Aerial Gamma-Ray Surveys in Alaska
Joseph S. Duval
U.S. Geological Survey Open-File Report 01-128
Please note: The data from this CD are now online, but some files are quite large. All data can be obtained from the Data directory.
Overview of These Data
These surveys were flown as part of the National Uranium Resource Evaluation (NURE) Program of the U.S. Department of Energy (USDOE), and were flown during parts of 1976, 1977, 1979, and 1980. For most of the surveys, the flightline spacings are 10 km and the lines are oriented east-west. North-south tie-lines were flown at intervals of about 40 km. The nominal altitude of the aircraft above the ground was 122 m. The gamma-ray detectors used were thallium doped sodium iodide detectors with a total crystal volume of about 50 L. The gamma-ray spectra were processed to produce corrected count rates for potassium, uranium, thorium, and total count. The corrections applied to the gamma-ray data included corrections for aircraft and cosmic radiation background, altitude variations, Compton scattering, and airborne bismuth-214. Most of the data were originally organized by the 250,000 scale topographic quadrangles in Alaska. For the purposes of this presentation of the data, the data were collected into a single database. Some of the aerial measurement systems were calibrated so that the data were represented as concentrations of potassium, uranium, and thorium in the rocks and soils but some systems presented the data as counts per second. Because of these differences, the data were further processed to remove calibration differences and to convert the count rates to concentration values. The database includes the original data values as well as the processed values.
After completion of data processing, the data were gridded and map presentation were prepared of the various grids. For the convenience of the user, a map showing a grid of magnetic data as prepared by Saltus, Hill, and others (1999) is also included as well as a map of the topographic data for Alaska. In order to view the maps and databases, you will need to install a free viewer that is included with the data. Access to the magnetic data via the world-wide web is provided through several web-sites (Connard and others, 1999; Saltus and Simmons, 1997; Saltus and others, 1999). The magnetic data are also available on CD-ROM (Saltus, Connard and Hill, 1999).
Aerial gamma-ray surveys measure the flux of gamma rays emitted as a result of the radioactive decay of the naturally occurring radioactive elements K40 (potassium), U238(uranium), and Th232 (thorium). Because different kinds of rocks and soils generally contain different amounts of potassium, uranium, and thorium, aerial gamma-ray data provide information that can be used to differentiate different types of rocks and soils (see e.g. Pitkin, 1964; Darnley and Grasty, 1971, Vulkan and Duval, 1993). Because geochemical processes can alter the amounts of potassium, uranium and thorium in rocks and soils, aerial gamma-ray data also provide information that can be used to aid mineral exploration (see e.g. Pitkin, 1991; Yeates, 1982). Other applications of gamma-ray data are reviewed by Duval (1980).
According to Adams and Gasparini (1970) uranium and thorium are chemically stable in the tetravalent ion state under reducing conditions, have similar ionic radii, equal coordination number (8) with respect to oxygen, and complete outermost electron shells. As a result they strongly tend to remain together in geologic processes that occur under reducing conditions. Because of their large atomic size, high valence and electronegativity, uranium and thorium cannot form isomorphic series that involve major rock forming minerals and occur mostly in accessory minerals. Under oxidizing conditions, thorium remains stable in the tetravalent state but uranium is oxidized to the hexavalent state and is soluble in aqueous solutions. Some of the accessory minerals that often contain uranium and/or thorium are apatite, sphene, zircon, allanite, monazite, pyrochlore, thorite, uraninite, and xenotime. Definitions for the various minerals were taken from the second edition of the Glossary of Geology.
Potassium is an alkali element and is one of the major rock-forming elements of the Earth's crust. It is present in almost all crustal rocks and is enriched in the last stage of magmatic differentiation. Potassium is locally enriched in metamorphic rocks during the process known as potassium metasomatism. Although potassium is relatively mobile at temperatures and pressures of rock formation, little is known about the kinetics of its transfer through solid rocks. During weathering of crystalline rocks, potassium is readily dissolved but is also easily removed from solution. Potassium in solution is readily adsorbed on clay minerals or may contribute to the formation of glauconite and sericite. Under certain conditions, potassium can precipitate from sea water as a chloride or sulfate (carnallite, kainite, sylvite). Common potassium minerals found in igneous and metamorphic rocks are potassium feldspar, leucite, muscovite, and biotite. For more information on potassium refer to Fairbridge (1972).
This CD-ROM publication contains data and maps that are derived from aerial gamma-ray surveys flown over parts of Alaska. Because of the volume of data contained on this CD-ROM, it is not practical to download the datasets, but the CD can be purchased from the address given below. The button bar at the bottom of this page links to additional information about the data contained on the CD-ROM.
This CD-ROM may be purchased from the following USGS source:
USGS Information Services,
Box 25286, Federal Center,
Denver, CO 80225
telephone: 303-202-4210; e-mail: firstname.lastname@example.org
See also http://store.usgs.gov for more information.
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Last modified 28-Jan-2009
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