Abstract
Several lines of evidence suggest that heterogeneity in sedimentary materials exhibits multiple scales. However, characterizing permeability heterogeneity in three dimensions over a range of scales has proven extremely difficult. Even the most extensive data sets on aquifer properties (e. g. Borden and Cape Cod) are sparse relative to the volume of the aquifer and scales of interest. Outcrop studies provide opportunities for high-resolution study, but the distribution of measurements is quasi-three-dimensional at best. The recently completed Experimental Earthscape Facility (XES) at the St. Anthony Falls Laboratory at the University of Minnesota provides an opportunity to create and study the heterogeneity of sedimentary deposits in three dimensions. The tank measures 13X6 m and, as the bottom subsides, the tank is fed with a mixture of water, sand, and coal. The sand and coal represent coarse- and fine-grained detritus, respectively, and are deposited by fluvial processes (sheetflows and channels). In addition to the inherent autocyclic variability, subsidence rates, sediment supply, and water discharge are varied to impose allocyclic variability. The sediments are analogous to those of an entire sedimentary basin measuring tens to hundreds of kilometers. Grayscale images of the sediments are interpreted as a mixture of fine- and coarse-grained materials. The permeability of the mixture is approximated using the geometric mean of the two end-members. Log-log variograms of the estimated permeability are anisotropic. The tranverse horizontal variogram indicates simple (monofractal) scaling over a range of scales. The vertical variogram on the other hand shows little evidence of fractal behavior. The observed scaling is consistent with the geologic observation of channel-fill deposits embedded in variably dipping inter-channel deposits. The XES basin provides a tool for better understanding important relationships between geologic processes, their associated depositional products, and the statistical character of those deposits.