Aerial gamma-ray data can be used to quantify and describe the radioactivity of rocks and soils. The majority of the gamma-ray signal is derived from the upper 20-25 cm of surficial materials (rock or soil). A gamma-ray detector is mounted in an aircraft that is flown over an area at a certain altitude, usually 120-150 m (400-500 ft). Equivalent uranium (eU) is calculated from the counts received by the gamma-ray detector in the energy window corresponding to bismuth-214. This technique assumes that uranium and its decay products are in secular equilibrium. A contour map of eU is then produced for the the area. The same technique is used to estimate potassium (K), from the K-40 energy window, and equivalent thorium-232, from the thallium-208 energy window. Total gamma exposure can be estimated by combining the data from the potassium, uranium, and thorium data channels. The primary source for aerial radiometric data in the United States is reports of the U.S. Department of Energy's National Uranium Resource Evaluation (NURE) program of the 1970s and early 1980s. These data have been integrated into contour maps of equivalent uranium, thorium, potassium, and total gamma radioactivity exposure for the conterminous United States.
These images were generated from NURE aerial gamma-ray data presented in United States Geological Survey Digital Data Series DDS-9, "National Geophysical Data Grids: Gamma-Ray, Magnetic, and Topographic Data for the Conterminous United States", by J.D. Phillips, J.S. Duval, and R.A. Ambrosiak, 1993.
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