76 



KWING AND NAFE 



[chap. 5 



in the sediments. The wave guide might be a low-velocity layer, as described 

 by Katz and Ewing, or it might be bounded above by a discontinuity and 

 below by a velocity gradient. In either case the coupling would require that the 

 up])or sediments have a lower velocity than the bottom water. 



'i'hc evidence cited above indicates two significant features of deep-ocean 

 sediments: (1) they have appreciable velocity gradients, particularly in the 

 upper few hundreds of meters, and (2) the velocity in the uppermost part of 

 the sedimentary column is equal to or lower than that in the bottom water in 

 most places. Direct measurement of sound velocities in sediments (Laughton, 

 1954; Hamilton, 1956; Hamilton ef a7., 195(): Shumway, 1956; Sutton, Berck- 

 hemer and Nafe, 1957) has shown velocities generally ranging between 1.45 to 

 1.80 km/sec. Most values for deep ocean sediments are in the lower range, in 

 good agreement with the seismic evidence. In the intermediate and shallow 

 areas, higher velocities are found by both types of measurement. A comparison 

 of velocity versus depth relationships in shallow-water and deep-water sediments 

 is given by Nafe and Drake in Chapter 29. 



3. Variable Angle Reflections 



The method used to determine the amount of gradient by seismic reflection 

 profiles is outlined briefly in Chapter 1 and has been described in more detail 

 by Hill (1!)52), Ofticer (1955) and Katz and Ewing (1956). It is a process in 

 which a family of theoretical curves is compared with the observational data 



1.0 ^ 



tt 



I 0.5- 



ffjo - 5 sec 



" =6 sec 



" = 7 sec 



o -Profile A , Rj^Q^ 5 sec 



D (sec) 



Fig. 2. Theoretical curves for i?.s-/i?i versus D. Observational data from two profiles 

 are plotted. Solid symbols represent arrivals observed in tirst-order reflections and 

 refractions; open symbols represent second-order observations. 



and the best fit determined. The curves shown in Fig. 2 (from Nafe and Ewing, 

 unpublished manuscript) give theoretical values of i?s - /?i versus range, D, 

 for three different values of water depth and gradient, where /?s is the travel 

 time of the wave refracted in the sediments, ^i is that of the bottom-reflected 

 wave and D is that of the direct water wave. These curves were computed for 

 linear velocity gradients, for which the formulae are 



