伏海底地形和倾斜界面等复杂构造的数值模拟。本项目研究了基于A-phi势的海洋可控源电磁（MCSEM）三维非结构自适应有限元正演模拟算法，采用能精确逼近任意起伏海底地形和复杂构造的非结构网格进行剖分，采用梯度恢复型后验误差估计指导网格自动细化，并对源点和接收点附近网格进行局部体积约束加密，以克服源点的奇异性和提高接收点处插值的精度；然后采用线性方程组直接求解器Pardiso进行求解，并采用MPI接口函数，实现了多场源多频率的MCSEM三维自适应有限元正演并行算法。模型试算结果表明，该正演算法具有较高的计算精度和计算效率，计算精度在1%以内。对于反演，我们采用伴随互易定理计算灵敏度矩阵，然后采用改进的Occam正则化反演方法，实现了基于非结构网格的MCSEM三维自适应反演。在寻求最小目标函数的模型中，兼顾了固定拉格朗日乘子和动态拉格朗日乘子搜索模型的方法，提高了计算效率；在网格设计中，反演网格采用粗网格，而正演网格由反演网格局部加密和自适应细化得到，在减少反演参数个数的同时，又保证了正演计算的精度。反演试算结果表明反演模型能稳定收敛到真实模型附近，目标拟合差减小到1左右。本项目研究成果为复杂海底地形下MCSEM三维高精度模拟和反演解释提供了一套切实可行的工具，具有较强的应用价值和潜在的应用前景。