Acoustic Properties of Sediments

Abstract

This is a summary of an experimental study to measure the acoustic properties of water-saturated sediments. The sediments used were laboratory prepared to allow control of physical parameters (such as grain size, volume concentration, compressibility, etc.) and to approximate natural sediments. Acoustic velocity and attenuation in the sediments were measured over the frequency range 4–600 kHz. Acoustic measurements were made at high frequencies by means of two probes inserted in the sediments, and at low frequencies by means of a specially constructed rigid-wall standing-wave tube. The data presented show the frequency dependence of attenuation and velocity in the laboratory-prepared sediments and the change in this frequency dependence with changes in physical parameters of the sediments. Sediments composed of pure kaolinite, or kaolinite and sand up to 15% (by weight), show an f1.37 frequency dependence of attenuation. Sediments with greater than 30% sand (by weight), including pure sand, exhibit an f0.5 frequency dependence of attenuation. The measured velocity dispersion approximately 2% over the frequency range 4–200 kHz. Velocity increases with frequency. All measurements reported are for sediments free of entrapped gas.

Viscous attenuation of acoustic waves in suspensions

“…This might have been a consequence of flocculation or settling, or both. The measured attenuations increase to about 10% concentration, when they level off and then begin to decrease, and thus it is inferred by Hampton (1967) that particle interactions begin to be significant at ¢ = 0.10. Urick (1947Urick ( ,1948 presented similar measurements of velocity and attenuation for a kaolinite suspension at a frequency of 1 MHz, and the attenuation and velocity display the same variations as a function of concentration as the 100 kHz data.…”

“…Kuster and Toksiiz (1974b) concluded that anelasticity of the polystyrene spheres made a significant contribution to the attenuation in the suspensions, and it is possible that the neglect of this factor does cause some of the error in the theoretical predictions. Hampton (1967) presented a summary of a fairly large amount of data from laboratory measurements in sediments containing water. Included was a set of measurements of velocity and attenuation in suspensions of kaolinite clays of varying concentration at 100 kHz.…”

“…Included was a set of measurements of velocity and attenuation in suspensions of kaolinite clays of varying concentration at 100 kHz. There is a fair amount of scatter in the attenuation data, which Hampton (1967) attributes to varying concentrations. This might have been a consequence of flocculation or settling, or both.…”

“…The source of the disagreement is unclear. A true estimate of the radius parameter is difficult to estimate, although Hampton (1967) presents a grain size cumulative distribution function by weight which gives a mode of approximately l/-Lm. The effective radius value determined by inversion will reflect not only the volume distribution with respect to radius, but also the strong dependence of attenuation on radius.…”

Attenuation of sound in concentrated suspensions: theory and experiments

“…Experiments in which the viscous losses are significant were carried out by Hampton (1967), but those were for clay particles which are highly non-spherical. To assess the theory for the viscous regime, we have measured attenuation for a suspension of small glass particles in water.…”