USING GIS TO CHARACTERIZE THE SPATIAL HETEROGENEITY OF FRACTURE SYSTEMS IN LAYERED ROCKS

Fractures are discrete planar features and thus by nature are heterogeneously distributed throughout the earth’s upper crust. Methods commonly used to evaluate fracture distribution at the outcrop, aquifer and reservoir scales rely upon calculating representative values from fracture populations. These singular values are useful in characterizing bulk fracture properties, but they are unable to account for the spatial variations inherent in fracture populations. GIS are ideally suited to capture the spatial heterogeneity of fracture networks because they can (1) easily incorporate and manipulate linear features from 2-D maps and remotely-sensed images, and (2) calculate values of fracture properties for each cell within a gridded map area. We analyzed photographs of two outcrops viewed in cross section, one a limestone from central Israel and the other an interbedded sandstone-shale sequence from western Turkey. Map areas were divided into 1cm by 1cm square cells and the fractures were digitized as linear traces. The fracture intensity was then calculated for each cell in the map by summing the lengths of all fracture traces within a circular area prescribed by a search radius centered on that cell. Results reveal distinct linear trends and zones of high fracture intensity that would remain unnoticed if quantified as bulk values. The effect of stratigraphic layering on fracture distribution is apparent in the example from Turkey, where beds of high fracture intensity (0.079 per cm) alternate with beds of low intensity (0.018 per cm), revealing a strong deviation from the overall fracture intensity of 0.035 per cm. In the Israel example values of high fracture intensity are aligned in a distinct linear trend normal to bedding, corresponding to a vertical fracture zone in outcrop. Average fracture intensity values within the fracture zone are 38% higher than the average value for the entire mapped area. Identifying zones of high fracture intensity, whether parallel or normal to bedding, has direct environmental and engineering applications because subsurface fluid flow is often localized within fracture zones. Thus, identifying fracture zones by analyzing the spatial heterogeneity of fracture intensity may contribute to the understanding of groundwater flow and contaminant migration through fractured bedrock.