EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



In the areas that we characterize by certain values of k, we 

 notice that typically the standard deviation in velocity determina- 

 tion is about 100 meters per second. I have not been satisfied with 

 that value; I thought we could probably do better. One of the 

 reasons we wanted to do the set of measurements in the Hatteras 

 abyssal plain was it gave us a chance to go to a localized area and 

 do several of these experiments to see how much the scatter in deter- 

 mining velocity resulted from real geological change and how much, 

 perhaps, resulted from some shortcoming in our method. 



It turned out that in the closely grouped measurements in the 

 Hatteras abyssal plain the velocity scatter did not appreciably change 

 over what we had derived from 30 or 40 measurements over the whole 

 Hatteras abyssal plain and part of the lower continental rise. This 

 result caused us to consider whether our treatment of these data was 

 paying enough attention to the details of the structure in the water 

 column. 



2 2 



We had initially treated the water column in the X - T calcu- 

 lations as though it were a constant velocity layer, figuring that 

 we were working mainly with rather steep ray paths for which the 

 constant velocity assumption should produce only a small error. In 

 our first attempt to improve this model, we divided the water layer 

 into several layers, but this did not seem to reduce the scatter in 

 the velocity versus depth determinations. A better water model 

 shifted the average somewhat, not surprisingly, but it didn't really 

 take the scatter out of these data. 



A scheme proposed by George Bryan and representing an effort 

 to escape the water layer model is demonstrated in Figure 2. It is 

 a very simple two-layer model, water and sediment with a reflector 

 at the bottom of each layer. The reflection curves and ray paths are 

 shown . 



255 



