The USGS has released a revised "long-wavelength (50–2,500 km) content of the regional magnetic anomaly compilation for the conterminous United States by utilizing a nearly homogeneous set of National Uranium Resource Evaluation (NURE) magnetic surveys flown from 1975 to 1981."
The authors "created a preliminary, full spectrum, 1.25-km magnetic anomaly grid from a database by combining short-wavelength magnetic anomalies from the North American Magnetic Anomaly Map (NAMAM) and long-wavelength anomalies from NURE using a Gaussian filter centered at 50-km wavelength,... useful for analyzing geodynamic aspects of the crustal and mantle magnetic field that require precise long-wavelength information."
The file can be downloaded from the ftp site below.
Ref: A Preliminary, Full Spectrum, Magnetic Anomaly Grid of the United States with Improved Long Wavelengths for Studying Continental Dynamics: A Website for Distribution of Data
By D. Ravat, C. Finn, P. Hill, R. Kucks, J. Phillips, R. Blakely, C. Bouligand, T. Sabaka, A. Elshayat, A. Aref, and E. Elawadi, Open-File Report 2009–1258, U.S. Geological Survey
ftp://ftpext.usgs.gov/pub/cr/co/denver/musette/tmp/outgoing/ravat/