Wisconsin Aeromagnetic Compilation
Aeromagnetic anomalies are due to variations in the Earth's magnetic field caused by the uneven distribution of magnetic minerals (primarily magnetite) in the rocks that make up the upper part of the Earth's crust. The features and patterns of the aeromagnetic anomalies can be used to delineate details of subsurface geology including the locations of buried faults, magnetite-bearing rocks, and the thickness of surficial sedimentary rocks (which are generally non-magnetic). This information is valuable for mineral exploration, geologic mapping, and environmental studies.
The Wisconsin aeromagnetic map is unusual among many of the larger states for its relative uniformity of flight line spacing: about 95% of the state was covered by surveys with a flight spacing of 1/2 mile or less. Much of the state is covered by recent surveys with excellent positioning accuracy (4205-4209; 4221-4223). Two areas are covered by surveys of lower resolution of about one-mile spacing (0392 and MI-07). Most of the surveys were flown at either 500 or 1000 ft above mean terrain (drape); three drape surveys were flown lower, 1133, WI-07, 4031. Two level surveys, 0349, and WI-12 were flown at 1800 and 2000 ft barometric, respectively. These levels are approximately 1000 ft above ground. The composite state grid was constructed from grids of 26 separate aeromagnetic surveys conducted between 1956 and 1999 (see data processing details). An aeromagnetic index map and data table shows the locations of the surveys and summarizes the specifications of those surveys: grids for 17 of these surveys are available for downloading.
This grid is an interim product. Considerable editing of digital flight-line data was undertaken for two surveys (1157 and WI-12) and strike filtering in the direction of the flight lines was necessary to reduce flight line anomalies for these surveys. Positioning errors are evident in one of the earlier surveys, WI-04. Corrections to this survey are not feasible because the data were obtained as a grid.
The Wisconsin Geological and Natural History Survey (WGNHS) played a large part in the acquisition of aeromagnetic survey data used in the compilation of the Wisconsin map. Michael Mudrey of the WGNHS acquired industry surveys WI-10, WI-11, and WI-12. Surveys WI-10 and WI-11 constitute the largest block of high-resolution (1/8 mile flight line spacing), public-domain data in the eastern United States (these surveys are colored pink on the aeromagnetic index map described above. John Karl of the University of Wisconsin, Oshkosh (Karl and others, 1993) single-handedly conducted the survey of north-central Wisconsin (WI-04).
This project was supported by the Mineral Resource and Geologic Mapping Programs of the USGS. Thanks to USGS colleagues Pat Hill and Robert Kucks for their assistance in preparing this report.
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