ELECTRICAL RESISTIVITY IMAGING OF AN AQUIFER IN WESTERN MASSACHUSETTS

Electrical resistivity tomography and ground penetrating radar were used to study the shallow subsurface in a metamorphic terrain in the Berkshire Mountains of western Massachusetts. Seven, 32-channel resistivity profiles were acquired using 10-meter probe spacing employing a Wenner-alpha array with an automated data collection system. Several additional profiles were acquired with 1 to 5 meter spacing and a variety of array configurations. Radar data were collected coincident with resistivity profiles and in a concentrated area of interest. Initial results indicate the study area is overlain by a thin veneer of Wisconsin-age glacial till covering metamorphic rocks associated with the Appalachian orogen. Resistivity data were inverted using RES2DINV with default parameters though inversions were completed using half-cell widths. Resistivity data image low resistivity in the shallow subsurface with an abrupt change to high resistivity at approximately 11-13 meters depth. Though resistivity generally decreases beneath that interface, those features are not easily observed in the raw output models. First and second derivatives of the resistivity models with depth better represent significant changes obscured in the inverted models. Gradient and gradient change data display slightly positive changes in the shallow section to approximately 10 – 12 meters depth. Resistivity gradients and gradient changes increase significantly at those depths from 15 to over 50. That high second gradient decreases slightly to approximately 30 meters in depth where the second derivative becomes negative at 23-28 meters depth. The negative trend extends to 46-50+ meters where another gradient change reversal is observed. A 2.5 dimension, geographically referenced image of the resistivity data displays a negative gradient change feature that is 17-24 meters thick. That negative gradient change area can be traced across the entire study area. It is interpreted to mark an increase in saturation that is perhaps associated with a partially entrained aquifer.