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The Marine Seismic System (MSS) and its performance during the Ngendei Experiment are described. The MSS is a digital, triaxial submarine borehole seismograph that comprises a borehole sensor package connected by coaxial cable to an ocean-bottom recording unit. Two shipboard recording systems provide redundant data logging and instrumentation monitoring capability. One of the shipboard consoles, the Teledyne system, monitored the borehole package. The other, the Gould system, emulated the functions of the ocean-bottom recorder. A submerged mooring facilitates the installation and recovery of the ocean-bottom recorder. The borehole package is emplaced by means of a special carriage tool employed at the bottom end of the drill string. The package rested undamped at the bottom of DSDP Hole 595B, 54 m within basement rock overlain by 70 m of sediments. The hole-lock and cable-isolating mechanisms of the borehole package were disconnected after they were found to damage the coaxial cable. One of the horizontal seismometers was apparently damaged and consequently yielded no usable data. Totals of 120, 83, and 43 hours of data were collected on the Teledyne and Gould shipboard recorders and on the ocean-bottom recorder. The ocean-bottom recorder failed after approximately 2 days of operation because of a water leak in one of its two battery spheres. INTRODUCTION The past decade has seen the first use of submarine borehole seismographs in the deep ocean. The earliest employment of such instrumentation was in seismic refraction experiments for elucidating the structure of the oceanic crust and uppermost mantle (see, e.g., Stephen et al., 1980, 1983). More recently, borehole seismographs have been of interest for use in nuclear discrimination studies because of a hypothesized signal-to-noise advantage gained by borehole emplacement of seismometers over ocean-bottom emplacement. To date, only two borehole seismographs possess sufficient sensitivity to test this hypothesis: the Ocean Sub-bottom Seismograph (OSS) of the Hawaii Institute of Geophysics (Duennebier and Blackinton, 1983), and the Marine Seismic System (MSS) funded by the Defense Advanced Research Projects Agency (DARPA) (Ballard et al., 1984). The OSS has been successfully operated twice, during DSDP Legs 67 (Carter et al., 1984) and 88 (this volume). A prototype of the MSS tethered to Glomar Challenger was successfully operated during DSDP Leg 78B (Ballard et al., 1984). This chapter describes the first deployment of the fully operational MSS during DSDP Leg 91, which composed a portion of the Ngendei Experiment. Menard, H. W., Natland, J., Jordan, T. H., Orcutt, J. A., et al., Init. Repts. DSDP, 91: Washington (U.S. Govt. Printing Office). 2 Addresses: (Adair, Orcutt) Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, CA 92093 (Adair, present address: Rockwell Hanford Operations, Energy Systems Group, P.O. Box 800, Richland, WA 99352); (Harris), Naval Ocean Research and Development Activity, NORDA Code 500, NSTL Building 1100, Bay St. Louis, MS 39529 (Jordan) Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139. MARINE SEISMIC SYSTEM DESCRIPTION The MSS is the culmination of a multicorporation development program coordinated by the Naval Ocean Research and Development Activity (NORDA) at the behest of DARPA. When fully operational, the MSS comprises a borehole sensor package connected by coaxial cable to an untended ocean-bottom recording system (Fig. 1). The borehole package and a companion shipboard recording console, referred to here as the Teledyne system, were developed by Teledyne-Geotech. The ocean-bottom recording system and its shipboard counterpart, referred to as Gould systems, were developed by Gould, Inc. Global Marine Development Corporation built a carriage for transporting the borehole package to the seafloor at the bottom end of the drill string. The Naval Civil Engineering Laboratory in Port Hueneme, California, designed and tested a submerged mooring that facilitates the installation and recovery of the ocean-bottom recording system. Transcriptions of the original MSS data are available on nine-track tape from DARPA's Center for Seismic Studies in Alexandria, Virginia. The Center for Seismic Studies was developed as a pilot data collection and analysis facility in compliance with nuclear arms limitation treaties, which call for the rapid dissemination of seismic data to be used in treaty verification (Berger et al., 1984). MSS Instrumentation Figure 2 depicts the cylindrical borehole sensor package. The instrumentation is housed in a 609.6-cm-long pressure vessel of 20.3 cm OD. The instrumentation in-

Description and Performance of the Marine Seismic System during the NGENDEI Experiment