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Aerial Gamma-Ray Surveys in Alaska
Joseph S. Duval
U.S. Geological Survey Open-File Report 01-128

Data Processing

Data Sources

The aerial gamma-ray surveys in Alaska were flown under contract to the U.S. Department of Energy as part of the National Uranium Resource Evaluation (NURE) Program.  The surveys were flown during the summer months of 1975, 1976, 1977, 1979, and 1980.  The figure below shows the index map of the aerial surveys by contractor and year flown.

Survey Index Map

Aero Service refers to Aero Service, Inc., T.I. to Texas Instruments, Inc., and LKB to LKB Resources, Inc.  You can click on parts of the index map to view the references for the surveys.  The surveys were flown with east-west flightlines spaced at about 10 km and north-south tie-lines spaced at about 40 km.  The nominal survey altitudes were 122 m above the ground with a detector volume of about 50 L of thallium doped sodium iodide detectors.  The data were corrected by the contractors for aircraft and cosmic background, altitude variations, Compton scattering, and airborne bismuth-214.  Some of the gamma-ray systems were calibrated such that the data were provided by the contractors as apparent surface concentrations of potassium, uranium, and thorium.  For the other surveys, the data were provided as counts per second.  The data were processed by the U.S. Geological Survey (USGS) to convert the count rates to concentration units.  This was done by comparing the data to calibrated surveys and assuming that the conversion only requires multiplication by a scaling factor.  In a similar manner the total count data were converted to a common scale by assuming that all of the gamma-ray systems used the same energy threshold for the total count measurement.  For large areas of Alaska, the count rates are very low and the data were often zero or negative.  Zero and negative values were arbitrarily set to a small value close to zero.  Because the count rates decrease at an exponential rate as the altitude increases, data for altitudes greater than about 250 m above the ground were set to a dummy value and were not used in the gridding process.  After completion of all corrections, grids of the various data channels were calculated using a minimum curvature algorithm and a grid cell of 5 km.  The latitude and longitude coordinates were projected using an Albers equal-area conic projection with the Clark 1866 ellipsoid, standard latitudes of 55 and 65 degrees North, a reference latitude of 0 degrees, and a reference longitude of 154 degrees West.

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Last modified 16-Apr-2001 (jsd)
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