MAGNETIC MAPPING OF ADIRONDACK LAKES

Magnetic and gradiometric maps were made of Piseco Lake and the southern arms of Indian Lake. Innovations in data collection permitted by the collection of spatially referenced magnetic data using so-called “walking” magnetometers and gradiometers permits the collection of vast amounts of data, over relatively short timeframes, using non-gridded surveys. A proof of concept over-water magnetic survey was carried out last summer on two Adirondack Lakes. The gradiometer (GEM Systems GSM-19) was mounted to the bow of a rubber inflatable boat (16 foot Sea Eagle) powered by a 20 hp Tohatsu four-cycle outdoor motor. Testing revealed that magnetic data collection was not affected by the metallic mass of the motor while stationary or the electromagnetic field when operational. Grid patterns (irregular grid) were driven at various speeds from 0-20 knots (boat planning) during data collection. Data was collected at cycle times of 0.2-2.0 seconds (1800-18,000 data points per hour). Post processing of the GPS signal results in location accuracy of 1.5 m. Magnetic data was imported into ArcGIS version 9.2 and contour maps were generated by kriging.

In both cases variations in over 1000 nanoteslas were measured across the area surveyed. In general, magnetic trends follow the structure (trend of foliation) of bedrock units and subtle lithologic variations (variably deformed granitic and charnockitic gneisses) in both areas. Piseco Lake shows a trough-like magnetic and gradiometer pattern with an abrupt drop in magnetic intensity along the western shore of the lake, likely coinciding with the bounding fault. In addition, the northern half of the lake shows lower magnetic values. These data are consistent with the proposed half-graben structure of the lake, likely tilted to the north, and the possible preservation of Paleozoic units or lake sediments of lesser magnetic signature within the northern part of the lake basin. In the southwest arm of Indian Lake, alternating bands of high and low magnetic signatures deflect in a left-lateral sense consistent with kinematic studies along the shore and islands within the mapped brittle fault zone.