EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



Figure 3 shows the distribution of our sonobuoy wide-angle 

 reflection measurements on a world basis. The boxes indicate areas 

 where we seem to have enough measurements in a geologically definable 

 province to characterize it. Figure 4 shows a value of k (gradient) 

 for each of these same areas. 



Do not pay any attention to the central equatorial Pacific area. 

 It indicates a very high value of gradient with a k value of 3.9. 

 Although the value is correct, it represents a special case of some 

 very thin, low-velocity sediments on top and some very high-velocity 

 limestone at the bottom. It more properly ought to be treated as 

 a two-layer case. The other numbers are the best values we can pro- 

 duce at present. Remember that the numbers represent k in the linear 



expression V = V + kT. 

 o 



Our methods of measuring from the surface are just not good 

 enough to determine with precision the uppermost sediment velocity 

 (< 100 meters thickness) , but some characteristics of these data 

 give us very good reason to believe that in the uppermost 100 meters 

 or so is a considerably steeper gradient than the value listed for 

 the entire section. 



I want to discuss now the distribution of sediments. This is 

 important because if negative bottom loss is a reality, it is be- 

 cause velocity gradients (and good sub-bottom reflectors) form, in 

 effect, an acoustic lens at certain ranges. The more sediment we 

 have, the more possibilities we have for acoustic lenses of various 

 characteristics, to say nothing of the smoothing effects of sub- 

 stantial thicknesses of sediments. So it is of some interest to us, 

 I think, to know the distribution of sediments around the world. 



Figure 5 gives the distribution for the Atlantic. Although you 

 cannot see the thickness contours, you can see the hatched region in 



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