Although the Prudhoe Bay data show Uttle difference in noise level, the noise com- 

 ponents at each site differ. The authors (ref 24) note that the noise levels above 8 kHz were 

 low. 



Tufts Point Dredging Site/Arnak Artificial Island Construction Site 



Figures 2, 3 and 4 show the noise generated from two construction locations in the 

 Beaufort Sea (ref 25). The sounds are from construction activities associated with develop- 

 ment of offshore operations. 



At the Arnak artificial island site, operating machinery included a suction dredge, a 

 tending tug, a clamshell shovel, and several crew boats. Figure 2 shows the noise components 

 from this site. The frequency band and amplitudes from the Tufts Point and Arnak sites are 

 similar. Data were not reported for frequencies below 250 Hz (ref 25). 



At the Tufts Point dredging site, noise sources included a suction dredge, crew boats 

 and tugs. Noise measurements were made at ranges from 90 to 4000 m in four different di- 

 rections from the site. An artificial breakwall extends northwest from the site and probably 

 limited noise from that direction (fig 3). The average noise levels from the other three direc- 

 tions are similar in frequency and higher than values measured from the northwest. 



Transient sounds also were recorded at the Tufts Point site. Noisy couplings in the 

 floating pipeline probably produced the short-duration sounds plotted in figure 4. 



Logistic Traffic Noise at the Tufts Point Site 



Figure 5 shows the noise generated from tugs, tugs pushing barges (empty and full) 

 and crew boats at the Tufts Point site. The frequency spectra and amplitudes are compar- 

 able to those in figure 2. The isolated sources shown in figure 5 also were included in the 

 composite sounds shown in figure 2. 



Semi- Submersible Platform in the North Atlantic 



Figure 6 shows source levels for low frequency component noise from a semi-sub- 

 mersible drilling platform in the North Atlantic (ref 26). These values are similar to those 

 shown in figure 1, but the amplitude varies less with frequency. The Atlantic measurements 

 are from a single, distant measuring site in deep water, and thus likely less variable than the 

 Arctic measurement. 



Data in figures 1-6 show noise from offshore oil and gas drilling activities is in the 

 frequency range from 10 Hz to 10 kHz, with peak source levels between 130 and 180 dB. 

 Signal-to-noise (S/N) ratios may approach 80 to 100 dB above background noise levels 

 (ref 27). Depending on the detection threshold of the receiver and the prevailing back- 

 ground noise levels, S/N levels of these magnitudes could be detected at considerable ranges 

 from the source. 



To estimate distances at which a marine mammal could detect a component of noise 

 with source levels shown in figures 1-6, a transmission loss model for deep or shallow water 

 propagation must be selected. Either model includes a number of assumptions concerning 

 the characteristics of the receiving system. (Information on the hearing for large whales is 

 discussed in the following section.) These assumptions are: 



-^ Urick, RJ, Principles of Underwater Sound, p 384, McGraw-Hill Book Co. 1967. 



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