SITE 1107

Hole 1107A

Position: 17º01.4180'S, 88º10.8497'E

Start hole: 2024 hr, 22 May 1998

End hole: 0730 hr, 28 May 1998

Time on hole: 131.10 hr

Seafloor (drill-pipe measurement from rig floor, mbrf): 1659.0

Distance between rig floor and sea level (m): 11.3

Water depth (drill-pipe measurement from sea level, m): 1647.7

Total depth (drill-pipe measurement from rig floor, mbrf): 2152.80

Penetration (mbsf): 493.80

Principal Results

After an 8-day transit we arrived on Site 1107. There was an ambitious program outlined in our prospectus, including the establishment of a borehole for future installation of a subseafloor observatory, a conventional logging and vertical seismic profile experiment, deployment of a test installation of the strainmeter module in preparation for Leg 186, and the Ninetyeast Ridge Observatory (NERO) offset seismic experiment (NOSE) in conjunction with the continuing expedition of the Seismic Investigation at Ninetyeast Ridge Using Sonne and JOIDES Resolution (SINUS). We originally scheduled 11 days to complete these objectives. However, our extended port call, lost shipment, and extended transit times all worked to shorten our operational schedule, paring away 17 of our original 26 days of total operational time. This reduced our time on location at NERO from 11 days to <6 days. Our optimistic estimate indicated that even given this radical reduction, if all went extraordinarily well, we could still complete the borehole (although to a significantly less depth of penetration than our original target of 100-200 m into basement) and have some time remaining for the two-ship experiment. Our restricted schedule, however, required that we allocate no time for the many other operations we had hoped to complete at NERO.

After we arrived on site, we deployed a beacon and ran to seafloor with 48.82 m of 16-in casing fixed to a reentry cone. This assembly was washed in, and subsequently we reentered the hole with a 14-in tricone bit to drill a large borehole to allow deployment of 10-in casing some 30-40 m into basement. We also deployed the OBS and installed the Lamont-Doherty Earth Observatory sensor sub on the drill string to conduct our second SWD experiment. Based on Leg 121 statistics, we had hoped to drill to basement in 12 or so hr, and to drill at least 30-40 m into basement over the next 10 hr. Drilling the sediment column took longer than we expected, probably because of the size of the hole we were drilling and resistive layers of volcanic breccia and tuff overlying basement. Basement drilling also proceeded somewhat slower then we expected, although penetration rates were quite variable in the subaerially emplaced lava flows. At about 410 mbsf, we encountered a relatively hard layer, and the ROP slowed to <2 m/hr. In light of the fact that drilling in basement had, up to this point, proceeded reasonably quickly, we envisioned this hard layer as an ideal position to anchor the bottom of the casing with cement. After drilling to 422 mbsf to ensure that any material wiped off the walls of the borehole during emplacement of the casing would have a place to go and not impede casing operations, we terminated deepening Hole 1107A as we had reached our target depth for casing of ~40 m into basement.

In our optimistic schedule, developed after recognizing we only had 5.5 days of operational time, we hoped to set aside about 48 hr of ship time for the two-ship experiment. This time included pipe trips, set up and rig down time, and preparation to get under way (as this was to be our last operation), which resulted in an estimated 29 hr of shooting time for the two-ship experiment. Any additional time was to be allocated to deepening the hole. At this point in our operations, however, individually minor but collectively significant delays caused by handling pipe in heavy seas, slowed ROP, and mechanical difficulties had pared more than 25 hr from our already drastically reduced timetable.

By the time our last casing operation was completed (10-in casing set to 414 mbsf), we recognized there would not be sufficient time to clean out the cement shoe in the bottom of the casing, drill through the cement, clean out the rathole underneath, and make 10 m of new hole below the casing string. This was the absolute minimum envisioned as necessary to establish a borehole for the observatory emplacement. In our estimation, completing the borehole and allowing time for even a short two-ship experiment would have resulted in a 24-hr delay in our arrival in Darwin, Australia. This was not possible given the program's tight operational schedule and that the leg had already been extended 2 days beyond the original schedule.

Even with the disappointment we all felt regarding cancellation of the two-ship experiment, we recognized that although we did not have sufficient time to prepare for and rig down after a two-ship experiment (at least 20-24 hr), because we already had a drilling bit in the bottom of the hole, we did have enough operational time to deepen the borehole. We elected to use a tricone bit, rather than a coring bit, to ensure we could penetrate through the casing shoe without delay. This bit, although not allowing coring of the material drilled, did allow rapid penetration through the formation in the few hours remaining. We continued drilling to a depth of 493.8 mbsf, which is just over 120 m into basement and almost 80 m below the casing shoe. This depth should be more than adequate to allow a successful installation of the Ninetyeast Ridge Observatory. Postcruise processing is required to interpret the data collected during our SWD experiment; however, our initial inspection of the data indicates this will be possible.