New England 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 aeromagnetic map of New England was digitally compiled from a large number of data sources, most of which were originally analog surveys and presented as analog contour maps. Prior aeromagnetic compilations in New England were by analog methods and include Zietz and others (1974, 1980) and Zietz and Gilbert (1981). An analog compilation of southern New England with geologic interpretation was given by Harwood and Zietz (1976). More recently, digital aeromagnetic compilations in the region include the Lewiston and Sherbrooke quadrangles in Vermont-New Hampshire-Maine (Bothner and others, 1985), the Hartford Early Mesozoic basin in Connecticut and Massachusetts (Bond and Phillips, 1988), and the Gulf of Maine (Macnab and others, 1990).
The advantages of digital compilation are many: joining of the surveys is more accurate and objective, details of the magnetic field are preserved at all scales, for most surveys a uniform Definitive Geomagnetic Reference Field (DGRF) removal is possible, continuation of individual surveys to a common altitude is easily done, and various kinds of filtering such as decorrugation can be applied to correct imperfections in surveys. Once a digital grid has been compiled, it is available for digital enhancements and derivative products.
The New England aeromagnetic compilation, described here, is one part of a national digital compilation by the U.S. Geological Survey. For prior state compilations offered as web sites see: http://crustal.cr.usgs.gov/crustal/geophysics/index.html#two. In order to make surveys and state compilations with a variety of drape and level altitudes look similar, a standard elevation of 305 m (1000 ft) above mean terrain (drape) was adopted. To conform to this national standard, each survey grid was analytically continued upward or downward to 305 m (1000 ft) above ground, as needed.
The composite magnetic anomaly grid of New England states and the Gulf of Maine was constructed from grids of about 80 separate aeromagnetic and ship borne magnetic surveys conducted between 1948 and 1982 (see data processing details). The locations of the surveys and the specifications of those surveys are shown by index maps and data tables. The composite grid of the New England states was compiled at a data interval of 500m. Individual aeromagnetic grids and maps of the 6 states are also available, each trimmed to the state boundaries.
The state aeromagnetic grids for Massachusetts, Connecticut, and Rhode Island were treated separately and recompiled at a grid resolution of 160m, a three-fold resolution advantage, because these states are nearly completely covered by surveys with a flight spacing of 1/2 mile at 500 ft above ground (drape). Although these states were initially flown at 1-mi spacing and later filled to 1/2 mi coverage, the results are reasonably good after some decorrugation. The separate aeromagnetic maps and grids of these three states were kept at the original 152m altitude to retain the maximum resolution. The coastal and offshore areas were flown digitally much later at 1500 ft barometric altitude at 1 or 2 mi spacing and parts of these are attached to the land grid after downward continuation and regridding.
These grids are interim products. Considerable editing of digital flight-line data was undertaken for many surveys and strike filtering in the direction of the flight lines was necessary to reduce flight line anomalies for many surveys.
This project was supported by the Mineral Resource and Geologic Mapping Programs of the USGS. Thanks to USGS colleagues Pat Hill and Robert Kucks for their assistance in preparing this report.
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Contact: David L. Daniels
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