Wyoming Aeromagnetic Data Processing

The assembly of 27 individual aeromagnetic surveys and grids to create the Wyoming state-wide compilation was done in several steps.

DATA PROCESSING STEPS

1. 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.
2. The Definitive Geomagnetic Reference Field (DGRF) calculated (Sweeney, 1990) for the date of the original survey was applied (in some cases this required the determination and addition of the original reference field removed before removal of the DGRF) .
3. The original survey grids were continued to 304.8 meters (1000 feet) above ground by converting  to draped as necessary. Upward continuation of the NURE and other draped surveys was by standard 2D FFT filtering techniques. Level-to-drape continuation was performed using the chessboard method (Cordell and others, 1992).
4. The survey grids were regridded, as necessary, to the final grid cell size of 500 meters using a minimum curvature algorithm.
5. The datum levels of the converted grids were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.
6. 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 adjacent state compilations in Utah (Bankey and others, 1998), Montana (McCafferty, 1998), and Colorado (Oshetski and Kucks, 2000).

ADDITIONAL PROCESSING STEPS OR INCONSISTENCIES

• Quadrangles of NURE surveys that consisted of 6 mile flight spacing (Cody, Arminto, Casper, And Preston) were plugged using minimum curvature interpolation to remove areas of no data within the Wyoming state boundary.
• Merging of NURE quadrangles at the 42^oN latitude generated lineations at the boundaries due to questionable compatibility of data sets.
• The border between Cody-Sheridan and Thermopolis-Arminto at the 44^oN latitude has a questionable merge boundary which may actually be geologically controlled.
• The Sierra Madre (5040) survey has extensive flight-line noise problems that are of unknown origin. The solution was to first remove any extremely offending lines and then to micro-level the remaining lines. There still exists some major shifts that appear to be in blocks of lines but they have been allowed to remain.
• Miscellaneous herringbones exist within the map where the data quality was beyond any reasonable recovery.

GRID PROJECTION SPECIFICATIONS

• Projection = Lambert conformal conic
• Central meridian = 107.5 degrees W
• Base latitude = 41 degrees N
• Standard parallels = 33 and 45 degrees N
• Semi-major ellipsoid axis = 6378206.4 m
• Eccentricity squared = 0.0067686579973

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