Magnetic anomalies are due to variations in the Earth's magnetic field caused by the uneven distribution of magnetic minerals (primarily magnetite) in 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 Michigan magnetic map is constructed from grids that combine information (see data processing details) collected in 25 separate magnetic surveys conducted between 1947 and 1999. 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. Most of the available digital data, (USGS Open-File Report 02-361), were obtained from magnetic surveys flown by or on contract with the U.S. Geological Survey (USGS) or were obtained from other Federal agencies. Many of the pre-1975 surveys were available only on hand-contoured maps from the (USGS) and through contractors and had to be digitized. These maps were digitized along flight-line/contour-line intersections, considered to be the most accurate method of recovering the original data. Digitized data are available in USGS Open-File Report 99-0557. Some of the magnetic surveys were flown at 305 m (~ 1000 feet). The other magnetic surveys were mathematically continued to 305 meters (~1,000 feet) above ground before all were merged together to form the State compilation. The merging parameters were designed to favor higher quality data. An index map shows the location of the original surveys, and a data table summarizes the detailed specifications of the surveys.

Grids for all Michigan magnetic and gravity maps are in Geosoft Oasis montaj binary format (downloadable free software for conversions to other grid formats is available at www.geosoft.com) and are available in the data directory. Oasis map files, help, and metadata files are also included in the data directory, as well as ASCII gravity data files.

Our priority in the construction of the State of Michigan magnetic compilation was always to acquire the highest resolution magnetic data sets for any given area (see the Michigan magnetic data index map). Where local high-resolution magnetic survey data were not available, in either digital or digitized format (mainly the southern peninsula of the State), aeromagnetic data collected by the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy were used. These NURE data are available in digital format and cover the entire State.  However, because magnetic surveying was not the primary objective in the design of the NURE surveys, these data are subject to certain limitations. Although the NURE surveys were flown at elevations close to the reduction datum level, the spacing between flight lines was 9,600 m (~6 mi). Therefore any magnetic anomalies that originate in surface rocks are poorly represented by widely spaced flight lines.  However, the areas where the NURE data were used in the compilation are underlain by thick sequences of sedimentary rocks, which usually have very weak magnetic properties.

An all-analog compilation of Michigan aeromagnetic surveys was published by Zietz and others (1974). Since that time, 13 additional magnetic surveys have been flown and are part of this new digital compilation.

Eastern Mineral Resources Team