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 (VSP) 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 completed 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 without use of any additional ship time. These data await shore-based processing to develop a plan for future deployments of this technology. Regrettably, we were unable to complete the conventional VSP 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.