Simulation of Fine-Scale Heterogeneity of Meandering River Aquifer Analogues: Comparing Different Approaches

Abstract

We compare different approaches to fine scale simulation of aquifer heterogeneity of meandering river depositional elements, based on the study of a 3-D quarry exposure of historical point bar-channel sediments of the Lambro River (Po plain, Northern Italy). The starting point is a sedimentological and hydrostratigraphic hierarchic model obtained after mapping of five quarry faces with centimeter-scale detail. The vertical facies maps show the shape and size of two superimposed composite bars, of their component unit bars and channel fills and the distribution of the individual facies within them. Textural and poro-perm analyses allowed the definition of the properties of four basic hydrofacies (Open Framework Gravels, Gravelly Sands and Sandy Gravels, Clean Sands, Sandy Silts and Clays), with permeability contrasts by at least one order of magnitude $(1{0}^{-9} < \mathrm{K} < 1{0}^{-1})$. The correlation of hydrofacies has been quantified after discretization of the maps with square cells (side 0.05 m), by both transition-probability geostatistics and variographic analysis, to support 3-D pixel-oriented simulation of the volume. We found a high level of correspondence between the semivariogram ranges and the experimental transition probabilities computed on the entire dataset. Several realizations of 3-D conditioned simulations, that honour the vertical facies maps, were computed using Sequential Indicator Simulation (SIS) and T-Progs (transition-probability geostatistics software). Both methods yield more realistic results if the highest rank depositional elements are simulated separately than if the sedimentary volume is simulated on the whole. Image analyses on random sections through selected realizations shows that, in this specific case, SIS yields the most realistic simulations. However, both techniques are not capable of accounting for trends of depositional features that determine a non-stationary behaviour at the facies scale.