Aerial gamma‐ray data provide estimates of the apparent surface concentrations of potassium (K), equivalent uranium (eU), and equivalent thorium (eTh). These data can be expressed as nine radiometric parameters: K, eU, eTh, eU/eTh, eU/K, eTh/K, eTh/eU, K/eU, and K/eTh. The U.S. Geological Survey (USGS) has developed a technique which combines any three of these parameters to form a composite color image. The color image provides a partial synthesis of the radiometric data that can be used to aid geologic mapping and mineral exploration. The sample data set, from the Freer area in south Texas, illustrates the use of the color images.

No abstract

The initial parameter to consider when planning an aerial gamma‐ray survey is whether the survey will obtain total‐count or spectrometric data. This decision is often controlled by the money available, because spectrometric surveys are at least three times more expensive than total‐count surveys. Other parameters important in the planning of an aerial survey are flight‐line spacing, survey altitude, and detector volume. These parameters are shown in graphs that provide a better understanding of their interaction and can be used in survey planning. The graphs are based upon the concepts of the infinite source yield and the circle of investigation. These concepts relate an aerial gamma‐ray detector to the ground area viewed by the detector. Concentric rings that show the ground area viewed by a stationary detector are constructed using the radius of the circle of investigation for fixed percentages of the infinite source yield as a function of detector altitude above ground level. The data permit calculation of the width of a strip of ground along the flight line viewed by a moving detector for several flight altitudes. Velocity was found not to affect the detection of finite sources seriously in calculated gamma‐ray data, and the measurement time interval was found to have effects similar to those of velocity. Graphs of percent ground coverage and estimated ground resolution provide means of determining flight‐line spacing and survey altitude. The effect on the measured gamma‐ray signal by an anomalous source within the circle of investigation is calculated using concentric rings that represent one sample period along the flight line. Required detector volume can be estimated from graphs that provide approximate count rates for given detector volumes for total count and spectrometric data. A table shows estimated errors for several detector volumes and portrays the statistical uncertainty that results from the smaller volumes. The various curves presented can be used to define survey design parameters. Hypothetical examples illustrate their use and the fact that frequent compromises occur during the planning process.