A weathering intensity index for the Australian continent using airborne gamma-ray spectrometry and digital terrain analysis

Weathering intensity largely controls the degree to which primary minerals are altered to secondary components including clay minerals and oxides. As weathering intensity increases there are changes in the hydrological, geochemical and geophysical characteristics of the regolith. Thus, once calibrated, weathering intensity can be used to predict a range of regolith properties. A weathering intensity index (WII) over the Australian continent has been developed at a 100 m resolution using regression models based on airborne gamma-ray spectrometry imagery and the Shuttle Radar Topography Mission (SRTM) elevation data. Airborne gamma-ray spectrometry measures the concentration of three radioelements — potassium (K), thorium (Th) and uranium (U) at the Earthʹs surface. The total gamma-ray flux (dose) is also calculated based on the weighted additions of the three radioelements. Regolith accounts for over 85% of the Australian land area and has a major influence in determining the composition of surface materials and in controlling hydrological and geomorphological processes. The weathering intensity prediction is based on the integration of two regression models. The first uses relief over landscapes with low gamma-ray emissions and the second incorporates radioelement distributions and relief. The application of a stepwise forward multiple regression for the second model generated a weathering intensity index equation of: WII = 6.751 + − 0.851 ∗ K + − 1.319 ∗ Relief + 2.682 ∗ Th/K + − 2.590 ∗ Dose. The WII has been developed for erosional landscapes but also has the potential to inform on deposition processes and materials. The WII correlates well with site based geochemical indices and existing regolith mapping. Interpretation of the WII from regional to local scales and its application in providing more reliable and spatially explicit information on regolith properties are described.