Hiroyasu Momma

PRELIMINARY RESULTS OF A THREE-YEAR CONTINUOUS OBSERVATION BY A DEEP SEAFLOOR OBSERVATORY IN SAGAMI BAY, CENTRAL JAPAN

Abstract A comprehensive long-term deep seafloor observatory was deployed at the plate boundary between the Philippine and North American plates in Sagami Bay, central Japan in 1993 in order to investigate the relations among geophysical parameters associated with earthquake swarms and eruptions of submarine volcanoes that have occurred repeatedly from at least 0.01 Ma to the present. It is also located within the habitat range of the vesicomyid clam, Calyptogena soyoae , the presence of which suggests seepage from below. The observatory, at a depth of 1174 m off Hatsushima Island, is equipped with multi-sensors, such as a seismometer, hydrophone, heat flow temperature probes, color video cameras, a CTD and a current meter. The data and power to and from the land station at Hatsushima Island are sent in real time through an 8-km-long electro-optical cable. More than 3 years of continuous geophysical and environmental data on the deep seafloor were collected up to 1996. In this paper, we describe the system and report on significant changes in ground temperature associated with three earthquake swarms in the adjacent area.

Monitoring system for submarine earthquakes and deep sea environment

Although more than 80 percent of earthquakes in Japan occur on the seafloor, the seafloor seismic network on the seafloor is sparse and insufficient. To increase the network, the Comprehensive Seafloor Monitoring System was deployed in Nankai Trough off Cape Muroto in March 1997. The prototype system is a combination of observatories with a cable and without a cable. The former system comprises of two seismometers, two tsunami pressure gauges and a seafloor observatory with multiple sensors and 125 km long optical cable. The data are sent in realtime to the land station at Muroto and they are also transmitted to JAMSTEC in Yokosuka and Meteorological Agency of Japan. The latter system, which could be deployed at any place, is comprised of a seafloor observatory with multiple sensors and four long-term digital ocean bottom seismometers. The data could be recovered once every month by releasing pop-up buoys to the surface through the satellite. The system with a cable was deployed on the landward slope of Nankai Trough off Muroto at water depths between 1286 m and 3572 m. The system without cable will be deployed 200 km off Muroto in Shikoku Basin at a depth of 4300 m in early 1998. Five similar systems will be deployed until the year of 2002.

Multidisciplinary geophysical measurements on the ocean floor using decommissioned submarine cables: VENUS project

To perform geophysical and multidisciplinary real-time measurements on the ocean floor, it has been attempted to reuse decommissioned submarine cables. The VENUS project reuses the TPC-2, which is one of these systems and runs across the entire Philippine Sea Plate between Guam Island and Okinawa Island. The VENUS system comprises an ocean floor observatory, a submarine cable, and a land system. The major components of the ocean floor observatory are geophysical instruments and a telemetry system. There are seven scientific instrument units including broadband seismometers and a hydrophone array. Digital telemetry using the old analog telephone cable obtains high data accuracy and real-time accessibility to data from a laboratory on land. The bottom-telemetry system and a part of sensor units were installed at a depth of 2157 m on the landward slope of the Ryukyu (Nansei-Syoto) Trench on August 29, 1999. The data from the hydrophone array and tsunami gauge have been correctly transmitted to the data center. The rest of the scientific instruments will be deployed by deep-tow equipment and a remotely operated vehicle. Using a decommissioned submarine cable will greatly reduce construction costs compared to using a new cable system.

The VENUS project-instrumentation and underwater work system

The VENUS project is a five-year plan between 1995 and 1999 to utilize the retired trans-Pacific telephone cable from Okinawa to Guam Islands for multidisciplinary geo-scientific studies. The first seafloor observatory off Okinawa Island was deployed in March 1998 in the landward slope of Nanseishoto Trench at a depth of 2,200 m. Among several ideas to transmit the data from sophisticated sensors into the cable, such as inductive data coupling system, direct data coupling system was adopted in the VENUS phase I in order to get sufficient power from the cable for the instruments. Integrated seafloor observatory is comprised of broadband and velocity-type seismometers, tsunami pressure gauge, magnetometer, hydrophone array, sub-bottom temperature probe, geo-potential probes, precision transponders for baseline measurement, digital camera, CTD, current meter, etc. These sensors were deployed precisely by JAMSTEC/deep tow and connected with the main cable by manned submersible Shinkai 6500 and ROV Kaiko. In order to carry out underwater works in situ, ROV matable coaxial, electrical and optical connectors were developed. Also, cable cutter and grip were developed for underwater operations by Shinkai 6500. The VENUS project will contribute greatly to expand global seismic network, and to reuse economically retired submarine coaxial and optical telephone cables over the world ocean floor. Also, a revolutional progress is expected in the deep sea technology and deep sea research.

A new approach to geophysical real-time measurements on a deep-sea floor using decommissioned submarine cables

In order to better understand earthquake generation, tectonics at plate boundaries, and better image the Earth’s deep structures, real-time geophysical measurements in the ocean are required. We therefore attempted to use decommissioned submarine cables, TPC-1 and TPC-2. An OBS was successfully linked to the TPC-1 on the landward slope of the Izu-Bonin Trench in 1997. The OBS detected co-seismic and gradual changes during a Mw 6.1 earthquake just below the station at 80 km depth on November 11, 1997. A pressure sensor co-registered a change equivalent to 50 cm sea-level change. This suggests a high possibility detecting silent earthquakes or earthquake precursors if they exist.A multi-disciplinary geophysical station has been developed for deep-sea floor using TPC-2 since 1995. The station comprises eight instrument sets: broadband seismometers, geodetic measurements, hydrophone array, deep-sea digital camera, CTD, etc.These activities are examples that decommissioned submarine cables can be great global resources for real-time cost-effective geophysical measurements on a deep-sea floor.

Missing of the ROV Kaiko vehicle - problem on the secondary cable

Kaiko has been the only full ocean depth survey system in the world since 1995. The Kaiko system comprises a launcher, the vehicle, primary cable, secondary cable, on board equipment and the R/V Kairei as a mother ship. In May 2003, the secondary cable of the Kaiko snapped during the 296th dive at a depth of 4675 m in the Nankai Trough, 130 km southeast off Cape Muroto on Shikoku Island, and the vehicle has been missing. The secondary cable of the Kaiko is an electro-optical cable, and its tension member is made of aramid fiber. It was found that the secondary cable snapped against a tension less than 400 kgf, which was far below the designed breaking strength of 3 tonf. The aramid tension members were investigated and found to have weakened by bending under extremely high hydrostatic pressure, and local fatigue of the tension members occurred at the cable termination near the vehicle.

VENUS PROJECT-submarine cable recovery system

The deep seafloor observatory for the VENUS project will be a first complex observatory which uses retired coaxial submarine cable. Installation of the observatory requires three procedures; cable amputation, cable recovery, and re-installation. For the VENUS project, the cable reinstalling operation requires precise cable position control to put the observatory to target area. To solve the problem, Japan Marine Science and Technology Center (JAMSTEC) developed a cable recovery system consisting of a cable cutter, cable grippers and a pump unit. The pump unit which is electrically powered from underwater vehicle, generates hydraulic pressure to actuate the cable cutter and cable gripper. This system has an ability to amputate the cable at the deep seafloor and pick out to the surface. Cable recovery system was designed and manufactured to fit for underwater vehicles in JAMSTEC and has a capability to actuate in full ocean depth. Performances of the components are confirmed by pressure tests, amputation tests, gripping tests and strain tests on land or in pressure test tank. To ensure the total system performance, sea trials will be conducted by manned submersible at 6500 m depth seafloor.

Observation of deep sea current and change of bottom shapes in the Suruga Trough

The Suruga Trough, located 150 km away to the south-east of Tokyo, Japan, is known to have turbidity and strong deep water currents. In 1989 and 1990, the authors measured the current from the bottom layer to the middle layer using moored current meters in order to understand the characteristics of the bottom current. Visual observation has been also done at the bottom using a 8 mm-video camera. The following results were obtained from the observations: i) There are steady changes of current with about 13 days cycle depending on tide. At the spring tide, current of SSW direction with a period of a day was found; maximum velocity is about 60 cm/sec. At the neap tide, the current is below 20 cm/sec with a period of a half day. 2) A turbidity current in the deep sea area caused by a typhoon was observed at the bottom layer in August 1990. The maximum velocity was over 70 m/sec. 3) Sediment transport processes associated with the bottom current was observed by the video images.<<ETX>>

Development of advanced secondary cable for the full ocean depth ROV Kaiko

Kaiko has accomplished more than 20 dives to 11000 m at the Challenger Deep in the Mariana Trench since 1995. On May 29th in 2003, Kaiko made 296th dive at a depth of 4675m to recover bore whole data from the A-CORK. After completion of the data recovery, the secondary cable of Kaiko was snapped, and the vehicle was lost. After the accident, secondary cables including the snapped cable were examined at JAMSTEC. The secondary cable was snapped at the end of the cable because of the structural defect of the cable termination. Also, it was made clear that the cable strength has weakened by 30% all along the cable. This is because of buckling by micro-bending of the aramid fibers under extremely high water pressure. In order to avoid the buckling, we knew that it was necessary to get a pressure balance between each fiber. For this purpose, JAMSTEC has developed two types of advanced secondary cables as first stage prototype for the Kaiko. In one of the secondary cables, aramid fiber was made up to FRP same with a strength member for the primary cable. In the other new secondary cable polyarylate fiber was just bundled.

Search and recovery of the H-II Rocket Flight No. 8 engine

In November 1999, the H-II Rocket Flight No. 8 was launched from Tanegashima Space Center. However, the first stage engine stopped in 4 minutes. Consequently, the engine dropped 380 nautical miles off Izu-Ogasawara Islands in 3000 m of water. The JAMSTEC was requested to search for the first stage engine in order to identify the cause of the failure. The search area was a box, 3.3 km in width and 26 km in length. First of all, the survey using 40 kHz side scan sonar on board the ROV Kaiko was carried out. After seven days of sonar survey, a probable contact was obtained. It was the engine section. However, the main engine was not discovered yet. Then, the JAMSTEC/Deep Tow sonar was deployed in the second cruise. On the second day, sonar contacts were obtained approximately 15 km southeast of the engine section. On December 24, 1999, the main engine was discovered by the deep tow camera. In the third cruise, the nozzle skirt of the main engine was discovered by the ROV Dolphin-3K. Finally, the main engine was recovered approximately two months after the launch.