Because rock is heterogeneous at all scales, it is often invalid to assign a single porosity, permeability, resistivity, or velocity to a volume in the subsurface. Well data demonstrate that these properties can vary appreciably between two points just a foot apart. The same happens at the pore scale. Consider, for example, a CT scan of a 1.3-mm Berea sandstone cube (Figure 1). The porosity of this sample, as calculated by counting the blue (pore space) pixels, is 0.164.