Recent work by Pride *et al.* (1992) using volume averaging to derive the form of the equations of motion
for sound traveling through a fluid-saturated porous medium (Biot, 1962)
has been restricted by the assumption that the solid part of such a solid/fluid
composite was microhomogeneous, *i.e.*, composed of only a single solid constituent.
Motivated by the results of Berryman (1992b) showing that at least two
solid constituents are needed to explain a variety of laboratory data
on fluid-saturated rocks,
we show in the present work how to generalize the results of Pride *et al.* (1992)
to multicomponent solid frames.
Some of the earlier approaches to volume averaging include those of
Slattery (1967), Whittaker (1969), and Burridge and Keller (1981).
The approach of Pride *et al.* (1992) and the closely related approach taken here
have much in common with the methods of Slattery and Whittaker.
However, those authors were studying fluid flow through a rigid solid
matrix, whereas the present approach necessarily includes the effects of
solid deformation.

It will also prove important to make connection with the definitions
of Brown and Korringa (1975), which are themselves based on the well-known
jacketed and unjacketed thought experiments of Biot and Willis (1957).
While generally similar ideas have also been presented by Rice (1975) and Rice and Cleary (1976),
we will stress the more detailed discussion presented by Brown and Korringa.
If the total volume of the porous sample is *V* and the pore volume contained
in that sample is (where the porosity is given by
), then Brown and Korringa (1975) define constants so
that

-V_V_ = p_dK_p + p_fK_.
The independent variables in these formulas are the changes in differential pressure
and pore-fluid pressure .The differential pressure is the difference between the external (confining)
pressure and the fluid pressure, so .The coefficients are written in terms of the jacketed (or frame) bulk
modulus *K ^{*}*, the unjacketed (or solid grain) bulk modulus

K_p = 1K^* - 1K_s,
assuming only that an energy density for the bulk deformations exists.
Measurements of *K*_{p} have been made by Zimmerman *et al.* (1986)
for some rocks.
One other important fact is that, if the porous solid frame is composed of
a single constituent (microhomogeneity), then -- and only then --
, where *K*_{m} is the bulk modulus of the single type
of mineral grain present.

In the second section of the paper, we briefly review the averaging theorem and the single solid component results. In the third section, we present the new results for volume averaging of multicomponent solid frames and derive the macroscopic equations of motion, which are just Biot's equations of poroelasticity with coefficients that must be found through an homogenization procedure.

11/12/1997