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Maximum noise fraction method reveals detail in aerial gamma-ray surveys

Low-noise images of the U/Th ratio can be obtained from aerial gamma-ray survey data by using the maximum noise fraction (MNF) method, which applies statistical linear algebra operators to most of the 256 channel raw spectral data of a survey. With reduced noise, U/Th data may be better used to map geological variations and explore for minerals such as U, Th and Sn. Data from a small (120 line-km) survey in north Queensland are used to demonstrate the method and confirm the reliability of the detail in the U/Th map. The map is compared with those derived using conventional processing methods and using noise adjusted singular value decomposition (NASVD). Statistical tests of selected areas and profile displays show all methods produce similar U/Th ratios on a broad scale, but only the MNF method enables small-scale features to be identified. A small dam in the study area, which being waterfilled has low count-rates, gives anomalously high U/Th ratios. This dam is seen in the conventionally processed U/Th image as a small area of high ratios, is difficult to see in the NASVD image, but is shown with its correct size and shape in the MNF image.