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

Report No. 6945
BBN Systems and Technologies Corporation
and similar high speed small craft during this season has reduced the SNC
rating for this class from highest to a tie for the with seismic array. Small
craft, the hypothesized large dredge, and trawlers are the secondary sources.
The pinniped species and harbor porpoise considered in the SER analysis
for this area are the same as in the spring - summer period since they are
generally resident throughout the year. The results of the SER analysis are
shown in Table 5.9B and Fig. 5.5B. As a result of the lower SNC values for
this season in the area, the SER values are also lower than those obtained for
the spring - summer period. The harbor porpoise SER ratings show a moderate
potential for acoustic influence by the sources considered with the highest
rating for the seismic array. Ratings for the other two species are generally
low.
The southbound gray whale migration is concluding during the early
portion of this period. No analysis was done for gray whales since the SER
results obtained for the whales present would be expected to be similar to
those obtained for the spring - summer period. The values for the smaller
sources would be somewhat reduced because of the smaller number operating.
The SER values which would be obtained for gray whales would be high, suggesting
that the transient gray whales are potentially more influenced by the
local noise sources than the resident smaller mammals.
The southern region of the Shumagin Planning Area is in deep water off
the edge of the continental shelf. Only the northern continental shelf region
was included in the general SNC model area estimate. The north edge of the
Shumagin Planning Area is traversed by ship and fishing vessel traffic using
the Unimak Pass. This results in a significant noise input to this area. The
sources used in the SNC analysis were selected to be representative of the
Unimak Pass traffic and local small craft activity. In addition, sources
associated with oil exploration and drilling operations were hypothesized to
be present. The results of the SNC Model analysis are shown in Table 5.10A
for the late spring - summer season in the Shumagin area. Large tankers are
the dominant sources in this area because of their high sound levels at low
frequencies. The seismic array, medium-sized cargo vessels (which compare
with the large Alaska ferries in acoustic output), outdrives, and tug/barges
are the secondary sources in this analysis. Three medium-sized cargo type
vessels were considered to be operating concurrently.
The northern portion of the Shumagin Area is in the Aleutian arc volcanic
zone and as a result the underwater ambient noise is influenced by sporadic
volcanic and seismic activity. This activity is particularly intense in the
Shumagin Island area (Fig. 3.7) where the probable ground acceleration levels
are about twice as high as on the northern side of the Alaska Peninsula. As
discussed previously for the North Aleutian Basin, area, marine mammals are
subjected to transient high level sounds at low frequencies when seismic
events occur. While no rating has been made in Table 5.10A for the seismic
noise in the Shumagin area, an estimation procedure is discussed in Sec. 6 as
a basis for further study.