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
and
Phillips, J.D.1 and Ravat,
D.4
2002
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.
U.S. DEPARTMENT OF THE
INTERIOR
U.S. GEOLOGICAL SURVEY
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 North American magnetic anomaly 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 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/.
The first grids (NAmag_origmrg.grd and USmag_origmrg.grd)
show the magnetic field at 305 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.
The six grids described in
this report are available for download using ftp (ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/pub/open-file-reports/ofr-02-0414)
or by purchasing this open-file-report on DVD through Open-File Services. Two metadata files, one for the North
American grids and one for the United States grids, are also included with the
gridded data.
References
Maus, S., Rother, M., Holme, R., Luhr , H., Olsen, N., and Haak, V., 2002,
First scalar magnetic anomaly map from CHAMP satellite data indicates weak lithospheric field,
Geophys. Res. Lett., v. 29, 10.1029/2001GL013685
Ravat, D., Whaler, M. Pilkington,
T. Sabaka, and M. Purucker, 2002, Compatibility
of high-altitude aeromagnetic and satellite-altitude magnetic anomalies over
Canada: Geophysics, v.
67, p. 546-554.
U.S. Magnetic-Anomaly Data Set Task
Group, 1994, Rationale and operational
plan to upgrade the U.S. magnetic-anomaly data base: National Academy
Press, Washington D.C., 25 p.