Terrestrial Radioactivity and Gamma-ray Exposure in the United States and Canada: Gridded geographic images

Aerial gamma-ray surveys measure the gamma-ray flux produced
by the radioactive decay of the naturally occurring elements
K-40, U-238, and Th-232 in the top few centimeters of rock or
soil. If the gamma-ray system is properly calibrated the data
can be expressed in terms of the estimated concentrations of
the radioactive elements. A ternary color-composite image was
generated by commercial software Geosoft Oasismontaj to map the
combination of all three elements.
During the period 1975-83, the U.S. Department of Energy carried
out the National Uranium Resource Evaluation (NURE) Program,
which included aerial gamma-ray surveys of most of the United
States. These data were reprocessed to generate this product.