HANNA: DESIGN OF TRANSMISSION LOSS EXPERIMENTS 



Dr. Flatte: I want to tell Chuck that you can't focus a plane 

 wave, you are right. 



Mr. Spofford: Thank you. 



Mr. Pedersen: I have a few comments to make on the measurement 

 of bottom loss that might help resolve some of these ambiguous 

 situations. 



When we make bottom-loss measurements, we perhaps run typically 

 out to the second or third convergent zone. We compare the loss per 

 bounce that we got by way of one bottom bounce, two bottom bounces, 

 and three bottom bounces. 



That is, reduce all the data to a common base and then, if 

 these don't agree, you have a self-consistency check right on the 

 spot. That is, you don't have to come back another time to measure 

 to see if it was consistent to the extent that the bottom is uniform 

 over this distance. 



You can make these comparisons and any errors in source level 

 always show up as a fixed displacement. That is, sometimes instead 

 of measuring the loss directly, you measure the difference between 

 the second bounce and the third bounce, or something like this. There 

 are certain fixed errors that can be removed in this fashion. 



The second point about this is the problem of measuring loss at 

 the low angles as you approach the convergent zone. The relationship 

 of where the bottom reflected angle is intersected by the zone doesn't 

 stay constant from zone to zone. Generally speaking, if you have a 

 case where you just have surface reflected rays, I believe that you 

 can penetrate down to lower angles by going into the, say, third 

 convergent zone than by going into the second zone. 



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