Finite Element Simulation of 3-D Marine Controlled Source Electromagnetic Fields in Anisotropic Media with Unstructured
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Pure and Applied Geophysics
Finite Element Simulation of 3-D Marine Controlled Source Electromagnetic Fields in Anisotropic Media with Unstructured Tetrahedral Grids RONGHUA PENG,1,2 XIANGYUN HU,1,2 Abstract—Marine controlled-source electromagnetic (CSEM) method has proved to be a powerful tool and found its increasing applications for offshore resource exploration and tectonic studies. However, bathymetric variations and anisotropic structures encountered in practical measurements have posed challenges for reliable interpretation of marine CSEM data collected in increasingly complex geologic settings. In this study, we present a Finite element (FE) based forward algorithm for simulating 3-D marine CSEM responses in geologic settings characterized with rough bathymetry and electrical anisotropy. Unstructured tetrahedral meshes are employed to permit precise descriptions of arbitrary seafloor topography. After validating the accuracy of the algorithm, we investigate the effects of bathymetric variations and complicated electric anisotropy separately and jointly on marine CSEM responses. Numerical results have demonstrated that both the bathymetry and electrical anisotropy can cause significant distortions on marine CSEM responses, but to different extent. The effects of full electrical anisotropy and bathymetry have to be taken into consideration for reliable interpretations of marine CSEM data in practice. Keywords: Controlled source electromagnetics, electrical anisotropy, bathymetric effect, finite element method, unstructured tetrahedral grids.
1. Introduction The marine controlled source electromagnetic (CSEM) method has found extensive applications for offshore hydrocarbon explorations over the last two decades due to its potential in distinguishing between hydrocarbon- and saline-filled reservoirs by providing electric conductivity structures on the subsurface (Ellingsrud et al. 2002; Constable and Srnka 2007;
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Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China. E-mail: [email protected] 2 Hubei Subsurface Multi-scale Imaging Key Laboratory, Wuhan 430074, China.
JIANHUI LI,1 and YAJUN LIU1 Darnet et al. 2007) MacGregor and Tomlinson 2014). The principle of conventional marine CSEM method involves the transmission of low-frequency EM signals generated by a high-powered source (e.g., horizontal electric dipole, HED) towed close to the seafloor and an array of electromagnetic receivers located on the seafloor to record the time varying induced signals (Edwards 2005). As the marine CSEM technique becomes increasingly sophisticated and the application of marine CSEM surveys in industry gradually matures, there has been considerable interest in prospecting for difficult hydrocarbon targets in more complex tectonic settings. However, reliable interpretation of CSEM data in these cases meets challenges posed by bathymetric variations, electrical anisotropy and other issues. Marine CSEM surveys are often conducted over complex geological areas with large bathymetric variatio
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