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Data Series 138: Nebraska, Kansas, and Oklahoma Aeromagnetic Data Processing

The assembly of 28 individual aeromagnetic surveys and their grids to create the Nebraska, Kansas, and Oklahoma State-wide compilations was done in several steps using almost exclusively 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 both in-house and Geosoft software.

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 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).

3. 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 1,000 m for the composite grid, are available for downloading and are listed below.

4. The original survey grids were continued to 304.8 m (1,000 ft) 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).

5. The survey grids were regridded, as necessary, to the final grid cell size of 1,000 m using a minimum curvature algorithm.

6. The datum levels of the converted grids were adjusted (by addition or subtraction of a constant value) to minimize differences at the boundaries.

7. These adjusted grids were combined into a single merged grid that shows the magnetic field measured or calculated at  304.8 m (1,000 ft) above ground. This level was selected in order to be compatible with recent USGS State compilations, such as those in Colorado (Oshetski and Kucks, 2000), Wyoming (Kucks and Hill, 2000), North Dakota (Sweeney and Hill, 2003), South Dakota (Kucks and Hill, 2002), New Mexico (Kucks, Hill, and Heywood, 2001), and Arizona (Sweeney and Hill, 2001).

INTERMEDIATE GRIDS AVAILABLE FOR DOWNLOADING

All grids are presented in either Geosoft binary grid format, with two files describing each of the grids (suffixes .grd and .gi), or Geosoft GXF format, an ASCII format described in the readme.txt file. Either of these formats can be converted to numerous other formats using the free conversion software offered at http://www.geosoft.com/.

1. neksok_final - Final Nebraska, Kansas, and Oklahoma merged aeromagnetic anomaly grid at 1,000 ft above ground and gridded at 1,000 m.

2. ne_1000ag - Final Nebraska composite aeromagnetic anomaly grid at 1,000 ft above ground and gridded at 1,000 m.

3. kansas_1000ag - Final Kansas composite aeromagnetic anomaly grid at 1,000 ft above ground and gridded at 1,000 m.

4. okmag_1000ag - Final Oklahoma composite aeromagnetic anomaly grid at 1,000 ft above ground and gridded at 1,000 m.

5. ks08mag_final - Joplin, Kansas, aeromagnetic survey KS08 gridded at 200 m.

6. wichita_lv - Wichita Mountains, Oklahoma, aeromagnetic survey 0116 gridded at 200 m.

7. neksok_cba - Nebraska, Kansas, and Oklahoma complete Bouguer gravity anomaly gridded at 1,000 m.

8. neksok_iso - Nebraska, Kansas, and Oklahoma isostatic gravity anomaly gridded at 1,000 m.

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