A marine deep-towed DC resistivity survey in a methane hydrate area, Japan Sea
Tada-nori Goto 1 8 Takafumi Kasaya 1 Hideaki Machiyama 1 Ryo Takagi 1 7 Ryo Matsumoto 2 Yoshihisa Okuda 3 Mikio Satoh 3 Toshiki Watanabe 4 Nobukazu Seama 5 Hitoshi Mikada 6 Yoshinori Sanada 1 Masataka Kinoshita 11 Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 2-15, Natsushima, Yokosuka, Kanagawa 237-0061, Japan.
2 Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan.
3 National Institute of Advanced Industrial Science and Technology (AIST), Geological Survey of Japan (GSJ), Institute for Geo-Resources and Environment (GREEN), AIST Tsukuba Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan.
4 Research Center for Seismology, Volcanology and Disaster Mitigation, Graduate School of Environmental Studies, Nagoya University, Furou-cho, Chikusa-ku, Nagoya 464-8602, Japan.
5 Research Center for Inland Seas, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan.
6 Department of Civil and Earth Resources Engineering, Graduate School of Engineering, Kyoto University Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan.
7 Present address: Japan Drilling Co. Ltd., Shin-Horidome Bldg. 6F 2-4-3 Nihonbashi Horidome-cho, Chuo-ku, Tokyo 103-0012, Japan.
8 Corresponding author. Email: tgoto@jamstec.go.jp
Exploration Geophysics 39(1) 52-59 https://doi.org/10.1071/EG08003
Submitted: 31 October 2007 Accepted: 14 December 2007 Published: 5 March 2008
Abstract
We have developed a new deep-towed marine DC resistivity survey system. It was designed to detect the top boundary of the methane hydrate zone, which is not imaged well by seismic reflection surveys. Our system, with a transmitter and a 160-m-long tail with eight source electrodes and a receiver dipole, is towed from a research vessel near the seafloor. Numerical calculations show that our marine DC resistivity survey system can effectively image the top surface of the methane hydrate layer. A survey was carried out off Joetsu, in the Japan Sea, where outcrops of methane hydrate are observed. We successfully obtained DC resistivity data along a profile ~3.5 km long, and detected relatively high apparent resistivity values. Particularly in areas with methane hydrate exposure, anomalously high apparent resistivity was observed, and we interpret these high apparent resistivities to be due to the methane hydrate zone below the seafloor. Marine DC resistivity surveys will be a new tool to image sub-seafloor structures within methane hydrate zones.
Key words: DC resistivity survey, methane hydrate, deep-towed system, high apparent resistivity, piston coring.
Acknowledgments
We are grateful for extensive support by the captain and ship’s crews on R/V Kaiyo, and by marine technicians and the Research Support Department in JAMSTEC. We thank Dr G. Snyder, H. Tomaru, and H. Lu for helping on the KY05–08 cruise. We also thank students at University of Tokyo, Kobe University, Chiba University, and Kyoto University. The forward code for DC resistivity calculations was originally developed by Dr K. Ushijima, Dr H. Mizunaga and Dr A. Kato of Kyushi University. Helpful comments about methane hydrate exploration were given by JOGMEC, especially Dr. T. Saeki. The manuscript was greatly improved by reviewers and editors. This project is partially supported by the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research, 16360449, 2006.
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