Role of clay particle electrostatics and the dielectric permittivity of suspended silt particles in sound attenuation in mud

Abstract

Sound attenuation in marine mud sediments is partly caused by viscous dissipation of acoustically induced flow past suspended silt particles. Clay particles in the surrounding lattice carry electrostatic charges, causing high porosity, so one asks why silt particles do not settle because of gravity to the bottom of the mud layer. Explanation of the suspension and the associated attenuation of sound proceeds from consideration of a quartz sphere immersed in mud. The somewhat-random electric field created by the clay particles causes an electric dipole moment to arise in the sphere because of its dielectric permittivity. This is proportional to the electric field and varies with position, and the result is an electrostatic force on the sphere, the force being proportional to the gradient of the electric field. In equilibrium, this force is balanced by a gravity force. There is a natural spring constant associated with deviations from equilibrium, and the resulting dynamical model is a fixed-mass sphere subjected to a spring force, the force of gravity, and the viscosity-associated force caused by the motion of the surrounding fluid and the no-slip condition at the sphere’s surface. Paper quantitatively discusses the model’s implications on the suspension theory of sound attenuation. Results suggest approximate validity for representative acoustic frequencies of model where sphere has same density as water. [Work supported by ONR.]