Electrical potential measurement by ocean cable network

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This three-year project has been carried out for the purpose of constructing a submarine cable network to measure electrical potential variations in the Pacific. For the first and second years, recording systems were installed at the international cable landing stations at Guam, Ninomiya, Okinawa, and Nakhodka. Observations of geomagnetic field variations have been also made at Guam, Vladivostok, and Muntinlupa.Using the data thus obtained, analyzes were carried out to study the electrical conductivity structure of the mantle in the western Pacific region. One-dimensional analysis indicated a presence of a highly conducting layr at about 150 km depth. A thin sheet modeling technique was applied to evaluate the effect of major surface heterogeneity such as ocean-continent distribution.At period range longer than several hours, induction effects by water flow across the cable become dominant in the measured voltage. A study on the time variation in Kuroshio flux has been done by using the cable voltage data between Okinawa and Taiwan where Kuroshio flows into East China Sea through a narrow channel.A simple theoretical work was done to calculate the induction effect on the electromagnetic variations originated from the Earth's core through the conducting mantle. This calculation showed that the electric field variations measured on the Earth's surface includes a component corresponding to the toroidal mode of the geomagnetic field variations which can never be observed by magnetic measurements. The calculation suggested that short term secular variations of a few decades' periods may be observed only by a cable voltage measurement. It also implied that a cable network may reveal not only the intensity but also information on the spatial distribution of the toroidal magnetic field near the core-mantle boundary.