Leg 179 Scientific Prospectus

ABSTRACT

Leg 179 is a two-part drilling program; first we will test the hammer drill-in casing system recently developed under the direction of the Ocean Drilling Program (ODP), then we will drill and case a hole for the Ninetyeast Ridge Observatory (NERO) project. The sea trials for the hammer drill-in casing system will take place near ODP Site 735, in the rift mountains of the Southwest Indian Ridge. We will evaluate the operational characteristics of the components of the hammer drill-in casing system, as well as the complete system. This test will also address operational limits of the system in terms of water depth, topography, and surface slope. Time permitting, we will recover cores through the cased reentry hole, to as great a depth as possible. Fifteen days on site have been allocated for these tests. Successful completion of the NERO project will fill a major gap in the global coverage of seismic, magnetic, and general geophysical monitoring. Currently, geophysical observatories are only present on continents and islands; thus, data collection is incomplete. Establishing a cased reentry hole into basement at ODP Site 757 will be the first step toward the installation of a Geophysical Ocean Bottom Observatory. This observatory will be part of the future network of seafloor observatories proposed in the International Ocean Network (ION) program for studying global geodynamics and earthquakes. A borehole observatory at Ninetyeast Ridge will enhance investigation of the dynamics of the Indian plate. This plate has a complex geologic history characterized by high intraplate seismicity and may contain a diffuse plate boundary between the Central Indian Ridge and the Indonesian Arc. A seismometer will be installed in the hole at a later date. Several additional seismic experiments are planned for the NERO site during and after drilling operations. These include (1) a vertical seismic profile, (2) a seismic-while-drilling experiment, 3) and a two-ship offset seismic experiment (OSE) using the German research vessel Sonne, which will shoot several seismic profiles of varying azimuth and along circles with different radii about the hole. Shots in the OSE will be recorded by both USGS and GEOMAR ocean bottom seismometers (OBS) situated about the hole and a three-component borehole geophone. This unique set of seismic experiments, together with the full suite of seismic experiments planned on board the Sonne, will help define the seismic structure in the vicinity of the borehole and will be used to assess how local influences will affect long-term measurements planned for the site. Ten days have been allocated for drilling, casing, and seismic experiments at the NERO Site.