An Experimental study of the Earth's Electric Field

点击翻译按钮获取中文摘要

This three-year research project aimed to develop a new observation system that enables us to record time variations of the electric field of the Earth at deep ocean bottom by using a long (10-100 km) cable.. The research result of our feasibility study concluded that the use of such a system is essential to detect the electric field of Toroidal magnetic mode in the outer core. The geophysical importance of the observation of this physical quantity is obvious because it is expected to give significant constraints on the magneto-hydrodynamics in the outer core,. Besides this signal from the outer core, the proposed observation system is considered to be useful for studying long-term variabilities of ocean flux and the electrical conductivity structure in the deep mantle.The system consists of the main recording unit and a 10 km long cable. It is designed to be installed by using a deep-tow system and to be recovered by using a ROV (Remotely Operating Vehicle). The system was designed by … More assuming DT-4000 and Kaiko-10K both of JAMSTEC for a deep-tow and ROV. During the first year, an instrument was built and on-land tests were performed. In the second year, we tried to install the system in the Philippine Sea (near Minami-Daito Island), but the installation was failed due to a break of the cable soon after the beginning of run. Causes for this failure were extensively examined and clarified. Several modifications were made to the system design and the improved system was built at the end of the second year of this project. This system was named EFOS-10 that stands for Earth's electric Field Observation System with a 10 km long cable. The EFOS-10 installation was successfully made by using DT-4000 on R/V Natsushima cruise in May, 2004. Its recovery was scheduled one year after the installation, i.e., in May, 2005. It is because of the availability of ROV of JAMSTEC (The vehicle of Kaiko-10K was lost in 2003, and its supplementary vehicle Kaiko-7K became available in May 2005).The recording unit was successfully recovered, form which 1-year-long time series record of electric variations was obtained. The data quality was examined and was confirmed to be satisfactory as it has been expected. It is concluded that the EFOS-10 can be useful for various electromagnetic investigations at sea. Less