Simulazione geostatistica e modellazione del flusso in un analogo di acquifero fluviale meandriforme

A 3-D quarry exposure of historical point bar-channel sediments of the Lambro River (Po plain, Northern Italy) permits to analyse meandering river depositional elements and to assess their hydrodynamic characteristics. The work follows three main steps: i) drawing and discretization of the geology starting from 5 vertical facies maps and estimates of conductivity of every facies; ii) 3D geostatistical simulation of facies heterogeneities, based on semivariogram and transition probabilities analysis; iii) application of a 3D numerical flow model to upscale the conductivity field ad to estimate connectivity. The starting point is a sedimentological and hydrostratigraphic hierarchic model obtained after mapping of 5 quarry faces with cm-scale detail. We have recognized two main units corresponding to the exposed parts of two composite point bars with minor channel fills on top: unit A, the lower one, includes Roman-Middle Age findings and shows a composite point bar to main channel fill lateral transition, whereas unit B, the upper one, includes Renaissance Age findings and is mostly represented by a composite point bar, with chute channel scour and fills on top . The erosion surface between them, named α surface, was mapped using GPR data. Units A and B are formed by a hierarchic arrangement of depositional units, that determines the architectural heterogeneity of the aquifer analogue. Textural and poro-perm analyses allowed to define the properties of 4 basic hydrofacies with permeability contrasts by at least one order of magnitude (10-9<K<10-1 m/s): 1) Open Framework Gravels, 2) Gravelly Sands and Sandy Gravels, 3) Clean Sands, 4) Sandy Silts and Clays,. After discretization of the maps with square cells (side 0.05 m), the correlation of hydrofacies has been quantified by both transition-probability and indicator variogram analysis, in view of 3-D, pixel-oriented simulation of the volume. Several realizations of 3-D conditioned simulations, that honour the vertical facies maps, were computed using Sequential Indicator Simulation and T-Progs (dell’Arciprete et al., 2008). Both methods yield more realistic results if highest rank depositional elements are simulated separately than if the sedimentary volume is simulated as a whole. Image analyses on random sections through selected realizations shows that SIS yields the most realistic simulations. However, both techniques cannot account for trends of depositional features that determine a non-stationary behaviour at the facies scale. The equivalent hydraulic-conductivity tensor has been computed with finite-difference modeling of groundwater flow for blocks of the aquifer analogue with size of 11.4  11.4  2.85 m3. At this scale the medium is anisotropic and heterogeneous if we observe the entire volume, but almost homogeneous within the units A and B. The volumes simulated with T-Progs result less permeable and objects are less connected in comparison with volumes simulated using SIS.