Aeroradiometric Map Background Information

The National Uranium Resource Evaluation (NURE) program was conducted by the U.S. Government between 1974 and 1983. The NURE program was administered by the Grand Junction, CO, office of the Department of Energy. The program included airborne gamma-ray spectrometry and magnetic data collection as well as extensive geochemical sample collection and processing. Aeroradiometric and aeromagnetic surveys of 98 1° by 3° quadrangles were flown in Alaska between 1975 and 1980. The data were flown in 15 surveys by Texas Instruments (T.I.), Lockwood, Kessler, and Bartlett (LKB), and AeroServices (Aero) under contract to the U.S. Government. A series of contractor reports document the surveys on a quadrangle by quadrangle basis. We list references to these reports on the detailed survey index pages accessible through the Survey Info page.

The Alaska NURE aeroradiometric surveys were flown with the following specifications:

1. Flight height: 400 ft (122 m) above ground.
2. Flight line spacing: 6 mi (10 km).
3. Direction of flight lines: E-W.

The data were measured using sensors containing 30-50 L of thallium activated sodium iodide crystals. The data were processed by the contractors to correct for aircraft background contamination, cosmic rays, altitude variations, airborne Bi-214, and Compton scattering. The sensors were calibrated using test pads maintained at Grand Junction, CO, and Lake Mead, AZ. The data are reported in four data channels:

1. Total count (energy between 0.4 and 2.8 MeV).
2. Equivalent Uranium (energy between 1.66 and 1.86 MeV) in ppm.
3. Equivalent Thorium (energy between 2.4 and 2.8 MeV) in ppm.
4. Potassium (energy between 1.37 and 1.57 MeV) in percent (%).

Because of the low flight height (400 ft) and relatively wide data spacing (6 mi), less than 4% of the ground surface was actually sampled by the measurements in the quadrangles surveyed. The measurements typically sample only the upper 2 ft (50 cm) of the Earth (Duval and others, 1971).

The aeroradiometric grids were produced from the raw NURE radiometric data tapes as follows:

1. Data were read and reformatted into a standard line format.
2. Data lines with gaps were identified and renumbered.
3. A 3-point median filter was passed over the data (to remove one-point spikes).
4. A Gaussian filter was run over the data (to reduce multi-point noise).
5. Data points exceeding minimum and maximum criteria for each channel (uranium, thorium, potassium, and total count) were eliminated.
6. 2.5 km grids were constructed from each data channel using the minimum curvature gridding algorithm.

This procedure was performed on 6 blocks spanning Alaska. The resulting grids were then merged together to produce the final data grids. As part of the merging process datum levels were adjusted between surveys to minimize obvious data shifts at survey boundaries. This process does not correct for all the discrepancies between the surveys. These differences are particularly noticeable in the normalized RGB maps because these maps amplify small variations in the data values. Care should be taken when interpreting the patterns on these maps to avoid attaching geologic significance to boundaries between surveys.

In addition to directly sensing surface uranium, radiometric data can be used to locate intrusive rocks or to map any rock unit with a distinctive radiometric signature. Potassium is commonly found in potash feldspars, microcline, and orthoclase or in micas such as muscovite and biotite. Uranium and thorium in igneous and metamorphic rocks are usually found in accessory minerals such as apatite, sphene and zircon or in the rarer allanite, monazite, pyrochlore, thorite, uraninite, and xenotime (Hoover and others, 1992). Uranium tends to be highly mobile in the near surface whereas thorium is relatively stable. Uranium is chemically active over a broad range of temperature and pH and moves readily in the groundwater. Thorium is much less soluble than uranium and potassium and does not move except by mechanical means such as wind and erosion processes. Both thorium and uranium content tend to be greater in felsic rocks and to also increase with alkalinity (Hoover and others, 1992).

The ratio between potassium and thorium is rather constant in most rocks, typically varying from 0.17 to 0.2 (K/Th in %/ppm, Hoover and others, 1992). Rocks with K/Th ratios significantly outside this usual range have been called potassium or thorium specialized (Portnov, 1987). Igneous rocks with potassium specialization have been related to gold-silver, silver-polymetallic, molybdenum, and bismuth deposits. Thorium specialized rocks are identified with tin, tungsten, rare-earth, and rare-metal deposits (Portnov, 1987).

Ternary radioelement maps are a useful way to display uranium, thorium, and potassium data (Broome and others, 1987). In many cases, particular rock types will have characteristic ratios of the three elements and this type of map display can highlight useful geologic trends and patterns. In addition to distinguishing lithologic variations, because of the contrasting chemical properties of the radioactive elements, radiometric data can provide information relevant to understanding of various geochemical and physical processes, including alteration and weathering (Duval, 1989). Aeromagnetic maps provide good complementary information for use in geologic interpretations.