ASSESSING THE ARCHITECTURE OF A DRUMLIN WITH THE USE OF ELECTRICAL RESISTIVITY TECHNIQUES, OSWEGO, NEW YORK

Central New York is host to one of the world's largest occurrences of drumlins. Although the typical drumlin consists of a wide variation of sediment types (clay, silt, sand, gravel, and boulders), the distribution of these materials is not necessarily homogeneous. The electrical properties of the subsurface are influenced by variations in sediment size and mixtures that control moisture and groundwater distribution. To assess the internal architecture of a drumlin at the Rice Creek Field Station (Oswego, NY), electrical resistivity experiments were conducted at various scales. Three different electrical resistivity, Wenner-array, experiments were conducted over an extended period of time. The initial surveys involved very long electrical resistivity lines containing 50 nodes spaced at 10 meters. These experiments had a penetration >100m and were used to constrain the depth to bedrock, and possibly the geometry of regional aquifers. The second experiment involved the repeated resistivity survey of a site (42 nodes, 1 m spacing) over a period of several years to calibrate the impact of precipitation events on the electrical structure of the drumlin till. The third experiment involved a grid of resistivity surveys (24 nodes, 3 m spacing) that traverse from the flank to crest of the drumlin, and intersect the sites of the first two experiments. With these data sets, it was possible to produce a model showing the material variation of the drumlin deposits. Calibration of the final surveys with the long-term observations refined the models to include parameters such as % saturation of the subsurface, and infiltration rate. Preliminary results show that the drumlin contains very complex resistivity patterns, with some consistent trends. Regions of high-resistivity correlate with surface exposure of boulders and gravel. As well, these deposits have the most rapid infiltration rates as determined by the tracking of resistivity anomalies in the subsurface following precipitation events. Areas of low resistivity correlate well with poorly drained portions of the drumlin and are interpreted to contain finer grained till. Sinusous and linear high-resistivity anomalies that occur on the flank of the drumlin are interpreted as gravel and boulder deposits, and may represent ancient streams that developed during glacial retreat.