A discussion on the effect of heterogeneity on the onset of convection in a porous medium

The effect of heterogeneity on the onset of convection induced by a vertical density gradient in a saturated porous medium is discussed. The distinction between moderate and high heterogeneity of permeability is emphasized. Topics discussed include the limitations of the use of the log-normal distribution for permeability, the different effects of vertical and horizontal heterogeneity, and the use of an electrical resistor analogy applied to an illustrative quartered-square example. In a postscript, a new approximate criterion for instability applicable to the case of strong heterogeneity is proposed.