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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). more...

It is essential to observe seismic activities on the sea floor just above the seismogenic zones where large earthquakes have been occurred. We have developed UT-OBCS, a new ocean bottom cabled seismometer for a high density seismic array network, with utilizing ICT (Information and Communication Technologies), i.e., IP (Internet Protocol) on the seafloor. UT-OBCS is a compact and low cost system. The system reliability is kept by a redundant transmission route design which is easily constructed using the ITC. In 2010, we started a real time seismic observation in the Sea of Japan by using four UT-OBCSs and a 25 km cable. more...

Geomagnetic field variations are often used for the study of the deep interior of the Earth. On the other hand, the geoelectric field has seldom been employed for these studies although it has sensitivity to phenomena that the geomagnetic field variations does not have. A reason for this is the smallness of the electric field of deep Earth origin if it is measured at the Earth's surface. Thousand kilometer scale trans-oceanic submarine cables are very attractive tool to measure such small but global scale geoelectric field, and the observation of the electric field from deep Earth is possible only when such long cables are used. In this paper, we show the capability of submarine cable electrical voltages to probe the Earth's deep interior, especially for the phenomena related with the toroidal magnetic field in the core. more...

The Japanese islands are positioned near the plate convergent zones, and large earthquakes have repeatedly occurred in marine areas around Japan. Although there are already approximately 2,000 land-based earthquake observatories in Japan, the number of permanent earthquake observatories in the oceans is quite limited. It is important to observe seismic activities on the seafloor just above these seismogenic zones for understanding the generation of large earthquakes. An ocean bottom cabled seismometers system (OBCS) is the most suitable observation tool for this purpose since data can be obtained in real-time. We have developed a new compact OBCS system. The first installation of the new OBCS system will be done in the Sea of Japan in August in 2010. The OBCS system has a cable length of 25 km and four observation nodes with 5 km spacing. This paper describes the new compact OBCS system and the Sea of Japan OBCS project. more...

To install seismometers and tsunami-meters on the sea floor is necessary for monitoring seismic activities of a seismogenic zone. An ocean bottom cable seismometers system (OBCS) is most suitable for this purpose since data can be obtained in real-time. Some systems have been installed over the past 25 years around Japan. Although the existing OBCS systems are useful for the study of seismic activities, the number of station is limited due to their costs. Therefore lower cost in both production and installation by development of a portable system is expected. We have developed a new OBCS system. Utilization of the up-to-date ICT (Information and Communication Technologies) and semiconductor technologies is essential to reduce a size of the observation node and a cost of production and installation. We have a plan to install the new OBCS system in Japan Sea. The cable system has total length of 25 km and 4 observation nodes with 5 km interval. At the present, we are choosing an area for the deployment. The first installation of our new OBCS system is planed in 2010. This paper describes the new compact OBCS system in detail and presents a plan for the first installation. more...

An ocean bottom cabled seismometer (OBCS) is the best solution for the monitoring of earthquake activity in a seismogenic zone where big or huge earthquakes have occurred repeatedly. However, the number of existing OBCS has been insufficient for high resolution observation of the seismogenic zone, due to the high costs involved. In order to lower the cost of OBCS and to meet requirements for flexibility and expandability of measurements, the latest Information and Communication Technologies (ICT) and JP (Internet Protocol) are adopted for use at the ocean bottom. This paper presents the objectives, architecture and system design, and the newly developed prototype of an OBCS. more...

An ocean bottom cable seismometers system (OBCS) is the best instrument to observe seismic activities on the sea floor because data is obtained in real-time. Therefore some systems have been installed over the past 25 years around Japan. Although the existing OBCS systems are useful for the study of seismic activities, the number of station is limited due to their costs. In order to lower the cost of OBCS and to meet requirements for flexibility and expandability of measurements, the up-to-date Information and Communication Technologies (ICT) and IP (Internet Protocol) are adopted for use at the ocean bottom. This paper presents a prototype of a new compact OBCS in detail and evaluation of obtained data by the prototype. more...

An ocean bottom cable system (OBCS) is the best solution for monitoring the tectonic activities in a seismogenic zone. However the number of seismometers of the existing OBCS is insufficient for high resolution observation of the seismogenic zone, because the cost of the OBCS has been very expensive. In order to solve this problem, the authors have studied new cost effective OBCSs with 40 seismic arrays which determine the location of seismic events over a 100 square kilometer area for long-term observation in a remarkable seismogenic zone based on the cost analysis of existing OBCS. more...

Trans-oceanic submarine cable of several thousands kilometers in scale can be a useful tool of geophysical measurement. One of the geophysical information expected be contained in very long baseline cable voltages is that originated due to the toroidal magnetic field at the core-mantle boundary (CMB). It is well-known that the toroidal field cannot be observed by magnetic field measurements at the surface of the Earth. Probably, indirect detection of the magnetic field via long baseline electric voltage measurements is the only way to constrain on the toroidal field observationally. In this paper, we present recent estimate of long-term variation of the electric potential observed using submarine cables. A trial to remove the motionally induced electric potential by ocean current is also presented. more...

An ocean bottom cable system (OBCS) is the best solution for monitor the tectonic activities in a seismogenic zone. However the number of seismometers of the existing OBCS is insufficient for high resolution observation of the seismogenic zone, because the cost has been very expensive. In order to lower the cost of OBCS, authors have studied cost analysis of existing OBCS in depth jointly with engineers of various fields, and specified the key factors for lowering the cost in system, and estimated the cost of new OBCS. Authors have designed two types of new cost effective OBCSs based on this study. more...

This paper deals with the time delayed diversity (TDD) system characteristics for mobile broadcast geostationary satellite orbit (GSO) system measured by using the GPS (Global Positioning System) satellite. To design digital mobile broadcast satellite system with TDD concept, it must estimate attenuation characteristics by urban building, electrical light poles and trees etc. Southern orbital arc GPS satellite signals with elevation angle about 40 degrees data are used for propagation characteristics measurements, rather than actual GSO satellite. To overcome urban signal attenuation, the effect of TDD system characteristics are examined from processing measured GPS data. more...

Electric voltage measurements using long submarine cables have been performed with one of the purposes to obtain the strength of the toroidal field and its variation at the core-mantle boundary. Several observational constraints on the toroidal field have been obtained but the physical plausibility of them was examined. We performed a simple but systematic computation of kinematic dynamo surrounded by electrically conducting mantle in order to examine the plausibility. It is confirmed that the dynamo can generate the strength of the toroidal field variation obtained observationally naturally. Detectability of the electric signal originated from the geodynamo against those from magnetotelluric or motional induced electric field is re-examined. Understanding of the decadal variation of the ocean flow is indispensable for the detailed discussion of the toroidal field using submarine cable voltages. more...

We present a model of 3-D electrical conductivity structure in the mid-mantle estimated by using voltage data by eight submarine cables as well as magnetic field data around the North Pacific. We assumed that 3-D heterogeneous conductivity distribution could be separated into radially symmetric (1-D) structure and 3-D perturbation. The 1-D model obtained by Utada et al. (2003) is employed as the reference model, and the distribution of the 3-D perturbation is inversely estimated so that the observation-calculation misfit is minimized. As the results, following features in the obtained 3-D conductivity structure are found to be similar to the seismic velocity structure: 1) conductive zone in the transition zone beneath Hawaii with a conductivity perturbation of factor of three; and 2) resistive zone in the uppermost lower mantle beneath Philippines with twice as resistive as the 1-D reference model. Assuming that the heterogeneities purely of thermal origin, temperature anomalies are estimated to be 200-300 degrees. more...

Efforts have been initiated to reuse a former submarine telephone cable ("TPC-1" retired in 1991) for undersea scientific measurements. TPC-1 runs from Ninomiya, Japan to Guam, U.S.A., and has a length of ca. 2700 km with the strike almost in the N-S direction. A prototype instrument package was successfully installed in northern Japan at a depth of 100 m using a similar coaxial cable. An instrumental package has been built and will be deployed near 30/spl deg/N next year. The instrument has three component accelerometers with 24 bit A/D converters, a hydrophone applying analog telemetry, and a quartz thermometer and a quartz pressure sensor with a data length of 16 bits. The data will be transmitted to the Ninomiya station in real-time. Other plans to reuse submarine telephone cables have been considered between Naoetsu, Japan and Nakhodka, Russia and between Okinawa, Japan and Guam. > more...

Real-time earthquake monitoring in the deep ocean is one of the urgent demands in Japan because Japan has been badly damaged by large earthquakes occurring in the surrounding area of the Japanese archipelago. To obtain real-time seismic data from the deep ocean bottom, a disused submarine coaxial cable (TPC-1) deployed in 1964 has been used. The Ninomiya, Japan, and Guam section is currently called the GeO-TOC cable whose length is 2,659 km. In this project, several technological questions arising from reuse of disused submarine cables for scientific purposes, have been solved. For example, there was a question on the remaining life of cables because the cable system uses vacuum tubes in its repeaters. In January, 1997, an ocean bottom seismometer unit was installed at 2,708 m depth on the Izu-Bonin trench slope by adding extra cables with a different diameter. The "IZU OBS" has several sensors including a three-axes accelerometer. Data from accelerometers are digitized by 24 bit A/D converters and are transmitted from the bottom unit to the shore station in Ninomiya through the GeO-TOC analog cable. The electrical power needed for the ocean bottom package is fed from the cable DC supply in Guam. Continuous data are currently processed jointly with land seismic data. more...