Development of Ocean Bottom Electromagnetic Measurement System and Three-dimensional Structure of the Transition Zone between the Ocan and the Continent

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Purpose of the present investigation is to develope an ocean bottom system which measures the variations of the three components of the magnetic field and two horizontal components of the electric field of the earth. Such a system is required to study the complicated electromagnetic structures beneath the Japanese Islands and the adjacent area. During the studying period of 1987 and 1988, the system named Obem (Ocean Bottom Electric and Magnetic field measurement apparatus) has been completed, and tested at Kakioka Magnetic Observatory of the Meteololgical Agency and Yatsugatake Magnetic Observatory of the University of Tokyo. Druing the development of the apparatus, magnetic sensor, electrode, pressure vessel, measureing circuits, power source, and acoustic release system have been reexamined to fit to the present purpose.The first sea floor measurement by using the presently developed Obem was made during june and November of 1988 at the southern-most part of the Juan de Fuca ridge. The project is a part of the international coopertive study (EMRIDGE project), which aimes to investigate the generating process of the oceanic plate from a detailed study of the electromagnetic structure beneath the ridge system. The present apparatus is suitable for the study, since one instrument can measure a complete set of the electromagnetic signals at the site. The sea floor apparatus including the present one was installed by hakuho-maru which belongs to the Ocean Research institute of the University of Tokyo, and were safely recovered by Canadian research vessel Parizeau.