ARCTIC ENVIRONMENTAL LF ACOUSTICS 

 MEASUREMENTS, MODELS AND PLANS 



Beaumont M. Buck 

 Polar Research Laboratory, Inc. 



Measurement of low frequency (LF) propagation and noise have 

 been made at two types of manned ice camps in the Arctic over 

 the past decade. Floe ice stations are the best acoustic plat- 

 forms but are short-lived (2 to 3 months in the spring) . Ice 

 islands provide long-term facilities, but man-made noise 

 levels are high and the large, thick "islands" have their 

 own environmental acoustic character that is not necessarily 

 representative of Arctic pack ice. The under-ice acoustic 

 environment varies considerably with both geographic loca- 

 tion and time and, since logistics costs of manned experi- 

 ments are high, only a limited sampling has been possible 

 to date. This paper highlights the current state of deep 

 Arctic LF acoustics empirical data, presents a simple 

 propagation model and describes an extensive ambient noise 

 experiment in preparation for 1975-77 that utilizes the 

 latest in unmanned remote stations. 



Arctic "deep" water is conveniently defined by the nature 

 of the propagation under conditions of the stable velocity 

 profile that obtains in the 1.5 million square miles deeper 

 than about 500 meters. Underwater explosives have been 

 used almost exclusively for transmission experiments. How- 

 ever, tame submarine targets have been periodically available 

 in the Arctic to test detection feasibility of the largest 

 circular array installed in any ocean. The ice itself 

 affords a unique opportunity for such installations and, 

 together with the extremely low ambient noise conditions 

 that prevail at times, an excellent testing ground for 

 underwater explosives as acoustic sources. 



Deep Arctic ray theory is now well comprehended and, with 

 measurement data, enables an understanding of many practical 

 aspects of propagation important to low- frequency sonar, 

 for example, optimum hydrophone and target depths for 

 maximizing or minimizing signal-to-noise ratios. Almost 

 all sound energy from long ranges arrives in the depression 

 angle sector of 5 to 17 degrees below the horizontal. At 

 ranges of over 100 nautical miles, the propagation is as 



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