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

Report No. 6945
BBN Systems and Technologies Corporation
whales, which occurred in the 24-29 April time period. On 1 May the visual
count was 20 whalesoduring a 7 hour period. The count during the time of
maximum passage had averaged 43 animals per day over a 3-day period. Some
fluctuations in count were clearly due to poor visual conditions (fog, wind,
rain), but these numbers indicate the trend in the whale count. On 2 May at
164300 toward the end of the oQservation period, seismic energy arrived from a
M = 6.7 earthquake in Coalinga, California--144 km away. During the following
24 hours a series of 15
,
aftershocks in the M = 3.5 - 5.1 range were reported
by seismic stations, on 4 May six shocks of M 3.4 were reported, with
nine events of M 2 3.4 reported for the 5th of May. On 3 May only a single ,
mother/calf pair was seen from the shore observation site (late in the
afternoon), and three pairs each day were seen on 5/4 and 5/5. Observation
conditions were good to fair on 5/2, excellent 5/3-5/4 and good to very good
on 5/5 when the observation work was terminated.
The sound measurement system used by BBN was overloaded by the main shock
on 5/2 and was not operating at the time of calculated aftershock arrival,
hence we do not have sound pressure level data available for comparison with
spectra in Fig. 3.2 or with predictions given in Fig. 3.8. Based on the known
overload limit of the hydrophone preamplifier, a received sound pressure level
of 176 dB will cause signal distortion; saturation should occur at a higher
level of about 186 dB. The overall received sound pressure level from the
main shock was expected to be about 195 - 206 dB (Fig. 3.8) .
Obviously, we do not know whether the underwater sound (fluctuating
compressional wave energy) f~om the main shock and from subsequent aftershocks
caused the gray whales to move further from shore (beyond visual observation
capability). Even though it is tempting to draw that conclusion, we may have
been observing a natural rapid cessation of the migration pulse. Nevertheless,
it is conceivable that marine mammals will change behavior temporarily
during the onset of earthquake short term events. There have been many
anecdotal observations of animal behavioral anomalies before and during
seismic disturbances (see, for instance Lee et al., 1976 and Stierman, 1980).
3.2.6 Ice noise
There are several dynamic processes associated with ice in arctic and
near-arctic regions which can contribute in a significant way to the natural
underwater background noise. Under-ice noise studies, notably by Milne
(1960), Milne and Ganton (1964), Greene and Buck ( 1964), and Buck and Wilson
(1986), and summaries (e.g., Urick, 1983) have demonstrated the high
variability of ambient noise levels in relation to such parameters as wind
speed and changes in temperature and pressure ridge activity. During calm
wind conditions and stable temperature, sound levels under a continuous ice
sheet are frequently below those measured in the open ocean under sea state=O
conditions. Environmental changes such as a decrease in temperature (causing
ice cracking) or an increase in wind speed can result in an increase in the
background noise by as much as 40 dB. Rising temperatures tend to stabilize
the ice and background noise levels drop. Wind-related effects have relatively
little influence on under-ice noise when there is solid ice cover, but
they become quite important when there are fractures in the ice with leads and
floes and sharp ice/water discontinuities at the edge of the ice pack or ice