NERO SITE

After an 8-day transit, we arrived at Site 1107. The specific NERO site was selected based on seismic data collected in support of Leg 121, investigating the geology and paleoclimatic history of Ninetyeast Ridge. We selected Site 757 as our target, because, while meeting the criteria for emplacement of a borehole observatory in the Indian Ocean, it was also in our most direct line of transit between the hammer drill test site and our end of cruise port call in Darwin, Australia. Hole 1107A is located in 1650 m of water at 17º1.42'S, 88º10.85'E (Fig. F12). There was an ambitious program outlined in our prospectus, including establishing a borehole for future installation of a subseafloor observatory, conventional logging and VSP experiments, deployment of a test installation of the strainmeter module in preparation for Leg 186, and the NERO offset seismic experiment (NOSE) in conjunction with a concurrent German expedition called Seismic Investigations at the Ninetyeast Ridge Observatory Using Sonne and the JOIDES Resolution (SINUS) during Leg 179. We had 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 devoted to the hammer tests and NERO and reducing our time on location at NERO from 11 days to less than 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 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 assembly on the drill string to conduct our second SWD experiment. Based on Leg 121 statistics, we had hoped to drill to basement in ~12 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 previously reported in the Leg 121 Initial Reports volume (Pierce, Weissel, et al., 1989). Basement drilling also proceeded somewhat slower then we expected, although penetration rates were quite variable in the subaerially emplaced lava flows. At ~410 mbsf, we encountered a relatively hard layer, and ROP slowed to <2 m/hr. In light of the fact the 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 would not impede casing operations, we terminated deepening Hole 1107A because we had reached our target depth for casing of ~40 m into basement.

In our optimistic schedule developed after recognizing that we only had 5.5 days of operational time, we had hoped to set aside ~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 because of handling pipe in heavy seas, slowed ROP, and mechanical difficulties had pared >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, make 10 m of new hole below the casing string, and still have time remaining for a two-ship experiment. A 10-m penetration below the casing string was the absolute minimum envisioned as necessary for establishing 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 at Darwin. 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 had elected to use a tricone bit rather than a coring bit to ensure that 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 hours we had 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 (Fig. F13). We hope this depth will allow a successful installation of NERO. During drilling through the sediment column and into basement, we again collected SWD data via OBS and the shipboard accelerometer. Postcruise processing is required to interpret these data; however, our initial inspection of the data indicates this will be possible.