New Jersey Aeromagnetic and Gravity Maps and Data

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.

[Summary provided by the USGS.]