基于局部加密非结构网格的海洋可控源电磁法三维有限元正演

3Dfinite element modeling of marine controlled-source electromagnetic fields using locally refined unstructured meshes

本文基于非结构网格实现了海洋可控源电磁法三维有限元正演模拟.该算法采用完全非结构网格剖分,可以模拟任意起伏地形和复杂地电模型.为了避免场源的奇异性,采用一次场/二次场分解算法,一次场由基于Schelkunoff势函数的一维解析公式得到.为了提高算法的精度和效率,采用对测点附近单元和异常体区域进行体积约束加密的方法,实现了非结构网格的局部加密.一、二维模型计算和分析表明,本文采用的局部加密方法能够明显地改善算法的精度,最大相对误差基本在1%以内.对三维模型计算及对比分析,说明了该算法对三维可控源电磁正演的实用性.复杂海底地形模型的正演模拟表明,海底地形对电磁场的影响很大,在进行海洋可控源电磁资料解释时,地形的影响有必要考虑在内.

We present a 3Dforward modeling algorithm of marine controlled-source electromagnetic（CSEM）fields using finite element method based on unstructured meshes.As the unstructured mesh is incorporated,the algorithm can easily accommodate complex structures,such as dipping interfaces and rough topography.Secondary coupled-potential formulation of Maxwell＇s equations allows to avoid the singularities introduced by the sources.The primary electromagnetic fields excited by electric dipole source in layered isotropic media are derived from the semi-analytical formulas based on Schelkunoff potentials.To improve the accuracy and efficiency of the algorithm,we propose an effective local mesh refinement strategy,where the elements near the receiver locations and in the abnormal areas are refined using the volume constraints.We validatethe finite element code against a 1Dand 2Dreservoir model and study the performance of the local mesh refinement strategy.The Canonical disc model illustrates the utility of the finite element method for 3DCSEM modeling.The bathymetry model study shows that the seafloor topography gives an important response which needs to be reproduced by numerical modeling to avoid the misinterpretation of measurements.