CAVANAGH: AMBIENT-NOISE MODELS 



This approach has been implemented by Bartberger (1969) at NADC and 

 also at AESD.* It can be extended for more realistic descriptions of 

 the ocean environment, namely those in which sound speed and bathy- 

 metry vary with range. 



The extension uses the invariance of action discussed by Weston 

 (1959) and Milder (1969) , and more recently applied by Preston Smith 

 (1974) . Define the action for a geometric ray as 



fm 



where the line integral is over a complete ray cycle, c is local 

 sound speed, 6 is ray angle. For an ocean which is slowly varying in 

 range (i.e., nearly constant over a ray's range period), J is an 

 invariant on a ray. Hence, by tracking the evolution of each ray 

 via J, we can efficiently estimate range-averaged transmission loss 

 or, more directly, calculate n with formula (6). 



In summary, given the present resolution of inputs to noise 

 models, a sensible alternative to the expensive point model is the 

 field model equipped with an efficient method of dealing with the 

 range-varying environment. 



* AESD Memo, "Fast Ambient Noise Model I (FANM I) ," 1974. 



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