= Effective permeability of complementary wedges
{title2=$k_{\rm eff}=(k_1-k_2)/\log(k_1/k_2)$}
In a slender layer, two parallel sublayers of fractions $x/L$ and $1-x/L$ have local effective permeability $K(x)=k_1x/L+k_2(1-x/L)$. Integrating their successive hydraulic resistance gives the logarithmic mean displayed above. A leading <pressure gradient> uniform across the thickness does not make parcel paths horizontal; a small transverse velocity preserves <mass conservation> as the cross-sectional conductance changes.
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