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
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 (Duval, Cook, and Adams, 1971). If the gamma-ray system is properly calibrated (for example, see Grasty and Darnley, 1971), the data can be expressed in terms of the estimated concentrations of the radioactive elements. The potassium concentration data are usually expressed in units of percent potassium (percent K), uranium as parts per million equivalent uranium (ppm eU), and the thorium as parts per million equivalent thorium (ppm eTh). The term equivalent is used because the technique actually measures the gamma-ray flux from the decay of bismuth (Bi-214), which is a decay product of U-238, and from the decay of thallium (Tl-208), which is a decay product of Th-232. Radioactive disequilibrium in the thorium decay series may cause the measured equivalent uranium and equivalent thorium to differ from the actual uranium and thorium present in the surface rocks and soils. Because Rn-222 is a daughter product of the U-238 decay series, the U-238 concentrations can also be used to estimate the amounts of Rn-222 in the near-surface soil gas. 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 conterminous United States. Although many of the airborne gamma-ray systems used to make these surveys were calibrated, many of the earlier surveys were done without calibration and conversion to the concentrations of the radioactive elements. Detailed examinations of the digital data available on magnetic tape also showed that many of the "calibrated" surveys do not match the data from other "calibrated" surveys of adjacent areas. For these reasons, the data must be corrected to obtain a consistent data base for the conterminous United States. Because uranium, thorium, and potassium concentration data are useful in geologic studies and because the NURE data are the only nationwide data base on the natural radiation environment, the U.S. Geological Survey (USGS) reprocessed the aerial gamma-ray data to produce maps showing surface concentrations of potassium, uranium, and thorium for the conterminous United States. These maps have been released as USGS Open- File Reports (Duval and others, 1989, 1990). Some of the reprocessed data have also been released in profile form (Duval, 1995). |
