-in
casing pipes were used.Site 1151 is ~48 km south of Site 1150 along the direction of the Japan Trench axis in a seismically inactive zone (Fig. F41). The contrast in seismic activity is the main reason for establishing a second borehole observatory for comparison at this spatial separation. Because many aspects and features of the two observatories are identical, we will describe the chief differences between Site 1151 and the Site 1150 observatory and avoid repetition of "Borehole Instruments Status" in the "Site 1150" chapter.
More than 1000 global positioning systems stations are distributed on Japanese islands, but there are no geodetic observatories offshore. The two observatories of strain, tilt, and seismic waves are expected to reveal the previously unknown nature of plate coupling at the seismogenic zone.
The installation of a geophysical observatory at Hole 1151B proceeded as shown in Table T19. Based on core recovery and lab measurements from Hole 1151A, we decided to emplace the strainmeter at 1097 mbsf (Fig. F41), where the core recovery was good in a relatively constant, higher density (~1.9 g/cm3) and P-wave velocity (~2000 m/s) section. The target depth is slightly shallower, but physical properties data suggest a more competent rock environment than at Site 1151.
Based on the first
experience, some changes were made to expedite the operation, which are detailed
in "Operations". The
strapping of the cables proceeded much more quickly than at Site 1150. Only one
centralizer was attached at every joint except at critical sections. Seas were
calmer than during the first installation, which prompted this decision. To
reach the target depth, 93 joints of 4-in
casing pipes were used.
The four cables were terminated in the subsea shop as before. Connectors were attached to cables cut 30 cm longer than the required lengths. This extra length is half of that used at Site 1150. Less excess length makes it easier to strap the cable onto the hanger below the MEG. All cable sections must maintain full contact with the pipe wall. ODP engineers and borehole researchers worked overnight to complete the terminations.
A final check of the MEG module connected to the cables via the underwater mateable connectors indicated that the power-up test failed on the CMG-1T ocean borehole seismometer (OBHS). Other sensors were functional; therefore, the MEG was retrieved from the hanger and checked in the downhole lab. The defective part of the power switching circuit distributing power to the OBHS was modified so that this problem will not recur. A surge protector was added to the circuit to damp large transient voltages. After about 3 hr the MEG was reseated in the frame attached to the hanger. After a final successful check, the casing hanger was lowered through the moonpool. The moonpool door opening had to be adjusted several times for the casing hanger to pass through safely.
Because we encountered
difficulty in lowering the instrument at Site 1150, some modifications were made
to the drilling plan. The open hole was shortened to 43 m and the upper part of
the hole was reamed with the 14
-in
bit before setting the 10-in
casing. Because it is critical for the strainmeter to have a good bonding to the
surrounding rock, the bottom portion was not reamed. The instrument string was
successfully inserted on the first attempt.
The bottom of the hole was filled with cement with a density of 1.9 g/cm3 and a volume of 12.7 m3 without displacing with mud beforehand. That volume will fill to above the open-hole section into the cased-hole section.
After successfully cementing the instruments, the power/access terminal (PAT) battery frame was made up in the moonpool. The assembly is the same as before, but improvements were made to take fewer steps to start lowering the PAT hung by the logging line. There was no time loss due to waiting for daylight. The ship's heave was smaller than before, at ~0.5 or less m.
The relative position of the MEG top connector plug to the remotely operated vehicle connector parking position on the PAT was similar but slightly nearer than at Site 1150. The disconnection of the J-tool to decouple the drill string went smoothly.
The array of emplaced instruments from the bottom to the top of Hole 1151B consists of the volumetric strainmeter, three-component broadband PMD-2123 seismometer sensor, two-component tiltmeter, and three-component CMG seismometer sensor (T1020) (see "Borehole Instruments" in the "Borehole Instrument Package" chapter). A 3-m-long stinger pipe with centralizers was attached to the strainmeter bottom. We had a spare unit for each sensor but all functioned well.
All the components were successfully emplaced. These are the power supply, data recorder, and the MEG to merge and digitize data and control the observatory (see "Seafloor Instruments" in the "Borehole Instrument Package" chapter). The MEG at this site has two fewer sets of 24-bit digitizer boards than those at Site 1150. This is because the strainmeter at this site has just one component.
The second seafloor borehole geophysical observatory, 'Neath Seafloor Equipment for Recording Earth's Internal Deformation (NEREID-2), was successfully installed. The vision of establishing borehole observatories beneath the seafloor has become a reality. Though only a first step, the two observatories installed during Leg 186 and the data that will obtained from them are expected to accelerate efforts for establishing similar observatories worldwide.
