The Ocean Seismic Network Pilot Experiment

R.A. Stephen*, J.A. Collins*, J.A. Hildebrand**, J.A. Orcutt***,
K.R. Peal****, F.N. Spiess**, F.L. Vernon***

* Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA
** Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, CA
*** Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, CA
**** Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA


      The goal of the OSN Pilot Experiment is to learn how to make high quality broadband (0.003-5Hz) seismic measurements on the sea floor in preparation for extending the Global Seismic Network and PASSCAL style experiments to the ocean basins (Forsyth et al., 1991; Montagner and Lancelot, 1995; Purdy and Dziewonski, 1988; Purdy and Orcutt, 1995). The experiment was carried out at the OSN-1 drill site (ODP Hole 843B) 225km southwest of Oahu (Figure 1). Operations were carried out on a deployment cruise on the R/V Thompson from January 3 to February 11, 1998 and on a recovery cruise on the R/V Melville from June 11 to 20, 1998. Three configurations of broadband seismic instruments were deployed: 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 data was acquired on three conventional Ocean Bottom Seismometers with 1Hz geophone sensors, on differential pressure gauges, on a conventional hydrophone and on a current meter. The locations of the various seismic systems at OSN-1 are shown in Figure 2. We also carried out a wireline temperature and caliper log of the hole, we tested a wireline borehole water sampler and we tested a wireline borehole packer that could be used to block fluid flow in the hole.

The broadband systems

      The schematic in Figure 3 outlines the procedure for installing the borehole seismic system in an existing borehole on the seafloor from a conventional, non-drilling, research vessel. The ship is maintained within a 10m watch circle by dynamic positioning and Global Positioning System navigation. Acoustic transponders on the seafloor assist in locating the lead-in-package relative to the re-entry cone. A camera and lights on the lead-in-package at the bottom of the borehole seismometer (Figure 4) also assist in locating the cone. Once re-entry is completed the system is lowered until the data recording package (Figure 5) lands in the cone. The Control Vehicle (Figure 6) has propulsion and additional navigational aids to manipulate the string near the seafloor. Once the system has been tested in place, the tether at the top of the recording package is released and the Control Vehicle returns to the surface. Figure 7 shows the data recording package sitting in the re-entry cone at OSN-1 after the tether has been disconnected. Recovery of the recording package is carried out with a grappling hook attached to the bottom of the Control Vehicle. At OSN-1 the water depth was 4407m, the re-entry cone is 5m across and the borehole seismometer was emplaced 248m below the seafloor.
      To ensure that the seafloor broadband seismometer did not sink into the sediments it was placed on a flat plate (Figure 8). The recording systems for both the seafloor and shallow buried broadband seismometers were essentially identical (Figure 8). The recording packages were separated from the sensors to reduce the effect of vibrations of the recording package frame contaminating the seismic signals. The buried broadband seismometer was pushed about 1m into the seafloor from a burial frame (Figure 9).

The data

      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) (Figure 10). Signal to noise ratios for earthquake events varied depending on frequency band, ambient noise conditions, and sensor design.
      Preliminary analysis indicates that broadband seafloor seismic installations can yield comparable quality data in terms of ambient noise levels and time series events to island and continental stations. In some frequency bands the seafloor stations are as quiet as any stations in the world. The results discussed below are based on observations from selected days. The ambient noise field on all sensors has a strong time dependence which is being addressed in a separate study. These results will also be site dependent and will vary depending on sea state conditions, sediment thickness, bottom type, the size of the ocean basin, etc (Webb, 1998).
     Our results from the OSN Pilot Experiment have refined some of our preconceived notions about seafloor ambient noise. Previously it was felt that seafloor stations would always be much noisier than island stations, particularly at short periods because of ocean wave noise (Webb, 1998). This was not observed on comparing OSN to Kipapa, at frequencies above 0.03Hz (Figure 11). The performance of the borehole sensor at very low frequencies (below about 0.02Hz) appears to be contaminated by installation noise, which was observed during testing on land, and which could be ameliorated by packing the borehole with sand or glass beads.
      Previously it had been thought that short period teleseismic body waves would be very difficult to observe on the seafloor in the Pacific. Prior to this experiment there were only two. We observed over ten, including mb's as small as 5.1 (Figure 12).
      Previously short period horizontal noise at seafloor stations was always observed to be much greater than vertical noise. This was not true for the seafloor borehole station (Figure 13).
      Previously it was thought that a borehole sensor would give at most 10dB improvement over a seafloor or shallow buried sensor. This was not observed at OSN-1. At short periods the borehole was quieter than the shallow buried by up to 20dB on verticals and up to 30dB on horizontals (Figure 14). The seafloor and shallow buried broadband sensors at OSN-1 are both sensitive to shear modes in the sediments. Above the microseism peak (about 0.1-10Hz), ambient noise levels are ???-???dB noisier on the sensors on or in the sediment compared to the borehole sensor in the basement. The shear modes also cause high frequency ringing or coda (about 0.1-1.0Hz) on events received on the sensors on or within the sediment. Below the microseism peak (about 0.01-0.1Hz), both ambient noise and earthquake signals are comparable between the buried and borehole sensors.
      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.


Acknowledgements

      The Ocean Seismic Network Pilot Experiment was a collaborative effort involving about twenty scientists, engineers and technicians from Woods Hole Oceanographic Institution and Scripps Institution of Oceanography. The deployment cruise was carried out on the R/V Thomas G. Thompson, run by the University of Washington, and under the command of Captain Glen Gomes. The recovery cruise was carried out on the R/V Melville, run by Scripps Institution of Oceanography and under the command of Captain Eric Buck. 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 a Mellon Grant from Woods Hole Oceanographic Institution. Material on the OSN Pilot Experiment is available on the internet at: "http://msg.whoi.edu/osn/OSNPE_intro.html"


REFERENCES

Forsyth, D., Sacks, S., and Tréhu, A., (1991). JOI/IRIS Ocean Seismic Network - U.S. Pilot Experiment Task Force Meeting Joint Oceanographic Institutions, Inc.,.

Montagner, J.-P., and Lancelot, Y. (Ed.) (1995). Multidisciplinary observatories on the deep seafloor (INSU/CNRS, IFREMER, ODP-France, OSN, USSAC, ODP-Japan, Marseille).

Purdy, G.M., and Dziewonski, A.M. (Ed.) (1988). Proceedings of a workshop on broad-band downhole seismometers in the deep ocean (Joint Oceanographic Institutions, Inc. and the JOI U.S. Science Advisory Committee, Washington, D.C.).

Purdy, G.M., and Orcutt, J.A. (Ed.) (1995). Broadband seismology in the oceans - Towards a five-year plan (Ocean Seismic Network / Joint Oceanographic Institutions, Inc., Washington, D.C.).

Webb, S.C. (1998). "Broadband seismology and noise under the ocean," Rev. Geophys. 36, 105-141.



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