SECT. 3] PHYSICAL PROPERTIES OF MARINE SEDIMENTS 799 



by the act of samjiling is shown by Ericson and Wollin (1956). They compare the 

 lengths of corresponding intervals of cores taken with and without a piston. 

 Nearly a 50% reduction in length occurs in the upper part of the core taken 

 with no piston. 



B. Compressional Wave Velocities and Attenuation 



The methods of seismic refraction have been described elsewhere in this book 

 and consequently will not be repeated here. It is sufficient to remark that 

 compressional velocities and thicknesses of principal layers, and sometimes 

 shear velocities, are obtained by this method. Since frequencies are typically of 

 the order of 100 c/s (wavelengths of the order of 20 m) and path lengths within 

 the sediment may be many kilometers, the resulting velocities are average 

 values over large volumes of material. 



Velocity measurements on sediments in situ were reported by Hamilton et al. 

 (1956). This was accomplished by insertion of two probes each with a barium 

 titanate transducer into the sea floor by a diver. The probes were 1 ft apart 

 and penetrated to a depth of 6 in. Velocities were then measured by direct 

 timing of 100 kc/s pulses. 



Sutton et al. (1957) measured sound velocities in freshly obtained cores by 

 direct timing of an acoustic pulse across the diameter of the core. A velocity 

 standard was used to obtain the origin time of the transmitted pulse. 



Velocity measurements by direct timing, though not on marine sediments, 

 have also been described by Hughes (1957), Paterson (1956) and Wyllie et al. 

 (1956). Laughton (1957) and Kershaw {in litt.) have employed direct timing 

 methods to study the effect of compaction on physical properties of marine 

 sediments. 



Shumway (1956, 1960) measured both compressional velocity and attenua- 

 tion by exciting axial modes of oscillation in sediment samples contained in 

 thin-walled plastic cylinders. The values of velocity reported by Shumway were 

 obtained by comparison of resonance frequencies for a given mode of a sediment- 

 filled container with that from a water-filled container. Attenuation was 

 measured by determining the Q of the system from the resonance widths at 

 half power points. Toulis (1956) has derived equations relating Q and the 

 resonance frequency to the attenuation and sound velocity respectively for this 

 system. He has also shown how to introduce corrections for finite thicknesses of 

 the cylindrical container. 



C. Shear-Wave Velocities 



Although there is no doubt that shear waves occur in marine sediments, there 

 are many practical difficulties to overcome in measurements and few reliable 

 observations have been made. The coefficient of rigidity is normally so small 

 that measurements by direct timing on recovered samples have been accom- 

 plished only under elevated pressure. Laughton (1957) measured shear-wave 



