As only the first ν right singular vectors appear in the approximant in eqn. 6, the best ν-dimensional subspace of 3D porosity images is thus spanned by the orthonormal set of vectors v 1 , v 2 , ..., v ν . Analogous to eigenfaces, we declare this set of vectors, which are fictitious 3D porosity images [well, with c subtracted and normalized to have magnitude unity] discovered by SVD, the eigencages. Algebraically, eqn 6 approximates the flattened 3D porosity image of cage k as: