V 2 . (^ r arT T o v an~l~ T o n -i) 



(Ton - T o n -l) (30) 



where V = interval velocity for nth laver 



If the hydrophone and sound source are separated hy a sufficiently 

 large distance, sound waves are refracted at the seafloor and underlying 

 interfaces. These refracted waves travel at greater speeds through the 

 rock and are again refracted through the water to the receiving hvdro- 

 phone. In practice, the extended hydrophone cable is gradually hauled in 

 while a record is being made. Initiallv, the refracted waves arrive before 

 the direct waves, hut the difference in arrival times becomes smaller as 

 the cable length is decreased. The reciprocal of the slope of the refrac- 

 ted arrival record, time versus distance, is the velocity. For a compre- 

 hensive discussion of this method, see a standard geophvsics text, as 

 Dobrin (1960). 



Another method for determining sound velocities through sediments is 

 the common depth point (CDP) method (Hempstead, 1966). This technique, 

 widely used bv the petroleum industrv, involves the recording of reflec- 

 tions at different surface detectors from different shot positions which 

 are chosen to maintain the same reflecting points on the subsurface reflec- 

 tors. The results vield a time-varying primary and multiple velocities. 

 Computers are generally required in processing the data. See Mayne (1965, 

 1962, 1967) for excellent discussions of this method. 



Manv other sophisticated techniques developed primarily for use in 

 petroleum exploration have been utilized, such as Automatic Velocity 

 Analysis (AVA) and Visual Interval Velocity Approximation (VIVA) (Pettv 

 Geophysical Engineering Company) . 



Thus, there are several methods to determine the compressional 

 velocity profiles in marine sediments. Of the methods discussed, it is 

 believed that wide-angle reflection profiling using sonobuovs is the 

 most attractive approach for engineering purposes. 



The measurement of shear wave velocities, whether in-situ or on cores, 

 is much more difficult, devices with probes (Shwwav , 1960) and bottom- 

 sitting seismometers (Auld et al. , 1969) have been used with some success. 

 This area requires much more research, in particular with emphasis upon 

 the engineering applications and interpretation of the data. 



Attenuation of Sound Velocity 



Knowledge of the attenuative properties of sediment types enables 

 the person interpreting a seismic record to "sav something" quantitative 

 about the nature of the material present on the basis of its being a 



19 



