SUMMARY
The two primary objectives, which consisted of (1) testing the hammer-in casing system recently developed by ODP on the Atlantis Bank, Southwest Indian Ridge, and (2) drilling a cased reentry hole into basaltic basement on the Ninetyeast Ridge to allow future installation of an ocean floor geophysical observatory, the Ninetyeast Ridge Observatory, were accomplished during Leg 179. Because of delays during the leg caused by a late departure from Cape Town resulting from a lengthy repair to the ship’s guide horn, slower than expected transits, and an errant shipment of hammer drill supplies requiring a resupply effort at sea, certain ancillary objectives were not accomplished and one unexpected contingency drilling site was cored on the Atlantis Bank, Southwest Indian Ridge.
The hammer testing at Sites 1104 and 1106 on the Atlantis Bank supplied the engineers with the enough data to suggest that the hammer is viable for the purpose of initiating hard rock holes on gentle or steep bare rock slopes. At the same time, modification to the bit design, in particular the underreaming arms originally developed for use on land, will be required to accommodate the significant heave conditions the ship experiences during drilling. A conventional simplified hammer bit design without underreaming capabilities was also tested and performed admirably with the hammer penetrating 8 m in 2 hr with the same strong heave conditions. This indicated that the underreaming bit design and its response to significant heave is the most serious problem and not the hammer’s basic ability to spud and rapidly penetrate hard rock on the seafloor. In general, sea states were uncooperative during the entire hammer testing, and this inhibited completion of the tests and shortened their duration. A new generation of the active heave compensator may also help to improve bit operational tolerances. The ODP engineers are now quite convinced that the hammer system with a modified or alternate bit design will be the answer to the significant problem of spudding and maintaining stable holes in a range of bare rock environments and will be one of the most significant engineering developments for future hard rock drilling by the Ocean Drilling Program.
Contingency drilling at Site 1105 was highly successful and drilling penetrated 158 mbsf over a 6-day period. This resulted in 118 m of core from a large gabbroic massif exposed along the transverse ridge of the Atlantis II Transform. The site was ~1.3 km from the highly successful Hole 735B, which was cored to 1508 mbsf. The core included gabbro, olivine gabbro, and abundant oxide gabbro, and preliminary investigation indicates that part of the pseuodstratigraphy may be similar to that recovered in Hole 735B. We also acquired an extensive suite of logging data including high-quality FMS borehole image data. Because the core and logging data provide a comprehensive data set, cross-hole correlations between two high recovery offset holes (Holes 1105A and 735B) drilled into the plutonic foundations of the oceanic crust will be possible for the first time. The data will also allow us to conduct high-resolution structural and igneous studies and gain understanding into the geometry of igneous layering and ductile shear zones in the cumulate sequences sampled. The drilling of Hole 1105A demonstrated the versatility of the JOIDES Resolution in a hard rock environment in the face of unplanned contingencies and the general ability to easily establish quality holes with good recovery on the Atlantis platform.
The primary objective of the NERO portion of Leg 179 was to prepare a seafloor borehole for future establishment of Geophysical Ocean-Bottom Observatory (GOBO). This objective specifically included drilling a single hole as deep as possible into basement, as well as installing a reentry cone and casing beyond the sediment\basement interface to prepare this site for an ocean-bottom observatory. The GOBO will be installed at a later time by submersible or surface ship and will be part of the future network of seafloor observatories proposed in the ION program. Hole 1107A was drilled in a water depth of 1648 m on the Ninetyeast Ridge. The sediment cover above the basaltic basement was drilled to ~372 mbsf; however, the drilling rates suggested that firm basement was not reached until 404 mbsf. Drilling reached a depth of 493.8 mbsf which was sufficient to create a 9-7/8-in borehole below casing for the downhole seismometer installation. The acceptable region of the borehole in basement for emplacement of a seismometer is the lower 71.8 m. The total penetration into basement was 122 m, and total penetration below casing reached 79.4 m. The hole is regarded as a success for a number of reasons. These include the excellent stability of the hole, the fact that the hole was cased to 43.4 m into basement as planned, and because a significant depth of the borehole below the casing was achieved, which otherwise have might been a source of potential noise as experienced at other ocean-floor seismometer installations.
Lastly, Leg 179 accomplished basic testing of the SWD concept at Sites 1104, 1105, and 1107 that should allow the feasibility of the concept to be evaluated. Other supplementary objectives of the leg, including coring and logging of the NERO hole, the offset seismic experiment, a VSP, and a strainmeter test could not be achieved because of the unfortunate and considerable amount of operational time lost.