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

Report No. 6945
BBN Systems and Technologies Corporation
The predicted radius of influence for the drillship example, which ranges
from 1.1 km to 1.8 km, is larger than that of the smaller vessel examples in
all of the area studies. However, when the potential areas of influence are
considered, the smaller mobile sources have larger values. Thus in an active
drill site area, the support vessels, which are generally moving around,
provide the primary noise disturbance potential. The tug/barge source used
here can be considered representative of a smaller supply vessel type.
The zone of influence estimates for the North Aleutian Basin area are
likely about equally valid for the northern part of the Shumagin area where
the water depth is less than 100 m. The IFD Model predicted similar sound
transmission conditions for the two areas, except at low frequencies where the
rocky bottom region in the Shumagin area showed somewhat less transmission
loss (see Appendix C). As a result, only the large tanker source was
considered explicitly for the Shumagin area zone estimates. The zone radius
for the tanker operating at 16 kts (30 km/hr) was predicted to be 27 km for
the summer propagation condition. This can be compared with the value of 20
km obtained for dredge operation (assumed to be the same as obtained in the
North Aleutian Basin). These zone radii are the largest predicted in the
study, considering that icebreaker operation in the North Aleutian Basin is
not generally required. The area of influence for the tanker over a two-hour
period is estimated to be over-four times as large as that for the dredge.
The probability is quite high that two or more large tankers or container
ships of this size are operating concurrently in the Shumagin area because of
the Unimak Pass ship traffic density. As a result noise levels due to
commercial shipping in this area are expected to be comparable to or higher
than those that may be produced by oil industry operations.
A map overlay showing the estimated zones of influence for the loudest
sources in each of the four primary study areas is located in an envelope
inside the back cover. This overlay can be used with the species distribution
maps in Section 2 and with the two other overlays showing general source
distributions.
5.3.2 Airborne Sources
Airborne sound from land vehicles, vessels, and aircraft has been
observed to cause disturbance reac'tions in marine mammals. Aircraft, because
of their mobility and wide use in Alaskan marine regions, are the most
dominant type of high level airborne source. Seals, walruses, and sea lions
that haul out on beaches and ice are the most sensitive species to disturbance
from aircraft sound (Sec. 2.4.1). No quantitative measurements of sound
levels observed to cause disturbance to these species have been reported. As
a result, it is difficult to define criterion sound levels for the onset of
probable disturbance reactions. However, several observations, described in
Sec. 2.4.1, have been made wherein estimates of aircraft type and slant range
were obtained for observed disturbance reactions of harbor seals and walruses.
These observations have been used to estimate a probable disturbance threshold
level for these two species.
The results of this analysis are shown in Table 5.13. A level 110 dB in
the dominant bandwidth of-a "light aircraft" (assumed to be a Cessna 185 or