Fracture Trace Analysis with a Geographic Information System "GIS"

Abstract

Lineaments on side-looking airborne radar (SLAR) and LANDSAT satellite imagery were mapped over the western two-thirds of Rhode Island for a study of fracture trace analysis. A total of 360 large (>1.6 km) and 13,000 small (< 1.6 km) lineaments were identified on SLAR imagery; 260 large and 8,000 small lineaments were mapped from LANDSAT imagery. The western part of the state consists mainly of brittle granites and metamorphic rocks, which yield to stress by faulting and fracturing. Most of the area is covered with glacial deposits. The Geographic Information System (GIS) with computer programs was used for storing, processing, and displaying geographic information including the lineaments, their lengths and orientations. Since the imagery is frequently slightly distorted, the location and orientation of the lineaments are corrected by a comparison of recognizable points on the imagery and a base map. One of the great advantages of GIS is the ability to update and edit files without having to start anew. Another important feature of GIS is the ability to combine different maps into one. In this project a geographic, geologic, and lineament map were combined into one map. Data associated with the geographic coordinates, such as strike and length of lineaments, can be placed into an ASCII file from where it can be imported into a LOTUS spreadsheet for further calculation and display. The main purpose of the fracture trace analysis was to assist in the search for new landfill sites. Because of the demonstrated correlation between lineaments and high permeability bedrock zones, it is recommended to place candidate sites away from lineaments, particularly if they coincide with known faults or rock boundaries. Only lineaments larger than 1.6 km were used, since the map showing all lineaments looked chaotic and left little space for a candidate site. The lineaments were categorized by their correspondence with topographic features. They also received attributes expressed in correlations with geophysical anomalies, geologic boundaries, and with areas of a high density of small lineaments. The fracture trace analysis is an integral part of a geological package which includes bedrock geology and surficial maps. Besides geologic data, legal and environmental data as well as economic and social impacts were considered. In a stepwise exclusion process a final list of 11 candidate sites were selected, which were filtered out from 115 original candidate sites. A geologic interpretation of the lineaments is tenuous at this time, as they suggest that many faults have not been mapped. The lineament orientations suggest a system of faults and fractures that was caused by complicated stress conditions.