Principal Investigator: Heather L. Caudill (Georgia State University)
Sponsor: GWRI
Start Date: 1995-06-01; Completion Date: 1995-06-01;
Keywords: Fracture Analysis, Fractured Aquifers, Lineaments, Groundwater
Description:
With the recent growth trend in the metropolitan Atlanta, there is an increasing need for viable sources of water. Well siting in the fractured crystalline rocks of the Piedmont has always been an arduous task, often resulting in positioning of wells of variable productivity within meters of one another. Careful siting techniques can greatly increase the probability of encountering a high yielding hydrogeologic zone. The primary purposes of this project are to analyze well productivity in fractured rocks of the Piedmont in the Atlanta area and to propose procedures to better and more consistently locate sites for high yield wells.
Orientation and other characteristic features of mesoscopic fractures at the outcrop scale and lineaments at the 1: 24000, 1: 100000, and 1: 500000 scales were integrated with well data collected by the United States Geological Survey to delineate highly productive zones in the Atlanta Metropolitan area. The project utilized scanline techniques to quantitatively describe and measure fracture networks at the outcrop scale. Dominant fracture sets present on the outcrop scale were also detected at topographic map and satellite image scales, suggesting a similar distribution of the fractures over 6 orders of magnitude (10.2 m to l04 m).
There are two dominant steeply-dipping sets of fractures with horizontal traces trending between 020°-070° and 300°-350°, and length and apertures that are greater than those of other steeply-dipping sets. These two major trends also occur as lineaments at the 1:24000, 1:100,000, and 1:500,000 scales. There is a third set of variably-oriented and gently-dipping fractures with large aperture and length relative to the two steeply-dipping sets. These pressure-relief fractures have horizontal traces that are subparallel to the two major trends and the trend of lineaments. The two dominant, steeply-dipping sets are sub-perpendicular to the gently-dipping set, and their intersection produces localized, high interconnectivity zones in the Piedmont.
An analysis of well data in the study area shows that well yield is a function of the following variables: local fracture lineament density and interconnectivity, well depth to tap the pressure-relief gently-dipping fractures, well distance from points of high intersection density of the steeply-dipping fractures, position of the well relative to the center of valleys, rock type, and lithostratigraphic contacts which control fracture/lineament densities. The Brevard fault zone and rocks belonging to the Sandy Springs Group and Clairmont Formation have the highest linear fracture and areal lineament densities at all the scales studied.
The results show that sites for high yield wells in the metropolitan Atlanta area within the Georgia Piedmont can consistently be located if geomorphologic, structural, and suatjgr.apJJk observations are made at several scales and integrated by analyzing the above-mentioned factors that control the well yield.