HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



Suppose we have two rays that bend together repeatedly , you 

 know, successive rays forming a caustic, there can be intensifica- 

 tions without any violation or conservation of energy or anything 

 like that. It's commonly known as focusing. 



All along, you see, we have had experimental evidence for years 

 that there is a strong intensification of an arrival that has to pass 

 through the bottom in some manner and arrive at ranges of the order 

 of 12-14 miles plus, and it continues strong for quite a few miles. 



This is the experimental reason why I have believed in the 

 possibility of negative bottom loss. But I have been left very 

 hungry by these various ray analyses, because always it was carefully 

 explained to me that the velocity gradient in the bottom was linear. 



Mr. Spofford: I think the point is we are after the plane wave 

 reflection coefficient of the bottom. This is what the models need, 

 this is why we are supposed to be out there measuring reflectivity. 



If you go to a range where you think you are observing a 5 degree 

 grazing angle on the bottom and you are really seeing a reflected 

 angle at 5 degrees, plus an angle which is going into the bottom at 

 20 degrees, transiting through the sediment, and coming up again 

 with a strong focus (which is certainly possible if the sediment is 

 deep enough and the curvature is strong enough) you are not measuring 

 the reflectivity at 5 degrees, you are measuring transmission loss at 

 that point in range. 



The point I think John and I are trying to get to is, there is 

 a specific mission in mind for these measurements which is bottom 

 reflectivity. If you put plane waves into the bottom at various 

 angles, you don't observe reflectivities greater than one. 



545 



