1. A method for modeling on a computer a physical region, wherein the physical region comprises a plurality of strata, the method comprising: receiving data that defines at least one physical characteristic of the physical region; providing a triangular mesh on a plane of a model of the physical region, wherein the mesh comprises a plurality of cells; coarsening the triangular mesh in a non-uniform manner such that cells that are less desirable are larger; and projecting the coarsened triangular mesh in a direction orthogonal to the plane in the physical region to form a prismatic grid, wherein each of the cells of the coarsened triangular mesh is separated into sub-cells according to the strata.