Stability analysis of the Biot/squirt models for wave propagation in saturated porous media

This work is concerned with the Biot/squirt (BISQ) models for wave propagation in saturated porous media. We show that the models allow exponentially exploding solutions, as time goes to infinity, when the characteristic squirt-flow coefficient is negative or has a non-zero imaginary part. We also show that the squirt-flow coefficient does have non-zero imaginary parts for some experimental parameters or for low angular frequencies. Because the models are linear, the existence of such exploding solutions indicates instability of the BISQ models. This result, for the first time, provides a theoretical explanation of the well-known empirical observation that BISQ model is not reliable ( not consistent with Gassmann's formula) at low frequencies. It calls on a reconsideration of the widely used BISQ theory. On the other hand, we demonstrate that the 3-D isotropic BISQ model is stable when the squirt-flow coefficient is positive. In particular, the original Biot model is unconditionally stable where the squirt-flow coefficient is 1.