SOUND-VELOCITY GRADIENTS 



The velocity of sound in the Mohole sediments just 

 below the water-sediment interface (1484 m/sec) is about 

 2.2 percent less than the velocity in the sea water just 

 above the bottom (1517 m/sec). (This is a commonly 

 observed phenomenon for this type of high-porosity deep- 

 sea clay. ) This value compares with the 2.4 percent com- 

 puted by Fry and Raitt in the area, and that measured 

 from the TRIESTE in the San Diego Trough (2.3 percent). 35 ' 5 



Vertical sound-velocity gradients in deep-sea sedi- 

 ments appear to lie between 0.5 and 2.0 sec" , with most 

 values between 0.9 and 1.4 sec -1 . 4 " The balance of 



evidence indicates that the corresponding velocity-vs-depth 

 relationship is parabolic or exponential rather than linear 

 in form, but there is insufficient evidence on which to make 

 a decision. Several constant gradients have been indicated 

 by dashed lines on figure 4. Apparently the vertical sound- 

 velocity gradient, as reconstructed by these samples, lies 

 between 0.7 and 1.4 sec -1 , with an average near 1.0 sec -1 . 

 If 1.0 sec -1 is used, an interval velocity of 1569 m/sec is 

 derived, which compares favorably with the value of 1.6 km/ 

 sec which was computed from the seismic reflection sur- 

 veys and the drilled thickness. 28 A linear velocity equation 

 for the area, of the form 



would be 



Velocity at depth, Z, V = V + KZ 



V = 1484 + 1.0(Z) 



where 



V = velocity at sediment surface in m/sec, 



o J 



K - velocity gradient, 



Z = depth in the sediment in meters. 



33 



