Ocean bottom measurements of the Earth's electric field using long cable installed by ROV

The materials in Earth's crust and mantle are classified as semiconductor in terms of electrical property. Therefore essential information to understand various kinds of geodynamic processes can be provided from measurements of the electric field of the Earth as well as the measurements of the magnetic field. Recently, we have developed a geophysical instrument for accurate measurement of Earth's electric field, named as EFOS (Earth's electric field observation system), which is installed by using ROV (remotely operated underwater vehicle). It is shown by recent experiments to provide data of much higher quality than conventional instrument, OBEM (ocean bottom electro-magnetometer).