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

Report No. 6945
BBN Systems and Technologies Corporation
generally consistent with those on migrating gray whales (Malme et al. 1986).
It is noteworthy that the threshold for distinct reactions to seismic pulses,
about 164 dB, was about 50 dB higher than their reaction thresholds for
continuous industrial noise such as that from drillships or production
platforms. In this respect their behavior was consistent with that of humans,
who are also more sensitive to continuous noise than to pulsed noise with an
equivalent peak level (Fidell et al. 1970).
Similarly, experiments have shown that bowhead whales react to strong
pulses of seismic noise by interrupting their normal activities and swimming
away (Richardson et a1 . 1985a, b, 1986 ; L jungblad et a1 . 1985, 1988). The
first obvious behavioral reactions were typically detected when the
approaching seismic ship was about 7-7.5 km away. In the Ljungblad et al.
experiments, first reactions were evident when received noise levels were
about 142-157 dB, and "total avoidance" (all whales moving away) was evident
at 152-178 dB. (Note: Received levels of seismic pulses reported during
studies of bowhead whales were instantaneous peak levels; those reported for
gray and humpback whales were averaged over the duration of the pulse and thus
would appear to be somewhat lower for the same actual level.)
Bowhead whales have frequently been observed engaged in seemingly normal
activities when exposed to seismic pulses with received levels up-to about at
least 158 dB re 1 vPa at distances beyond about 6 km from seismic vessels.
However, statistical analysis has found significant reductions in surfacing
and dive durations and number of blows per surfacing when bowheads are exposed
to noise from seismic vessels 6-99 km away (Richardson et al. 1986; Koski and
Johnson 1987), consistent with changes observed when bowheads are strongly
disturbed by closer seismic vessels. A similar pattern of change in surfacing,
respiration and dive cycles has been noted in summering gray whales
exposed to seismic noise (Malme et al. 1986, 1988). Thus, it is likely that
bowheads are often subtly affected by seismic noise at distances well beyond
those at which strong avoidance becomes evident, and at correspondingly lower
received noise levels.
Humpback whale reactions to seismic noise have been studied in less
detail. They, like bowhead and gray whales, tolerate noise pulses from
distant sources, but exhibit startle responses at the onset of noise.pulses
with received levels of 150-169 dB (Malme et al. 1985).
In summary, baleen whales seem to be quite tolerant of noise pulses from
marine seismic exploration. They usually continue their normal activities
even when exposed to pulses with received levels as high as 150 dB, and
sometimes higher. Such levels are 50 dB or more above typical ambient noise
levels. However, subtle behavioral effects are suspected to occur at least
some of the time at received levels less than this. At least in bowheads and
gray whales, strong avoidance is common when received levels reach 160- 170 dB.
Such levels typically are found several kilometers from a vessel operating a
full-scale array of airguns.