Clark and Yarnall 



1) An analytical result proves that geometrical spread- 

 ing loss for all SRBR rays exceeds that of any RBR ray. 



2) A comparative study indicates that bottom reflection 

 losses are greater in the SRBR model. 



3) The SRBR rays will be subject to scattering losses 

 at the surface which will not be experienced by 

 RBR rays. 



4) The RBR model provides a reasonably good fit to measur- 

 ed values of travel time dispersion. 



A decision has been made to neglect the SRBR contribution in 

 this discussion, and consider the signal at H43 a superposition of 

 RBR arrivals only. 



It should be noted that the model makes no provision at all 

 for mixed rays of the type illustrated in Figure 7c,* As will be seen, 

 the effects of the sea surface are evident in the acoustic data, but 

 are not a dominant influence. It is necessary to emphasize that the 

 models under discussion can make no attempt to account for the full 

 complexities of the actual propagation path between Fowey Rocks and 

 Bimini. Indeed, more thorough studies might lead to the conclusion 

 that a model composed exclusively of mixed rays is the most adequate 

 physical description possible with ray theory. 



Proceeding with the RBR calculations, we are now in a po- 

 sition to perturb the model in various ways which will provide a 

 simulation of certain large scale changes in the medium. Our purpose 

 is twofold: 1) to study the acoustic response to the changes, in order 

 to gain insight into the nature of the measure of medium variability 

 provided by acoustic amplitude and phase, and 2) to obtain theoreti- 

 cal order of magnitude numbers associated with such changes, for 

 comparison with experimental data. It is possible, for example, to 

 simulate the effect due to tidal changes in water depth by increment- 

 ing the water depth sinusoidally, and adjusting the sound speed gradi- 

 ent for the effect of the pressure change at each increment. 



This simulation has been studied in some detail, and has 

 contributed to the conclusion that tidal water depth changes cannot 

 account for the full range of observed tidally related phase varia- 

 tions at H43. Over a water depth change equivalent to the tidal 

 range in the Straits the first ray arrival (N=9 in Fig. 7b) suffers 

 negligible change in amplitude, and 260° change in phase. Subsequent 



* These are selected rays from computer calculations performed 



with line segment approximations to the bottom and to the sound 



speed profiles (Refs. 1 and 4), The technique does not permit 

 source to receiver calculations. 



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