In view of the unavailability of a data base of direct behavioral 

 observation as described above, this study is taking an alternative approach, 

 using the source-path-receiver model. In this approach, the underwater noise 

 is measured at a known distance from the oil platform, or other noise source, 

 a sound propagation path is assumed, and the sound pressure level is 

 calculated for various distances from the source. Estimates of the hearing 

 sensitivity of various species of animals are then used to predict the range 

 of audibility of the various sounds emitted from OCS operations under various 

 weather and oceanographic conditions. Such estimates of detection range 

 provide initial guidelines of maximum expected ranges of influence, the 

 implicit assumption being that if the animal is unable to hear the sound, the 

 animal will be unaffected by it. Of course, it must be pointed out that the 

 fact that the animal is able to hear the sound (at ranges shorter than the 

 detection range) does not assure a reaction. In fact, it is likely that 

 unless the sound has an extremely threatening meaning to the animal, overt 

 responses to the sound may not occur until its level is substantially above 

 the threshold of detectability. The need for actual observational data on 

 response thresholds of the animals to various OCS oil and gas platform-related 

 sounds must be reiterated here. Field observations of animal behavior under 

 conditions of known acoustic stimuli are essential, and should be strongly 

 supported, but until they are available, maximum range estimates of the type 

 presented -in this report can serve as rough estimates of maximum ranges of 

 possible influence. Although the type of response to be expected from animals 

 within the maximum detection range is highly uncertain, the expectation of 

 zero influence at distances beyond this range may be a '^ery useful 

 consideration in environmental planning. 



G-7 



