FRACTURE TRACE ANALYSIS FOR SMALL DOMAIN WELL SITING, NORTHWESTERN NORTH CAROLINA, USA

Fracture trace analysis, a widely used technique for locating water wells in fractured bedrock aquifers in large domains such as cities and states, yields large data sets. In contrast, small domain studies for individual homesites or subdivisions of less than 5000 m² typically have limited data sets. In these studies potential well sites are constrained by many factors, including property boundaries and topography. The Pin Point Method of small-domain well siting (described here), is based on field measurements of fracture orientations plus locations of wells and springs. Lineaments from traditional lineament analyses are integrated into a single set of reproduced lineaments for comparison with field data. We collect client property and drilling history data, measure attitudes of all observed fracture sets in outcrops in the vicinity, and locate and plot springs and existing wells. In NW North Carolina, a typical data set contains 50 to 200 measurements on six to 18 joint sets. Arithmatic averages, stereonets, or rose diagrams provide average fracture azimuths, which, combined with the reproduced lineaments, are used as mean fracture orientations in the analysis. We use the number of springs and wells to weight individual joint traces on the map. Individual fractures with multiple existing wells and springs along them are considered to be transmissive fractures and those that coincide with reproduced lineaments are given more weight. Using known well and spring locations as "pin points," we draw lines representing the azimuths of transmissive fractures through the pin points, thus generating a grid of intersecting lines. Potential well sites are selected where two or more transmissive fractures intersect on the client property and these are rank ordered based on the weights assigned. In NW North Carolina, this method has successfully located wells at 92% of the drilled sites we selected. Two case studies demonstrate the use of the method.