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We now consider four scenarios of progressively greater
complexity. For the first pair we assume the mineral *K*_{m} and
drained *K*_{d} bulk moduli of all layers are uniform, and therefore
that the effective stress coefficient ()is the same in each layer. Furthermore, the overall
volume effective stress coefficient is also the same
[a fact that follows from (20)].
Although specific in many respects, this model still permits some
flexibility in the choice of pore fluids and their spatial distribution.
The other main freedom we have left is to assume that each layer's
shear modulus is as sensitive or more sensitive than its bulk modulus to
irregularities in the pore space (Makse *et al.*, 1999). So, the shear modulus can vary significantly from layer to layer,
which will be important to our main discussion.
The second pair of scenarios allow the bulk modulus to vary in the
layers, and again study both uniform and patchy pore-fluid saturation.