standard linear solid 



elastic ~ [1 + w2t T 



o'l 



1 + oj^Ti' 



1 + 



1 + 



(T, 



Ti) 



1 + (n A T T 1 



1/2 



(25) 



where ui = angular frequency 



An equation of the above type may be capable, according to Meidav, 

 of accounting for the changing values of elastic moduli with frequency, 

 which may be troublesome in unconsolidated materials, and for the fact 

 that dynamic moduli are often higher, sometimes considerably higher, than 

 the static values. Swain (1962), for example, calculated a Poisson's 

 ratio of 0.275 from laboratorv tests on a core; and 0.343 for an in-situ 

 value using compressional and shear wave velocities. Nafe and Drake (1961) 

 indicated that seismic refraction data commonly yield values of Poisson's 

 ratio higher than 0.25. 



Whitman (1969), however, commented that measurements of elastic 

 moduli in-situ using geophysical techniques have, in a number of cases, 

 been found to be in excellent agreement with those values determined by 

 laboratory tests. 



Further, geophysical measurements are made under very small strains; 

 and sediments may experience nonlinear behavior, as shear strain in soil 

 increases above 10~° inch/inch (Whitman, 1969). Thus, a value of Poisson's 

 ratio calculated from seismic techniques may not be adequate for founda- 

 tion calculations where considerablv larger shear strains are involved. 



It is not intended to discuss comprehensively the rheological proper- 

 ties of marine sediments. The reader is referred to such works as Meidav 

 (1960) for further details. It is, however, evident that much more study 

 is needed to determine the extent to which the moduli of marine sediments, 

 particularly clay, are frequency and level-of-shear-strain dependent. 



Meanwhile, seismic methods do provide an approximate value of Poisson's 

 ratio which would be of value in assessing the in-situ behavior of materials 



Determination of Sound Velocity in Sediments . The techniques most 

 commonly used to determine the velocities of sound in unconsolidated sedi- 

 ments and rocks are the wide-angle reflection, seismic refraction, and the 

 common depth point techniques. 



In the wide-angle reflection technique, the separation of the sound 

 source and the hydrophone is steadily increased so that the angle between 

 the incident and reflected waves progressively increase. Recently, expend- 

 able sonobuoys have been used so that sediment velocities can be deter- 

 mined while reflection profiling (Le Pichon et al., 1968). The method of 

 determining the interval velocities by the wide-angle reflection profiling 



17 



