【摘要】 地层宏观电性各向异性会对可控源电磁响应产生重要影响.由于海底地层电性结构常表现为电导率各向异性,若仅对海洋可控源电磁(MCSEM)数据进行常规各向同性反演,有可能无法获得准确的反演解释结果,从而削弱MCSEM技术的可靠性.本文实现了电导率垂直各向异性(VTI)条件下频率域海洋可控源电磁数据三维反演算法.其中,三维正演采用基于二次场控制方程的交错网格有限体积法,并利用直接矩阵分解技术来求解离散所得的大型线性方程组,有利于快速计算多场源的响应.反演采用具有近似二次收敛性的高斯牛顿算法对目标函数进行最优化.最后,对具有VTI电性各向异性特征的盐丘构造模型的MCSEM合成数据分别进行了电导率各向同性和垂直各向异性三维反演,结果表明:各向同性三维反演算法无法对受VTI介质影响的MCSEM数据进行正确的反演解释,而垂直各向异性三维反演能够获得更为可靠的地下电阻率结构和异常体分布,展现出对海底电性各向异性结构更为优良的反演解释能力.更多还原

【Abstract】 Electrical anisotropy of geologic formations has a significant impact on controlled source electromagnetic(CSEM)field responses.It is now well recognized that sedimentary formations in the marine environment are usually characterized by electrical anisotropy due to strong sedimentation.In the presence of such electrical anisotropy,the interpretation of marine CSEM data based on the assumption of isotropic media may produce misleading resistivity images,which will undermine the power of MCSEM technique.To solve this problem,we develop an efficient three-dimensional(3 D)anisotropic inversion algorithm for marine CSEM data affected by structures with vertical transverse isotropy(VTI).The forward engine is based on the scattering-field formulation to mitigate the numerical singularity in the vicinity of the transmitters,and employs a direct matrix factorization method to solve the system of linear equations arising from finite volume discretization.The direct solver facilitates the reuse of the matrix factorization,which makes the solutions for multiple transmitters with little additionaleffort.The inverse algorithm is based on the Gauss-Newton method with a quasi-quadratic convergence rate,which reduces the number of expensive matrix factorization required.Finally,numerical experiments on synthetic marine CSEM data show that 3 Danisotropic inversion can produce more reliable resistivity images,demonstrating better interpretation capacity than conventional isotropic inversion of marine CSEM data in the presence of electrical anisotropy.更多还原