Bearing Stake Exercise: Sound Speed and other Environmental Variability
2006
Fenner, Don F. | Cronin, Jr, William J.
The BEARING STAKE exercise was conducted at five sites in the northwest Indian Ocean during January through April 1977 (northeast monsoon). Oceanographic data from the exercise consisted mainly of expendable bathythermograph (XBT) observations that were converted to sound speed using the equation of Wilson (1960) and a salinity field derived from exercise deep ocean station data. At all exercise sites except site 4 (Somali Basin), acoustic propagation was bottom-limited in respect to the high frequency (about 18-m) and low frequency (about 91-m) sources. Although site 4 was bottom-limited in respect to the high frequency source, at least 200 m of depth excess occurred for the low frequency source. At site 4 and throughout the western Somali Basin, interleaving of intrusive water masses (including Red Sea and Antarctic Intermediate Waters) caused complex and highly variable sound speed profiles to depths of at least 1800 m. At site 1B (Gulf of Oman) sound speed profiles were up to 5 m/sec lower than those observed one month earlier (site 1A), probably due to increased northeast monsoon upwelling after the site 1A occupation. Maximum temporal and spatial sound speed variability throughout the exercise area occurred between about 100 and 150 m, just below the depth of the low frequency source. At all sites, wind speeds and sea/swell heights were low to moderate and should not have markedly influenced ambient noise levels above 200 Hz.
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