On the evening of March 2, 1991, the research vessel JOIDES Resolution departed from Honolulu harbor for a 17-day mission that reflects an important new trend in the international Ocean Drilling Program (ODP). The primary objective of ODP Leg 136 was to prepare a seafloor site for future experiments needed to develop the Ocean Seismographic Network (OSN). The focus of OSN is much deeper than the strata reached by the drill bit of the Resolution, and this cooperative effort will enable the drilling ship's resources to be shared with a community whose interests extend to the inner core of the Earth.
During Leg 136, ODP Hole 843B (proposed Site OSN-1) was established on the Hawaiian Arch, approximately 225 km south-southwest of the island of Oahu (Figure 1). The hole was drilled through 242 m of sediment and 70 m into the basaltic basement. It will provide a site for performing borehole seismometer experiments that include noise measurements, recording data from teleseismic events for comparison with an existing high-quality station on Oahu, and testing new broad-band sensors and other instrumentation for long-term deployment. A significant part of this report will be devoted to providing the reader with the background and objectives of the OSN.