This paper describes an automated cable-laying system that can automatically pay out a submarine cable on the seafloor while keeping balance with the ground speed. We have installed DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) at Kumanonada and a ROV (Remotely Operated Vehicle) was successfully used to lay the cables which connect observatories to the nodes of the DONET. However, such a cable-laying work was regarded as one of the hardest works since it takes approximately 10 hours (about 10km distance) and is manually carried out by a ROV operator(s). The automated cable-laying system can contribute to reduce the physical and mental burdens on the ROV operator and speed up the construction of DONET2 (the second phase of DONET). In this paper, the components and cable-laying control strategies are described and some experimental results are presented. In addition, we report on the first mission to connect the borehole observatory C0002 to the Node D of the DONET. This achievement has made it possible to receive the data of the borehole observatory in real-time. The paid out cable is 8470m and the travelled distance is approximately 8000m.
DONET is a flexible and expandable submarine cabled seafloor observation infrastructure for mega-thrust earthquake research and disaster prevention. The system which consists of a backbone looped submarine cable system, five science nodes and 20 set of state of art earthquake and tsunami observatories was developed and constructed in the program started from 2006 and fully in operation from 2011 at the hypothesis region of To-Nankai earthquake. Simultaneously with the construction of original DONET, the development and deployment of second DONET system (DONET2) was planned and started in 2010 to subject for the hypothesis region of Nankai earthquake. Since this region spreads out in the wide area about 2 times rather than the region of the To-Nankai earthquake, deployment of a large-scale observation network system is required. Expansion of scale of observatory means the increase of power distribution capacity and the confliction of network topology, it will need to solve the several engineering subject to expand the design of original DONET to large scale system DONET2. This paper describe several indispensable technologies of submarine cabled seafloor observation system DONET (power consumption control, power blanching and time synchronization), then introduce the novel engineering approach for high voltage power distribution control and large scale network topology management for DONET2 off Kii channel observatory network.
Abstract A remotely operated vehicle (ROV) can install submarine cables along a planned route accurately, thereby deploying cabled seafloor observatories at the intended locations. In this study, we developed an ROV-based cable-laying system. Its manual and automated versions have been successfully used for the installation of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). The automated cable-laying system shows many advantages; for example, the cable-laying speed increased by approximately 0.1 knot from 0.5 knot, and the number of operators was reduced by half. Thus, the physical and mental burdens on the shipboard members including ROV operators were relieved. In this paper, we share our experiences when laying cables using an ROV during DONET installation. First, the issues that should be addressed to complete a successful cable-laying operation are discussed; subsequently, our manual and automated cable-laying systems are introduced. Then, the observations made during field operations for DONET installation are presented, which provide useful tips for developing an ROV-based cable-laying system.
Existence of fluid on seismogenic zones has a key role on great earthquakes. The electrical conductivity structures obtained by electromagnetic survey across the great earthquake zones show that the seismically locked zones correspond to the low conductive zones. The low conductivity is possibly interpreted as relatively low fluid content. For more discussion on the role of fluid to earthquake occurrence, we have just started an electromagnetic and seismological monitoring by using long submarine cables off Toyohashi, the southwest Japan Island. The cables are located on the Tokai seismogenic zone, where both slow-slipping and locked zones are obvious by GPS observation. Here, we introduce the recent and upcoming situations of the project.
Seafloor observation system using the underwater cable is one of the most reliable ways to provide high-speed data transmission and continuous power feeding to underwater devices. Recent evolution in the optical submarine cable and Internet technology make it possible to develop a versatile scientific submarine cable network. IEEE OES (Institutes of Electrical and Electronics Engineers, Oceanic Engineering Society) Japan chapter has conducted the feasibility study on the new scientific study on the new scientific submarine cable network. The proposed scientific cable network named ARENA has the following feature: (1) mesh-like cable network configuration covering vast research area with 3,600 km of total cable length; (2) over 66 observation nodes with 50 km intervals; (3) robustness against failures; (4) wideband optical transmission system capable of transmitting plural HDTV (high definition television) signal and synchronizing time signal with accuracy of one microsecond; (5) system extensibility; and (6) exchangeability of sensors. This paper will describe the outline of ARENA.
As a new approach for low cost long-term seafloor monitoring, the Mobile Seafloor Observatory, which has a depth capability of 6000 m, was developed and tested at sea. The observatory comprises of a multi-sensor Mother Station and four Satellite Stations. The Mother Station is equipped with a velocity type seismometer, a Tsunami pressure gauge, a digital camera with flash, a CTD, an electromagnetic current meter, two heat flow temperature probes, a hydrophone and 16 pop-up buoys. Part of the data in Mother Station could be monitored monthly through satellites by releasing pop-up buoys. After one year long observation, all the data in 8 GB hard disc is retrieved by recovering the station. Each Satellite Station comprises of a long-term velocity type digital ocean bottom seismometer (OBS) which could be recorded three-component seismic data continuously for more than 50 days. The data in 2 GB Exabyte tape could be retrieved by recovering the station. They are deployed around the Mother Station at separations between 20 and 50 km. The observatory was tested at a depth of 1377 m for four months, and will be deployed onto the Shikoku Basin at 4300 m deep to carry out comprehensive seafloor monitoring.
We used ambient noise records observed by DONET seafloor seismometer and LTBMS borehole seismometers to monitor seismic velocity structure along the Nankai Trough subduction zone. Seismic interferometry, which can produce zero offset pseudo shot records by calculating auto- and cross-correlation function from ambient noise records, was used. We applied the seismic interferometry method to the ambient noise records observed by DONET and LTBMS seismometers from June to Sep. 2016. We also applied stretch interpolate technique to detect temporal change of seismic velocity during the period. Results suggest that DONET data with seismic interferometry method can detect less than 0.5% velocity changes below seafloor and LTBMS data can also detect much smaller velocity change which may occur during SSE. These results imply a possibility for this technique to be used as seismic structure monitoring tool without any artificial seismic sources.
