High-frequency Acoustic Properties of Water-Saturated Sediments: Laboratory Study.

This study was performed in controlled laboratory conditions using several broadband transducers to cover a wide frequency range from 200 kHz to 3MHz. Various types of sediments were used: natural medium and coarse sand, and glass beads with corresponding mean sizes. Sound speed and attenuation were measured and compared with predictions of Biot theory. The theory failed to predict both high attenuation and negative dispersion of sound speed observed at very high frequencies (above 500 kHz for medium sands), which can be assumed an effect of strong grain scattering. All the measurements were performed at different times to ensure the repeatability of the results. A comparison of results obtained for natural sands with those for artificial sediment (glass beads) of the same mean size, allowed observing the influence of the grain shape on the reflected and backscattered signals. Also, the influence of the transducer directivity diagram on the sediment reflectivity was observed. This effect is discussed and shown to be essential in the case of strong incoherent scattering observed for coarse sediments. Finally, the time-frequency contents of the reflected signal were analyzed. It was found to be very sensitive to the granular structure of the sediment. [Work supported by CNRS and ONR.]