CHARACTERIZATION OF FRACTURED BEDROCK USING ELECTROMAGNETOTELLURICS

Understanding the geologic and hydrogeologic characteristics of the fractured crystalline bedrock that is overlain by unconsolidated materials is a challenge for ground water supply exploration and remediation. Geologic methods traditionally used to identify favorable drilling targets include fracture trace analysis combined with the mapping of structural features in bedrock outcrops as well as the use of various surface geophysical methods. These remote sensing and field mapping techniques are important to understanding the geology and tectonics of an area. Once the well has been drilled, borehole geophysical methods and other in situ methods are used to support the remote sensing and field data.

The remote sensing analysis provides information on the x and y locations of the photolinears as well as the intersections of these features. The traditional geophysical methods, such as very low frequency geophysics, resistivity, and electromagnetics, also provide information on the x and y locations of subsurface anomalies and to a limited extent the depth of the anomaly.

ElectroMagnetoTellurics (EMT) provides information in the z direction. The EMT method is a passive method that measures the electric and magnetic fields generated by naturally occurring telluric currents that flow through the subsurface. The collection and analysis of EMT data over a potential well location provides a “view” into the subsurface as to the depth and frequency of predominant fracture zones in the underlying fractured bedrock.

The application of EMT to several existing bedrock sites as well as ground water exploration programs for the development of new public water supply wells in crystalline bedrock aquifers will be presented for the Seacoast area of New Hampshire.