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

Report No. 6945
BBN Systems and Technologies Corporation
lower than the maximum level. This has the effect of reducing the predicted
average range to 30 km. When the 30 dB S/N criterion is applied using the
maximum 1/3 octave band level, a predicted range of 35 km is obtained.
The smallest zone of influence for the icebreaker, within the areas
studied, is expected to occur in the Chukchi Sea during winter 100% ice cover
conditions where a radius of 5.4 km is predicted for the Lr = 120 dB
criterion. This is a factor of 7.4 smaller than the zone radius predicted for
the North Aleutian Basin. The 30 dB S/N criterion predicts a 11.7 km radius
of influence for icebreaker operation during 100% ice cover conditions. The
greater radius predicted by the latter criterion is a result of the low
ambient noise levels observed during these conditions.
The predicted radius of influence for the seismic array can be seen to be
largest for operation in the Chukchi area and smallest for operation in the
North Aleutian Basin - opposite to the findings for the icebreaker. This
results from propagation predictions for higher low frequency losses in the
Norton Basin and in the North Aleutian areas than were inaicated by transmission
loss measurements in the Chukchi Sea. However the Chukchi transmission
loss data (Greene 1981) did not cover the shorter ranges considered in
the present modeling results. As a result extrapolation errors may be present
and caution should be used in interpreting the zone of influence predictions
for the low frequency seismic array signal.
The area of the zone potentially influenced by the icebreaker operating
in the Chukchi Sea is shown in Table 5.12 to be 92 km2 in the winter and
200 km2 in the summer. While this can be seen to be larger than the area
influenced by any other single source, if several smaller sources were operating
concurrently the total area influenced by them may be greater than that
for a single icebreaker. For example, if three outdrives or similar high
speed fishing vessels were operating concurrently with non-overlapping zones
of influence, the tota area potentially influenced within a 2-hour period is
estimated to be 201 km' - comparable to that of the icebreaker. In the other
areas the zone of influence of the outdrive can be seen to be larger than in
the Chukchi because of estimated better sound transmission conditions at high
frequencies.
The zone of influence for the dredge can be seen to be comparable in the
Chukchi and Norton Basin areas, with a somewhat smaller radius for the summer
condition in Norton Basin. When the estimated zone areas are compared, tug/
barge and small craft activities can be seen to have similar or greater
potential influence areas than the dredge example, particularly if several
sources are operating concurrently. Thus the ongoing gold dredging activity
near Nome may not be the dominant noise source during active cargo shipping
and fishing seasons. However, it is not known how similar the noise level
1
from the gold dredge BIMA is with respect to the dredge noise levels used in
this analysis. The dredge source can be seen to have a considerably larger
predicted radius of influence in the North Aleutian Basin than in the other
two areas because of the estimated better sound transmission conditions at
high frequencies. In the North Aleutian Basin the estimated zone of influence
area for the dredge during summer conditions is more than 10 times larger than
the zone areas for the tug/barge, trawler, or outdrive.