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Data Series 135: Iowa Magnetic Compilation

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Magnetic 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 magnetic anomalies can be used to delineate details of subsurface geology, including the locations of buried faults and magnetite-bearing rocks and the depth to the base of sedimentary basins. This information is valuable for mineral exploration, geologic mapping, and environmental studies.

The Iowa magnetic map is constructed from grids that combine information (see data processing details) collected in nine separate magnetic surveys conducted between 1953 and 1972. The data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. Every attempt was made to acquire the data in digital form. Unfortunately, all of the available data were in analog map form and had to be digitized. These maps were digitized along flight-line/contour-line intersections, which is considered to be the most accurate method of recovering the original data. Digitized data are available in USGS Open File Report 99-0557. All survey grids have been continued to 305 m (1,000 ft) above ground and merged together to form the State compilation. An index plot shows the location of the original surveys and a data table summarizes the detailed specifications of the surveys. A final grid with a 1,000-m (3,280-ft) interval is in Geosoft Oasis montaj binary format (downloadable free software for conversions to other grid formats at www.geosoft.com) and available in the data directory as 'magia.grd'. A 'README.txt' file located there further defines the source of the format.

This project was supported by the Mineral Resource Program / Crustal Imaging & Characterization Team of the USGS.

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