National Geophysical Data Grids: Gamma-Ray, Gravity, Magnetic, and Topographic Data for the Conterminous United States
by
Jeffrey D. Phillips, Joseph S. Duval, and Russell A. Ambroziak
1993 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director
Magnetic anomalies are produced by variations in the distribution of iron minerals, usually magnetite, in the rocks of the Earth's crust. Igneous and metamorphic rocks can be very magnetic. By comparison, sedimentary rocks are usually nonmagnetic. Magnetic anomalies therefore provide a way of mapping exposed and buried crystalline rocks. The grid of magnetic anomaly data for the conterminous United States and adjacent marine areas (Godson, 1986) was created from digitized contours of the east half of the Composite Magnetic Anomaly Map of the United States, Part A (U.S. Geological Survey, 1982), and the Composite Magnetic Anomaly Map of the Conterminous United States West of 96 Degrees Longitude (Bond and Zietz, 1987),with additional data used in the compilation of the Magnetic Anomaly Map of North America (Geological Society of America, Committee for the Magnetic Anomaly Map of North America, 1987). A regional gradient present in the 1982 map was removed by using a corrected geomagnetic reference field (Godson, 1986). The data, originally gridded on a 2-km interval using the spherical Transverse Mercator projection of the Magnetic Anomaly Map of North America, were reprojected to the Albers projection used on this CD-ROM and regridded on a 2-km interval using a minimum curvature gridding program (Webring, 1981). An interpretation of the 1982 anomaly map was presented by Hinze and Zietz (1985). |
