基于微增模型的海洋可控源电磁法三维非结构化矢量有限元数值模拟

3D MCSEM modeling using an edge-based finite element method based on an unstructured grid and incremental model

为了分析海洋中含金属矿的储油气砂层的成分、结构对于海洋可控源电磁场的影响特征,引入多相微增等效介质模型,结合非结构化网格实现了海洋可控源电磁三维矢量有限元正演.首先对多相微增模型进行了介绍,并分析不同参数影响下的等效电导率变化特征.而后,详细推导了频率域可控源电磁法非结构化矢量有限元方程.接着,利用预处理的IDR(s)迭代算法求解线性方程组.最后设计典型的海洋地电模型,通过正演计算,验证了本文算法的正确性和有效性,同时研究了海洋储油砂层中的金属矿的含量、孔隙度、含水饱和度等参数对于可控源电磁场响应特征的影响.

The aim of this study is to reveal characteristics of the three-dimensional marine electromagnetic field produced by a controlled source in pyrite-rich sedimentary rocks.We present an edge-based finite element method for 3 D marine controlled-source electromagnetic modeling in the frequency domain based on an unstructured grid and 4-phase incremental model.The theoretical incremental model is introduced firstly. Then the characteristics of the effective medium resistivity are analyzed considering the influence of different parameters.To avoid the source singularity problem for MCSEM modeling,we adopt a secondary field formulation.The sparse finite element system of equation is solved using an IDR（s）with incomplete LU decomposition results as a preconditioner to speed up the convergence.The effects of key parameters,i.e.,pyrite content,porosity,pyrite conductivity,grain aspect ratio and water saturation on theelectrical conductivity of pyrite-bearing sandstones and their corresponding CSEM responses are comprehensively studied.The results are expected to help the CSEM data interpretation when a pyrite-bearing sandstone reservoir is encountered in the future.