Abstract
We have been developing ocean bottom hybrid joint box system for real-time communication and monitoring in deep seabed. The joint box can provide high voltage power and large data transmission used in seabed as a part of seafloor cable network system. We installed first generation joint boxes in the Nankai Trough area, Japan as a part of DONET (Dense Oceanfloor Network System for Earthquake and Tsunamis)project. 12 joint boxes have been deployed in seabed with depth of up to 4000 m from 2010 to 2016. Backbone submarine cables, which provide main power and communication lines from land station to seabed joint boxes, have been deployed before the installation of the joint boxes. Each port of joint box can supply 45 Watts/port and transmit 50 Mbps data for geophysical instruments including broadband seismometer, pressure gauge (tsunami meter), thermometer, etc. 53 seafloor observation systems were connected to the joint boxes for real-time monitoring in the wide area, approximately 200 × 250 km. Observed long-term data confirmed that the joint box has high stability and reliability. There are almost no missing data except for during maintenance. We also applied a data processing method, seismic interferometry method, to long-term seismic data observed by broad-band seismometer. This method can estimate time variation of velocity structure below each seismometer by calculating auto- and cross-correlation. Obtained results confirmed that small change of seismic velocity, which would be generated by earthquake events, is detectable. Since 2014, we have developed modified joint box, which can provide high power and large data transmission, 250 Watts/port and Giga bps rate, respectively. The new joint box can transmit high density information, for example, real-time video image, and operate large seismic observation system with several hundred channels.
