As soon as the NEREID-191 system was installed in Hole 1179E, the SWB started charging the accumulator (see "Power Supply"). The downhole OBH sensors (see "Borehole Instruments"), however, did not start functioning until an ROV arrived at the site to activate the NEREID-191 system.
On 29 October 2000, Kaiko (Fig. F29), an ROV designed by the Japan Marine Science and Technology Center (JAMSTEC) to operate in water depths of up to 10,000 m, visited Site 1179 to activate the WP-2 observatory. Before the ROV dive, another SAM housed in a titanium sphere was deployed by free fall from the mother ship, Kairei. The ROV completed the following operations:
(See "Seafloor Instruments" for MEG and SAM information; see "Power Supply" for PAT and DL information.)
In the original plan, the SAM-191 unit was to be installed on the PAT at the time of the installation of the WP-2 observatory at Hole 1179E. During Leg 191, it was decided, however, not to lower the SAM-191 down to the seafloor with the PAT because there was a significant probability of poor sea conditions around Site 1179 in October 2000, when the ROV was scheduled to visit the site. Poor sea conditions might have prevented the ROV from diving to activate the NEREID-191 system during its planned visit to Site 1179. If the NEREID-191 system had been activated during Leg 191, the SAM-191 would have started to work and consumed its internal battery while the OBH sensors and the MEG-191 were inactive. To avoid the wasteful consumption of the battery, we decided to install the SAM-191 at the time of the ROV visit in October 2000.
After the SAM-191 is installed on the PAT, the NEREID-191 system activates as soon as the electrical connection between the MEG-191 and the PAT is made using the UMCs. Power is supplied to the OBH sensors, the MEG-191, and the SAM-191 from the accumulator. Once this connection is made, data from the OBH sensors start to flow into the SAM-191 on the PAT, which can store 72 GB of data, sufficient to record 590 days of observations.
The ROV can check the status of the system via a SAM/ROV interface that allows RS232C communication between the SAM-191 and the ROV. If there are no problems, the ROV needs only to swap the SAM-191 at least once every 1.5 yr for no loss of data continuity.
The SWB battery life is somewhat unpredictable because it is strongly dependent on such factors as bottom-water currents and dissolved oxygen. The NEREID-191 system at Site 1179, which is different from the previous ones installed at Sites 1150 and 1151 (Sacks, Suyehiro, Acton, et al., 2000), has PCS and DL (see "Power Supply") units that can monitor and record the voltage and the current provided from the SWB to the system. The pressure vessel of the DL unit was set on top of the PAT. We were able to retrieve the DL unit with an ROV. Power-supply data recorded in the DL give us important information of the exact battery life of the SWB at Site 1179.
Because the performance of power generation of the SWB could not be assessed until recovery of the DL, we prepared another SAM that has lithium battery cells during the first dive of the Kaiko and decided to connect the SAM with lithium batteries to the MEG-191 for the first observation period in 2000. The SAM is housed in a titanium sphere with five lithium battery cells. The lithium batteries have a total capacity of 1300 Ah and supply the power to the whole system of the WP-2 observatory. A UMC is mounted on top of the titanium sphere to make a connection directly to the MEG-191.
The seafloor downhole observatory is still at the pilot study stage. There is no routine setup that one can rely on. In our design, the OBH sensor string is unrecoverable for the reasons given in "Installation Techniques". This necessitates that the string be composed of highly reliable instruments (see "Borehole Instruments"). On the other hand, the seafloor components are virtually all replaceable and serviceable by an ROV. The SAM-191 or the titanium-sphere SAM is replaced at each ROV visit. The MEG-191 can be pulled out of its seating frame and reinserted by an ROV, although the operation is more complex than other tasks.