A definition for the specific storage coefficient Ssis given which is unambiguous for general isotropic three-dimensional aquifer elasticity. In every representative elementary volume, S s is the fluid volume released from storage per unit decline in hydraulic head, per unit bulk volume, under conditions such that there is no strain in two orthogonal directions, and the total normal stress in the third orthogonal direction is constant. The specific storage coefficient is a point property of the aquifer and is defined independently of problem domain stress and head boundary conditions. The expression for S s in terms of aquifer and fluid compressibilities is identical to the familiar forms obtained assuming zero horizontal strain and constant overburden in an aquifer, although it is not restricted to these conditions. As a point property of the fluid-saturated material, the specific storage coefficient is one of four constants in the general constitutive poroelastic equations relating three-dimensional aquifer stress and strain to fluid pressure and dilatation. Written in terms of Ss, these equations show that pore fluid mass diffusion is governed by a diffusivity equal to the ratio of hydraulic conductivity to specific storage under arbitrary boundary conditions. It is shown that S s controls slow compressional body wave velocity in the low frequency limit and that the uniaxial aquifer compressibility a is not necessarily related to the vertical direction.
A theoretical calculation of the excess acoustic attenuation due to hydrodynamic interactions in colloidal suspensions, when the suspended particles are spheres or plates, is presented. Our model is based on the fluid flow shearing between suspended particles during the passage of a longitudinal acoustic wave. To incorporate the many-body effects of the system, the nearest-neighbor distribution function for finite-size particles is introduced. The results of the modeling are compared to available experimental results. The main features of the experimental curves (e.g., attenuation maxima as a function of concentration and an increase in attenuation with frequency) are reproduced and it is shown that the attenuation due to hydrodynamic effects is a significant contribution to wave damping in high-concentration suspensions.
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