Airborne gamma-ray spectrometry has been used since the late 1960s primarily for the detection of uranium deposits. Since then, with progressive improvements in instrumentation and data quality, it is now used extensively in geological mapping and mineral exploration.
Bedrock is the primary source of radioelements 'seen' by gamma-ray spectrometric mapping. As discussed earlier, K is common in many major rock forming minerals whereas Th and U are present in much smaller amounts and are commonly associated with resistate and accessory minerals. The abundance of K, Th and U generally increases from basic to acid igneous rocks. Variations in the proportions of the three radioelements can be used map different rock types. Gamma-ray imagery, together with magnetics, aerial photography and multi-spectral imagery, is one of the most useful datasets in geological mapping. Harris (1990) used cluster analysis techniques on individual radioelement bands and ratio bands to generate maps of major radioelement and lithological associations. For many other examples see the reference list and the Canadian Geological Survey website: http://gsc.nrcan.gc.ca/gamma/survey_e.php). Gamma-ray imagery and various derivative datasets over the Pilbara in northern WA illustrate the wealth of geological information that can be extracted from the analysis gamma-ray datasets (see Pilbara 3D interactive model).
In addition to mapping bedrocks, there are many examples where airborne spectrometry has been used to target bedrock alteration associated with mineralisation. Ratio maps, in particular the Th/K ratio, have been used to highlight K alteration associated with mineralisation in Canada. For example the Casino Au-Cu-Mo porphyry in Canada was mapped by elevated K values associated with a potassic alteration zone consisting of K-feldspar and biotite (Bower et al. 1995). In general, low Th/K ratios are excellent indicators of K alteration because Th is not usually associated with the K alteration process (Shives et al. 1995.) In Australia, Dickson and Scott (1977) showed that a variety of mineralisation processes can affect the distribution of radioelements in different ways and that recognising responses associated with mineralisation requires an understanding of the responses of bedrock and regolith materials.