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Digital data grids for the magnetic anomaly map of North America

By North American Magnetic Anomaly Group (NAMAG): Viki Bankey¹, Alejandro Cuevas², David Daniels¹, Carol A. Finn¹, Israel Hernandez², Patricia Hill¹, Robert Kucks¹, Warner Miles³, Mark Pilkington³, Carter Roberts¹, Walter Roest³, Victoria Rystrom¹, Sarah Shearer¹, Stephen Snyder¹, Ronald Sweeney¹, Julio Velez², Phillips, J.D.¹, and Ravat, D.⁴

¹ U.S. Geological Survey
² Consejo Recursos Minerales de Mexico
³ Geological Survey of Canada
⁴ Southern Illinois University, Carbondale, Illinois

The 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). This integrated, readily accessible, modern digital database of magnetic anomaly data is a powerful tool for further evaluation of the structure, geologic processes, and tectonic evolution of the continent and may also be used to help resolve societal and scientific issues that span national boundaries. The map derived from the digital database provides a comprehensive magnetic view of continental-scale trends not available in individual data sets, helps link widely separated areas of outcrop, and unifies disparate geologic studies.

This open-file report presents three unique, gridded data sets used to make the magnetic anomaly map of North America. Subsets of these three grids that span only the conterminous United States were also created, giving a total of six grids. Details on the data processing and compilation procedures used to produce the grids are described in the booklet that accompanies the North American magnetic anomaly map. All three grids have 1-km spacing and are projected to the DNAG projection (spherical transverse mercator, central meridian of 100 o W, base latitude of 0o, scale factor of 0.926 and Earth radius of 6,371,204 m.) More details are given in the metadata files that accompany the gridded data files. These grids are presented in Geosoft binary grid format, with two files describing each of the six grids (suffixes .grd and .gi). This format can be easily converted to numerous other formats using the free conversion software offered by this company at http://www.geosoft.com/. (Important note: because of size considerations, the current version of the free conversion software (Geosoft interface) is not complete using the initial download as recommended. The "data exchange format" software (which is still free) can be downloaded using the Software Manager within the interface after installation. See the Geosoft web site for any updated procedures).

The first grids (NAmag_origmrg.grd and USmag_origmrg.grd) show the magnetic field at 1,000 m. above terrain.

For the second grids (NAmag_hp500.grd and USmag_hp500.grd) we removed long-wavelength anomalies (500 km and greater) from the first grid. This grid was used for the published map. Although the North American merged grid represents a significant upgrade to older compilations, the existing patchwork of surveys is inherently unable to accurately represent anomalies with long (greater than roughly 150 km) wavelengths, particularly in the US and Canada (U.S. Magnetic-Anomaly Data Set Task Group, 1994). The lack of information about long wavelength anomalies is primarily related to datum shifts between merged surveys, caused by data acquisition at widely different times and by differences in merging procedures. Therefore, we removed anomalies with wavelengths greater than 500 km from the merged grid to reduce the effects caused by the spurious long wavelengths but still maintain the 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.

In addition to the 500-km high pass filter, an equivalent source method, based on long-wavelength characterization using satellite data (CHAMP satellite anomalies, Maus and others, 2002), was also used to correct for spurious shifts in the original magnetic anomaly grid (Ravat and others, 2002). These results are presented in the third grids (NAmag_CM.grd and USmag_CM.grd), in which the wavelengths longer than 500 km have been replaced by downward-continued satellite data. The steps used to create the third long-wavelength-corrected grid are:
     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 and 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 satellite data, with the aeromagnetic data weighted very low, was used to produce a stabilized downward continuation of the satellite data.
     3. The inverted data were interpolated to 0.05 degrees and again low-pass filtered using the same Gaussian 500-km filter to remove short-wavelength artifacts.
     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-degree aeromagnetic 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.

Two metadata files, one for the North American grids (NAmag_meta.txt) and one for the conterminous United States grids (USmag_meta.txt), are also included with the gridded data.

Finally, three files in Adobe Acrobat PDF format are included here. The map PDF file (mag_s.pdf) shows the published map. Two files in PDF format reproduce the booklet that accompanies the map; one is a Rehabilitation Act Section 508-compliant version (mag_book_508.pdf) whereas the other includes photographs along with the text (mag_book_s.pdf). Installer programs for several operating systems for Adobe Acrobat Reader are available at http://www.adobe.com/.

Listing of grid file names:

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.