HYDROLOGIC AND GEOLOGIC IMPLICATIONS OF TWO-DIMENSIONAL RESISTIVITY MODELS BASED ON AUDIO-MAGNETOTELLURIC ELECTRICAL SOUNDINGS COLLECTED NEAR LEETOWN, WEST VIRGINIA

The Leetown Science Center, a Department of Interior and Department of Agriculture funded aquatic and fish research facility, requires high-quality cold water from a shallow (<250 m) multi-formation strataform limestone/dolomite aquifer. Recent drought conditions (during 1999) prompted an assessment of the ground-water resource and hydrogeologic framework. Regionally, the geologic strike is N 20º E, however, thrust faults, folds (anticlines and synclines), cross faults, fracture zones and nearby contact with the siliciclastic stratigraphicly overlying Martinsburg Formation add complexity. To improve subsurface information audio-magnetotelluric (AMT) electrical soundings, were collected. A 2D electrical section generated from the soundings produced an image of anticlines and synclines and identified a low-resistivity zone. When drilled and tested this zone provided the highest open-hole specific capacity (440 L/min/m) in the area. Based upon the success of the first AMT section, additional resistivity data were collected and corresponding sections, maps, and derivatives were generated and eventually integrated into the hydrologic modeling and geologic mapping. A total of 50 AMT electrical soundings were sited on and near the 500-acre Leetown facility to identify geologic and hydrologic features. 2D resistivity maps generated for three depths were deemed useful for integration into the hydrogeologic model. Further work with the 2D resistivity data is consistent with a shallow epikarst model. 3D resistivity models based upon the epikarst model and tested against the field data yielded mixed results. 3D electrical problems and noise from rural infrastructure may bias the results. Further phase-sensitive skew analysis of the AMT data may resolve the 3D electrical modeling problems.