BUCK: ARCTIC ENVIRONMENTAL LF ACOUSTICS MEASUREMENTS, 

 MODELS AND PLANS 



good or better than in open ocean areas, but only in the 

 very low frequency spectrum; above about 100 Hz, it is 

 considerably worse. Transmission loss can be closely 

 approximated with a simple low-pass filter model that 

 has been derived empirically. 



Deep Arctic ambient noise is dominated by ice dynamics 

 and can vary in level from close to Wenz ' s "lower limit" 

 to that of the open ocean under near-hurricane conditions. 

 Local wind force correlates highly with ambient levels, 

 as does ice speed. Long-term median levels are highly 

 dependent on geographic location with the quietest loca- 

 tion in the ocean being the center of the Pacific Gyre. 

 Under high noise conditions the noise is highly aniso- 

 tropic. High correlation between noise-level time series 

 at stations separated 150 miles has been observed. There 

 is evidence that low-frequency noise is vertically direc- 

 tional and the predominant energy is close to or slightly 

 below the dominant long-distance signal ray arrivals. 

 Therefore, the usefulness of a vertical array is doubtful. 

 Signal-to-noisewise, a horizontal seismometer frozen into 

 the ice is comparable to a 100- foot deep hydrophone and 

 slightly better at the lowest frequencies. 



Preparations are under way for a large-scale central 

 Arctic ambient noise experiment in conjunction with 

 Project AIDJEX. Up to 24 unmanned stations collecting 

 noise level and weather data at the synoptic times every 

 3 hours for up to 2 years is planned. Satellite and HE 

 telemetry will be used in this experiment which will be 

 the most extensive and complete low- frequency ambient 

 noise experiment ever conducted in any ocean area. 



The stable sound velocity profile of the ice-covered Arctic is 

 characterized by a steep positive gradient to around 500 meters 

 followed by a more gentle positive gradient to the bottom. This 

 causes a concentration of shallow RSR rays that travel in the upper 

 500 meters and suffer a relatively large number of reflections per 

 unit distance and a group of more widely spaced, deep rays that 

 reflect less often (see Figure 1) . The bottom of Figure 1 is the 

 signature of a shot from 780 nautical miles showing the distinct 



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