1. A method executed by a computer of providing a simplified subterranean model of a subterranean structure, comprising: selecting a first grid size for the simplified subterranean model, wherein the first grid size is coarser than a second grid size associated with a detailed subterranean model; populating the simplified subterranean model with subterranean properties according to the selected first grid size, wherein a plurality of realizations of the simplified subterranean model are provided for different sets of values of the subterranean properties; and ranking the realizations of the simplified subterranean model based on comparing outputs of simulations of the realizations with measured data associated with the subterranean structure.