Two-dimensional inversion of airborne radiometric data for source concentrations: Application to Cu-Au targeting at Mara Rosa Magmatic Arc, Brazil

Airborne radiometrics (radiometric surveying) has been used in many applications, such as geological mapping, environmental studies, mineral exploration and lithology mapping. The standard processing of gamma-ray spectrometry data provides good results when the acquisition conditions such as the flight and sensor orientation are constant. In practice, abrupt changes in flight height when flying in mountainous terrain are common, and standard processing neglecting this factor can lead to erroneous interpretation. The primary cause is the fact that the successive corrections applied to radiometric data in standard processing do not consider the effective sampled area of a survey (i.e. the field of view), which can have significant and variable overlaps between adjacent samples due to changing observation height. For this reason, standard height and sensitivity corrections may lead to incorrect estimates of the concentrations of the radioelements on the ground. To ameliorate this problem, we have developed a two-dimensional inversion-based processing method that incorporates the aircraft height, replaces the sensitivity correction and accounts for a positivity condition using a logarithmic barrier. Some minerals associated with potassium, uranium and thorium allow the mapping of hydrothermal alteration, especially potassium which is an important constituent of hydrothermal fluids. We demonstrate the new methods by comparing the standard processing results with the inversion-based processing of airborne gamma-ray spectrometry data in the gold deposit-rich area of Mara Rosa Magmatic Arc, Brazil. The inversion-based processing results enhance the anomalies and suppress the interpolation artefacts as well as increase the signal-to-noise ratio. The application of airborne radiometric transformed maps also improved the footprint knowledge around Cu-Au mineralization and highlighted potential areas for further studies.