EWING: ACOUSTIC PROPERTIES OF THE SEA FLOOR 



interfaces. If in one province we get enough such measurements, we 

 can construct a plot of interval velocity versus depth in the sedi- 

 mentary coliomn. 



We usually plot the depth function in one-way travel time, in 

 seconds. The inset in Figure 1 shows a velocity/depth plot for the 

 western North Atlantic rise. Here, as in most other places, we find 

 that we can fit these data with a function that is linear in time 

 although not quite linear in depth. If we express V = V + kT, where 



T is one-way travel time vertically through the sedimentary section, 



2 

 k in this equation is in units of km/sec . Most physicists, I think, 



tend to think of sound-speed gradients in depth rather than time, 



which are typically expressed in terms of kilometers per second per 



kilometer or just in seconds . In most of the velocity range that 



we are dealing with in soft marine sediments, these two types of 



gradients turn out to be only about a factor of two apart. In other 



words, a gradient of about one per second corresponds to a k of 



approximately two kilometers per second per second. 



In a lot of our measurements of this type from around the world, 

 we characterize different areas in terms of this value, k, which, in 

 fact, is characterizing the sea bottom in terms of velocity gradient 

 in the sediment. Before I summarize these measurements, refer again 

 to the inset in Figure 1. I pointed out that there is quite a lot 

 of scatter in these data. The reason we can get this many data 

 points is that the geology changes even in a rather local region. At 

 one place we may see a reflector at some depth below the sea bottom; 

 in other places we may be measuring it at half that depth or twice 

 that depth. So if we make enough measurements, we get a fairly good 

 distribution of layer thicknesses, and, therefore, we get several 

 values of T. For each value of T, which is a measure of depth in the 

 section, we calculate interval velocity so we can get a good distribu- 

 tion of velocities versus depth. 



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