A small ocean bottom electromagnetometer and ocean bottom electrometer system with an arm-folding mechanism (Technical Report)
Takafumi Kasaya 1 3 Tada-nori Goto 1 21 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan.
2 Now at: Department of Civil and Earth Resources Engineering, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8540, Japan.
3 Corresponding author. Email: tkasa@jamstec.go.jp
Exploration Geophysics 40(1) 41-48 https://doi.org/10.1071/EG08118
Submitted: 31 October 2008 Accepted: 21 January 2009 Published: 27 February 2009
Abstract
Natural magnetic fields are attenuated by electrically conductive water. For that reason, marine magnetotelluric surveys have collected data at long periods (1000–100 000 s). The mantle structure has been the main target of seafloor magnetotelluric measurements. To ascertain crustal structure, however, electromagnetic data at shorter periods are important, e.g. in investigations of megathrust earthquake zones, or in natural resource surveys. To investigate of the former, for example, electromagnetic data for periods of less than 1000 s are necessary. Because no suitable ocean bottom electromagnetometer (OBEM) has been available, we have developed a small OBEM and ocean bottom electrometer (OBE) system with a high sample rate, which has an arm-folding mechanism to facilitate assembly and recovering operations. For magnetic observation, we used a fluxgate sensor.
Field observations were undertaken to evaluate the field performance of our instruments. All instruments were recovered and their electromagnetic data were obtained. Results of the first experiment show that our system functioned well throughout operations and observations. Results of other field experiments off Tottori support the claim that the electromagnetic data obtained using the new OBEM and OBE system are of sufficient quality for the survey target. These results suggest that this device removes all instrumental obstacles to measurement of electromagnetic fields on the seafloor.
Key words: arm-folding mechanism, fluxgate magnetometer, ocean bottom electromagnetometer.
Acknowledgements
We appreciate the extensive support offered by the respective captains, ships’ crews, and marine technicians of the R/V Natushima, Tansei-maru, Wakatori-maru, and Seifu-maru. We are grateful to Professor Oshiman, Dr Toh, Dr Shiozaki, Dr Yoshimura, Dr Fujii, Dr Yamasaki, Professor Shimoizumi, Mr Nakao and Professor Shingai for EM data acquisition off Tottori prefecture. Technical ideas for our instruments offered by Dr Baba, Dr Ichiki, Dr Utada and Dr Seama are gratefully acknowledged. Dr Tada provided valuable comments on the draft manuscript. OBEMs and OBEs are carefully manufactured and modified by Tierra Technica and CloverTech, respectively. I thank Mr Okabe for many useful ideas for an arm-folding mechanism. I thank Dr A. D. Chave for the computer code used for time series analysis. We also thank Dr Suyehiro, Dr Kinoshita and Professor Mikada for their helpful comments and support for OBEM and OBE development. The manuscript was greatly improved by the reviewer and editor. Development of our system was supported from the JAMSTEC budget. The EM data acquisition was partially supported by a Ministry of Education, Culture, Sports, Science and Technology Grant-in-Aid for Scientific Research, 19340127, 2007. Several figures were prepared using the GMT software, produced by Wessel and Smith (1998).
Baba, K., Chave, A. D., Evans, R. L., Hirth, G., and Mackie, R. L., 2006, Mantle Dynamics Beneath the East Pacific Rise at 17°S: Insights from the MELT EM Data: Journal of Geophysical Research 111, B02101.
| Crossref | GoogleScholarGoogle Scholar |
Kasaya, T., Goto, T., Mikada, H., Baba, K., Suyehiro, K., and Utada, H., 2005, Resistivity image of the Philippine Sea Plate around the 1944 Tonankai earthquake zone deduced by Marine and Land MT surveys: Earth, Planets, and Space 57, 209–213.
| Crossref | GoogleScholarGoogle Scholar | as ~0.1, 0.5, 10, and 20 nT at periods of 1, 10, 100, and 1000 s, respectively. If we use these values as B 0, we can estimate Bsf and Vsf at various periods and water depths (Table A1).
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References
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