situation. For example, as shown in Table 1, Case IA3, the semi-submersible 

 drilling rig is audible under quiet ambient conditions out to the tremendous 

 range of 1230 nautical miles with cylindrical spreading loss, but with the 

 more conservative spherical spreading, it is audible only to the moderate 

 range of 1,2 nautical miles. 



The ambient noise condition at the location of the receiving animal has 

 important influence on detection as may be noted in Table 1, Case lAl vs. Case 

 IA3, and Case IIAl vs. Case IIA3. Case I (optimal propagation) shows that for 

 high ambient noise, the detection range is reduced to 13 nautical miles as 

 compared to 1230 in low ambient. Case II (conservative propagation) shows a 

 range of only 190 yards under high ambient noise, compared to 2400 yards (1.2 

 nautical miles) for low ambient conditions. 



It is important to note that the above estimates are intended to provide 

 initial guidelines of maximum and minimum ranges as upper and lower limiting 

 conditions for general planning. The upper limit, Case IA3, is an extreme 

 situation, highly unlikely to be met in practice. Reflection losses at the 

 surface and bottom result in propagation which will in general fall between 

 the Case I and Case II curves of Figure 3, protM!)ly more often nearer the 

 spherical spreading of Case II. For example, an estimate of propagation loss 

 out to 50 nautical miles (101 kiloyards) for the continental shelf off the 

 northern coast of Alaska is 80 to 120 dB for a rroqiioncy of 100 hertz 

 (Underwater Systems, Inc., 1974). This is mucfi greater than the 50 dB shown 

 for cylindrical spreading in Figure 3, and, in fact, brackets the 100 dB shown 

 for spherical spreading. 



G-32 



