Infrasonic Ambient Ocean Noise Spectra from Freely Drifting Sensors

摘要

Self-contained Swallow Floats can record very low frequency (VLF) ambient ocean noise over extended periods of time. The autonomous buoys measure and record the components of particle velocity in the 1-10 Hz band. They can be ballasted to neutral buoyancy at a desired depth. The buoys generate and receive high frequency acoustic signals which may be used to determine their relative positions. The deployment of several of these units forms a freely drifiting array of sensors. Individual element time series may be combined coherently off-line using a beamformer, e.g., the MPL dynamic Beamformer. The Swallow float design minimizes self-noise which can limit accurate ambient ocean noise measurements. The floats drift freely and are not subject to flow noise or cable strumming. They measure particle velocity and are therefore insensitive to variations in local pressure. MPL has conducted Swallow float deployments annually since 1982. The 1982 deployment tested only the acoustic positioning system. Between 12 and 14 July 1983, three Swallow floats were deployed at a location approximately 50 miles west of San Diego. California and southeast of San Clemente Island (32'N, 118'W). Sea state during the deployment varied between 1 and 2. Water depth was approximately 1900 meters. Two of the floats deployed to a depth of about 1400 meters and the third deployed to the bottom. Measurements recorded by float 3 which deployed to 1400 meters have been analyzed and results are reported in this thesis.