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

本研究的目的是研制一种海底测量系统，它测量地球磁场的三个分量和两个水平分量的变化。要研究日本群岛及其邻近地区的复杂电磁结构，就需要这样的系统。在1987-1988年的研究期间，OBEM(海底电场和磁场测量仪)系统已经完成，并在气象机构的Kakioka磁观测站和东京大学的Yatsugatake磁观测站进行了测试。在研制过程中，对磁传感器、电极、压力容器、测量电路、电源和声释放系统进行了重新检查，以适应目前的用途。1988年6月至11月在胡安德富卡海脊最南端用目前研制的Obem进行了第一次海底测量。该项目是国际合作研究(EMRIDGE项目)的一部分，其目的是通过详细研究海脊系统下的电磁结构来研究大洋板块的形成过程。由于一台仪器可以在现场测量一整套电磁信号，因此本仪器适合于研究。包括本装置在内的海底装置是由东京大学海洋研究所所属的Hakuho-Maru安装的，并由加拿大科考船Parizeau安全打捞上来。

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.