Abstract
The controlled source extremely low frequency (CSELF) electromagnetic method is characterized by extremely long and powerful sources and a huge measurement range. Its electromagnetic field can therefore be affected by the ionosphere and displacement current. Research on 3D forward modeling and inversion of CSELF electromagnetic data is currently in its infancy. This paper makes exploratory attempts to firstly calculate the 1D extremely low frequency electromagnetic field under ionosphere-air-earth coupling circumstances, and secondly analyze the propagation characteristics of the background electromagnetic field. The 3D staggered-grid finite difference scheme for solving for the secondary electric field is adopted and incorporated with the 1D modeling algorithm to complete 3D forward modeling. Considering that surveys can be carried out in the near field and transition zone for lower frequencies, the 3D Limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) inversion of CSELF electromagnetic data is presented (in which the sources, or primary fields, are included), with the aim of directly inverting the impedance data, regardless of where it is acquired. Derivation of the objective functional gradient is the core component in the inversion. Synthetic tests indicate that the well-chosen approximation to the Hessian can significantly speed up the inversion. The model responses corresponding to the coexistence of conductive and resistive blocks show that the off-diagonal components of tensor impedance are much more sensitive to the resistivity variation than the diagonal components. In comparison with conventional scalar inversion, tensor inversion is superior in the recoveries of electric anomalies and background resistivity.
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Acknowledgments
We express our gratitude to Prof. Dai Shikun, Prof. Yan Liangjun, and Editor-in-Chief Fan Weicui for their constructive comments. We would also like to thank the Associate Editor for the hard work involved in preparing this manuscript.
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This research is jointly supported by the National Natural Science Foundation of China (No. 41374078), and geological survey project of Ministry of Land and Resources of China (No. 12120113101300).
Cao Meng is a doctoral candidate at the School of Geophysics and Information Technology, China University of Geosciences (Beijing). He received his bachelor’s degree in geophysics from the China University of Geosciences (Beijing) in 2011. His main research interests are 3D Electromagnetic Forward Modeling, Inversion Theory, and Programming.
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Cao, M., Tan, HD. & Wang, KP. 3D LBFGS inversion of controlled source extremely low frequency electromagnetic data. Appl. Geophys. 13, 689–700 (2016). https://doi.org/10.1007/s11770-016-0585-6
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DOI: https://doi.org/10.1007/s11770-016-0585-6