13:45 〜 15:15
[SEM14-P18] Development a modified deep-sea electric field observation system for electromagnetic sounding of mantle transition zone
キーワード：マントル遷移層、電気伝導度、海底観測、電場観測
Water is one of key elements that controls dynamics of the mantle and evolution of Earth. Existence of water near the Earth’s surface enables Earth-like plate motion, and mantle convection with subducting plate can cool the underlying core efficiently. This cooling enables generation of a magnetic field in the core, and the presence of the magnetic field reduces the escape of water from the Earth's surface into space. Studies of planetary formation show that proto-Earth collected and contained a large amount of water, as much as 10-100 mass of present ocean, in it when it was accreted. Estimating the amount of water in the current Earth will enable us to discuss the water budged, and this contributes for the discussion of Earth’s evolution.
Observationally determined electrical conductivity of the upper mantle and transition zone is often employed to estimate the amount of water in these regions since the conductivity of the materials is sensitive to the amount of water. Electromagnetic responses in the frequency domain calculated from observed fields are used to estimate the electrical conductivity. Although the electric field observed using established Ocean Bottom Electro-Magnetometer (OBEM) is suitable to study the upper mantle. However, its sensitivity, which is determined by the antenna length of about 5 meters, is not sufficient to observe the electric field variations with a period of one day or longer that are necessary for estimating the conductivity in the transition zone.
To observe electric field variation applicable to study the conductivity of mantle transition zone, a deep-sea electric field observation system (EFOS) that is composed of a 1-10 km long cable and recording unit has developed [Utada et al., IEEE, 2003]. Joint analysis of the electromagnetic field and seismic data enabled us to constrain the upper bound of the amount of water in the transition zone beneath northwestern Pacific [Matsuno et al., EPSL, 2017]. Although the advantage of EFOS to constrain the conductivity in the transition zone has been proven, the system was used only by our group since its deployment and recovery required complicated operations by a specific ROV. To increase the opportunity of long-baseline electric field observations at the seafloor, we are developing a modified electric field system, EFOS-X, that can be deployed and recovered by manned submersible with much simpler operations than that of EFOS. In this presentation, we are going to introduce the overview of EFOS-X and two candidate ways of deployment of the system by Shinkai 6500.
Observationally determined electrical conductivity of the upper mantle and transition zone is often employed to estimate the amount of water in these regions since the conductivity of the materials is sensitive to the amount of water. Electromagnetic responses in the frequency domain calculated from observed fields are used to estimate the electrical conductivity. Although the electric field observed using established Ocean Bottom Electro-Magnetometer (OBEM) is suitable to study the upper mantle. However, its sensitivity, which is determined by the antenna length of about 5 meters, is not sufficient to observe the electric field variations with a period of one day or longer that are necessary for estimating the conductivity in the transition zone.
To observe electric field variation applicable to study the conductivity of mantle transition zone, a deep-sea electric field observation system (EFOS) that is composed of a 1-10 km long cable and recording unit has developed [Utada et al., IEEE, 2003]. Joint analysis of the electromagnetic field and seismic data enabled us to constrain the upper bound of the amount of water in the transition zone beneath northwestern Pacific [Matsuno et al., EPSL, 2017]. Although the advantage of EFOS to constrain the conductivity in the transition zone has been proven, the system was used only by our group since its deployment and recovery required complicated operations by a specific ROV. To increase the opportunity of long-baseline electric field observations at the seafloor, we are developing a modified electric field system, EFOS-X, that can be deployed and recovered by manned submersible with much simpler operations than that of EFOS. In this presentation, we are going to introduce the overview of EFOS-X and two candidate ways of deployment of the system by Shinkai 6500.