海洋可控源电磁与大地电磁三维数据空间法联合反演

Marine electromagnetic (EM) methods have been widely used for marine resource exploration and tectonic investigations. The marine controlled source electromagnetic (CSEM) method and the marine magnetotellurics (MT) differ in their resolution capabilities. The former is generally more sensitive to shallow and local electrical structures, whereas the latter is better suited to recover large scale regional electrical structures. The combination of the two methods tends to have better abilities than individual ones, and joint inversion is the key of the combined application. In the proposed project, we focus on developing a program for three dimensional (3D) joint inversion of marine CSEM and marine MT data. The direct solver, which is generally not only much more stable but also more accurate than the iterative solver, will be employed to solve the resulting large linear system, which could greatly benefit the multi-transmitter CSEM forward modeling. An appropriate weighting scheme will be used to balance the two datasets to make sure that joint inversion provides significantly better resolution of subsurface structures than the individual inversions. For minimization of the objective function, the data-space version of the Gauss-Newton (GN) method is preferred due to its lower computational cost compared with the traditional model-space GN method. In addition, a multilevel parallelization will be implemented to speed up the inversion by efficiently making use of computation resources. In summary, the goal of this study is the potentially developed joint inversion program can make full use of strengths of both methods, so that complex and multi-scale subsurface structures can be better imaged.

海洋电磁法广泛应用于海底资源勘探和洋壳构造研究。海洋可控源电磁法（CSEM）和大地电磁法（MT）具有不同的勘探能力，前者对小尺度的局部电性结构分辨能力强，后者则更适合于恢复较大尺度的区域电性结构，但对高阻异常不够敏感。若将这两种方法结合起来应用，有可能取得比单一方法更好的效果，其关键在于数据的联合反演。本课题拟研究海洋频率域CSEM和MT的三维联合反演算法。正演大型线性方程组的求解采用矩阵直接分解法，将比迭代解法更稳定、精度更高，并且有利于快速计算多场源的响应；联合反演需对两种数据的拟合项进行适当加权才可能使得其反演效果优于单独反演；对目标函数的最优化使用数据空间的高斯-牛顿法，其计算量将比传统的模型空间方法更小；使用消息传递接口对三维反演实行多层次的并行化，以高效利用计算资源来加速计算。最终实现实用化的三维联合反演算法，能充分发挥两种方法的优势，更准确地探测海底复杂的多尺度电性结构。

本项目主要研究了频率域可控源电磁法（CSEM）和大地电磁法（MT）的三维各向同性和任意各向异性正演算法、三维各向同性联合反演算法。正演中充分利用各向同性和各向异性电导率离散化、CSEM和MT方法的共同点，将算法完全表达为矩阵/向量的形式，形成了广泛适用的频率域电磁三维拟态有限体积正演算法。通过联合雅可比矩阵计算、基于数据点个数的加权实现了CSEM与MT的并行化三维高斯牛顿-共轭梯度联合反演算法。模型的测试结果表明：所实现的三维正反演代码具有很高的并行化计算效率；各向同性和各向异性正演算法均具有很好的计算精度；联合反演相比独立反演能够更好地恢复出多尺度地电模型的特征。项目的研究成果可为海洋和陆地矿产资源勘探、深部地球结构和动力学研究提供技术支撑。