Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
causing the broad effervescence sound spectrum. Urick (1971) also discusses
$" this phenomenon in the context of Greenland icebergs. The other curve
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regarding glacier noise shown in Fig. 3.9 is the spectrum of a glacial seismic
- event, also recorded in Glacier Bay. Miles and Malme (1983) reported the
p results of an experiment in which a two element hydrophone array was used to
3 . obtain direction of arrival of a series of these events. That information,
coupled with estimates of seismic path and water path travel times, showed
that the source of these events was the upper portion of Reid Glacier (rather
than the lower area where calving occurs). It has been hypothesized that the
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cause is stick-slip action at the ice/rock interface, generating enough energy
in the rock to be equivalent to a M = 1-2 earthquake. Others (Weaver and
Malone, 1979 and Van Wormer and Berg, 1973) have reported similar seismic
events associated with Mt. Rainier and Mt. St. Helen's glaciers.
3.2.7 Summary of ambient noise components
Figures 3.1, 3.2, and 3.9 provide typical underwater and under-ice
background noise spectra associated with a variety of sources likely to be
encountered in the Alaskan outer continental shelf and near-shore regions.
Any attempt to list them in order of importance would be misleading since the
associated sound levels vary considerably with frequency as well as with such
environmental conditions as wind, tide, ice cover, rainfall rate and proximity
to glaciers. Sound sources considered in this study are:
Wind and sea state conditions
Rain and sleet
Distant shipping
Surf
Turbulence due to tidal or other strong currents
Seismic noise '
Ice cracking and pressure ridging
Glacial activity
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Glacial ice effervescence.
Generally, if we accept that all of these sources can occur in or affect
coastal areas, the dominant sources for various frequency ranges can be
identified. In the very low frequency range of 1-10 Hz, tidal current
turbulence effects .and natural seismic events (which tend to be tens of
seconds in duration) would dominate, frequently causing 1/3 octave band sound
levels of 140 dB. In the 10-100 Hz band, the dominant sources of noise are
earthquakes and other seismic events (135 dB or more depending on distance)
and distant shipping (102 dB). From 100-1000 Hz, surf noise with peak levels
of about 120 dB (depending on distance), ice pressure ridge noise (116 dB),
glacial ice effervescence (115 dB), distant shipping (100 dB) and heavy wind