EXAMINING THE SPATIAL SCALE OF VARIATIONS IN TERRESTRIAL GAMMA-RAY BACKGROUND OVER INDIVIDUAL BEDROCK GEOLOGIC UNITS

The 2011 nuclear disaster at the Fukushima Daiichi nuclear power plant in Fukushima Prefecture, Japan has highlighted the need to better understand and be able to predict the distribution of natural radioisotopes on the surface. As the disaster unfolded, scientists working to determine the areal extent of contamination struggled to differentiate between low levels of contamination and the natural background caused by geochemical variations in the rocks and soil. While the distribution of anthropogenic radioisotopes can be determined using spectral means, their contribution to exposure rates can only be quantified once a suitable calibration has been performed. The ability to estimate the pre-existing radiological background and exposure rate would be particularly valuable in emergency situations where resources are spread thin and time is of the essence. Furthermore, having a means of estimating pre-release radiation levels and communicating those to the public can reduce concerns raised by members of the public who are engaged in amatuer radiation measurements. To the best of our knowledge no one has attempted to forward model background radiation based on geology and bedrock geochemistry of K, U and Th (the radioisotopes that dominate natural background). However, the ground-based component of the natural background should be predictable based on a sufficiently detailed understanding of the geochemistry of the rocks and soil on the surface. Based on several study areas in the western US we have developed a successful technique for extrapolating low resolution (1-20 km line spacing) aerial gamma-ray survey data measured by the National Uranium Resource Evaluation using the distribution of bedrock geologic units augmented by remote sensing imagery. In order to generalize this technique to larger geographic areas we need to understand the scale of within-unit variations in K, U and Th. Here we present an analysis of the spatial scale of within-unit radioisotope variations for several widespread stratigraphic units including the Navajo Sandstone, Kayenta Formation and the Pierre Shale.

DOE/NV/25946--2560