PREDICTION OF REGIONAL GROUNDWATER FLOW AND PERMEABILITY ANISOTROPY USING FRACTURED OUTCROP EXPOSURES, EDWARDS AQUIFER, CENTRAL TEXAS

Christopher K. Zahm, Randall Marrett and John M. Sharp, Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, U.S.A. (present address of Zahm: Department of Geology, Colorado School of Mines, Golden, CO. 80401, U.S.A

For presentation at the IAH/AIH conference in Las Vegas

Fluid flow within a fractured aquifer requires knowledge of the fracture geometry to predict anisotropy of the permeability tensor. A newly introduced fracture anisotropy factor uses outcrop measurements of fracture length and orientation to predict regional permeability anisotropy. Fracture maps were generated at scales of 4 m², 100 m² and 100 km². The fractures patterns have similar attributes of fracture length and orientation at all three scales. These maps were used to develop length-weighted rose diagrams of fracture segments that quantify the distribution of the fracture orientations. The rose diagrams illustrate similar mean orientation and fracture intensity at the scales considered. Normalization of the petals of the rose diagram was done to determine the newly introduced fracture anisotropy factor (<Omega> f). Using <Omega> f with the cubic law, groundwater flow vectors were estimated. This technique provides a tool to predict regional groundwater flow vectors from outcrop measurements of fractures a priori to drilling wells.