Installation and operation of ICT seafloor cabled seismic and tsunami observation system in source region of Tohoku-oki earthquake

A seafloor cabled system is useful for study of earth science and disaster mitigation, because real-time and long-term observation can be performed. Since a number of sensors on seafloor should be increased, a new system with low costs for production, deployment and operation is expected. In addition, the new system should have sufficient for flexibility of measurements after installation. To achieve these demands, we started development of a new system using Information and Communication Technologies (ICT) for data transmission and system control. A software-base system is enable us to make a compact observation node. Reliability of the system is kept by using redundant system which is easily constructed using the ICT. The system based on this concept was developed as Ocean Bottom Cabled Seismometer and tsunami-meter (OBCST) system which has both seismometers and tsunami gauges. Each observation node has a CPU and FPGAs. The OBCST system uses standard TCP/IP protocol with a speed of 1 Gbps for data transmission, system control and monitoring. IEEE-1588 (PTP) is implemented to synchronize a real-time clock, and accuracy is less than 300 ns. The node can be equipped an external port for additional observation sensor using Power over Ethernet interface. Deployment of the OBCST system was carried out in September 2015 by using a commercial telecommunication cable ship off Kamaishi, northeastern Japan. The OBCST system off Kamaishi has three observation nodes with an interval of 30 km. Each observation node has three-component accelerometer as a seismic sensor and a precise pressure gauge as a tsunami-meter. It is found that the noise levels at the OBCST system are low at frequencies greater than 2 Hz and smaller than 0.1 Hz. This level of ambient seismic noise is close to a typical system noise. From the pressure data, pressure gauge has a resolution of less than 1 hPa, which corresponds to a change of water height of less than 1 cm, and data from all the pressure gauges are consistent. The data from the system are distributed by using a network system in Japan. From the installation in September 2015, the system is collecting data continuously.