Elastic wave propagation in fluid-saturated porous media II

Recently, a new method of deriving the coefficients in the strain energy functional of Biot’s theory for elastic waves in fluid-saturated porous media was proposed. The results of this approach have been criticized as being unphysical in the limit K*/Kf → 0, where K* and Kf are the frame and fluid bulk moduli. We show that an error arose due to the literal interpretation of the term ef as the fluid dilatation whereas this term actually differs substantially from the fluid dilatation in some situations. We also show that by lumping part of the solid together with the fluid (conceptually) to form a fictitious fluid suspension, the corresponding fictitious ef may be correctly interpreted as the dilatation of the fluid suspension. This trick leads to a transformation which takes the recently published formulas into standard form. The standard formulas are found to be invariant under the same transformation. New insight into the significance of the standard formulas for the coefficients is obtained.