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.

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