Utah Aeromagnetic Data Processing
The assembly of 20 individual aeromagnetic surveys and grids to
create the Utah state-wide compilation was done in several
steps.
DATA PROCESSING STEPS
- Grids were constructed from the original aeromagnetic survey data
with a cell size of between 1/3 and 1/5 of the flightline spacing
of the survey, using a minimum curvature gridding algorithm.
For digitized contour line data,
the initial grid was constructed using a minimum curvature
algorithm and a spacing appropriate for the scale of the digitized map.
- The Definitive Geomagnetic Reference Field (DGRF) calculated for
the date of the original survey was applied (in some cases this required the
determination and removal of the original reference field applied).
- The survey grids were regridded, as necessary, to the final
grid cell size of 1000 m using a minimum curvature algorithm.
- To create the composite aeromagnetic grid, the datum levels of
adjacent surveys were adjusted (by addition
or subtraction of a constant value) to minimize differences at the
boundaries.
- The original survey grids were upward
continued to 12,500 ft barometric elevation and converted from
drape to level as necessary.
Upward continuation of the NURE surveys was by standard 2D FFT
filtering techniques. Level-to-drape continuation
was performed using the method described
in Cordell and others, 1992).
- The datum levels of the converted grids were adjusted
to minimize differences at the boundaries.
- These adjusted grids were combined into a single merged grid
that shows the magnetic field measured or calculated at 12,500 ft
barometric elevation.
- Two regional aeromagnetic lines flown by the Navy's Project Magnet
(one east-west at approximately 38.5 degrees and one north-south at
-112.3 degrees) and one long NURE line (east-west at 39.5 degrees)
were used to check for datum shifts across the merged map, using
the method by Grauch (1993). Profiles were extracted from the
merged data along these three lines and compared to the regional
anomalies to search for datum shifts between surveys. These shifts
result from removing incorrect constants before merging. Where
line comparisions show a datum shift, the correct constant was
removed, even though in some places this resulted in visible
merge boundaries. These lines directly crossed parts of surveys
A, B, H, I, J, L, M, N, O, and Q.
- Conversion of the merged grid from level to draped surface was
performed using two different methods:
chessboard (Cordell and others, 1992) and xia-m (Phillips, 1996).
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conformal conic
- Central meridian = 112 degrees W
- Base latitude = 0 degrees N
- Standard parallels = 33 and 45 N
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
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Bankey
