A Preliminary Full Spectrum Magnetic Anomaly Database of the United States With Improved Long Wavelengths for Studying Continental Dynamics

Under an initiative started by Thomas G. Hildenbrand of the U. S. Geological Survey, we have improved the long-wavelength (50-2500 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 surveys were flown in quadrangles of 2° of longitude by 1° of latitude with E-W flight-lines spaced 4.8 to 9.6 km, N-S tie-lines variably spaced, and a nominal terrain clearance of 122 m. Many of the surveys used base-station magnetometers to remove external field variations. NURE surveys were originally processed with IGRF core-field models, which left behind non- uniform residual trends in the data and discontinuities at survey boundaries. In this study, in place of the IGRF/DGRF, we used a spatially and temporally continuous model of the magnetic field known as the Comprehensive Model (CM), which allowed us to avoid discontinuities at survey boundaries. The CM simultaneously models the core magnetic field and long-wavelength ionospheric and magnetospheric fields, along with their induced components in the earth. Because of the availability of base-stations for removing external fields, we removed only the core-derived geomagnetic field based on CM4 (spherical harmonic degree 13) for our compilation. The NURE data have short-wavelength (less than 30 km) noise due to cultural sources, base-station offsets, and residual external field effects. It is possible to reduce and even remove these defects by identifying and editing them and by applying leveling and micro-leveling. There are also many high resolution individual surveys over the U.S. which could be incorporated into the improved NURE database; however, this could take a few years. Therefore, we have created a preliminary full spectrum magnetic anomaly 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. We call this product the NURE-NAMAM2008 magnetic database. NURE- NAMAM2008 is useful for analyzing geodynamic aspects of the crustal and mantle magnetic field that require precise long-wavelength information; e.g., estimating Curie-temperature depths and constraining lithospheric temperatures. Preliminary studies show that the corrected long-wavelength components in NURE- NAMAM2008 lead to more realistic Curie depths for the average western U.S. crust.