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

Report No. 6945
BBN Systems and Technologies Corporation
The analysis of noise sources found in the Alaskan marine environment
includes natural, industrial, transportation, and cultural sources.
Information on their output spectra is presented in graphs and tables of 1/3
octave source level (dB re 1 gPa at 1 m). When available, information on the
temporal characteristics of the sources is also included.
Acoustic transmission loss characteristics are obtained from measurements
and model predictions. These characteristics, along with the above source
level data, are used to estimate the effective acoustic ranges of sound
sources. Both airborne and underwater transmission loss characteristics are
required. However, empirical information on underwater acoustic transmission
loss in Alaskan marine environments is sparse. As a result, it was necessary
to use sound propagation models to obtain estimated transmission loss
characteristics for several areas studied.
Information on species distribution was combined with information on
source distribution, source level, and transmission loss to determine the most
significant sources in terms of their acoustic ranges and the numbers of
mammals potentially affected. This was done by developing a Standardized
Noise Contribution Model which is based on the acoustic energy density
contributed to the environment by a specific type of source in a defined
reference area. The source rating is combined with a Standardized Exposure
Rating Model for a specific species. The latter model takes into account the
degree of matching between the source bandwidth and the species' hearing
sensitivity, and the number of animals present in the reference area. The
output of this procedure provides an indication of which source - species
combinations have the highest potential for acoustic interaction in a given
area. Zones of influence for the loudest and most widely distributed sound
sources, as determined by the modeling procedure, are estimated for four
selected OCS planning areas of high current interest - Chukchi Sea, Norton
Basin, North Aleutian Basin, and Shumagin.
Study Results
The ldudest sound sources in the Alaskan marine environment are seismic
arrays (both air gun and vibroseis), icebreakers, large ships, and dredges.
Sound levels produced by the smaller vessels used for cargo hauling, fishing,
and recreation become significant when several vessels are operating in a
relatively small area. Earthquake events produce high underwater sound levels
sporadically in active seismic areas such as the Aleutian arc. Sound produced
by aircraft is the loudest airborne noise component. The primary impact of
this noise is near airports and landing strips and along routes where low
level operations are prevalent.
Baleen whales are believed to have hearing sensitivity characteristics
which include the frequency ranges of most of the man-made sources described
above. As a result the exposure model showed that the gray, bowhead, fin, and
humpback whales which frequent Alaskan waters are species with high
probabilities of acoustic interaction with most of the sound sources
studied. The model predicted that killer whales, harbor porpoise, Dall's
porpoise, harbor seals, and fur seals would be influenced primarily by the
loudest sources since their hearing sensitivity does not extend to the low