Disclosed is a method of modelling a subsurface volume, and a corresponding computer program and apparatus. The method comprises defining a regular planar grid of regularly arranged locations in the two dimensions substantially parallel to the surface, and calculating a plurality of one dimensional arrays, each one dimensional array extending from a different one of the regularly arranged locations, in the direction from surface to subsurface. Each of the one dimensional arrays comprises a plurality of one dimensional cells, wherein the cells are delineated according to actual geological features of the subsurface volume.