MAGNETIC GRADIENT ANOMALIES ON ONONDAGA LAKE, NEW YORK: ARE THEY GEOLOGIC OR ANTHROPOGENTIC?

A high-resolution magnetic field strength survey was completed on Onondaga Lake, NY (~18 sq km), to identify geologic and search for anthropogenic sources for anomalies. The survey was completed using an Overhauser gradiometer attached to an inflatable motorboat. Onondaga Lake was surveyed using a zig-zag track across the width and length, and at an average speed o f 21 km/hr that resulted in readings with 1.2 m spacing. Measurements were made at more than 32,400 locations in less than 3 hours, with the bottom sensor providing total magnetic field strength (nT) and both sensors providing the vertical gradient (nT/m). The total field strength data was examined for diurnal variation. Subtraction of the earth’s average magnetic field for the center of Onondaga Lake produced the magnetic anomaly data set that was used for mapping. There is a higher to lower magnetic anomaly gradient from northwest to the southeast ends of the lake (~7.5 km). However, there are also small-scale (up to several 100 meters wide) variations in the overall gradient. The lake-scale horizontal gradient is most likely due to the geology beneath Onondaga Lake. Isolated and relatively small anomalies, deviate from the regional magnetic gradient by 500 to >2000 nT. These anomalies are most likely due to man-made objects that have a strong influence on the local total magnetic field such as sunken boats or pieces of boats, anchors, or other discarded or lost materials containing metal. In 1955 a military jet crashed into Onondaga Lake and the main wreckage was never salvaged. It is interesting that one of the larger isolated anomalies closely corresponds to the crash location.