A second factor which makes unlikely the extreme ranges calculated for 

 low ambient noise is the upward trend in ship noise during the last few 

 decades (Ross, 1976). At low frequencies the present levels of shipping make 

 it highly unlikely the light shipping noise shown in Figure 4 will be 

 experienced, except in yery remote locations. The moderate curve serves as a 

 much more probable lower limit to low-frequency ainbient noise. 



The above considerations suggest that a realistic interpretation of 

 maximum expected ranges in Table 1 would best depreciate the possibility of 

 the extreme ranges associated with Case I (cylindrical spreading) and ambient 

 noise condition 3 (low). This suggests that the more probable limiting ranges 

 would fall between Case IIA2 (0.22 nautical miles) and Case IA2 (99 nautical 

 miles). 



Another important consideration is the apparent wide variation in amount 

 of underwater noise radiated by different platforms, both drilling and 

 production. Some, like those used for the calculations in this report, are 

 quite noisy, particularly at the low frequencies in the 10 to 1000 Uz range 

 considered important to whales, while other platforms, with no highly obvious 

 mechanical differences, appear to be quiet underwater. Tlie fact that such 

 differences occur suggests that it is possible to construct and operate oil 

 drilling and production platforms such that they do not produce great 

 underwater noise levels. It is highly important that the differences between 

 the noisy and quiet platforms be carefully analyzed to determine the critical 

 noise-determining factors in design, machinery type and mounting, and 



G-33 



