HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



Mr. J. I. Ewing (Lamont-Doherty Geological Observatory of 

 Columbia University): That's right, too. There is a critical 

 distance involved. In this situation there is a single ray path, 

 but beyond this distance there are two families. At an increased 

 range one family dives deeper into the bottom than the critical ray 

 does, and the other family goes shallower than the other ray does. 



Beyond that critical distance you have two distinct paths. 



One thing I object to is the neglect of sub-bottom reflections 

 in your treatment. Beyond the low grazing angle you are likely to 

 have rays reflecting off of sub-bottom interfaces at very favorable 

 angles of incidence to return the amount of energy. 



Dr. Hanna: Okay, I should make it clear at this point that I 

 have, indeed, not included those possibilities in the problem and I 

 am not suggesting for a moment that they aren't out there in some 

 real case of interest to us. 



Mr. Ewing: I agree when you are out there near the 30-mile 

 range you probably only have one refracting ray. The deeper ones 

 have probably already been intersected by either some sedimentary 

 reflector or by the basement rock. 



Mr. Pedersen: It depends on where you cut off this positive 

 gradient layer there with your sedimentary bottom. But if you just 

 imagine continuing that on indefinitely, you see that branch has to 

 come back out again in range. 



Dr. Hanna: That sounds like almost an academic thing to do, 

 though. That is to say, this sedimentary layer already is 500, 550 

 fathoms deep and, in any event, I think it is unlikely that any 

 energy that penetrates this deeply, if there is any absorption in 

 the problem, is going to come back to haunt me again, anyway. 



542 



