studies From 1974 to 1977 



Very little experimental data are available on the attenuation of shear waves. The 

 available data are almost all in the fields of geotechnical (soil mechanics) engineering and 

 earthquake research. The available data were collected, studied and reported by Hamilton 

 (1976d) and the abstract of this report follows. 



The objectives of this report are to review selected, published measurements of the 

 attenuation, or energy damping, of low-strain shear waves in surficial water-saturated sands 

 and silt-clays (mud) that might occur as marine sediments. In various computations, a 

 hnear viscoelastic model is favored in which velocity dispersion is negligible, linear attenua- 

 tion is proportional to the first power of frequency and the specific dissipation function, 

 1/Q, and the logarithmic decrement are independent of frequency. The logarithmic decre- 

 ment is favored as a measure of energy damping because of research in soil mechanics. The 

 very sparse data indicate that in water-saturated sands and silt-clays, the logarithmic decre- 

 ments are mostly between 0.1 and 0.6. If approximate values of shear wave energy losses 

 are required for generalized computations, it is suggested that a value for the logarithmic 

 decrement of 0.30 ±0.15 be assumed for sands and 0.2 ± 0.1 for silt-clays. Measured 

 logarithmic decrements of compressional waves in sands average about 0.10 ± 0.03; in silt- 

 clays about 0.02 ± 0.01 . The average values of the ratio of compressional- to shear-wave 

 logarithmic decrements, using the above average values, would be 0.3 for sands and 0.1 for 

 silt-clays. 



SOUND VELOCITY-DENSITY RELATIONS IN SEA-FLOOR SEDIMENTS 

 AND ROCKS 



Introduction 



Continuous acoustic reflection surveys are rapidly delineating the sediment and rock 

 layers of the sea floor. Wide-angle reflection and refraction measurements (as with expenda- 

 ble sonobuoys) yield velocities in these layers. This allows true layer thicknesses to be 

 computed. Further, the new velocity data frequently can be linked to sediment and rock 

 types by geologic reasoning and by direct linkage to the boreholes of the Deep Sea Drilling 

 Project. Therefore, it would be useful to establish relationships between velocity and density 

 in the various sediment and rock types in the sea floor. This would allow prediction of 

 density (a prime requirement) to correspond to measured sound velocities for purposes of 

 modeling the sea floor for underwater acoustics studies. Additionally, density profiles can 

 be constructed from these data or from densities derived from velocities computed from 

 equations of velocity versus travel time or depth. 



Studies From 1974 to 1977 



Naval Ocean Systems Center measurements of density and velocity in marine sedi- 

 ments were combined with infonnation from the literature and a report pubhshed which 

 relates density and velocity for common sediments and rocks (Hamilton, 1978). Figure 14 

 is a summary of individual curves. The abstract of the report follows. 



19 



