Engineering topics
Publications and source records attributed to Marsac, Kara.
Presentation describes project to use aerial radiological mapping to refine geologic maps using available geological and geochemical data to predict radiological backgrounds.
Aerial gamma ray surveys have many applications in geology and science in general, such as locating mining prospects, defining radioactive plumes, and detecting nuclear weapons. Unfortunately there is currently no simple way to separate the natural gamma radiation of soil and rocks from that of contaminants such as radioactive plumes. This project used geochemical data (uranium, potassium and thorium concentrations) collected from national databases, private companies, and the NURE (National Uranium Resource Evaluation) Survey, to create forward models of exposure rates measured by aerial gamma ray surveys. We developed these techniques using an area in north central Arizona known as the Navajo Mines area, chosen for its optimal conditions for aerial gamma ray surveys and readily available survey data. Models based on geochemical analyses from databases were not found to be successful, in part due to lack of data for some units. Models based on NURE data sorted by geologic unit were successful at replicating the aerial gamma ray survey, though units with Uranium mineralization proved difficult. ASTER visualizations were effectively used to create subunits of similar exposure rate within the Chinle Formation which contains multiple lithologies. For alluvial units, models based on drainage basin were attempted with success. With these models we are developing techniques to estimate the natural radiation generated by the rocks and soils of an area, making aerial ray surveys more effective.
Radiation occurs naturally in bedrock and soil. Gamma rays are released from the decay of the radioactive isotopes K, U, and Th. Gamma rays observed at the surface come from the first 30 cm of rock and soil. The energy of gamma rays is specific to each isotope, allowing identification. For this research, data was collected from national databases, private companies, scientific literature, and field work. Data points were then evaluated for self-consistency. A model was created by converting concentrations of U, K, and Th for each rock and soil unit into a ground exposure rate using the following equation: D=1.32 K+ 0.548 U+ 0.272 Th. The first objective of this research was to compare the original Aerial Measurement System gamma ray survey to results produced by the model. The second objective was to improve the method and learn the constraints of the model. Future work will include sample data analysis from field work with a goal of improving the geochemical model.