Studies From 1974 to 1977 



In an earlier report (Hamilton, 1971a), the presence and causes of rigidity and shear 

 waves in marine sediments were reviewed. Hamilton with Bucker and his colleagues at the 

 Naval Undersea Center (Hamilton et al, 1970) reported in situ measurements of compres- 

 sional wave velocity, density, and velocities of Stoneley waves (from which shear waves can 

 be determined). In these reports the variation of shear wave velocity with depth in the sedi- 

 ments was not considered. 



A short study and review of shear wave velocity versus depth in marine sediments 

 was issued by the Naval Undersea Center as TP 472 (Hamilton, 1975b), and later pubhshed 

 in Geophysics (Hamilton, 1976e). The abstract of this report follows. 



The objectives of this report are to review and study selected measurements of the 

 velocity of shear waves at various depths in some principal types of unlithified, water- 

 saturated sediments and to discuss probable variations of shear velocity as a function of 

 pressure and depth in the sea floor. Because of the lack of data for the full range of marine 

 sediments, data from measurements on land were used and the study was confined to the 

 two "end-member" sediment types (sand and silt-clays) and turbidites. 



The shear velocity data in sands included 29 selected, in situ measurements at depths 

 to 12 meters. The regression equation for these data is: V^ = 128D^-^ , where Vg is shear 

 wave velocity in m/s and D is depth in meters. The data from field and laboratory studies 

 indicate that shear wave velocity is proportional to the 1/3 to 1/6 power of pressure or 

 depth in sands; that the 1/6 power is not reached until very high pressures are applied; and 

 that in most sand bodies the velocity of shear waves is proportional to the 3/10 to 1/4 power 

 of depth or pressure. The use of a depth exponent of 0.25 is recommended for prediction 

 of shear velocity versus depth in sands. 



The shear velocity data in silt-clays and turbidites include 47 selected, in situ 

 measurements at depths to 650 meters. Three linear equations are used to characterize the 

 data. The equation for the to 40 meters interval (Vg =116-1- 4.65D) indicates the gradient 

 (4.65 sec~^) to be four to five times greater than is the compressional velocity gradient in 

 this interval in comparable sediments. At deeper depths, shear velocity gradients are 

 1.28 sec~l from 40 to 120 meters and 0.58 sec~^ from 120 to 650 meters. These deeper 

 gradients are comparable to those of compressional wave velocities. These shear velocity 

 gradients can be used as a basis for predicting shear velocity versus depth. 



Two figures reproduced here from Hamilton (1976e) illustrate shear velocity versus 

 depth in sands (Figure 12) and in silt-clays (Figure 13). 



ATTENUATION OF SHEAR WAVES IN MARINE SEDIMENTS 



Introduction 



When a compressional wave is reflected at some impedance mismatch within the sea 

 floor, some of the energy is converted to a shear wave and this converted energy is rapidly 

 attenuated. 



In some sophisticated mathematical models of sound interaction with the sea floor, 

 the attenuation of shear waves is a required input (see Part II, this report). 



