HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



did, and you get another ray that goes quite a lot deeper than that, 

 but coming to the same path. 



I believe this is, in effect, your lens. When you also intro- 

 duce reflectors in the sub-bottom, you can, at the right angle of 

 incidence, produce, almost, unit reflectivity. 



Dr. P. W. Smith (Bolt, Beranek, and Newman, Inc.) : What we are 

 after is tabulated bottom losses for prediction of changes in the 

 environment which are independent of the bottom. We do not want 

 those changes in the environment independent of the bottom to change 

 the parameters by which we classify the bottom. This particular 

 focusing feature is peculiar to the environment. We want a charac- 

 terization of bottom reflection that will be useful for sampling. 

 The problem is a very complex one. How do we take this apart and get 

 a nimiber that we can usefully use for transmission loss prediction. 



Mr. Swing: 1 fail to see how the energy that is returned from 

 the sediment is not part of the problem, because it is energy 

 returned into the water. A very large amount of it is returned to 

 the water. 



Dr. I. Dyer (Department of Ocean Engineering of Massachusetts 

 Institute of Technology) : The analogy might be that if for some 

 reason the model makers forced us to neglect the lower 2,000 meters 

 of the water column, and replaced the lower 2,000 meters of the water 

 column by an effective bottom water reflectivity and we find we have 

 convergent zones and we say, "Ooops! A negative reflection loss." 



It seems to me that the problem here is no different. The 

 bottom is part of the column, and any attempt to put an artificial 

 line there and describe it by a simple number in this frequency 

 range is bound to fail. 



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