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

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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
Report No. 6945 BBN Systems and Technologies Corporation floes: Greene and Buck (1964) demonstrated 10-15 dB fluctuations in under-ice 50 Hz noise levels; these were well correlated with changes in wind speed over the 2-28 knot range. As pointed out by Urick (1983), for a given wind condition, ambient noise levels are 12 dBlor more higher near a sharp ice edge than in open water, and 20 dB higher than the levels measured under the ice sheet well away from the ice edge. In areas where tidal glaciers exist, icebergs and bergy bits generate very high levels of broadband noise due to an effervescence effect and glacial movement on bedrock causes high level seismic impulsive noise. The following brief summary discusses five of the more important sources of ice-related noise. 3.2.6.1 Pressure Ridge Noise and Ice Cracking Buck and Wilson (1986) have reported data which they acquired in the Eurasian Basin of the Arctic Ocean during ice breakup and pressure ridge formation. They were able to deploy two hydrophones approximately 100 m from the ridge zone and at a depth of 30 m separated by 61 m to provide a two element array. A "lead pressure ridge1' was formed when 1-m thick re-frozen lead ice fractured and started to build up due to horizontal forces. A "floe pressure ridge1! was formed after the lead ice was forced onto the 4-m thick floe ice (where the camp was located) causing a build-up of ice load and fracturing of the floe ice. A pressure ridge and fractured keel were formed at the impact zone. Noise spectra acquired during the two stages of the pressure ridge formation are given in Fig. 3.9. Early in the pressure ridge formation (lead pressure ridge), 1/3 octave band sound pressure levels in the 100-400 Hz range were 93-94 dB. During the more forceful portion of the ridge formation (floe pressure ridge) the sound levels increased by about 19 dB to 111-113 dB. Falling temperature causes ice fracturing which results in an increase in underice noise levels. Milne and Ganton (1964) provided data obtained while temperature dropped from -12OF to -38OF in February 1963 during underice experiments in the Canadian Archipelago. Their data converted to 1/3 octave band levels are shown in Fig. 3.9. Probably by coincidence, the low frequency portion of their ice-cracking data coincide very closely with the Buck and Wilson lead pressure ridge formation curve, with peak levels of about 95 dB occurring at 200-300 Hz. 3.2.6.2 Glacial Ice and Glacial Activity Noise During BBN1s field study in Glacier Bay National Park in 1981 (Malme et al. 1982), it was necessary to derive a quantitative description of the acoustic environment at various locations within the park, including sites near tidewater glaciers where a large quantity of broken glacier ice covered the water surface. Ambient noise levels in the vicinity of the glacial ice averaged 50 dB higher than ambients recorded in other areas of the region where no glacial ice was present. The sound spectrum shown in Fig. 3.9 is broadband in nature and is capable of totally dominating other sources of noise. Close inspection of ice specimens reveals myriads of bubbles frozen into the ice which have been compressed to an elliptical or flattened crosssection through increasing pressure during glacier formation. Ablation of the ice causes the compressed gas in the bubbles to vent when at the ice surface
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FIG. D. 1 HUMAN VOCALIZATION AND AU
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FIG. D. 3 ESTIMATED HEARING CHARACT
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