ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

4pEA2. Underwater acoustic measurements with a digital ocean bottom seismometer.

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.