North Dakota 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 North Dakota aeromagnetic map is constructed from grids that combine information (see data processing details) collected in 13 separate aeromagnetic surveys conducted between 1978 and 2001. 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 were obtained from aeromagnetic surveys flown by the U.S. Geological Survey (USGS), flown on contract with the USGS, or were obtained from other federal agencies and state universities. Some of the 1980 data are available only on hand-contoured maps 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 as USGS Open File Report 99-557. All surveys have been continued to 304.8 meters (1000 feet) above ground and then blended or merged together. An index plot gives an overview of the original surveys, and a data table summarizes the detailed specifications of the surveys. The final aeromagnetic map can be downloaded as a grid with a 1000 meter grid interval.
Also included are maps and grids of some of the individual surveys at their original flight elevation that were generated with a finer grid spacing due to a finer flight line spacing. These were then regridded to the final grid spacing of 1000 meters for use in the state merge.
Our priority in the construction of the state of North Dakota aeromagnetic compilation was always to acquire the best resolution aeromagnetic data sets for any given area. (See the North Dakota aeromagnetic data index map.) 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 is available in digital format and covers 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 ranged from 4800 meters to 9600 meters (3 to 6 miles). The wide spacing between flight lines flown at low altitudes over surface rock units having high magnetizations causes anomalies with short spacial wavelengths to be elongated between flight lines, producing lineations perpendicular to the flight-line direction and 'pearl string' anomalies along the flight line. Also, problems related to the reduction of navigation control and inconsistent datum levels between NURE surveys can cause herringbone features which are sometimes observed within and at the boundaries of these surveys.
This project was supported by the Mineral Resource Program/Crustal Imaging and Characterization Team of the USGS.
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For more information about this report contact: Ron Sweeney
