LEG OBJECTIVES
There were two primary and several ancillary objectives outlined in the scientific prospectus for Leg 179. The two primary objectives can be summarized as (1) engineering tests of the hammer drill-in casing system, and (2) establishing a borehole, cased and cemented to basement and deepened as far as possible for future deployment of an ocean-bottom observatory.
Supplementary objectives included deepening a hole through a hammered-in casing, a proof-of-concept test for acquiring seismic data while drilling, and a conventional vertical seismic profile to complement an offset seismic profile experiment in conjunction with the Sonne. We had also hoped to attempt deployment of a test borehole strainmeter in preparation for Leg 186.
As a result of an extended delay in port because of ship repairs and loss of equipment in shipment, 17 of the 26 operational days scheduled for primary and ancillary objectives were lost. Although 2 additional days were added early in the leg to ensure completion of as many primary and ancillary objectives as possible, both of these days were lost to longer than expected transits. Consequently, in terms of primary objectives, the hammer test was incomplete as envisioned in the scientific prospectus, but still a success in that a detailed evaluation of the tests we were able to manage will result in modifications to various components of the system. As with most engineering endeavors, our test of the hammer drill-in casing system was a proof-of-concept experiment, and based on the results of these tests we remain confident in the viability of the system. In contrast, despite the abbreviated operations schedule, our second primary objective, establishing a borehole for the ION program, was wholly a success.
One consequence of the reduction in drilling time was less success in achieving supplementary objectives. We were, however, able to collect seismic-while-drilling (SWD) data at both the hammer drill test site and at NERO. These data await shore-based processing to develop a plan for future deployments of this technology. Regrettably, we were unable to complete the conventional vertical seismic profile experiment, the two-ship offset seismic experiment, logging and coring of the NERO hole, or the test deployment on the borehole seismometer. An unanticipated but overwhelmingly successful additional contingency program resulted from the delay we suffered waiting on a ship-to-ship transfer of hammer drilling supplies. We chose to invest this time in bare rock drilling at the hammer drill test site. What resulted was superb recovery from 158 m penetration in the gabbroic massif at the Atlantis II platform at a distance far enough from previous drilling to potentially allow correlation on a scale yet unfathomed in marine research. Additionally, a full suite of downhole logs was collected, which will surely aid in that endeavor.
Hammer Drill-in Casing Test Results
Hammer drill testing was carried out on the Atlantis II Bank, located east of the Atlantis II Fracture Zone which offsets the Southwest Indian Ridge between latitude 31°50'S and longitude 33°40'S at 57°E (Fig. 1). The platform is a flat-topped bench, ~9 km long and 4 km wide. This massif was successfully cored Legs 118 and 176 to a depth in excess of 1500 mbsf. This location was chosen primarily as an area of opportunity, to coincide with a point in the hammer drill development when sea trials were in order. As an additional benefit, the shallow water was envisioned to facilitate efficiency in our early operations. The flanks of the massif offered additional targets at greater depth and with topographic slopes, should the engineering tests advance beyond the initial objectives.
Site 1104
Site 1104 is located at a water depth of 731 m on the east rim of the Atlantis II Fracture Zone, ~200 m northwest of Hole 735B (latitude 32°43.32'S, longitude 57°15.85'E; Fig. 2). This site was selected based on a video survey beginning at Hole 735B, by which we sought to find a reasonably flat, large outcrop to initiate the first spud tests of the hammer drill. During the transit from Cape Town, the water hammer was successfully deck-tested. The initial assembly of the drill string including just the SDS hammer and a concentric arm bit to test the spudding capability of the system. A frequency analyzer for monitoring hammer operations was built and installed during the transit.
After a 4-hr video survey starting at the Hole 735B guide base, we selected a location with extensive, relatively flat-appearing outcrop and set the bit down on the outcrop to see how the hammer functioned without rotation. Several spud tests indicated the hammer was performing as expected, so we decided to pull the vibration-isolated television (VIT) frame and begin hammer drilling Hole 1104A. After ~45 min, it appeared from rig floor observation that we had made ~1.5 m of penetration, so we ran in the VIT to inspect the hole. We had also noted excessive vibration of the stand pipe and derrick during hammer operations. A clean, circular hole was apparent on the video image, so we pulled the camera and initiated a second test hole (Hole 1104B). After a couple of hours rotation, we had made ~2 m of penetration but also noted increasing torque and slower rate of penetration (ROP). Another camera trip revealed a second clean, circular hole, but some apparent obliquity indicated the hole had been initiated on a small local slope. We pulled the camera and attempted to spud a third hole, but the hammer would not fire, so it was pulled to the surface.
Inspection of the concentric arm bit indicated the reaming arms were damaged, and a valve had cracked in the hammer. The hammer was rebuilt, a new concentric arm bit installed, and we ran the assembly back to the seafloor. After a short video survey to inspect the site, we pulled the camera and spudded Hole 1104C. In less than two hours, although we noted ~2 m of penetration, there was also indication of high erratic torque and ROP effectively ceased. We attempted to initiate another hole (Hole 1104D) but made no advancement and experienced high and erratic torque, so this hammer test was terminated and we pulled the drill string. During the pipe trip we deployed the two United States Geological Survey (USGS) oceanbottom seismometers (OBS), 100 and 300 m, respectively, from our drill site to monitor the noise transmitted through the outcrop which was generated by the hammer.
When inspection of the second concentric arm bit indicated once again that the underreamer arms had experienced excessive wear, a bit was modified by trimming the concentric arms to match the outside diameter of the pilot bit. After this modification, however, the bit did not appear robust enough to cut through the hard rock so this modification was abandoned. A second modification removed the concentric arms, cut the bit shank, and welded the interval where the arms had been closed. We had hoped to test the drilling capability of the bit without the added challenge of attempting to ream out the hole. Unfortunately, during the modification of the bit, a crack developed and the bit was set aside. We then modified a third bit by welding the concentric arms closed. This bit was tripped to the seafloor and we initiated Hole 1104E. After about an hour we had made ~1.5 m of penetration, but the bit stuck in the hole. We were able to free the bit with left-hand rotation, indicating that the arms had broken free and were causing the bit to stick. Having exhausted all the bits we had on board for hammer testing, and with the promise of delivery of a different bit design in a few days from a supply vessel, we chose to commit to conventional rotary coring while we waited for the equipment transfer. The OBS and positioning beacons were recovered, thus ending operations at Site 1104.
Site 1106
After delivery of the new bits from the supply vessel, we returned to the location of Site 1104 in anticipation of continued hammer testing. Despite the same coordinates as Site 1104 (latitude 32°43.32'S, longitude 57°15.85'E, Fig. 2), because a new beacon was deployed and we wanted the record to indicate the next phase of hammer testing, this location was assigned as Site 1106. The hammer was tested on the deck in preparation for deployment, but the continued deterioration of sea state prevented any further transfer from the supply vessel.
The hammer was run, and after a brief seafloor survey, Hole 1106A was initiated. After ~2 m of penetration, the hammer ceased activity, and we tripped it back to the surface. Once again a valve had broken in the hammer, potentially because of excessive heave during the continuing poor sea state. Hole 1106B was initiated on the ensuing pipe trip, which included the second of the three bits that we acquired on the transfer (coincidentally the last bit capable of drilling an overgauge hole) because the first bit was worn after Hole 1106A. Only about one-half a meter of penetration was realized before the hammer ceased activity again, necessitating another pipe trip. Again the bit was worn, so it was replaced with the last of the bit configurations we had available, a flat-faced drilling bit. Our decision to run this bit was based on the assumption that if we could demonstrate the ability to make a hole, we could use this information in future bit design.
Hole 1106C was spudded and we drilled ~1 m in less than an hour before the hammer stalled again. On the ensuing pipe trip, the piston in the hammer was replaced and the flat-faced bit was run back to the seafloor. Hole 1106D was attempted, but the hammer would not start, so it was pulled and rebuilt once again. Hole 1106E was initiated, and the hammer drill system performed admirably, cutting an 8-m-deep hole in less than 2 hr. The pressure transducer on the stand pipe chose this time to give way, so the pumps had to be shut down for repairs. Once the repair was completed, the driller noted no pressure buildup and was able to slowly lower the drill string to the total depth (TD) of the hole + 4 m, indicating that we had lost some of the bottom-hole assembly. The subsequent VIT trip indicated that the bit and hammer were indeed missing, and because we could not see them on the seafloor, we assume they are still in Hole 1106E. Weather conditions had still not improved, and we did not have a clear idea which of the several holes within a few meters radius was Hole 1106E, so a fishing attempt was unrealistic. Given that we had exhausted all the bits and hammer spare parts, we declared the hammer test for Leg 179 complete, and got under way for Ninetyeast Ridge.
In summary, although a detailed summation of all the data relevant to the hammer testing awaits postcruise development, we do have some preliminary impressions. We are encouraged by the performance of the hammer and will be able to use this series of tests for optimal design improvements. Despite the less than desired performance of the bits, again we are optimistic, particularly based on the last test where we made 8 m of penetration in less than 2 hr, that bit design improvements will yield improved performance in the future. Finally, as with all our operations, sea state appears to be a primary control if not on the success of an operation, at least on its duration and ease of completion.
Site 1105
Because of delays in the resupply ship that inhibited resumption of hammer tests near Hole 735B, Hole 1105A was drilled on Leg 179 for a period of 6 days. The hole was located ~1.3 km east-northeast of Site 735 on the Atlantis platform along the eastern transverse ridge of the Atlantis II Transform (latitude 32°43.13'S, longitude 57°16.65'E; Fig. 2). The site is along a ridge-axial trend with respect to Hole 735B, but more distal from the north-south trending Atlantis II Transform that lies to the west. The site was chosen to avoid a duplication of Hole 735B efforts that might occur by drilling at proximal Site 1104. At the same time, we wanted to utilize Hole 735B as a reference section to attempt lateral correlation of large-scale igneous units, structural features, and geophysical characteristics over the broader distance represented by the offset in holes in the direction approximately parallel to the former ridge axis. In addition, the site was chosen to help constrain the overall structure of the massif exposed on the platform. If successful, the correlation experiment could yield a minimum measure of the dimensions of subaxial magma chambers and continuity of structure and processes along strike of the ridge axis at a very slow-spreading center. If correlations are unsuccessful we can limit the dimensions of igneous units, former magma chambers, or structures to be smaller than the scale of the experiment. Correlation will be attempted on the basis of detailed and integrated data sets including core descriptions and subsequent shore-based laboratory analyses to establish cryptic chemical and mineralogical variations, and the alteration and structural framework in the core. A full and highly successful logging program that was completed after the cessation of drilling will aid in the correlation attempts.
The hole penetrated to a depth of 158 m, and the cored interval measured 143 m, starting 15 m below the seafloor. Core recovery included 118.43 m of gabbroic rock for a total recovery of 82.8%. Together with logging results, this recovery provides complete coverage of the rock types and a comprehensive view of pseudostratigraphy in the gabbroic section cored (Fig. 3). Shipboard results now indicate a high probability that specific units, structures, and/or geophysical characteristics from Holes 735B and 1105A may indeed be correlated.
The cores recovered record a wide variety of rock types ranging from gabbro (Fig. 4), oxide gabbro with up to 20–25 modal percent Fe-Ti oxides (Fig. 5), and olivine gabbro (Fig. 6) to scarcer troctolitic gabbro, gabbronorite, and felsic rocks such as trondjhemite. Described within the core are 141 intervals that have been defined on the basis of distinct changes in mode, modal proportions, grain size, and/or texture. Well-defined igneous layer contacts or structural boundaries to these intervals are preserved in many sections of the core (Fig. 7). The highly layered nature of the gabbroic rocks documented within the core is supported by high-quality continuous Formation MicroScanner (FMS) logs of the borehole (Fig. 8), as well as other logs and whole-core magnetic susceptibility measurements (Fig. 9). The scale of the layering in the core varies from a few centimeters to meters. On a broader scale, the intervals define four basic units from top to bottom consisting of (1) a gabbroic unit characterized by more primitive rock types and by a scarcity or lack of oxide gabbro, (2) a gabbroic unit characterized by a high abundance of oxide gabbro and oxide-bearing gabbro, (3) a gabbroic unit characterized more primitive rock types and a lack of oxide gabbros, and finally (4) another unit rich in oxide gabbro and oxide-bearing gabbro. Rocks are crosscut by millimeter- to decimeter-sized veins of leucocratic gabbro, quartz diorite, trondhjemite, and irregular pegmatitic gabbro intrusions. Irregular veins and bands of oxide minerals have also been observed.
Thin sections indicate typical cumulate textures in the majority of samples that range from adcumulate to orthocumulate and show variable amounts of core-to-rim zoning in plagioclase. Poikilitic textures are also common with pyroxene as the oikocryst phase and plagioclase as the chadocryst phase (Fig. 10). Igneous laminations were observed in several samples but are generally scarce or may be overprinted by crystal-plastic fabrics. Preliminary bulk rock geochemical results show a wide range in the chemistry of gabbroic rocks with Mg numbers varying from ~0.80–0.23, Fe2O3 from ~3.5–24.0 wt%, P2O5 from ~0.01–4.1 wt%, Y from 7–192 ppm, Nb from 1–10 ppm, and Cr from 1–1066 ppm.
Alteration of the primary igneous mineralogy in the core is generally low, but varies on the scale of a thin section to meters. Alteration of olivine to chlorite, tremolite-actinolite, and talc is the most common manifestation of alteration, whereas plagioclase and clinopyroxene tend to be less altered. It is common for clinopyroxene, where altered, to be partially replaced by patchy brown amphibole, but alteration generally does not exceed 1%–2%. A portion of this brown amphibole is likely to be of magmatic origin. Where alteration is extensive, clinopyroxene is replaced by assemblages of actinolite and chlorite. Plagioclase is generally fresh. Actinolite and chlorite are also the most common vein assemblages, although scarce high-temperature brown amphibole and low-temperature smectite and carbonate veins have also been observed.
The structure of the core is complex, and structural styles and intensities range from brittle to ductile. Most of the gabbroic samples cored possess igneous textures, but there are several parts of the core that display crystal-plastic fabrics. Mylonitic zones characterized by high oxide-mineral content were observed at ~53 and 71 mbsf (Fig. 11). Coarser grained centimeter- to decimeter-thick zones of ductile shear are present in the upper 90 m of core, whereas thicker zones of ductile deformation with weak to strong crystal-plastic fabrics become more prevalent at depths in excess of 90 mbsf. Intervals of penetrative ductile deformation in the lower portion of the core locally exceed 2 m in thickness. Zones of ductile deformation are commonly oxide rich, as are the contact regions between undeformed and ductily deformed rocks. Oxide-gabbro rich zones tend to be strain localizers because many, but not all, of the crystal-plastic shear zones are rich in oxide minerals. Inclination of the ductile foliations vary from ~18° to 75° in the cored intervals and averages ~30°–35°. Thin sections show a range of textures from strictly igneous to slightly deformed igneous to dynamically recrystallized metamorphic textures with crystal-plastic fabrics. As deformation intensity increases, the effect can be most easily observed in plagioclase, where a progression from strain-free plagioclase to plagioclase with deformation twins, undulose extinction, kink bands, and dynamic recrystallization to neoblasts along grain margins progresses to porphyroclastic textures with small neoblasts of plagioclase and highly strained and kinked plagioclase, pyroxene, or olivine porphyroclasts. Olivine appears to have recrystallized to neoblast grain sizes because pyroxene, which tends to be preserved as the dominant porphyroclastic phase unless the intensity of deformation is most severe. Brittle fractures are generally filled with vein material such as actinolite and chlorite, but no large faults zones were recovered in the core. There were several regions of low recovery that could correspond to fault zones based on temperature, sonic, resistivity, and porosity logs. These regions of poor recovery generally sampled little intact core, although gabbroic rocks that were recovered were altered to smectite and contained carbonate veins.
Preliminary analysis of the downhole geophysical measurements from core and logging data yields a wide variety of information. Magnetic data indicate that the core possesses a single coherent magnetic direction with an average inclination of ~67°. This is compared with an inclination of –52° expected for the site. As in Hole 735B, these results indicate a consistently reversed polarity for the section and may indicate a significant block rotation of the massif similar in magnitude to rotations interpreted from Hole 735B (15°–20°). The consistency of the magnetic inclination downhole suggests that any relative rotations along ductile shear zone in the section must have occurred before cooling below the blocking temperature and are necessarily high temperature in nature. Magnetic susceptibility measurements clearly define zones of oxide gabbro and oxide-bearing gabbro documented in the core. Likewise, it provides a direct downhole comparison for the FMS logs, which measure resistivity. Oxide-rich zones are conductive whereas oxide-free zones have high resistivity. Magnetic intensity on split cores ranges from ~0.2–5 A/m.
Lastly, an SWD experiment was conducted at Hole 1105A using two USGS OBS. These data, together with accelerometer data from the drill rig, will be employed to test the feasibility of SWD during drilling operations of the JOIDES Resolution.
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 17°1.42'S latitude, 88°10.85'E longitude (Fig. 12). There was an ambitious program outlined in our prospectus, including establishing a borehole for future installation of a subseafloor observatory, conventional logging and vertical seismic profile 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 the continuing expedition of the Seismic Investigation at Ninetyeast Ridge using the 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 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 14-3/4-in tricone bit to drill a large borehole to allow deployment of 10-3/4-in casing some 30–40 m into basement. We also deployed the ocean-bottom seismometers 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 hours, 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 as were 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 less than 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 hours 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 more than 25 hr from our already drastically reduced timetable.
By the time our last casing operation was completed (10-3/4-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 few 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. 13). We hope this depth will allow a successful installation of the Ninetyeast Ridge Observatory. 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.