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

Report No. 6945
BBN Systems and Technologies Corporation
North Aleutian Basin
The sources used in the SNC Model analysis for the North Aleutain Basin
area are representative of existing fishing, transportation, and cultural
activities plus hypothetical oil industry operations. As shown in Table 5.8A,
the dominant sources in this area for the late spring - summer season are the
tug/barges, outdrives, and hypothesized seismic array. Important secondary
sources are small craft, trawlers (in transit) and hypothesized large dredge.
Outdrives and other high-speed small craft are dominant sources, because of
the large number in operation during peak fishing seasons. The larger tugs
and fishing trawlers, while not as numerous, contribute significantly because
they generally have relatively high RPM propellers which produce cavitation
noise for most vessel operating conditions.
The southern portion of this area along the Alaska Peninsula is in an
active volcanic zone (Sec. 3.2.5). As a result, sporadic low frequency noise
is produced by bottom motion during earthquake events. During these events,
the noise levels at frequencies below 50 Hz can be signicantly louder than the
source levels of the industrial sources listed in Table 5.8A, with the possible
exception of the seismic array. Sporadic seismic noise has not been
given a rating in Table 5.8A because high level events are relatively infrequent,
typically less than 1 per year of Magnitude 4 or greater (Fig. 3.5).
Four species - gray whale, harbor seal, harbor porpoise, and Steller sea
lion - were considered, together with the five sources just discussed, in the
SER model analysis for this area. The results in Table 5.8B and Fig. 5.5A
show that the gray whale - seismic array encounter again has the highest SER
rating, followed by the gray whales encountering the tug/barges and outdrives.
The model was not run using the seismic array or trawler in relation
to the other species since the pinnipeds and harbor porpoises were assumed to
be generally located near shore during this season. As shown by the SER
values, harbor porpoise have a higher potential for acoustic interaction than
the other small marine mammals as a result of their more sensitive low
frequency hearing.
There is a resident population of sea otters in the southern part of this
area. No hearing sensitivity data are available for this species. It is
expected that their hearing is optimized for airborne rather than underwater
sound since they spend most of their time at or above the surface. Observations
of the behavior of sea otters in the presence of an operating air gun
and support vessel were made as part of an acoustic disturbance study of
migrating gray whales (Malme et al. 1984). No significant changes in behavior
were observed for operation as close as 900 m (Riedman 1984).
The SNC analysis for the late fall - early winter season in the Nprth
Aleutian Planning Area is shown in Table 5.9A. The group of sources used for
the previous analysis was considered again after making changes in TL values
and in the expected number of sources. The SNC ratings for all of the sources
are reduced somewhat compared to those for the spring - summer season due to
higher short-range TL and an estimated smaller number of operational vessels
and small craft. The hypothesized seismic array along with tug/barges and
outdrives are the dominant sources. The estimated smaller number of outdrives