The Idaho map was compiled from a synthesis of digital data
acquired from fifty-six separate aeromagnetic surveys flown at different
times with varied flight elevations, flight-line spacings, and data-reduction
procedures. Figures 1a, 1b, 1c and table 1 illustrate and describe the
surveys used in the map compilation. Flight-line elevations ranged from
120 m above terrain to 4,300 m barometric (400 ft to 14,000 ft) ; flight-line
spacings ranged from 0.8 km to 9.6 km (0.5 mi to 6 mi). Survey aerial coverage
ranged from a 15' x 15' quadrangle to a 1 x 2 degree quadrangle. The data
were projected onto a cartesian coordinate system using a Lambert projection
with standard parallels of 33°N. and 45°N., a central meridian
of 114°W., and a base latitude of 0N.
Each survey was interpolated to a square grid using a minimum-curvature
algorithm (Webring, 1981); grid spacing
was typically 1/4 to 1/3 the original flight-line spacing. The magnetic-anomaly
grid [total field intensity minus the Definitive International Geomagnetic
Reference Field (DGRF)] was calculated (Sweeney,
1990) for the appropriate time of year and elevation of the original
survey. If an obsolete regional field other than the DGRF had been removed,
as was the case with much of the digitized data, the outdated geomagnetic
reference field was added back and the DGRF was subtracted from the grid.
An elevation of 305 m (1,000 ft) above terrain was selected
as the reduction datum level in order to be compatible with adjacent regional
compilations in Nevada (Hildenbrand and Kucks,
1988a, 1988b) and the Basin and Range province (Hildenbrand
and others, 1983), Utah (Bankey and others,
1998) and a regional compilation covering most of Idaho and southwest
Montana (McCafferty, 1992). Surveys flown
in draped mode (constant elevation above terrain) above or below this datum
level were analytically continued upward or downward (Hildenbrand,
1983) so that the data would be consistent with adjacent surveys. For
surveys flown at a constant barometric elevation, the related data were
analytically continued to the draped surface of 305 m above ground using
the method of Cordell (1985b). If the survey's
data had to be continued more than two grid intervals downward, the data
were regridded to a coarser interval prior to continuation to minimize
short-wavelength noise enhanced by the method. After reducing the data
to a common level, each survey was regridded to a 1-km interval and merged
to adjoining surveys using a cubic-spline method (Cordell
and others, 1992).
Every attempt was made to acquire the data in digital form.
Most of the available digital data were obtained from aeromagnetic surveys
flown by the U.S. Geological Survey (USGS), flown on contract with the
USGS, or were obtained from other federal agencies and state universities.
Much of the pre-1975 data are available only on hand-contoured 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. Table 1 specifies availability of digital data.
The entire study area is covered by aeromagnetic data collected
as part of the National Uranium Resource Evaluation (NURE) program of the
U.S. Department of Energy. These data are available in digital form and
provided the framework for the map compilation. 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 ranged from 4.8 km to 9.6 km, with the exception of
part of the Challis 1° x 2° quadrangle, which was flown at 1.6
km flight line spacing. The wide spacing between flight lines flown at
low altitudes over surface rock units having high magnetizations (basalts
of the Snake River Plain, for example) causes anomalies with short spacial
wavelengths to be elongated between flight lines, producing lineations
perpendicular to the flight-line direction and 'pearl string' anomalies
along the flight line. This problem was especially severe over the Snake
River Plain and Columbia Plateau basalt fields. Consequently, data from
surveys other than NURE were incorporated into the framework of NURE surveys
wherever possible.
