LOW-FREQUENCY PROPAGATION IN THE 

 ICE-COVERED ARCTIC OCEAN 



Henry W. Kutschale 



Lamont-Doherty Geological Observatory 

 Coliiinbia University 

 Palisades , New York 



The two features peculiar to the polar environment that 

 most strongly influence underwater sound are the permanent 

 ice cover, which averages about 12 feet in thickness in 

 the central Arctic Ocean, and the velocity structure in 

 the water. Ice movement generates ambient noise, and the 

 ice modifies propagation, particularly at high frequencies, 

 by scattering waves from the rough ice boundaries. Signals 

 are detected by hydrophones at water depth and geophones in 

 the ice. In deep water of the central Arctic Ocean, sound 

 velocity is generally an increasing function of depth from 

 the surface to the bottom. Such a velocity profile is 

 found only in polar waters. The sound velocity structure 

 is remarkably uniform as a function of both location and 

 time of year. Sounds are transmitted to great ranges in 

 this sound channel by upward refraction in the water and 

 repeated reflection from the ice canopy. The surface 

 sound channel of the Arctic is the polar extension of the 

 deep sound channel or SOFAR channel of the nonpolar oceans, 

 but the Arctic signals often differ markedly in character 

 from those observed in the deep channel, largely because 

 of the predominance of low-frequency waves (& to 40 Hz) 

 in the Arctic. The Arctic signals are dispersive with 

 low-frequency waves traveling faster than high-frequency 

 waves in each normal mode. At a range of 350 nautical 

 miles from an explosion up to eight normal modes are com- 

 monly observed with a suggestion of still higher modes. 

 In contrast to the uniformity of the velocity structure 

 in the water, the bottom and surface topography of the ice 

 is highly variable in both space and time. This vari- 

 ability of the ice cover is the major cause of corresponding 

 variations of propagation loss observed in different areas 

 during nearly simultaneous experiments over deep water. 

 Recent experiments conducted in cooperation with O. I. 

 Diachok of the Naval Oceanographic Office are described to 

 illustrate recent advances in experimental techniques . 

 Surface ice roughness was measxired continuously by laser 



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