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

Report No. 6945
BBN Systems and Technologies Corporation
ABSTRACT
A better understanding of the numbers, locations, and acoustic intensities
of the wide variety of man-made and natural noise sources in the Alaskan
marine environment is needed in order to determine the normal levels of
natural ambient noise and the "normal" levels of human noise to which marine
mammals are exposed in their usual habitats. The purpose of this study is to
provide an up-to-date compehensive synthesis of available information that
compares the relative magnitudes and effects on marine mammals of noise from
oil and gas industry activities with noise from other sources in Alaska OCS
and coastal waters. The study procedure incorporates the receiver, source and
path concepts generally used in acoustic analysis. The receiver characterization
includes a review of marine mammal distribution in Alaska and a map of
the distribution of each major species. (Scientific names of marine mammal
species discussed in this report are presented in Appendix F.) Information on
sound production, hearing sensitivity (when known), and observed responses to
noise sources is also included. The analysis of noise sources found in the
Alaskan marine environment considers natural, industrial, transportation, and
cultural sources. Acoustic transmission loss characteristics obtained from
measurements and model predictions are used to estimate the effective ranges
of the noise sources using available source level information. 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 range and the numbers of mammals potentially
affected. This was done by developing a Standardized Noise Contribution Model
combined with a Standardized Exposure Rating Model for various specific
species. This procedure provides an indication of which source - species combinations
may have the highest potential for acoustic interaction in a given
area. In terms of their potential effects on marine mammals, the loudest
sound sources in the Alaskan marine environment are seismic arrays (both air
gun and vibroseis), icebreakers, large ships, and dredges. Sound levels
produced by smaller vessels and boats become significant when several of these
sources are operating concurrently in a small area. Earthquake events produce
high underwater sound levels sporadically in active seismic areas such as the
Aleutian arc. Baleen whales are considered to have hearing sensitivity Zharacteristics
which include the frequency range 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 the species
with the highest probability 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
frequency range estimated for baleen whales. The other species studied,
including walrus, white whale, and Steller sea lion, were all predicted to
have medium to low probability of acoustic influence from the sources considered.
This is primarily a result of the fact that their optimal hearing
sensitivity is at frequencies above the dominant output frequencies of most
man-made sources.