The assembly of 85 individual aeromagnetic surveys and grids to
build the Alaska state-wide compilation was carried out in two
stages. (1) A compilation for interior Alaska (spanning from
61 to 66 N latitude and 144 to 159 W longitude) was completed
in 1994, (2) the rest of the state-wide compilation was
completed in August 1997.
Data processing for the interior Alaska aeromagnetic data compilation was
done by Paterson, Grant and Watson, Ltd, under contract to the
State of Alaska, Division of Oil and Gas with cooperation from the USGS.
Data from 25 aeromagnetic surveys were compiled.
Paper maps at a scale of 1:500,000 were published by the USGS
(USGS Map GP-1014; reference list ).
These maps are available from USGS Map Sales
(goto USGS Map Sales ).
Data processing for the state-wide compilation was done by
Northwest Geophysical Associates under contract to the USGS.
Data from an additional 60 surveys and previously processed grids
were added to the interior Alaska compilation. In addition,
some modification of the interior Alaska compilation was done to
improve the merge to adjacent surveys in the northwest and southeast
corners.
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 bidirectional gridding algorithm when necessary
due to wide flightline spacing. 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.
- Data quality problems were addressed in consultation with the USGS.
- The Definitive Geomagnetic Reference Field (DGRF) was applied for
the date of the original survey (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 1 km using a minimum curvature algorithm.
- The datum levels of adjacent surveys were adjusted (by addition
or subtraction of a constant value) to minimize differences at the
boundaries.
- These adjusted grids were combined, leaving a 1 cell gap between
surveys, into a composite grid (one of the final products).
- The original survey grids were upward or downward
continued and converted from level to drape as necessary to
produce a consistent survey specification of 1000 ft above ground.
Upward continuation of the NURE surveys was by standard 2D FFT
filtering techniques. Downward continuation and level-to-drape
was performed using the COMPUDRAPE technique (as implemented in
the GEOSOFT software package).
- The datum levels of the converted grids were then adjusted
to minimize differences at the boundaries.
- These adjusted grids were combined into a single merged grid (with no
data gaps inserted between surveys).
GRID PROJECTION SPECIFICATIONS
- Projection = Albers conical equal-area
- Central meridian = 151 W
- Base latitude = 55 N
- Standard parallels = 55 and 65 N
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
