High-Frequency Sound Reflection by Water-Saturated Sediment Interfaces

Sound reflection by water-saturated sands and glass beads with a flattened surface was studied under controlled laboratory conditions in a wide frequency range, from 200 kHz to 7 MHz. In the ¿low-frequency¿ domain and in the case of medium sand, the reflected sound level was found to be in good agreement with both classical sonar measurements and classical theories of reflection developed for fluid porous media (this reflected level is practically independent of the frequency); as the frequency increases, a large decrease in the reflected level occurs, possibly due to incoherent scattering. In the very high-frequency regime (>3 MHz), the sound level measured was more than 20 dB below the classical level, and it remained constant at higher frequencies. Similar experiments were carried out with coarse sand to study the effect of grain size on the reflection loss. The same behavior with only a frequency shift was observed. These effects were confirmed by repeating the experiments with glass beads of two sizes. The anomalies observed in the reflected levels measured seem to be directly connected to the ratio between the grain size and the wavelength. One of the main conclusions reached in this study was that for very coarse sand and gravels, the effects of the granular structure of the bottom should not be neglected, even at the usual sonar frequencies.