GEOSTATISTICAL ANALYSIS OF LITHOFACIES IN THE AQUIFER OF MEXICO CITY

The hydrogeological system in Mexico City is composed by a variety of lithologies. The first 300 meters are composed by lacustrine sediments underlied by alluvial sediments and volcanic rocks (tuff, basalt and andesitic igneous rocks). In most applications these lithologies are typically grouped and modeled as layers. This approach fails to address not only the heterogeneous nature of the rocks, but also the uncertainty in their contacts. Here we analyze lithologic records from water-wells in a portion of Mexico City to characterize the spatial variability of the first 300 meters of the hydrogeologic system. We use an indicator approach to describe the spatial variability of lithologic categories and compute experimental variograms. We explore separately the spatial correlation structures in the upper lacustrine sediments and the alluvial and volcanic rocks. We use indicator kriging to construct three-dimensional models of the hydrogeologic system where the kriged indicator values represent the probability that the lithologic category is present in a specific location.