HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



I think you are just arguing whether or not you interpret that 

 increase as a negative bottom loss or focusing. 



It really doesn't matter what you call it, but it is possible 

 to get the same effect. 



Dr. Hanna: The computations of intensity, that is, the rms sum 

 of those four paths that I showed, were made using a ray tracing 

 program which had that strong negative gradient in it. That is how 

 the rays were traced. It was not assumed that the rays reflected 

 from that bottom boundary, they refracted through it in that calcu- 

 lation. 



Dr. Weinberg: Maybe in the particular example that you looked 

 at, what you are saying is exactly right. But it's easy enough to 

 construct another example where you can get focusing into that 

 bottom region. 



Mr. Ewing: The way I look at it is that you have a gradient in 

 the water, and maybe you change the value of the gradient in the 

 sediment, but I think it can still even be linear, Brackett. The 

 simplest case is not to assume a discontinuity there. Consider an 

 infinitely thick section of sediment that just has a gradient, and 

 for the moment, let's forget about any possibility of reflection here. 



I believe it is proper to say that you don't hear anything at 

 your receiver until you get to some critical distance from the 

 source, at which the value of the gradient permits a ray to be bent 

 around and get back to the surface. That happens at some specific 

 depth below the interface. 



Then, if you imagine just one step beyond that, what you get 

 is one limiting ray that does not get quite as deep as the first one 



548 



