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

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Report No. 6945 BBN Systems and Technologies Corporation Studies of gray whale (Malme et al. 1983) and bowhead whale (Ljungblad et al. 1985) response to noise from air guns have shown that much higher effective peak pressure levels are tolerated before a 50% avoidance probability is reached when compared with the results from constant level playback studies. This is believed to be the result of mammalian hearing characteristics, as discussed earlier. The 50% avoidance probability has been found to occur for an average peak .pressure level of 170 dB for gray whales and 160 dB for bowheads. These responses are for transient signals having a spectrum peak at about 100 Hz and a duration typically less than about 50 msec. The 50% avoidance criterion for air guns has been determined as an average of the overall peak pressure levels for the pressure waveform. The air gun array and vibroseis array signals presented in the industrial noise data base are given in terms of peak level in a 1/3 octave band to be consistent with the other data sets. As a result, it is necessary to specify the overall peak pressure level of air gun signals in terms of the peak 1/3 octave band pressure spectrum to determine an equivalent criterion level for the zone of influence estimate. Measurements of the air gun array operation on the seismic survey vessel WESTERN POLARIS (Miles et al. 1987) showed that the ratio of the average peak pulse pressure to the pressure obtained from a power sum of the peak levels in the dominant 1/3 octave bands was 12 dB. Therefore, when using peak 1/3 octave spectra instead of the pulse pressure waveform, an effective received level of 158 dB is used as the gray whale 50% avoidance cl-iterion for air gun array signals. Moving sources may have a zone of influence which extends beyond the limits determined by the range at which the received level drops to the 50% probability of avoidance criterion. The behavioral response model incorporates a reference response time of 2 hours as the integrazion period in determining Leq. The total energy of all sounds received within a two hour period is considered as potentially influencing a behavioral response. As a result of this concept, moving sources can be considered to leave a trail or "footprint" which remains along the path of the source for a period of two hours. The effective zone of influence becomes elongated and has an area of: where RZ is the range at which the sound level is equal to 50% probability of avoidance criterion level (km) S is the speed of advance of the source (km/hr). On the other hand, as a result of the 2-hr averaging used in determining Leq, the estimated radius of influence around a moving source may be reduced from that expected if the same sound were present for the entire 2-hr period. In effect, the zone of influence would be determined by the range at which Leq equals the criterion level rather than by the range at which the maximum received level (Lr) equals the criterion. Since the concepts of acoustic response time and equivalent level estimation as applied to marine mammal hearing and behavior need further study, for the present, maximum received level values are used to estimate zone of influence radii for both fixed and .
Report No. 6945 BBN Systems and Technologies Corporation moving sources. Zone estimates include both range and area values. For moving sources the area values are determined using Eq. (28). Estimated zone radii are also given using L values where appropriate. eq 5.3.1 Underwater sources By using the measured or modeled TL data for each of the four selected planning areas together with source level spectra, it was possible to obtain plots showing average received level spectra versus range for each of the major industrial sources as determined by the SNC analysis. The zone of influence criteria for gray whales were applied to these plots to obtain estimated zone ranges for each of the planning areas and sources. The 30 dB S/N criterion developed for bowhead whales was also used since measured or estimated ambient noise spectra for the areas were available. This was done to obtain a comparison between this type of criterion and the constant received level criterion. Spectra were estimated at range intervals corresponding to approximately 10 dB decrements in received level in the mid-frequency bands. An example is shown in Fig. 5.7 for the Explorer I1 drillship hypothetically operating in the Chukchi Sea during summer conditions. In this egample the statistical spread of ambient noise levels and the 30 dB S/N avoidance criteria are also shown to facilitate the zone of influence estimation procedure. The information used to develop this figure was based on transmission loss data reported by Greene (1981). Computer assisted interpolation and extrapolation of the data were employed to obtain complete 1/3 octave transmission loss spectra for the range of 31.5 Hz to 1.6 kHz. These spectra were computed for 5 dB TL increments and applied to a measured source level spectrum for the drillship. Selected received level spectra obtained from this procedure are shown in the figure. For the other areas of interest where measured transmission loss data were not available, the results predicted by the IFD Transmission Loss Model (discussed in Section 4.2) were used as the basis for the transmission loss spectra synthesis. A complete set of received level plots for the mador sound sources operating in the four selected study areas is included in Appendix E. The zone of influence ranges and zone of influence areas were estimated. using these plots and summarized in Table 5.12. The zone ranges were determined using both maximum source level and L values, which consider the effective duty cycle of intermittent or fluctuate!g sources. The estimates concern potential gray whale response to underwater sound sources. Predicted responses of other species to airborne sound sources are discussed in the next sect ion. The icebreaker can be seen to have the largest estimated zone of influence in all of the areas where it may be operating. Although ice conditions in the North Aleutian area do not often require icebreaker operation, this area was also included in the zone of influence estimates. The sound transmission conditions in the North Aleutian area, which have relatively low losses at long ranges, provide the largest potential zone of influence. An effective range of 40 km is predicted if the maximum output level is considered. For the measured effective time fraction of 0.5, the L is 3 dB eq
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FIG. 3.2 NOISE SPECTRA IN SHALLOW W
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Adapted From K.H. Jacob (1986) Cali
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FIG. 3.8 ESTIMATED OVERALL SOUND LE
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FIG. 3. Q UNDERWATER NOISE SPECTRA
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FIG- 3.1 1 STATISTICAL ANALYSIS OF
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FIG- 3-12 STATISTICAL ANALYSIS OF C
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-- Alr Gun Array + Vlbrosels -* Ice
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FIG. 3.14 REPRESENTATIVE SHIPS AND
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FIG. 3-18 REPRESENTATIVE CULTURAL S
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FIG. 4.1 CORRECTION TO 300 M REFERE
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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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