EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



As you can see from the model, it is possible to take pairs of 

 rays in which the rays from the bottom reflection and from the sub- 

 bottom reflection are parallel to each other in the water column. If 

 we differentiate these reflection curves, the derivative tells us 

 the slope of the curve, of course, and, physically, the inclination 

 of the ray at the sea surface at that point. In effect, if we go 

 along these reflection curves and find pairs of derivatives that are 

 the same, we are finding pairs of rays (of which one is a bottom 

 reflection and one is a sub-bottom reflection) that have traveled 

 parallel, and presumably equal, time paths through the water. Thus, 

 each pair of common derivatives gives us a AX and a AT associated 

 with the path through the sediment layer, as shown in the diagram. 



We then carry out this procedure over a wide range of AXs and ATs, 



2 2 

 plot an X - T profile and get a value of interval velocity for the 



sediment layer. 



We treated a substantial amount of our data in this way and we 

 still have a lot of scatter — more than I like. This treatment 

 should take account of the water structure, but, of course, it only 

 takes account of a fixed water structure. No matter how you analyze 

 these data, the water layer is a part of the model and if it changes 

 significantly during the course of the experiment, you still have 

 a problem. 



We plan to put our entire experiment on the bottom of the ocean 

 as one way to answer the question for certain whether our scatter in 

 velocities versus depth results from the water column or from geology. 

 I'd be very surprised and disappointed if there were no geological 

 effect. But I have yet to be convinced that all of the variations 

 are geological ones. 



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