North Dakota Aeromagnetic Data
Processing
The assembly of 13 individual aeromagnetic surveys and their grids
to create the North Dakota state-wide compilation was done in several
steps, using almost exclusively both a Unix and a PC version of
Geosoft/OASIS montaj. References listed below are those known to be
used as part of this package. The Definitive Geomagnetic Reference
Field corrections (see step 2/ below) were calculated using in-house
software.
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 (Webring, 1982). 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
removed (in some cases this required the determination
and addition of the original reference field removed before removal of
the DGRF) (Sweeney, 1990).
- Much of the data required microlevelling, using OASIS montaj,
before continuation to our reference surface. This procedure
uses decorrugation, and attempts to remove any artificial
trends or anomalies due to residual flight line noise
remaining in the data. The grids resultant from this step
(whether or not microlevelling was performed) whose grid spacing
is less than the final grid cell size of
1000 meters for the composite grid are available for
downloading, and are listed below.
- The original survey grids were continued to 304.8 meters (1000 feet)
above ground by draping as
necessary. Upward continuation of the NURE and digitized draped surveys was by
standard 2D FFT filtering techniques. Level-to-drape
continuation, and surface-to-drape continuation where the
true measurement elevations were known, were performed using the chessboard method
(Cordell and others,
1992).
- The survey grids were regridded, as necessary, to the
final grid cell size of 1000 meters using a minimum curvature
algorithm.
- The datum levels of the converted grids were adjusted (by
addition or subtraction of a constant value) 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 304.8 meters (1000 feet) above ground. This level
was selected in order to be compatible with
recent
USGS state compilations, such as those in Montana
(McCafferty, 1998),
Colorado (Oshetski and Kucks, 2000),
Wyoming (Kucks and Hill, 2000),
and South Dakota (Kucks and Hill, 2002).
INTERMEDIATE GRIDS AVAILABLE FOR DOWNLOADING
All grids are presented in Geosoft binary grid format, with two files
describing each of the grids (suffixes .grd and .gi), and also in
Geosoft GXF format, an ASCII format described in the readme.txt file.
Either of these formats can be easily converted to numerous other
formats using the free conversion software offered by this company at
http://www.geosoft.com/.
- nd1000ag - Final North Dakota composite grid at 1000 ft.
above ground & gridded at 1000 m.
- 4251m100 - NE North Dakota (4251) gridded at 100 m.
GRID PROJECTION SPECIFICATIONS
- Projection = Lambert conformal conic
- Central meridian = 100 degrees W
- Base latitude = 45 degrees N
- Standard parallels = 33 and 45 degrees N
- Semi-major ellipsoid axis = 6378206.4 m
- Eccentricity squared = 0.0067686579973
- Horizontal datum = NAD27
- Ellipsoid = Clarke 1866
Top ||
North Dakota Mag ||
References ||
Crustal Team ||
Geology ||
USGS
For
more information about this report contact: Ron
Sweeney
