SECT. 3] 



PHYSICAL PROPERTIES OF MARINE SEDIMENTS 



811 



G. Depth Dependence of Compressional Velocity 



Evidence for depth variation of compressional velocity in marine sediments 

 is derived from reflection and refraction measurements. Arrivals reflected at a 

 sub-bottom interface or refracted in the upper part of the sedimentary column 

 commonly show the effects of gradients ranging from 0.5 to 2 sec~i. Hill (1952) 

 observed gradients of the order of 2 sec~i in the eastern Atlantic. Officer (1955) 

 reported reflections at a location northeast of Bermuda that indicated an average 

 gradient of about 1.0 sec-i. Katz and Ewing (1956) reported gradients of about 



2 - 



Depth (km) 



Fig. 7. Compressional velocity as a function of depth of burial. Data were obtained from 

 seismic refraction measurements. The solid curve was derived by Laughton (19.54) 

 from the artificial compaction of Globigerina ooze. It is a rough lower limit to the 

 observations. 



0.6 sec~i at a number of stations in the western Atlantic. Evidence from both 

 sub-bottom reflections and from refracted arrivals was summarized by Nafe and 

 Drake (1957). In Fig. 7 are plotted observations of velocity against thickness of 

 sediment overlying the refracting horizon for a number of stations in the 

 western Atlantic. Points of the figure are not distinguished as to water depth. 

 The only discernible pattern is shown by comparison of the measurements with 

 velocities found by Laughton (1954) for artificially compacted Globigerina ooze. 

 Laughton's velocity-depth curve, based on an assumed parabolic gradient, is an 

 effective lower limit to the points measured by the refraction method. This is to 



