Leg 91 was dedicated to the deployment of the Marine Seismic System (MSS) at a site in the Southwest Pacific Basin, approximately 1000 km east of the Tonga Trench. The MSS is a downhole seismometer system developed by the Naval Ocean Research and Development Activity (NORDA) and the Defense Advanced Research Projects Agency (DARPA) to monitor regional seismicity in the deep-ocean environment for potential use in verifying compliance with nuclear test-ban treaties. In its Leg 91 configuration, the MSS consisted of four basic components: (1) a borehole instrument package (BIP) comprising two vertical-component and two horizontal-component seismometers, state-of-health sensors, and related instrumentation; (2) a bottom processing package (BPP) capable of providing power to the BIP and recording data from it for a 45-day period; (3) an electromechanical (EM) cable connecting the BIP to the BPP or to on-deck devices; and (4) an installation, recovery, and reinstallation (IRR) system for deep-water mooring of the MSS. The MSS deployment strategy was to emplace the BIP in a hole drilled by Glomar Challenger into the basaltic basement that underlies the oceanic sediments, which has several potential advantages over locating seismometers within the sediment column or directly on the seafloor. Ambient seismic noise levels on the seafloor are known to be high, presumably because noise generated by pressure fluctuations within the water column is trapped as evanescent Stoneley waves propagating along the sediment/water interface and as locked modes within the low-rigidity sediment layer (Bradner et al., 1965; Latham and Sutton, 1966, Latham and Nowroozi, 1968). Model calculations involving complete transfer functions for realistic crustal structures indicate that noise levels below the sediment layer should be substantially reduced. Burial also isolates the seismometers from the motions caused by vortex shedding and current fluctuations within the benthic boundary layer, which have been shown to produce noise on ocean bottom seismometers (OBS) with significant cross sections in the water column (Sutton et al., 1980). Therefore, the signal-to-noise ratio for
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