V (e at - 1) 

 h = ° 



where h = sediment layer thickness (km) 



V = velocity of sound in sediments at the 

 water-sediment interface (km/sec) 



a = average vertical sound velocity gradient 

 within sediment (sec - -'-) 



t = one-wav sound travel time within sediment 

 (sec) 



(16) 



For deep sea sediments 



0.5 < a < 2.0 sec 



-1 



with average, a, near 1.0 sec . Most values, however, lie between 0.9 

 and 1.4 sec - ! (Hamilton, 1965). 



Some researchers have described the variation of acoustic velocity 

 with depth with a linear equation of the form 



V(Z) = V + aZ (17) 



where V(Z) = compressional velocity of sound at sediment 

 depth Z (km/sec) 



Z = depth in sediments (km) 



Examples of such equations are: 



shallow water sediments: V(Z) = 1.70Z +1.70 



Nafe and Drake (1957) (18) 



deep water sediments: V(Z) = 0.43Z 4- 1.83 



Nafe and Drake (1957) (19) 



Besides calculating the sediment-layer thickness, a knowledge of the 

 acoustic compressional velocities is useful in inferring certain physical 

 properties of the sediment and rock. For example, in an analysis of sedi- 

 ments from Lake Erie, Morgan (1969) arrived at the following equations: 



V = 2.380 - (2.197±1.208)n + (1. 333±0.982)n 2 r = 0.90 (20) 



V = 2.232 - (1.168±1.103)p + (0. 451±0. 333)p 2 r = 0.90 (21) 



12 



