BUCK: ARCTIC ENVIRONMENTAL LP ACOUSTICS MEASUREMENTS, 

 MODELS AND PLANS 



1,200 feet were separable from the deep rays in a typical shot signa- 

 ture, allowing separate energy analyses of the two. Figure 10 shows 

 the results of this type analysis at close to 800 nautical miles. 

 Both left and right plots are of percentage energy versus frequency. 

 The left plot averages all shot depths but separately graphs four 

 propagation paths by mean bottom depths. For example, at 20 Hz over 

 the path of mean bottom depth 9,200 feet, 58 percent of the total 

 arriving energy has travelled in the RSR rays shallower than 1,200 

 feet and 42 percent is in the deep rays. 



The right-hand plot averages all of the four paths but separates 

 the percentage energy by source depth. For example, at 20 Hz, for the 

 200-foot shot depth, 25 percent of the shot signal energy is in the 

 shallow rays and 75 percent is in the deep. 



It is concluded that in the frequency bank of 10 to 20 Hz, and 

 for source depths of 200 to 800 feet, only 2 to 6 dB signal is lost 

 for a path of 1,200 feet minimiim depth relative to the deepest possible 

 long-range path in the Arctic. 



Figure 11 demonstrates that the causes of noise in different 

 parts of the Arctic can be quite different. The three sets of plots 

 are for the shore-fast ice of the Canadian Archipelago (top) , an ice 

 island (middle) , and a floe (bottom) . The top of each set is 

 temperature versus time and the bottom is 150 to 300 Hz band noise. 

 In the Archipelago there is very strong correlation between the two 

 time functions but none at the Central Arctic sites. Thermal ice 

 cracking is the major source of noise in static shore-fast ice but 

 this effect, while present, is far overshadowed by gross ice dynamics 

 in the Central Arctic. 



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