Frequency dependence of phase speed, group speed and attenuation in water-saturated sand: laboratory experiments.
Résumé
Sound propagation in water-saturated sandy sediments was studied under controlled laboratory conditions in the 0.1 to 1.3 MHz frequency range. In the “low frequency domain” [100-200 kHz], the results obtained were consistent with classical Biot theory, which predicts a positive phase speed dispersion, but at higher frequencies (f>500 kHz), a strong negative dispersion of the phase speed was observed, which is in apparent contradiction with the Biot theory predictions. In addition, the attenuation coefficient was found to increase nonlinearly with frequency from 0.5 to 1.3 MHz, whereas Biot theory predicted an almost linear increase of this coefficient. The unexpected behaviour of the phase speed and attenuation coefficient observed in these sandy sediments in the very high frequency domain may be attributable to scattering by the sand particles, which becomes the predominant mechanism involved when the size of particles is comparable to the wavelength.