OPERATION

As soon as the NEREID systems were installed during Leg 186, the SWBs started charging the accumulators (see "Power Supply"). The downhole sensor strings, however, do not start functioning until an ROV arrives at the site to start the system (i.e., to connect the modules in the PAT electrically to the MEG). In the present NEREID design, the MEG and the downhole sensor string are physically connected by 4-in casing pipe so that they are installed with electrical connections made up, whereas the PAT is not electrically connected. This is because the PAT cannot be lowered with the 4-in casing pipe assembly because the reentry of the sensor string into the cone requires the use of the vibration-isolated television (VIT) camera and visibility to the stinger. The PAT would block the view, making reentry impossible. Furthermore, both the VIT camera and PAT must be suspended by cables, which could become tangled if deployed simultaneously.

The NEREID system operation first requires an electrical connection to be made between the PAT/SAM and the MEG. This was the mission of the Dolphin 3K of the Japan Marine Science and Technology Center in early September 1999 (see "Borehole Instruments" in "Borehole Instrument Status" in the "Site 1150" chapter) (Fig. F24). Once this connection is made, the data from downhole start to flow into the SAM on the PAT, which can store 72 GB, enough for ~1 yr of data from all the sensors (see "Borehole Instruments"). The ROV can check the status of the system via a BOB module, which seats on the SAM and communicates through a sapphire window using IR communication. If there is no problem, the ROV needs only to replace the SAM at least once every year to prevent loss of data continuity.

A seafloor downhole observatory is still at pilot-study stage. There is no set routine on which to rely. In our design the sensor string is unrecoverable for the reasons given in "Installation Techniques". Therefore, the string must be composed of reliable instruments (see "Borehole Instruments"). On the other hand, the seafloor components are replaceable and serviceable by a submersible vehicle. The BOB module may be considered a communication extension of the ROV. The SAM is replaced at each ROV visit. The MEG can be pulled out of its seating frame and reinserted by the ROV, although the operation is more complex than other tasks.

The battery life is somewhat unpredictable because it depends on the bottom-water current, dissolved oxygen, and the actual power consumption of the system over a long period of time. Since the battery operates only in the water, separate tests were conducted for the battery and for the rest of the system. We expect a 2- to 3-yr life span, within which time period we will have collected and analyzed the data quality and obtained preliminary scientific results.

In June 1996 an underwater cable system was constructed off Sanriku (northeast Japan) by the University of Tokyo Earthquake Research Institute to continuously monitor earthquakes and tsunamis. The cable was deployed to be perpendicular to the trend of the Japan Trench and is landed at Kamaishi, Iwate Prefecture, at ~39°15´N. The cable system covers the area where water depth is <2500 m. The eastern end of the cable is near Site 1150.

Three seismometers and two tsunami gauges are connected to the cable system. The seismometers are servo-type accelerometers and the tsunami gauge is a crystal oscillator type. Data from the sensors are digitally transmitted using optic fibers in the underwater cable to the land station at Kamaishi. From there the data are distributed to universities and governmental institutions in Japan through Tohoku University.

The multioptic-fiber type of cable system is extendable with six unused cables. Currently there is a plan to connect an extended 200-km-long cable at the end of the present cable and to add six seismometers and two tsunami gauges to the cable system. The planned cable runs east and south from the end of the present cable. We plan to connect NEREID-1 at Site 1150 and NEREID-2 at Site 1151 using these spare optic fibers, thus supplying power to the NEREID observatories and enabling real-time transmission of signals from the NEREID sensors to land.

The deployed seafloor seismometers and tsunami gauges in pressure vessels are directly connected to the optic cable, but the NEREID system requires a branch cable from the main optic cable. Because a long branch cable causes loss of electronic power, a branch cable should be no longer than a few kilometers. The suggested cable route to connect the NEREID systems is shown in Figure F25. By deployment of a branching unit (BU) near the NEREID system, a maximum of 30 W can be supplied from the cable system. The present NEREID system requires ~18 W, so we may add more sensors on the seafloor to the present system.

The MEG of the NEREID system has a UMC to communicate with the NEREID system and to supply power. In the present system, the MEG is connected to the PAT which has SWBs and to the SAM through the ROV cable. We can supply power to the whole system and can control the system through the UMC on the MEG. Data from all sensors can also be retrieved through the UMC. Therefore, we can connect the underwater cable to the UMC on the MEG in place of the PAT. We do not need any changes of the MEG or the sensors to connect to the cable system. We do need, however, an optical/electric conversion unit (OEC) (Fig. F26) to connect the NEREID system to the cable system because the control of the system and retrieval of data are performed using an RS-232C line. An ROV can connect a bidirectional OEC and the MEG using an ROV cable with the UMC.

Of the 12 optic fibers in the cable, six are already used for deployed seismometers and tsunami gauges and four are reserved for future cable extension (Fig. F27). Two fibers are available for real time data transmission from the NEREID systems. The BU includes a branching circuit that divides the optic fiber. In addition, each circuit has a function to select signals for itself and transmit the signals to the NEREID system. For data transmission from the NEREID system, the branching circuit makes a multiplexed data transmission. An optic fiber with a large bandwidth can handle the multiplexed data transmissions of two NEREID systems.

Connection of the NEREID system to the cable system has many advantages: (1) data are distributed in real-time, (2) system settings can be changed at any time, (3) power is supplied continuously, and (4) there is no limitation of observation time. After confirmation of data quality from the present NEREID system, we will begin designing each unit of the cable system for connection between the NEREID system and the cable system. The data from the NEREID systems will be distributed via the Internet.