Digital data grids for the magnetic anomaly map of North America

Online link https://geo-nsdi.er.usgs.gov/metadata/open-file/02-414/metadata.faq.html
Description A digital magnetic anomaly database and map for the North American continent is the result of a joint effort by the Geological Survey of Canada (GSC), U. S. Geological Survey (USGS), and Consejo de Recursos Minerales of Mexico (CRM). The database and ma
Originators U.S. Geological Survey
Publication U.S. Geological Survey Open-File Report 02-414

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Overview of attributes

This metadata file describes three individual but
related grids.  The first grid contains the residual
total intensity of the Earth's magnetic field.  The
second grid was made by removing a 500-km high-pass filtered
from the total intensity data.  The third grid is the result
of removing a magnetic field that was calculated from
satellite magnetic data by Tiku Ravat of Southern Illinois
University.
Each grid has an accompanying file with the suffix .gi that
gives projection information.  The file names are:
>NAmag_origmrg.grd = 232,371 KB
>NAmag_origmag.grd.gi = 21 KB
>
>NAmag_hp500.grd = 232,753 KB
>NAmag_hp500.grd.gi = 22 KB
>
>NAmag_CM.grd = 232,557 KB
>NAmag_CM.grd.gi = 22 KB

http://pubs.usgs.gov/sm/mag_map/mag_book_s.pdf

NAmag_origmrg.grd

Residual total intensity of Earth's magnetic field

Label Definition
origmrg grid cell value The total magnetic value minus a geomagnetic reference field (GRF), which is a long-wavelength regional magnetic field. The most commonly used reference field is determined from a model developed by the International Association of Geomagnetism and Aeronomy (IAGA). The International Geomagnetic Reference Field (IGRF), is a predictive model adopted at the beginning of a model period (e.g. in 1989 for 1990-1995). After the model period, a revised definitive model is adopted, the DGRF. This is the preferred model to use for removing regional magnetic fields

NAmag_hp500.grd

500-km high-pass filtered grid calculated from residual magnetic grid

Label Definition
hp500 grid cell value Because wavelengths greater than roughly 150 km are unreliable in the compilation, applying a high-pass wavelength filter would appear to be a viable solution to remove these unreliable wavelengths. However, removing wavelengths less than 500 km from the merged grid creates artifacts, such as spurious separation of continuous anomalies. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the erroneous long wavelengths but maintaining continuity of anomalies. The correction was accomplished by transforming the merged grid to the frequency domain, filtering the transformed data with a long-wavelength cutoff at 500 km, and subtracting the long-wavelength data grid from the merged grid.

NAmag_CM.grd

Equivalent-source corrected grid

Label Definition
CM grid cell value An equivalent source method based on long-wavelength characterization using satellite data was used to correct for long-wavelength shifts. We produced an aeromagnetic grid in which the wavelengths longer than 500 km have been replaced by downward-continued CHAMP satellite data. Steps 0 and 6 were performed by Bob Kucks. Steps 1-4 were performed by Tiku Ravat. Step 5 was performed by Jeff Phillips. 0. The North American 1 km merged grid was decimated to 5 km. 1. This 5 km grid was converted to a 0.05 degree grid. This grid was low-pass filtered using a Gaussian filter with a 500 km cutoff, then decimated to 1 degree. 2. A joint inversion of this 1 degree low-pass aeromagnetic grid and CHAMP satellite data, with the aeromagnetic data weighted very low, produced a stabilized downward continuation of the CHAMP data. 3. The inverted data were interpolated to 0.05 degree and again low-pass filtered using the same Gaussian 500 km filter to remove 4. The low-pass grid from step 1 was subtracted from the original 0.05 degree aeromagnetic grid to create a 500 km high-pass aeromagnetic grid. This grid was added to the low-pass inverted grid from step 3 to get a corrected 0.05 degree aeromagnetic grid. 5. The corrected 0.05 aeromagnetic degree grid was projected to the DNAG projection and regridded to 5 km. This was subtracted from the decimated 5 km aeromagnetic grid to generate a 5 km correction grid. A matched filter was used to remove short-wavelength artifacts resulting from the projection and regridding process. 6. The resulting 5 km correction grid was regridded to the original 1 km grid and subtracted from the original 1 km aeromagnetic grid to generate the final 1 km corrected aeromagnetic grid.