Levelling airborne and ground gamma-ray spectrometric data to assist uranium exploration

Full text: detection of ore bodies. The gamma-ray spectrometric method is an efficient method for the regional assessment of uranium potential and the detection of surface mineralization. However, the full potential of the method can only be realised when the data are adequately standardised. In this paper, we present examples of this standardisation at both regional and local scales. At a regional scale, we show how the levelling of airborne gamma-ray spectrometry data over Australia increases the value of the resulting data, and on a local scale, we demonstrate a geometrical correction for ground gamma-ray spectrometry in shallow holes that improves the accuracy of measurements. Most of Australia has been systematically surveyed using the gamma-ray spectrometric method over the past 40 years. However, many of the surveys were either inadequately calibrated, or the results were not reported in units of concentrations of K, U and Th. This limits the usefulness of these data, as it is difficult to compare anomalies detected on separate surveys, or to combine surveys into larger compilations in order to interpret regional features in the data. To solve these problems, Geoscience Australia flew an airborne gamma-ray spectrometric survey over the entire Australian continent with a 75 km line spacing to serve as a radioelement baseline for all current and future airborne gamma-ray spectrometric surveys in Australia. The survey was first back-calibrated to the International Atomic Energy Agency's (IAEA) radioelement datum, and then used to bring all the public-domain airborne gamma-ray spectrometric surveys in Australia to this datum. Interpreters can now use the levelled data to reliably compare the radiometric signatures observed over different parts of Australia and to interpret large-scale regional features in the data. Ground gamma ray spectrometry surveying with portable gamma ray spectrometers are carried out using specific source-detector geometries. Examples include a flat earth (ω = 2π sr), in shallow holes (ω > 2π sr), or at low altitudes above the surface of the earth. The latter is often the case with dynamic measurements. Since portable gamma ray spectrometers are calibrated for an infinite half-space source-detector geometry (ω = 2π sr), K, U, and Th radioelement estimates derived from portable spectrometer measurements with other source-detector geometries will be either positively or negatively biased. A field investigation of the K, U and Th response of a portable multichannel gamma ray spectrometer to measurement in shallow hole to a depth up to 0.4 m, and to the measurement above the Earth's surface up to the height of 3 m was conducted in an area of crystalline rocks of medium radioactivity in the Czech Republic in 2008. Estimates of the geometry corrections enable the compilation of radioelement maps and the comparison of anomalies from ground measurements taken under a variety of source-detector geometries. (author)