The techniques used to measure auditory thresholds of mammals have been reviewed 

 (ref 23). Both behavioral or electrophysiological methods have been used to measure the 

 hearing thresholds of marine mammals. Although an audiogram (ie, a measurement of hear- 

 ing sensitivity as a function of frequency) describes an animal's hearing limits and regions of 

 maximum sensitivity, it does not describe the animal's ability to hear a signal in the presence 

 of background noise. To determine such detection ability, critical band or critical ratio data 

 are required. 



Audiograms indicate that cetaceans and pinnipeds are capable of hearing noise from 

 offshore drilling activities. Data concerning marine mammals' reactions to such sounds are 

 incomplete and essentially lacking. 



SUMMARY OF PUBLISHED DATA 



Source levels (dB re 1 uPa. at Im) for six offshore drilling activities are shown in 

 figures 1 through 6. Estimated source levels were computed by taking the absolute received 

 level measured at the hydrophone and applying propagation loss for the distance from the 

 source so as to estimate the absolute level 1 m from the source. 



Transmission loss in shallow water is sensitive to the environment, eg, sea surface, 

 water depth and bottom type; therefore, spherical spreading loss (20 log R) is not appro- 

 priate. Reference 24 (figure 1) cites 40 log R to approximate sound propagation in the 

 shallow water of Prudhoe Bay. Reference 25 (figures 2 through 5) approximates transmis- 

 sion loss as (20 log R + XR) + S^. For figure 2, X = .0045 dB, and for figures 3 through 5, 

 X = .0075 dB. Spherical spreading (20 log R) was used to approximate the transmission loss 

 in computing source levels for figure 6 (ref 26). 



The source levels of specific frequency components contained in the noise spectrum 

 shown in figures 1 through 3 (ref 24, 25) are based on maximum received levels measured at 

 several distances from the source; therefore, the data in these figures are plotted as average 

 source levels. Source levels shown in figures 4 and 5 (ref 25) and in figure 6 (ref 26) are 

 based on maximum received levels measured at a single distance from the source. The data 

 in these figures are plotted as maximum source levels. 



ESTIMATED SOURCE LEVELS 



Offshore Drilling Activities in the Prudhoe Bay Area 



Figure 1 shows the major noise components from two drilling sites in the Prudhoe 

 Bay area: the NIAKUK 3 well, on a man-made gravel island, and the Reindeer Island Cost 

 Well, on a natural barrier beach island (ref 24). The source levels plotted are averages for 

 received levels measured at ranges from 1000 to 1600 m. 



23 



Francis, RL, Behavioral Audiometry in Mammals: Review and Evaluation of Techniques, Symp Zool Soc 



Lond, 37, p 327-280, 1975. 



Bolt Beranek and Newman Inc Tech Memo 513, Measurements of Underwater Acoustic Noise in the Prud- 

 hoe Bay Area, by CI Malme and R Mlawski, p 16, 1979. 



25 



Ford, J White Whale Offshore Exploration Acoustic Study, Report submitted to Imperial Oil Co, FF 



Slaney and Co, Ltd, Vancouver, Canada, p 21, 1977. 

 ^^ Bell Laboratories, APEX Final Report, by SA Kramer and IE Wing, 1976. 



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