The establishment of the seafloor borehole seismological observatory (NEREID-191) was successful. An almost identical borehole observatory was set up at the Boso peninsula of Japan in August of 1998 as a prototype. The first (NEREID-I) and second (NEREID-II) seafloor borehole geophysical observatories were installed during Leg 186 on the inner wall of the Japan Trench in July and August of 1999, respectively (Sacks, Suyehiro, Acton, et al., 2000). Site 1179 is the third seismological observatory and is an important component of ION on the northwest Pacific abyssal seafloor. The design concept of the system was almost the same as those successfully installed systems of NEREID-I and -II. Although the borehole sensors are physically inaccessible, the seafloor components are designed to be replaceable by a remotely operated vehicle (ROV). The multiple-access expandable gateway (MEG-191) data controller can be replaced. The storage acquisition module (SAM-191) must be replaced as the data disks are filled up after ~1.5 yr of recording (72 GB). Batteries can be revived by replacing the magnesium anodes. The system status can be checked by a hardwire link to the SAM-191 using underwater mateable connector (UMC) cables. Site 1179 is located ~1600 km from Japan, and there are no nearby ocean cables to utilize for data recovery and power. It is our plan to connect our observatory to Japan via an acoustic satellite communication link with a surface buoy anchored above the site. Although the speed of our underwater acoustic communication link between the bottom and sea surface is 9600 bits per second (bps), the communication speed will be lowered to the 2400 bps of satellite data telemetry, which is available at present. Then the data can be disseminated in almost real time.
A major problem encountered during the seismometer installation was that logging data were not available in order to decide the depth of emplacement of the seismometers in Hole 1179E. Logging at Hole 1179D had to be terminated at a depth of ~300 mbsf owing to bridging of the hole (see "Downhole Measurements"). Basement rocks recovered in Hole 1179D consist mostly of fresh aphyric basalts in massive flows, pillows, and breccia with a minor amount of interpillow sediments (see "Igneous Petrology"). Rates of penetration at Hole 1179E were very consistent throughout the basement interval, averaging ~2.0 m/hr. Based on drilling data, Hole 1179E appeared to be in excellent condition for installation of the seismometers. Based on core descriptions from Hole 1179D and drilling data from Hole 1179E, it was decided to emplace the two seismometers into the basaltic basement at depths between 458.5 and 462.7 mbsf or between 87.5 and 91.7 m below the inferred top of the basaltic basement. Basement rocks at that depth were considered to be fresh aphyric basalts forming massive flows or pillows. From measurements of physical properties, average densities and velocities in the basalt section were 2.754 g/cm3 and 5002 m/s, respectively.
At 1445 hr on 21 August, installation of the seismic borehole observatory began in Hole 1179E, and at 1445 hr on 24 August, the J-tool was released from the riser/hanger after a long installation operation of 3 days. At that time, the seismological observatory was successfully installed at Hole 1179E. A timeline of instrument deployment operations is given in Table T20.
The borehole instrument assembly (BIA) (Fig. F68) was positioned between a 4.5-in casing and the stinger pipe (Fig. F69) and was then lowered down to the moonpool level using the 4.5-in casing elevators. Each of the two instrument cables were fed off of their respective reels, over sheaves (600 mm diameter × 84 mm groove width) hung below the rotary table, and connected to the two Guralp CMG-1T broadband seismometers (Fig. F70). After testing the seismometers through the instrument cables, the 4.5-in casing pipe (API-J55-STC, 10.5 lb/ft, and ~11.7 m length each) and cable deployment began at 1830 hr on 21 August.
The cables were attached to the casing pipe by tie wraps covered by duct tape (Fig. F71). Approximately every 1.5 m, a 4.5-in casing centralizer (9.625 in OD × 3 in height) was attached (Fig. F72). The 4.5-in casing was assembled at ~5 joints/hr. It took longer than normal drill-pipe handling because the "iron roughneck" could not be used.
The termination of the two instrument cables began at 0500 hr on 22 August and lasted for 17 hr. During the cable termination, it was discovered that an O-ring of one of the canister connections had been mislabeled and had to be replaced. This resulted in an additional 5-hr delay.
The MEG-191 was installed into the MEG frame on the riser/hanger, and the instrument string assembly associated with electrical checks was completed by 0130 hr on 23 August (Figs. F73, F74). Then the drill string was lowered, and Hole 1179E was reentered for the fourth and final time at 1115 hr (Figs. F75, F76A). The instrument package was lowered into the hole without incident, and, at 1500 hr, the riser/hanger landed at the correct depth (Fig. F76B).
The bottom of the hole was filled with cement having a density of 1.9 g/cm3 and a volume of 8.0 m3. Using theoretical hole volumes and displacements, the top of the cement slurry should have reached a level of ~112 m above the 10.75-in casing shoe. For the CMG-1T broadband seismometers, cementing provides the best coupling to the surrounding rocks compared to other methods that utilize mechanical arms, pads, or sand. Furthermore, cementing prevents water motion and temperature fluctuation from becoming sources of noise.
After successfully cementing the instruments, the battery frame (power access terminal [PAT]) was made up in the moonpool (Fig. F77). The PAT was hung at three points by three cables from the dual acoustic releases (Fig. F78). The drill pipe passes through the center of the PAT and the guide sleeve of the VIT frame. Upon release, three small glass sphere buoys attached to the three cables pull the cables up. Because the operation was complicated, involving many transfers of weight while the ship's heave was a few meters, the actual lowering began after sunrise at 0415 hr on 24 August (Fig. F79). The frame was lowered using the logging cable, allowing precise depth measurements and good heave compensation, and placed successfully on the reentry cone after a 4.75-hr trip. The PAT was released from the bridle by an acoustic command.
The VIT/subsea television system was deployed to survey the platform installation and observe the J-tool release from the riser/hanger. Proper platform installation was verified, and the J-tool was released. The installation of the seismic borehole observatory at Hole 1179E was successfully completed at this time and awaited the ROV Kaiko visit in November to start the system.
The BIA consists of two sets of three-component CMG-1T broadband seismometers (top: SN T1036 and bottom: SN T1037). A 3.2-m-long "stinger" pipe with one centralizer was attached to the BIA bottom. The length of the stinger was chosen to optimize safe reentry.
Each of the sensors was checked two times through the MEG-191 and the SAM-191 during the installation and was confirmed to perform perfectly.
The cable link between the hole bottom and the seafloor was to be supported by the 4.5-in casing pipes. This way, the cables can be protected, the installation depth is precisely predetermined, and the casing string does not heave inside the hole during the cementing because the hanger/riser is coupled to the reentry cone. The centralizers further protect the cables.
A total of 38 joints of casing pipe hangs the instruments. The cables were cut at the moonpool 2.3 m longer than required to connect to the MEG-191 bottom stab plate in case there is a need for retermination. The cable termination required six people working on two cables in parallel for ~17 hr.
The seafloor instruments were successfully emplaced. The components are the PAT power supply, the SAM-191 data recorder, and the MEG-191 to merge data and control the observatory. The SAM-191 recorder unit was not installed on the PAT frame, but the installation was completed during the ROV visit in October. During Leg 191, the MEG-191 was slid into its holder attached to the hanger/riser pipe connected to the two cables at its bottom side. On the top of the battery frame, an ROV dummy receptacle was installed instead of the SAM-191 to avoid plug contamination. We decided at the last moment not to attach a stopper that prevents unwanted vertical upward motion of the MEG-191, which may disconnect the system. The stopper was not attached, because it needs to be removed in order to unplug the MEG-191 from the riser/hanger, which added an extra task for the ROV.
Zinc anodes were attached to the PAT frame, which is made of steel, to prevent corrosion. The anodes were not attached to other components that were made of titanium. Stainless steel canisters containing the accumulator, power control system (PCS), and data logger (DL) of the seawater battery (SWB) assembly were painted with epoxy-based anticorrosive paint.
The PAT had four SWB units and can generate a total electric power of 24 W, according to the manufacturer's specifications. The observatory constantly consumes 9 W of electric power. It is estimated that the SWB assembly will have enough capacity to provide electric power to the observatory for 5 yr. After the 5-yr period, the four magnesium anodes can be replaced by an ROV.
The output end of the oil-filled four-conductor UMC cable from the SWB via the PCS is laid on the top of the PAT (Fig. F79). An ROV must disconnect the end of the cable from the parking position and connect it to the top of the MEG-191 after removing the dummy UMC receptacle. Upon inspection with the VIT camera, the relative positions of the MEG-191 and the UMC were found to be farthest apart, 180° around the center (Fig. F76C). This arrangement of the MEG-191 and the UMC will force an ROV to perform a time-consuming operation in order to make a connection between the PAT and the SWB. Table T21 depicts the tasks to be accomplished by the ROV.