지구물리와물리탐사 (Geophysics and Geophysical Exploration)

  • 제12권1호
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  • Pages.41-48
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  • 2009
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  • 1229-1064(pISSN)
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  • 2384-051X(eISSN)
한국지구물리·물리탐사학회 (Korean Society of Earth and Exploration Geophysicists)
권호 표지

팔-접힘 구조를 가지는 소규모 OBEM과 OBE시스템 (기술보고서)

A small ocean bottom electromagnetometer and ocean bottom electrometer system with an arm-folding mechanism (Technical Report)

  • Kasaya, Takafumi (Japan Agency for Marine-Earth Science and Technology (JAMSTEC)) ;
  • Goto, Tada-nori (Japan Agency for Marine-Earth Science and Technology (JAMSTEC))
  • 발행 : 2009.02.28
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초록

자연적인 자기장은 전기 전도도가 높은 물에서는 감쇠한다. 이러한 이유로, 해양 지전류 탐사법에서 장주기(1000 - 100000 초) 자료를 측정하였나. 맨들 구조가 해저 지전류측정의 주된 적용 대상이었다. 반면에 지각구조를 밝히기 위해서, 예를 들어, 메카트러스트 지진지역이나 천연자원조사와 같은 분야에서는 보다 짧은 주기의 전자기장에 대한 자료가 중요하다. 예를 들어 전자의 조사를 위해서는 1000초 보다 자은 주기를 가지는 전자기장 자료가 필요하다. 그러나, 이러한 용도를 위한 적절한 해저 전자력계 (OBEM)가 없기 때문에 높은 샘플링 속도와 조립과 회수 과정을 용이하게 하기 위한 팔-접힘 구조를 가지는 소규모 OBEM과 해저 전위계 (OBE)를 개발하였다. 자기장 측정을 위하여 플럭스게이트 센서를 이용하였다. 개발된 장비의 현장 성능을 평가하기 위한 현장측정이 이루어졌다. 모든 장비는 전자기 측정자료를 획득한 채로 회수되었다. 첫번째 실험의 결과, 개발된 시스템은 동작과 측정 모두 그 기능이 정상적으로 잘 이루어지고 있음을 확인하였다. 돗토리 근해에서의 또 다른 현장실험 결과는 새로운 OBEM과 OBE 시스템으로 획득된 전자기 자료가 탐사 목적에 충분한 양질의 자료라는 주장을 뒷받침해 주었다. 이러한 견과들은 개발된 장비들이 해저면 전자기장 측정에 대한 장비적인 장애를 모두 해결하고 있음을 확증하고 있다.

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.

키워드

참고문헌

  1. Baba, K., Chave, A. D., Evans, R. L., Hirth, G., and Mackie, R. L., 2006,Mantle Dynamics Beneath the East Pacific Rise at $17^{\circ}$S: Insights from the MELT EM Data: Journal of Geophysical Research, 111, B02101. doi: 10.1029/2004JB003598
  2. Chave, A. D., Thomson, D. J., and Ander, M. E., 1987, On the Robust Estimation of Power Spectra, Coherences, and Transfer Functions: Journal of Geophysical Research, 92, 633–648. doi: 10.1029/JB092iB01p00633
  3. Constable, S. C., Orange, A. S., Hoversten, G. M., and Morrison, H. F., 1998,Marine Magnetotellurics for Petroleum Exploration, Part I: A Sea-Floor Equipment System: Geophysics, 63, 816–825. doi: 10.1190/1.1444393
  4. Goto, T., Kasaya, T., Machiyama, H., Takagi, R., and Matsumoto, R., et al.,2008, A marine deep-towed DC resistivity surevey in a methane hydrate area, Japan sea: Exploration Geophysics, 39, 52–59. doi: 10.1071/EG08003
  5. Hamano, Y., Yukutake, T., Segawa, J., Asaoka, T., Utada, H., Nakagawa, I.,and Sasai, Y., 1984, Seafloor measurement of natural electric field by use of newly developed instrument, Proceedings of conductivity anomaly research symposium, 259–265. (in Japanese)
  6. 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. doi: 10.1007/3-540-28519-9
  7. Kasaya, T., Goto, T., and Takagi, R., 2006, Marine electromagnetic observation technique and its development – For crustal structure survey: Butsuri Tansa, 59, 585–594. [in Japanese with English abstract] https://doi.org/10.3124/segj.59.585
  8. Key, K., and Constable, S. C., 2002, Broadband MarineMTexploration of theEast Pacific Rise at $9^{\circ}$50'N: Geophysical Research Letters, 29, 2054. doi: 10.1029/2002GL016035
  9. Seama, N., Goto, T., Nogi, Y., Ichiki, M., and Kasaya, T., et al., 2003, Preliminary Report of KR02–14 Kairei Cruise: JAMSTEC Journal Deep Sea Research, 22, 89–106
  10. Seama, N., Baba, K., Utada, H., Toh, H., Tada, N., Ichiki, M., and Matsuno, T.,2007, 1-D electrical conductivity structure beneath the Philippine Sea: Results from an ocean bottom magnetotelluric survey: Physics of the Earth and Planetary Interiors, 162, 2–12. doi: 10.1016/j.pepi.2007. 02.014
  11. Segawa, J., Yukutake, T., Hamano, Y., Kasuga, T., and Utada, H., 1982a, Seafloor measurement of geomagnetic field using newly developed ocean bottom magnetometers: Journal of Geomagnetism and Geoelectricity, 34, 571–585 https://doi.org/10.5636/jgg.34.571
  12. Segawa, J., Yukutake, T., Hamano, Y., and Utada, H., 1982b, A new model of ocean bottom magnetometers: Journal of Geomagnetism and Geoelectricity, 35, 407–421
  13. Tada, N., Seama, N., Goto, T., and Kido, M., 2005,1-D resistivity structures ofthe oceanic crust around the hydrothermal circulation system in the central Mariana Trough using Magnetometric Resistivity method: Earth, Planets, and Space, 57, 673–677. doi: 10.1007/3-540-28519-9
  14. Toh, H., Goto, T., and Hamano, Y., 1998, A new seafloor electromagnetic station with an Overhauser magnetometer, a magnetotelluric variograph and an acoustic telemetry method: Earth, Planets, and Space, 50, 895–903 https://doi.org/10.1186/BF03352185
  15. Toh, H., Hamano, Y., and Ichiki, M., 2006, Long-term seafloor geomagneticstation in the northwest Pacific: A candidate for a seafloor geomagnetic observatory: Earth, Planets, and Space, 58, 697–705 https://doi.org/10.1186/BF03351970
  16. Weitemeyer, K.A., Constable, S.C., Key, K.W., and Behrens, J.P., 2006, Firstresults from a marine controlled-source electromagnetic survey to detectgas hydrates offshore Oregon: Geophysical Research Letters, 33, L03304.doi: 10.129/2005GL024896
  17. Wessel, P., and Smith, W. H. F., 1998, New, improved version of Generic Mapping Tools released: EOS Transactions American Geophysical Union, 79, 579. doi: 10.1029/98EO00426