New Jersey 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 New Jersey aeromagnetic map in this report is constructed from grids that combine aeromagnetic data (see data processing details) collected in eight separate aeromagnetic surveys flown between 1950 and 1979. 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) may vary between surveys depending on the purpose of the project and the technology of that time. All of the pre-1976 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. All surveys have been continued to 304.8 meters (1000 feet) above ground and then blended or merged together.  The merging of grids and production of images were created using a PC version of Geosoft/OASIS montaj software.  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 opening page of this web report.

This grid is an interim product. Considerable editing of digital flight line data was undertaken for survey 3144 to reduce leveling inconsistencies between adjacent flight lines, most notably in the southern part of the state.  Anomaly resolution is only fair in the northern portion of this survey, which was flown at one-mile flight line separation, where the source rocks are at or near the surface.  In these areas of this survey where the anomalies run roughly parallel to the flight lines, the gridding process produces a 'string of pearls' effect.  Improved resolution can only be rectified by new surveys with more closely spaced flight lines. Heavy strike filtering in the direction of the flight lines was necessary to reduce flight line striping for two digital surveys (5004 and 6027). 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 generally ranged from 4.8 to 9.6 km (3 to 6 mile).  In some areas of the U.S., detailed NURE surveys were flown with a finer line spacing, usually at a 0.4 km (0.25 mile) interval.  In New Jersey, the NURE program flew the Reading Prong (5004) at this interval.

This New Jersey 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 New Jersey compilation were flown at either 122 m (400 ft) or 152 m (500 ft) above terrain.  To conform to the national standard, the entire State grid was analytically continued upward to 305 m (1000 ft) above ground (Hildenbrand, 1983).

This aeromagnetic compilation supercedes a prior report (Snyder, 1992) releasing the same data as three separate grids on 5.25" floppies.  The same data have since been reprocessed to produce better results.

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.