CHARACTERIZATION OF DRUMLINS USING ELECTRICAL RESISTIVITY TOMOGRAPHY, CENTRAL NEW YORK

The landscape of central New York is dominated by thousands of drumlins of varying morphology and sediment content. The drumlins range in size and shape, and earlier researchers have described how variability is related to formation processes. This investigation has characterized the electrical “structure” of select drumlins in central New York using electrical resistivity tomography in an effort to develop ERT facies for this important depositional system. This presentation will focus on two drumlins with vastly different shapes located in Oswego, NY, but similar dimensions. Both drumlins have local relief of approximately 30 meters. One drumlin (A) is 1.8 km long, 0.5 km wide, with a generally smooth oblong outline, and the long axis trends 160. The adjacent drumlin (B) is 2.1 km long, 0.7 km wide, with the widest area at the northern end, an overall taper to the south, and gently curved long axis with trend that varies from 140 to 160 from N to S. Electrical resistivity surveys were run along the crest of the axis of both drumlins. The ERT surveys were completed using a IRIS Syscal Pro Switch system with 48 electrodes spaced at 10 m. Surveyed distance along the crest of the drumlins was 470 m, using the Schumberger method, and depth of about 105 m. The survey of drumlin-A resulted in ER range of 5 to 200 ohm-m and an ERT model was completed using R2. A moderately steep ER gradient from 5 to 100 ohm-m occurs across the pseudosection at a depth of about 35 m. This gradient is the base of the drumlin on the fractured sandstone bedrock. This boundary has gentle relief that drop about 10 m from N to S along the distance of the survey. A continuous ER anomaly that ranges from 150-200 ohm-m spans the entire the survey and is interpreted as the homogeneous sediment core. The uppermost 10 m of the pseudosection is a semi-continuous ER anomaly of about 100 ohm-m. The survey of drumlin-B had produced similar overall results except that the ER values were about 100 ohm-m higher throughout the drumlin and there are isolated high (200 ohm-m) and low (10’s ohm-m) anomalies 10 m in diameter that disrupt the upper 10-20 m of the survey. Except for these anomalies, drumlin-B has the same ER general pattern as drumlin-A. The small anomalies may represent erratics (higher ER) in the till, or may represent perched aquifers (low ER) that occur above non-porous erratic or lenses of clay.