3-D Adaptive Finite-Element Modeling of Marine Controlled-Source Electromagnetics with Seafloor Topography Based on Secondary Potentials
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We present a solution of using adaptive nodal finite-element (FE) method to solve the marine controlled-source electromagnetic (CSEM) problem for 3-D earth models in the frequency domain. The forward problem is solved based on the secondary Coulomb-gauged electromagnetic (EM) potentials. We implement an adaptive mesh refinement algorithm according to an a posteriori error estimator based on a gradient-recovery operator of the secondary EM potentials. To increase the quality of the mesh at the receiver locations, the elements containing the receiver locations are constrained by assigning them a maximum volume for the initial mesh. An unstructured tetrahedral mesh used in our approach can provide an accurate description of complex structures such as dipping layers and rough topography that are not accurately fitted using structured meshes. We first validate the adaptive FE approach and demonstrate the convergence of the adaptive grid refinement procedure using a 1-D layered model. The canonical disc model example illustrates the capability of the adaptive FE approach for 3-D CSEM modeling. The bathymetry model shows that the algorithm is well suited to deal with complex seafloor topography, which needs to be simulated exactly to avoid the misinterpretation of marine CSEM data sets.
KeywordsMarine electromagnetic adaptive finite element unstructured mesh seafloor topography electromagnetic potentials
The authors give special thanks to Hang Si for the open source code TetGen which is used in this paper. The editor Prof. Farquharson is acknowledged for his valuable suggestions. The anonymous reviewers are thanked for detailed comments, which greatly improve the clarity of the paper. This study was jointly supported by the National Natural Science Foundation of China (41774078, 41774080, 41604086 and 41704075), the Natural Science Foundation of Jiangxi Province (20161BAB211027), the Natural Science Foundation of Shandong Province (ZR20161DQ15) and the Open Foundation of Fundamental Science on Radioactive Geology and Exploration Technology Laboratory (RGET1605). Professor David Nobes of East China University of Technology has proofread and checked the manuscript.
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