A model for high-frequency acoustic backscatter from gas bubbles in sandy sediments at shallow grazing angles

Abstract

A model for acoustic backscatter from trapped bubbles in sandy sediments was developed. The model combines a Biot acoustic penetration model with a resonance scattering mechanism from trapped bubbles. The bubble size distribution is assumed to mirror the size distribution of the fluid pores that exist between sand grains. An estimate of the pore size distribution is constructed from the grain size distribution, based on the known pore structure between dense random packings of hard spheres. The principle of acoustic reciprocity is employed to compute backscattered acoustic pressure in terms of the incident pressure and the scattering cross section of a bubble distribution. The model is applied to data from experiments recently taken at sea. It is concluded that trapped gas is often a likely cause of observed backscatter from sandy sediments. Very small amounts of gas appear to be sufficient to produce significant backscatter. ¸

A Multivariate Correlation Analysis of High- Frequency Bottom Backscattering Strength Measurements With Geotechnical Parameters

“…Our data and the data from literature are in good agreement and they show that there is virtually no relation between scattering strength and grain size [6]. Boyle and Chotiros [23] suggest that the absence of a correlation between scattering strength and grain size is due to gas bubbles trapped in the sediment.…”

Plane Wave at the Interface Between Water and a Gassy Poroelastic Layer With Underlying Elastic Seabed

“…In the limiting case for a complete saturation, the motion equation (1) can be reduced to a simpler form of classic Biot model, which was discussed by Chen et al [22]. Among these four bulk waves, P1 wave travels fastest with lowest attenuation, which is similar to the fast P wave in the Biot theory.…”

“…The above-mentioned literatures modeling the ocean floor as layered seabed sediments with an underlying half-space seldom took the moisture (or gas) variations of marine sediment layer into account. The model of Boyle & Chotiros [22] found that very small amounts of gas were sufficient to dominate other wave scattering mechanisms from the ocean floor. In fact, Biot theory did not involve the case when two different immiscible phase fluids coexist in the interstice of porous sedment.…”

Modelling acoustic scattering, sound speed, and attenuation in gassy soft marine sediments

“…However, their model leaves out the formulation of damping, sound speed and attenuation relevant to bubble pulsations and does not provide a route for acoustic inversion. The notable contribution of Boyle and Chotiros [49,50] also formulated the propagation of two compressional waves in sediment containing gas bubbles, although the viscoelastic damping was attributed to the pore water viscosity only, which may result in underestimated attenuation caused by bubbles.…”