Electrical potential measurement by ocean cable network

这个为期三年的项目的目的是建造一个海底电缆网络，以测量太平洋的电位变化。在第一年和第二年，在关岛、二宫、冲绳和纳霍德卡的国际电缆登陆站安装了记录系统。在关岛、符拉迪沃斯托克和蒙廷卢巴也进行了地磁场变化的观测。利用这些数据，对西太平洋地区地幔的电导率结构进行了分析研究。一维分析表明，在大约150公里深的地方存在一个高导电层。采用薄片模拟技术对海洋-大陆分布等主要表面非均质性的影响进行了评价。在超过几个小时的周期范围内，通过电缆的水流的感应效应在测量电压中占主导地位。利用冲绳与台湾之间的电缆电压数据，对黑潮通量的时间变化进行了研究。黑潮是通过一条狭窄的通道流入东海的。对地核通过导电地幔产生的电磁变化的感应效应进行了简单的理论计算。计算结果表明，在地球表面测量到的电场变化包含了一个与磁场变化的环面模相对应的分量，这是磁场测量无法观测到的。计算表明，只有通过电缆电压测量才能观察到几十年周期的短期长期变化。这也暗示了电缆网不仅可以揭示环向磁场的强度，还可以揭示环向磁场在核幔边界附近的空间分布信息。

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.