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

Report No. 6945
BBN Systems and Technologies Corporation
encounters. All of the other sources studied gave a medium to low SER ranking
for the odontocetes and pinnipeds. They all have hearing characteristics
which are most sensitive at high frequencies above the dominant output
bandwidth of most of the man-made sources in the marine environment. The
results for walrus, Steller sea lion, and Dallls porpoise are based on the use
of hearing characteristics for California sea lion and harbor porpoise and may
be incorrect if the actual hearing sensitivities are greatly different from
the values assumed.
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It may be important that the low-frequency sensitivity of odontocetes and
pinnipeds has not been determined as precisely as would be desirable.
Measurement difficulties in small test tanks have made it impractical to
measure the low frequency hearing of most species. Some of the estimates that
have been published may underestimate the hearing abilities of pinnipeds and
toothed whales at frequencies below a few kilohertz. Thus, they may be somewhat
more sensitive to industrial noise than the model estimates suggest.
Predicted zones of influence
The range at which a 50% probability of avoidance would be expected fcr
gray whales (the "zone of influencet1) was estimated for the major noise
sources in each of the four OCS planning areas studied in detail. The
predicted ranges were based on calculated acoustic propagation charcteristics
in all of the areas except the Chukchi Sea, where a limited set of measured
data are available.
The largest estimated zones of influence are produced by large tanker
operation in the Shumagin area where a radius of 27 km is predicted and by
dredge operation in the North Aleutian area which is predicted to have an
effective zone radius of 20 km. Icebreaker operation in the North Aleutian
area is probably infrequent, but if icebreakers are used in this area, a zone
of influence radius of 40 km is estimated because of the predicted efficient
mid-frequency sound transmission in this area. Sound transmission losses are
estimated to be higher in the Norton Basin area. Because of this, the predicted
zone of influence for icebreaker operation in Norton Sound is reduced
to 12 km. The transmission loss data for the Chukchi Sea provide an estimated
icebreaker zone of influence of 8 km during summer conditions and 5.4 km
during winter conditions.
No quantitative measurements of sound.levels observed to cause disturbance
of marine mammals are available for airborne sound sources. Specific
disturbance criteria are not therefore available. Some reported disturbance
observations of harbor seals during aircraft overflights were used to obtain
general estimates of minimum slant range distances for probable disturbance of
this species. These overflight distances varied from 120 m for a light single
engine propeller aircraft to about 420 m for a Boeing 727. Analysis of
observations of walrus disturbance showed that these animals have highly
variable response and may be disturbed by visual cues as well as acoustic
noise levels. Their apparent sensitivity to intrusive sounds is considerably
greater than harbor seals.