Georgia 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.
An aeromagnetic map of Georgia was published by Zietz and others (1980a, 1980b), which was constructed by an all analog process. In this process, aeromagnetic contours were manually traced off the map for each survey and combined, after photo-reduction and adjustment for level differences. The compilation of magnetic surveys in the present report uses the same aeromagnetic surveys, but combines the data digitally. The advantage of the digital product is not only the increased accuracy of the product, but also, being digital, it can be digitally plotted in a variety of scales, projections, enhancements, combinations, and is available for digital analytical tools favorable for interpretation of the data.
This Georgia aeromagnetic compilation is one part of a national digital compilation by the U.S. Geological Survey. Certain characteristics are common to all of the State compilations. Whereas surveys are typically flown either at a constant elevation above sea level or draped to a constant mean terrain clearance, the standard selected for this national compilation is a survey elevation of 305 m (1000 ft) above mean terrain. All of the surveys used in the Georgia compilation were flown at 152 m (500 ft) above terrain. To conform to the national standard, the entire State grid was analytically continued upward to 305 m above ground (Hildenbrand, 1983).
The Georgia aeromagnetic map in this report was constructed from grids that combine magnetometer data (see data processing details) collected in 14 separate aeromagnetic surveys flown between 1958 and 1981. The data from these surveys are of varying quality. The design and specifications (flightline separation, flight direction, analog/digital recording, navigation, and reduction procedures) may vary between surveys depending on the purpose of the project and the technology of that time. An index map and data table gives an overview of the original surveys and summarizes the specifications of the surveys. The resulting grid has a data interval of 500 m and can be downloaded. A color-shaded relief image of the grid is shown on the first page. Another image of the aeromagnetic map that shows more detail, may also be downloaded.
This grid is an interim product. Considerable editing of digital flightline data was undertaken for two surveys (3048B and 3097). Heavy strike filtering in the direction of the flight lines was necessary to reduce flightline anomalies for four surveys (307, 328, 3048B, and 3097). Anomaly resolution is only fair for surveys flown at one-mile flightline separation where the sources rocks are at the surface, as is the case for the Piedmont terrane, or at shallow depths beneath the inner Coastal Plain. Improved resolution can only be rectified by new surveys with more closely spaced flightlines. Any positioning errors may also be eliminated by new surveys with better navigation control.
This project was supported by the Mineral Resource and Geologic Mapping Programs of the USGS. Thanks to the following USGS colleagues: Pat Hill, Richard Saltus, and Robert Kucks for their assistance in preparing this report, Mike Webring, Tom Hildenbrand, Rob Bracken, and Jeffrey Phillips, for creating the USGS software used in the compilation, and Esther Castellanos for digitizing analog survey 3018 (Daniels and Castellanos, 1994).
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