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DS 128: Missouri Magnetic Data Processing

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The assembly of 25 individual magnetic surveys and grids to create the Missouri State-wide compilation was done in several steps using the commercial software package Geosoft Oasis montaj. References listed are those known to be used as part of the package.

DATA PROCESSING STEPS

1. Grids were constructed from the original magnetic survey data with a cell size of between 1/3 and 1/5 of the flight-line 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 vertical-intensity data, after conversion to total-field, were gridded at 1,000 m (3,280 ft).

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).

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 (3,280 ft) for the composite grid are available for downloading as indicated on the "Detailed Aeromag Maps" page.

4. The original survey grids were continued to 305 m (1,000 ft) above ground as necessary. Upward continuation of the NURE, vertical-intensity converted, and digitized draped surveys was by standard 2D FFT (two-dimensional Fast Fourier Transform) filtering techniques. Level-to-drape continuation and surface-to-drape continuation, where 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 (3,280 ft) 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 depicts the magnetic field measured or calculated at 305 m (1,000 ft) above ground. This level was selected in order to be compatible with recent USGS State compilations.

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