The Acoustic properties of gassy sediments due to gas-content fluctuations

Abstract

Aiming at the problem that the distribution of gas-bubbles in the actual seabed sediments is not necessarily uniform, which will lead different acoustic properties and affect the acoustic propagation in gas-bearing sediments. In this paper, the perturbation theory is applied to the corrected equivalent fluid density model in gassy sediments, which is a theoretical model for predicting sound speed and attenuation coefficient in gassy sediments. The analytic expression of the equivalent wavenumber of gassy sediments is derived under gas-bubble void fraction fluctuations. Assuming that the statistics of the heterogeneities are described by exponential covariance functions, numerical analysis is carried out for sound speed, attenuation coefficient, and normal reflection coefficient at a interface connecting water and gassy sediment. The numerical results show that the scattering caused by slightly random fluctuation of gas-bubbles in sediment has no obvious effect on the velocity of the medium, but has obvious effect on the attenuation coefficient. The acoustic scattering caused by random distribution of gas-bubbles induces an significant increase in attenuation coefficient for the insonifying frequencies below the gas-bubble resonance frequency and leads to a significant spike for insonifying frequencies above the gas-bubble resonance frequency. By changing the simulation parameters of the gas-bubble, one can find that when the bubble content is constant, the smaller the gas-bubble size, the greater the influence of the acoustic scattering of the gas-bubbles on the acoustic attenuation of the gas-bearing sediments. If the inhomogeneity is enhanced, it will lead to obvious resonance peak splitting, which indicates that the influence of the uneven distribution of gas-bubbles must be taken into account when using the measured acoustic characteristics to invert the gas-bubble size distribution.