SUMMARY from IRS poster on OSN-1



SUMMARY

      The primary goal of the Ocean Seismic Network (OSN) Pilot Experiment was to learn how to make high quality broadband (0.003-5H z) seismic measurements on the seafloor in preparation for extending the Global Seismic Network to the ocean basins. The experiment was carried out at the OSN-1 drill site (ODP Hole 843B) 225km southwest of Oahu. On the deployment cruise, on the R/V Thompson from J anuary 3 to February 11, 1998, we deployed three broadband seismic instruments: a borehole seismometer, a sensor buried surficially in the sediments, and a sensor resting on the seafloor. The borehole seismometer was a Teledyne KS54000 similar to the sensors used in the global IRIS/IDA and GSN networks. It was placed in the borehole using the MPL/JOI Wireline Reentry System. The seafloor and shallow buried sensors were Guralp CMG-3T seismometers. In addition to the broadband seismometers we deployed three conventional Ocean Bottom Seismometers with 1Hz geophone sensors, differential pressure gauges, a conventional hydrophone and a current meter. We also carried out a wireline temperature and caliper log of the hole and we tested a wireline borehole packer that could be used to block fluid flow in the hole. On the recovery cruise, on the R/V Melville from June 11 to 20, 1998 we also carried out a wireline temperature and caliper log of the hole and tested a wireline borehole water sampler. All three of the broadband instruments recorded data continuously and autonomously on the seafloor from the time they were deployed in early February until late May or early June, at least 115 days. Over fifty earthquakes were observed on the broadband systems ranging from a 4.5Mb event at 44 epicentral distance (2/14/98 2:15:03) to the 7.9Mw Balleny Islands earthquake at 91 epicentral distance (3/25/98 3:12:26). Signal to noise ratios for earthquake events varied depending on frequency band, ambient noise conditions, and sensor design. Preliminary analysis indicates that the seafloor borehole broadband seismic installation provided comparable quality data to similar continental and island stations over the band from 0.001 to 10Hz. The shallow buried broadband system compared very favorably with the borehole system for signals in the frequency band from 0.01 to 0.07Hz. Material on the OSN Pilot Experiment is available on the world wide web at: "http://msg.whoi.edu/osn/OSNPE_intro.html" It is our intention to compare the results from the three broadband oceanic sensors with the response of other seismometer systems on or near the Hawaiian Islands, for a range of ambient noise conditions and earthquake sources. We can study the behavior of the oceanic seismic noise as functions of deep sea currents, surface weather conditions and sea states. These results will help us address the trade-off between the cost of installing sensors below the seafloor, either buried in the sediments or placed in boreholes, and any improved data quality that may result from lower noise levels and/or improved coupling to true ground motion. The data collected during this experiment will be made available to the community via the IRIS Data Management Center. This work is sponsored by the National Science Foundation with additional support from Incorporated Research Institutions for Seismology (IRIS), Joint Oceanographic Institutions, Inc. (JOI), Scripps Institution of Oceanography, and the Woods Hole Oceanographic Institution.