Virginia 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.
Aeromagnetic maps of Virginia were previously published by Zietz and others (1977a) as a black and white contour map at a scale of 1:500,000 and by Zietz and others (1977b) as a color map at a scale of 1:1,000,000, with a contour interval of 100 nanoteslas. Both maps were prepared by an all analog process, where aeromagnetic contours were manually traced off a source map for each survey and combined, after photo-reduction and adjustment for level differences. The compilation of magnetic surveys in the present report uses a majority of the same aeromagnetic surveys, plus nine newer surveys, seven of them digital, and has the advantage of the data being combined digitally. The advantage of a digital compilation is not only increased accuracy, but also, being digital, it can be plotted at various scales and projections and is available for digital enhancements and derivative products.
The Virginia aeromagnetic map in this report is constructed from grids that combine aeromagnetic data (see data processing details) collected in 23 separate aeromagnetic surveys flown between 1948 and 1994. The data from these surveys are of varying quality. The design and specifications (terrain clearance, flight line separation, flight direction, analog/digital recording, navigation, and reduction procedures) varies between surveys depending on the purpose of the project and the technology of that time. All of the pre-1975 data are available only on hand-contoured analog 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 (USGS, 1999). An index map gives an overview of the original surveys, and data table summarizes the specifications of the surveys.
This Virginia 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. Since most of the surveys used in the Virginia compilation were flown at either 122 m (400 ft) or 152 m (500 ft) above terrain, an elevation of 152 m (500 ft) above terrain was initially selected as the starting point of the state compilation. Two surveys (VA04 and VA10) were flown at 1524 m (5000 ft) barometric and were continued downward to 152 m (500 ft) above terrain using equivalent source techniques (Phillips, 1996, Xia and others, 1993). One survey (VA19) was flown at 305 m (1000 ft) above terrain and was downward continued to 152 m (500 ft) above terrain. All of the data were then merged together at 152 m (500 ft) above terrain, and to conform to the national standard, the entire State grid was analytically continued upward to 305 m (1000 ft) above terrain (Hildenbrand, 1983). Grids at both elevations are available for downloading.
The data processing and merging of the grids were done using in-house software (Phillips, 1997) and the images were created using Geosoft's OASIS montaj software. The resulting grids have a data interval of 500 m and can be downloaded in several formats at either of two elevations - the grids at 500 feet above terrain are called vamag_500ft.*** and the grids at 1000 feet above terrain are called vamag_1000ft.***. A color-shaded relief image of the grid at 1000 feet above terrain is shown on the opening page of this web report.
This project was supported by the Mineral Resource and Geologic Mapping Programs of the USGS. Thanks to the following USGS colleagues: Pat Hill, 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 of the aeromagnetic data.
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