Leg 179 represents an endeavor aimed at two long-standing gaps in our understanding of the nature of the solid earth. First, we set out to test a new drilling technology that might enable deeper drilling and eventually higher core recovery than ever before possible from the deep ocean crust. Our second goal was to prepare a site where researchers can establish a long-term geophysical ocean-bottom observatory (GOBO) as part of the International Ocean Network (ION) program. Both of these primary objectives were accomplished. Initial tests of the hammer drill-in casing system yielded data that will allow future development and implementation of this technology on a regular basis. A nearly 500-m-deep borehole-cased and cemented into the hard-rock basement, and left with a reentry cone-was established at the Ninetyeast Ridge. Installation of a downhole observatory at this location will fill one of the six major gaps in global seismic monitoring coverage. Given potential delays in port because of repairs to the drillship, the advisory structure of ODP prioritized the supplementary objectives for Leg 179. The highest priority for these objectives was given to the two-ship offset seismic experiment in coordination with the Sonne. Unfortunately, time constraints prevented completion of this supplementary objective.
Drilling the igneous foundation of the ocean crust has always been a challenging undertaking. Since the inception of the Deep Sea Drilling Project and its successor, the Ocean Drilling Program (ODP), one of the principal objectives of the science community has been to penetrate an entire section of the ocean crust and to reach the boundary between the Earth's crust and mantle. Attempts at accomplishing this objective have been limited by drilling technology that was originally designed for recovering sediments by the oil industry but adapted by ODP for sampling igneous basement. There have been spectacular successes in this endeavor, witness Holes 504B and 735B, but challenges confronted in establishing, maintaining, and reentering boreholes in fractured, hard rock have been more common.
In response to these challenges, ODP has embarked on the development of a new technology, the hammer drill-in casing system, which will allow us to initiate a hole, then simultaneously deepen that hole and stabilize its walls with casing. This system is an adaptation of pneumatically driven drilling systems that have successfully drilled in environments not unlike those that present our greatest challenge. Owing to the water depths in which we operate, however, pneumatic power is not an option. This innovative design employs a hydraulically actuated hammer, which drives a drill bit into the ocean floor. Following the bit is a string of casing to stabilize the borehole walls and improve our ability to clean drill cuttings from the hole. The bit design allows it to be withdrawn through the casing system, after deployment of a reentry funnel, such that most of the documented problems associated with drilling hard rock in the ocean's basin are alleviated. This new technology not only solves the technological problems, but it also reduces our dependency on site specific surveys, because the hammer can initiate a hole without regard to local topographic variability, thin sediment cover, debris, or rubble lying on the surface.
HDS testing was undertaken adjacent to the Atlantis II Fracture Zone along the Southwest Indian Ridge (SWIR) on an uplifted platform where two other ODP legs had successfully cored in hard rock using conventional drilling technology (see Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999). We hoped that the shallow but variable water depth and locally flat but regionally rugged topography would both adequately emulate other environments where the HDS might be employed and test the limits of the system. By choosing this location, we also had a proven record of our best performance in hard-rock penetration rate and recovery for comparison.
Geophysical observatories currently operating worldwide share a common attribute and shortcoming in that these stations are only emplaced on continents or islands. Inasmuch as the world's oceans cover more than two-thirds of the planet's surface, the sporadic coverage allowed by observatories on oceanic islands is woefully incomplete. There are six major gaps in global seismic coverage, defined by vast expanses of sea without a land surface on which to establish an observatory. During Leg 179, ODP engaged in preparation of the first deep ocean global seismic observatory in one of these gaps, along the Ninetyeast Ridge in the east Indian Ocean.
Over the past decade, our understanding of deep Earth processes has improved owing to the development of new generations of global monitoring networks. Although the quantity and quality of data have radically increased, these new data have revealed large departures from lateral homogeneity at every level within the Earth from surface to core. Additionally, absolute plate motions cannot be accurately determined without precise geodetic measurements that are conventionally monitored on land. Extrapolating the Earth's magnetic field to the core/mantle boundary is challenged by gaps in coverage, particularly in the Indian and Pacific Oceans. Images of the velocity heterogeneity of the interior of the Earth, related to thermal and chemical convection, are aliased by the lack of control in observation sites. As the technology to deploy observatories to monitor these types of phenomena is under development, a borehole cased and firmly attached to basement at the Ninetyeast Ridge will provide an ideal deployment structure.