USGS Data Series 321: Illinois, Indiana, and Ohio Magnetic and Gravity Maps and Data: A Website for Distribution of Data

Illinois, Indiana, and Ohio Magnetic Compilation

Magnetic anomalies are caused by variations in the Earth's magnetic field that result from 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, the distribution of 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 Illinois, Indiana, and Ohio magnetic map is constructed from grids that combine information (see data processing details) collected in 19 separate magnetic surveys conducted between 1947 and 1994. 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, released in 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. Much of the pre-1975 data are 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, 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 surveys have been continued to 305 m (~1,000 ft) above ground and merged together to form the States compilation. Index maps show the location of the original surveys, and a data table summarizes the detailed specifications of the surveys. The final magnetic anomaly grid with a 500-m (~1,640-ft) interval can be downloaded from the data directory as mag_ilinoh.grd.

A section for higher resolution surveys is included in this report where data that were determined to justify a grid interval finer than the State compilation were created. Included is a map and grid of survey 4163 at its original flight elevation, which was generated with a grid spacing of 100 m due to a 448 m (~1,478-ft) flight-line spacing. Although survey IL05 would qualify as a high-resolution survey due to its flight-line spacing, the quality of the data was judged unworthy of this process. Survey 4163 was then regridded to the final grid spacing of 500 m (~1,640 ft) for use in the State compilation. Grids for this map are 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. A 'README.txt' file located there further defines the format.

Our priority in the construction of the States of Illinois, Indiana, and Ohio magnetic compilation was always to acquire the best resolution magnetic data sets for any given area (see the Illinois, Indiana, and Ohio magnetic data index maps). Where local high-resolution surveys were not available in either digital or digitized format, we used aeromagnetic data collected by the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy, which are available in digital format and together 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). The wide spacing between flight lines flown at low altitudes over surface rock units having high magnetizations causes anomalies with short spatial wavelengths to be elongated between flight lines, producing lineations perpendicular to the flight-line direction and "pearl string" anomalies along the flight line.

This report is an update of earlier compilations of the State of Illinois for the Illinois Geological Survey by the USGS (Hildenbrand and others, 1992) as Illinois State Geological Survey Geophysical Map no. 2, and of the State of Ohio by the USGS (Hildenbrand and Kucks, 1984) as USGS publication Geophysical Investigation Map GP-961.

This project was supported by the Mineral Resource Program of the USGS.