Lineaments are remotely sensed linear features on the Earth's surface that may or may not have a geological basis. Three analytical techniques are described in this report to identify those lineaments that may represent fracture-related features. Fracture-correlated lineaments may indicate the locations of zones of fractured bedrock that could serve as ground-water conduits to Great Bay, N.H. The U.S. Geological Survey, in cooperation with the Civil Engineering Department at the University of New Hampshire, the Cooperative Institute for Coastal and Estuarine Environmental Technology, and the U.S. Environmental Protection Agency, is assessing ground-water flow to Great Bay.
Techniques of fracture correlation include the buffer analysis, domain analysis by discrete-measurement analysis, and spacing-normalized analysis. The buffer-analysis technique was used to identify lineaments that have a coincident trends with nearby bedrock fractures. Buffer-correlated lineaments are related spatially to specific neighboring outcrops. Domain-analyses techniques were used to identify lineaments that coincided with fracture families in different regions of the bay.
Analyses results of these techniques indicate that (1) the buffer technique identified the smallest number of fracture-correlated lineaments (53) but the largest percentage (37 percent) of those analyzed by a given technique, which included lineaments within 305 m of an outcrop; (2) domain discrete-measurement analysis identified more fracture families and fracture-correlated lineaments (120 lineaments) than domain spacing-normalized analysis; and (3) domain spacing-normalized analysis identified the lowest percent of fracture-correlated lineaments (identifying 19 percent, or 77 lineaments) with these bedrock data. Almost half (47 percent) of the lineaments mapped in the Great Bay study area that fall within a domain cell or near an outcrop were fracture correlated by one or more of the techniques. Only 15 percent of lineaments analyzed for fracture correlation were fracture correlated by more than one technique.
Principal fracture-peak trends were identified through domain-discrete-measurement analysis that are coincident with the principal trends of open and vug-filled fractures. These principal trends also are coincident with bedding in the Kittery Formation. Spacing-normalized analysis identified principal fracture-peak trends that are coincident with the trends of closely spaced fracture sets. Principal fracture-peak trends were identified in most of the domains that are coincident with the principal trends and fracture trends of diabase dikes.
High concentrations of fracture-correlated lineaments are identified in areas with many outcrops. Fracture-correlated-lineament density and orientations also vary with bedrock type and contacts. In the northwestern part of the study area, approximately east-west trending fracture-correlated lineaments in the Kittery Formation are parallel to the trend of the bedding, and the strike of open and vug-filled fractures. In the Exeter Diorite, just inland and to the south along the western shore, fracture-correlated lineaments trend roughly north, from north-northwest to north-northeast, and trend roughly parallel to its contact with the Kittery Formation. North-northeast trending fracture-correlated lineaments are identified in the area east of the bay, which parallel the trends of the foliation and bedding in the Eliot Formation and the contact between the Kittery and Eliot Formations. On the south shore of the bay, most of the fracture-correlated lineaments identified in the Eliot Formation roughly parallel the contact with the Kittery Formation and the peak trends of diabase dikes and sills mapped at the bay.
Fracture-correlated lineaments on the west side of the bay, in the relatively high-yielding Kittery Formation, may represent fracture zones that could have an increased hydraulic connection with Great Bay. Fracture zones on the west side of the bay, however, are not likely to be connected to major sources of recharge. Fracture-correlated lineaments on the east side and possibly the south side, which may represent fracture zones, are more likely to represent hydraulic connections to large sources of recharge in stratified-drift aquifers, and result in points of concentrated fresh ground-water discharge to the Great Bay.
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