HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



amplitude can be at most A (neglecting spreading losses) and, thus, 

 will not exceed the energy of a path for the case where only one of 

 these two is present. Such a mechanism, then, cannot give rise to 

 negative reflectivity. 



While the above argument concludes that the presence of both 

 bottom-refracted and bottom-reflected paths cannot produce apparent 

 negative reflectivities, their simultaneous presence will certainly 

 produce interference patterns in the reflectivity as a function of 

 frequency. Figure 17 shows the relative arrival time structure for 

 the bottom-refracted and bottom-reflected paths for the case being 

 discussed here. Note that the reflected counterpart of each 

 refracted path arrives earlier; the fact that these paths arrive 

 simultaneously at maximum range is a direct consequence of no velocity 

 discontinuity at the bottom. The maximum travel-time difference 

 between these paths is about 15 msec for the geometry considered here; 

 the period of the corresponding variation with frequency of the 

 reflectivity will be 66 Hz or greater. Thus, the 1/3-octave filters 

 discussed above will give essentially the coherent combination of 

 these paths. 



Finally, note that the need for dealing with four inseparable 

 paths could be avoided by getting the source and receiver away from 

 the ocean boundaries, but this in general will increase the inter- 

 ference of refracting fields. Conversely, the influence of the 

 refracting fields diminishes near the ocean surface, but the need 

 to deal with the four paths increases. This qualitative trade-off 

 suggests that low grazing angle measurements may always pose a 

 significant problem. 



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