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Abstract:
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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 . |