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

Report No. 6945
BBN Systems and Technologies Corporation
same band (Greene 1985, 1987b). However, playback experiments using recorded
sound from that same drillship showed that some bowheads initiate weak
avoidance reactions when exposed to drillship sounds no stronger than those
tolerated by the bowheads observed several kilometers from actual drillships.
Taken together, results of drillship and dredge playback tests indicated that
most bowheads do not react overtly unless the received noise levels are about
110-120 dB, or 20-30 dB above ambient levels in the corresponding band and 20-
30 dB above the assumed threshold of hearing sensitivity (Miles et al. 1987;
Richardson et al. MS). Thus, the radius of responsiveness around a drillsite
is apparently considerably smaller than the radius of potential audibility.
Recently, migrating bowheads were monitored as they passed an operating
drillship in the Alaskan Beaufort Sea. There was clear evidence that the
whales avoided the area within 10 km of the ships, and some reactions were
evident at greater ranges (Koski and Johnson 1987). The average level of
broadband industrial noise 10 km from the drillship was 114 dB (Greene 1987a).
Reactions of migrating gray whales have been studied when the whales were
exposed to underwater playbacks of drillship, semisubmersible, drilling
platform, and production platform sounds (Malme et al. 1983, 1984). Avoidance
reactions to all of these sounds were noticed. Received sound levels at which
50% of the whales exhibited avoidance ranged from 117 to 123 dB, depending on
the type of noise. These sound levels corresponded to the received levels
that one would expect to find 1100 m from the actual drillship if it were
operating off the California coast, and 4-20 rn from the other three sources
(Malme et al, 1984). Larger radii of influence would be predicted if the same
noise sources were operating in the Bering or Beaufort Sea, where sound
attenuation rates are lower (Miles et al. 1987).
In summary, cetaceans exhibit avoidance reactions and other behavioral
effects when exposed to moderately intense levels of drilling or production
sounds. Whales seem most sensitive when the sound level is increasing or when
a noise source first starts up. The limited available data suggest that
stationary industrial activities producing continuous noise result in less
dramatic reactions by cetaceans than do moving sound sources, particularly
ships. There are indications that cetaceans may partially habituate to
continuous noise. At least in the case of white whales, habituation may
result in greatly reduced sensitivity. Cetaceans are often observed close
enough to drillsites to be within the zone where they are expected to be able
to hear industrial sounds emanatfng from those sites. Thus, the radius of
avoidance by cetaceans appears to be considerably smaller than the radius-of
audibility.
Virtually no information is available about the reactions of pinnipeds to
drilling or production operations.