Laboratory measurements of sound speed and attenuation of water-saturated granular sediments
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The propagation of acoustic waves through water-saturated granular sediments has been widely studied, yet existing propagation models can not adequately predict the speed and attenuation of sound across the range of frequencies of interest in underwater acoustics, especially in loosely packed sediments that have been recently disturbed by storms or wave action. Advances in modeling are currently dependent on experimental validation of various components of existing models. To begin to address these deficiencies, three well-controlled laboratory experiments were performed in gravity-settled glass beads and reconstituted sand sediments. Sound speed and attenuation measurements in the 0.5 kHz to 10 kHz range are scarce in the literature, so a resonator method was used to investigate a reconstituted sand sediment in this range. The literature contains laboratory and in situ measurements of sound speed and attenuation at higher frequencies, but existing models can not predict both the speed of sound and attenuation simultaneously in some sediments. A time-of-flight technique was used to determine the speed of sound and attenuation in monodisperse water-saturated glass beads, binary glass bead mixtures, and reconstituted sediment samples in the frequency range 200 kHz to 900 kHz to investigate the effect of sediment inhomogeneity. The effect of porosity, independent of changes in other sediment physical properties, has not been demonstrated in the experimental literature. Therefore, a fluidized bed technique was used to independently vary the porosity of monodisperse glass bead samples from 0.37 to 0.43 and a Fourier phase technique was used to determine the speed and attenuation of sound. Collecting these results together, measured sound speeds showed positive dispersion below 50 kHz while negative dispersion was observed above 200 kHz for some samples. Attenuation measurements showed an approximately f⁰̇⁵ dependence in the low frequency regime and an approximately f³̇⁵ dependence for large-grained samples in the high frequency regime. The laboratory experiments presented in this work demonstrate that both sound speed and attenuation in idealized loosely packed water-saturated sediments can not be simultaneously predicted by existing models within the uncertainties of the model input parameters, but the independent effect of porosity on sound speed can be predicted.