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. The intent of the study was to relate existing theories to the limited in situ measurements by means of suitable empirical relations. [A review of pertinent publications is presented.] Acoustic velocity and attenuation in the sediments were measured over the frequency range 4 to 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 f[superscript 1.37] frequency dependence of attenuation. Sediments with greater than 30% sand (by weight), including pure sand, exhibit an f[superscript 0.5] frequency dependence of attenuation. The measured velocity dispersion is approximately 2% over the frequency range 4 to 600 kHz. Velocity increases with frequency. All measurements reported are for sediments free of entrapped gas. Evidence is presented that supports the fact that even small quantities of gas can dominate the acoustic properties of sediments. [This work was sponsored by the U.S. Navy Bureau of Ships.].