E. Araki

The DONET: A real-time seafloor research infrastructure for the precise earthquake and tsunami monitoring

A project of submarine cabled seafloor observatory network development has been carried out in Japan since 2006. This research has aimed to establish the technology of a large scale real-time seafloor research infrastructure development for earthquake, geodetic and tsunami observation. The project scheduled to install 20 sets of cabled earthquake and tsunami observatory to an active seismogenic zone with mega-thrust earthquakes called Nankai trough. This paper describes design concept of a novel ocean observation system and the key technologies for reliable and extendable real-time seafloor monitoring.

A Design Concept of Seafloor Observatory Network for Earthquakes and Tsunamis

The reinforcement of monitoring capability on seafloor is important to understand and forecast the mega-thrust earthquake activity near plate boundary accurately. The deployment of high density real-time seafloor observatory network is suggested to realize this requirement. The maintenance of large scale system extend over a long period of time is one of a great challenge of underwater technology. The conventional high reliability system design of real-time seafloor observatory is not suit for keeping up a large scale system long time. A noble system design concept and practical system management is necessary to make the seafloor observatory network turn into a reality. This paper describes a design concept of the seafloor observatory network that scheduled to install to the Nankai trough in the next several years.

Long-term monitoring at C0002 seafloor borehole in Nankai Trough seismogenic zone

The C0002 long-term borehole observatory installed during IODP Expedition 332 in December 2010 have been successfully connected to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) this January 2013 during the KY13-02 cruise by the R/V Kaiyo. We confirmed from the DONET landing station that all the borehole instruments was properly functioning and finally started long-term borehole monitoring at Site C0002 in Nankai Trough seismogenic zone.

Vortex induced vibration suppression of the drill pipe for the long-term borehole monitoring system installation

This study proposes a Vortex Induced Vibration (VIV) suppression method for the Long-Term Borehole Monitoring System (LTBMS) installation in areas of strong ocean currents such as Kuroshio. One of the primary challenges in realizing LTBMS was to install high-precision, sensitive sensors into the borehole without damaging them. Two field tests were performed using accelerometers attached on instrument carrier and/or drill pipes to investigate the characteristics and causes of drill pipe VIV. This test demonstrates that the reduction of the drag on circular drill pipes and tubings and the vortex suppression can be achieved by the suppression ropes and the drill collars. They were suggested to be attached on the drill pipe above the sensor assembly for the actual LTBMS installation. From the VIV monitoring during the installation over a period of several days, the following three points can be drawn for the further VIV suppression: 1) The bottom hole assembly should be lowered in the low current area, with the relative current speed being as low as possible, 2) the drifting speed should be kept well below 1 knot, and 3) the drifting angle between drifting direction and sea current should be kept as small as possible (definitely less than 45°). The results show the VIV amplitude was further reduced to less than 0.5G, which led to the success of the first LTBMS installation.

The development and evaluation of sensors for long-term borehole monitoring system

In Integrated Ocean Drilling Program (IODP), the Long Term Borehole Monitoring System (LTBMS) is planned to be installed to sea bottom boreholes in the Nankai Trough area. LTBMS sensors are very sensitive instruments for collecting broadband dynamics with wide dynamic range for understanding earthquake mechanism. However, in IODP Exp.319, it was apparent that the strong ocean current “Kuroshio” cause vortex induced vibration (VIV) that gives damage to sensors in installation scheme. Thus, the LTBMS sensors have to be not only high sensitive but also rugged to survive against VIV. For this purpose, sensors with anti-vibration mechanism were developed by JAMSTEC project team. After development, noise evaluation test and vibration and shock tests that simulate vibration and shock in installation scheme were conducted for confirming the mechanism was working well. Furthermore, before and after the vibration and shock tests, power Spectral Density analysis was carried out using background noise recorded in low noise location, Matsushiro Seismological Observatory. As a result of this analysis, it was observed that the responses of sensors were not changed by the vibration and shock tests. Finally, all of these sensors were loaded to D/V Chikyu for installation to C0002 observatory in Exp.332.

Outline of New Cabled Observation System off Toyohashi

A new cabled observation system off the coast of Toyohashi in central Japan is now being developed. The system uses a couple of former underwater optical telecommunication cables of about 60 kilo-meters long. The authors will use the cable in two ways simultaneously. One is to build a new observatory at the end of the cable. Underwater sensors including a broadband seismometer, a precise water-pressure sensor and an electro-magnetometer will be connected to the junction unit using underwater mateable connectors. The other is to use the same cable simultaneously as a long emitting antenna to monitor the electro-magnetic property of the earth crust. We have developed a new time synchronization system. It provides precise 1PPS signal, clock and NMEA data to underwater sensors. In this paper, we will describe the outline of the system. The longterm monitoring will start in this April.

Development of DONET2 — Off Kii chanel observatory network

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.

Shock and vibration tests on sensors for long-term borehole monitoring systems

Shock and vibration tests on electrical devices have been carried out. Here the devices mean sensors combined with telemetry units and these are the same ones that have been installed in C0002 riserless hole in Nankai area, Japan in December 2010. Acceptable upper limit of shocks and vibrations on the devices have been confirmed through these tests.

Deployment of mobile and real-time deep seafloor observatory

Mobility, expandability, replaceability, and real-time data transmission capability are key functions for the prospective observatory on seafloor to realize interdisciplinary in-situ continuous ocean observations with a variety of spatial and temporal scale coverage. Put this new approach on a firm footing, JAMSTEC developed a mobile seafloor observatory test bed called AOS (Adaptable Observation System) as an equipment fit in to the branching interface equipped in the latest submarine cable connected earthquake observation system. In 2001, the first AOS sea trial was carried out by JAMSTEC research vessels and underwater exploring system (such as towed vehicles and ROVs). The mobile observatory was successfully set up in the predetermined area which approximately 6km far from a branching interface in the backbone cable system. The components of AOS was recovered on land for inspect and maintenance after the end of first trial, and set it up again to another observation site in 2004. The evaluation of reliability of observatory and verification of major seafloor scientific instruments related technologies are being implemented in this trial. The engineering approach and observation technique demonstrated in these sea trials are essential to realize sophisticated 4-dimentional seafloor observation network in the future. This paper summarized the results of trials and discusses the approach to establish the solution for these engineering difficulties.