A method is disclosed for using a three-dimensional seismic image of a subsurface earth volume to construct a geologic model specifying the spatially-varying grain size distribution, porosity, and permeability throughout the volume. The method applies to earth volumes composed of water-lain clastic sedimentary deposits and involves, in one embodiment, (a) identifying the outline forms of geologic bodies in geologic data; (b) using the outline forms of the geologic bodies to determine the spatially-varying grain size distribution within the bodies, guided by assumptions about the nature and behavior of the paleoflow that deposited the bodies; (c) determining rock properties such as, porosity and permeability within the geologic bodies based on grain-size distribution, mineralogy and burial history information.