Abstract
A dual-ship-towed marine electromagnetic (EM) system is a new marine exploration technology recently being developed in China. Compared with traditional marine EM systems, the new system tows the transmitters and receivers using two ships, rendering it unnecessary to position EM receivers at the seafloor in advance. This makes the system more flexible, allowing for different configurations (e.g., in-line, broadside, and azimuthal and concentric scanning) that can produce more detailed underwater structural information. We develop a three-dimensional goal-oriented adaptive forward modeling method for the new marine EM system and analyze the responses for four survey configurations. Oceanbottom topography has a strong effect on the marine EM responses; thus, we develop a forward modeling algorithm based on the finite-element method and unstructured grids. To satisfy the requirements for modeling the moving transmitters of a dual-ship-towed EM system, we use a single mesh for each of the transmitter locations. This mitigates the mesh complexity by refining the grids near the transmitters and minimizes the computational cost. To generate a rational mesh while maintaining the accuracy for single transmitter, we develop a goal-oriented adaptive method with separate mesh refinements for areas around the transmitting source and those far away. To test the modeling algorithm and accuracy, we compare the EM responses calculated by the proposed algorithm and semi-analytical results and from published sources. Furthermore, by analyzing the EM responses for four survey configurations, we are confirm that compared with traditional marine EM systems with only in-line array, a dual-ship-towed marine system can collect more data.
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Acknowledgments
We are grateful to Gao Zonghui and Zhang Wenqiang from the EM research group of Prof. Yin Changchun for their help.
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This research is financially supported by the Key National Research Project of China (Nos. 2016YFC0303100 and 2017YFC0601900), the Key Program of National Natural Science Foundation of China (Nos. 41530320 and 41774125).
Zhang Bo received a BS (2012) and a Ph.D (2017) in exploration technology and engineering from the Jilin University. He is currently a postdoctoral fellow at Jilin University engaged in 3D airborne and marine EM modeling and inversion.
Ren Xiu-Yan graduated from Jilin University with a B.S. in Exploration Technology and Engineering (2013). She is currently a graduate student at the Jilin University engaged in 3D airborne and marine EM modeling and inversion.
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Zhang, B., Yin, CC., Liu, YH. et al. 3D forward modeling and response analysis for marine CSEMs towed by two ships. Appl. Geophys. 15, 11–25 (2018). https://doi.org/10.1007/s11770-018-0659-8
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DOI: https://doi.org/10.1007/s11770-018-0659-8