David M. F. Chapman
John C. Osler
W. Cary Risley
Defence Res. Establishment Atlantic, P.O. Box 1012, Dartmouth, NS B2Y 3Z7, Canada
D. Jackson Dodds
GeoAcoust., Inc., Aurora, ON L4G 4J9, Canada
A digital ocean bottom seismometer (OBS) has been constructed comprising a
hydrophone, three 4.5-Hz geophones in a symmetric orthogonal configuration,
24-bit sigma--delta analogue-to-digital conversion, and a radio telemetry link
to a ship-board monitoring and recording laboratory. Careful attention to
self-noise floors and OBS/seabed coupling has resulted in an instrument capable
of accurately measuring ambient noise on both hydrophone and geophone sensors
in the 1--50-Hz range. A vibrator mounted in the OBS, controllable from the
receiving platform, allows in situ calibration of the OBS response to seabed
motion. The instrument's design, construction, deployment, and operation will
be described briefly, followed by some examples of the unusual ambient noise
data sets that have been collected. At one thinly sedimented site, a
very-low-frequency banded structure was observed in the ambient noise on the
horizontal geophone channels only; at many sites, a strong 6-Hz signal
associated with offshore drilling activity was observed intermittently in all
the geophone signals but was not seen in the hydrophone signals.