Velocities of compressional and shear waves in marine sediments determined in situ from a research submersible

Abstract

In situ measurements of the velocity and attenuation of compressional waves, and velocities of Stoneley waves from which shear-wave velocities were computed, were made at three stations in the sea floor off San Diego, California, from a research submersible; a fourth station was occupied by divers. Sediment types and water depths ranged from medium sand at 20 meters to clayey silt at 986 meters. Sediment densities, porosities, and grain sizes were measured in sediment samples taken at each station. These unique data allowed tentative evaluations of models and equations, and computations of constants, for elastic and viscoelastic saturated, porous media. For the range in sediment types, results included:Property Sediment type Sand Clayey siltRatio, Vp sediment/Vp bottom water 1.20 0.98Attenuation, db/m at 14 kHz 7 1Sediment density, g/cm3 2.01 1.33Shear-wave velocity, m/sec 197 88Rigidity modulus μ × 1010 dynes/cm2 0.08 0.01Lame's constant λ × 1010 dynes/cm2 6.34 2.80 It is concluded that either Hookean-elastic or viscoelastic equations can be used to compute compressional-wave and shear-wave velocities and other elastic constants. To account for absorption in sediments, a ‘viscoelastic’ solid model is used in which frequency-independent, complex Lame constants, (μ + iμ′) and (λ + iλ′) replace μ and λ in the equations of elasticity; with this model and equations, and data taken on the four stations, μ, μ′, λ and λ′ were computed. In this viscoelastic model, the mechanics of absorption are not specified, and the assumption of frequency-independent, complex Lame constants leads to a linear dependence of attenuation on frequency and no dispersion of velocity with frequency in the range of frequencies of interest in underwater acoustics and marine geophysics. The evidence of this study and others indicates that almost all open-ocean sediments have finite regidities and transmit shear waves.