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,
** 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
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
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
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
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.
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:
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.,
Webb, S.C. (1998). "Broadband seismology and noise under the ocean,"
Rev. Geophys. 36, 105-141.
This page was created by Tom Bolmer.
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Last Modified 9/30/99