Digital data grids for the magnetic anomaly map of North America

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 map represent a substantial upgrade from the
previous compilation of magnetic anomaly data for North
America, now over a decade old.
This report presents three unique, gridded data sets used to
make the magnetic anomaly map of North America. All three
grids have 1-km spacing and are projected to the DNAG
projection. These grids are provided in Geosoft binary grid
format, with two files describing each of the grids (suffixes
.grd and .gi).
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been replaced by downward-continued satellite data.