This map was extracted from the US. Geological Survey Digital Series DDS-9 CD-ROM (Phillips, J.D.; Duval, J.S.; Ambroziak, R.A. National Geophysical Data Grids: Gamma-Ray, Gravity, Magnetic, and Topographic Data for the Conterminous United States. US. Geological Survey Digital Series DDS-9; 1993). The primary measurements were taken as part of the NURE (National Uranium Resource Evaluation) project of the USDOE. Aerial gamma-ray surveys were conducted along east-west grid lines separated by 3 to 6 miles over most of the conterminous US. The observed flux of the 214 Bi line at 1764 keV is interpreted to yield an equilibrium equivalent 238 U content in parts per million (ppm eU).
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