PRELIMINARY CORRELATION OF FRACTURE TRACE ANALYSIS, VERY LOW FREQUENCY GEOPHYSICS, STRUCTURAL FIELD MAPPING, AND DOWNHOLE OPTICAL TELEVIEWER RESULTS IN BEDROCK GROUND WATER EXPLORATION

Most of the glacially-formed sand and gravel aquifers in the northeastern United States are small in size, highly susceptible to drought conditions, and have largely been fully explored and exploited as a source of ground water for public, private, and industrial uses. Because of the increasing demand for ground water in this area, exploration has begun to focus on the underlying bedrock aquifers as “new” sources of ground water. The use of bedrock aquifers in this area has traditionally been for residential wells. These wells typically yield 5 to 10 gallons per minute and are installed on building lots of 0.4 to 2 hectares in size. There are limited permitting requirements for the installation and use of these residential wells.

The new ground water withdrawal needs are for hundreds of gallons per minute. As ground water flow in bedrock aquifers is largely fracture-controlled, understanding the geologic characteristics of bedrock as well as the regional tectonic setting are critical to identifying favorable areas to site large ground water withdrawal wells. Geological methods traditionally used to identify favorable drilling targets include fracture trace analysis (a remote sensing method), mapping of structural features in bedrock outcroppings, especially fracture sets, and the use of various surface geophysical methods.

A ground water exploration program was performed along the seacoast of New Hampshire to identify drilling sites for high yield bedrock wells. Traditional exploration methods as well as downhole geophysical methods (i.e. optical televiewer logs) were performed to identify and correlate subsurface structural features with the results of structural mapping and remote sensing data. Aquifer pumping tests were also performed to correlate field mapping and optical televiewer logs. The preliminary results of the ground water exploration program and correlation of surface, subsurface, and pumping test data will be presented.