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

Report No. 6945
BBN Systems and Technologies Corporation
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ambient noise exist in the ocean, particularly in the shallow waters of
continental shelf areas. Deep open ocean ambient noise levels are quite
predictable above 500 Hz based on knowledge of wind and sea state conditions
and below 500 Hz based on knowledge or assumptions regarding distant ship
traffic conditions. On the other hand, in shallow water along the continental
shelf and in near-shore areas, ambient noise is frequently highly variable
from site-to-site and generally fluctuates considerably with time. Nevertheless,
as presented in the literature (e.g., Wenz, 1962 and Urick, 1983)
reasonable trends or estimates of shallow water ambient noise levels can be
presented for known important sources of sound, with the associated levels
varying as a function of definable parameters. Specific attention is given
here to those non-biological sources of noise which are expected to be major
contributors to ambient conditions along the Alaskan continental shelf.
Emphasis has been placed on the four Department of the Interior (Minerals
Management Service) Lease Sale areas of most interest to this study:
Shumagin, North Aleutian Basin, Norton Basin and Chukchi Sea.
The major sources of ambient noise that need to be considered in order to
understand the underwater acoustic environment of marine mammals inhabiting
the Alaskan Continental Shelf regions are:
wind, rain and sleet
distant shipping
surf
turbulence effects due to tidal or other strong currents
seismic noise (earthquakes, volcanic activity)
ice cracking and pressure ridge activity
glacial activity
glacial ice effervescence.
Typical average noise spectra due to these sources are presented. Most of
these exhibit a continuous but fluctuating time history and some are short
term or nearly transient in character. A 1/3-octave band sound pressure level
format has been selected for this study since it has been-established for
several marine mammals (as well as land mammals such as man) that background
noise which has a significant effect on detection of a sound signal is thenoise
occurring within a band roughly 1/3 octave wide, centered at the
frequency of the sound signal (see Section 3.1). Similarly, noise signatures
*In the underwater sound and oceanographic scientific communities, shallow
water is commonly defined as ocean depths of less than 100 fathoms (183
meters). The continental shelf break frequently occurs at about that depth,
although in Alaska, particularly along the Beaufort Sea coast, the shelf
break occurs at depths of about 50 to 70 m (27-38 fm).
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