When the distribution of radioelements within the bedrock and regolith are understood the gamma-ray imagery can be used to infer often quite specific soil properties. For example, low radioelement responses over granitic rocks in Cape York Peninsula invariably corresponded to sandy-textured soils with very low cation exchange capacity (Wilford et al. 1997) (see Ebagoola virtual field trip) . Dickson and Scott (1998) used K, Th and U concentrations to distinguish aeolian materials in soil developed from in situ weathering of bedrock. Bierwirth (1996) demonstrated the use of gamma-ray images for mapping soil properties including textural and geochemical characteristics in the Wagga Wagga region of NSW. In Western Australia (WA) Cook et al. (1996) used gamma-ray associations to separate soils developed on granitic, lateritic and doleritic parent materials. Roberts (2003) interpreted high resolution ground-based gamma-ray imagery and electromagnetics (EM) to compile detailed soil maps for use at local farm scales over part of the Boorowa catchment in NSW (Figure 25 and 26). Wilford (2003) used relationships between mineralogy and texture to map soils with different textural characteristics on depositional landscapes in the mid north-west of South Australia (see Jamestown 3D interactive model). Analysis of high-resolution gamma-ray data over the Wyalkatchem catchment in WA proved to be useful in mapping a variety of soil properties, specifically shallow soil on bedrock, gravel lags and percentage clay in the upper part (0-10 cm) of the soil profile (Taylor et al. 2002). Martz and De Jong (1990) demonstrated that variations of natural radionuclides of prairie soils in Canada were correlated with soil texture and leaching processes. Gamma-ray imagery, together with terrain attributes and other explanatory variables, have been used in quantitative approaches to predict soil attributes (Gessler et al. 1995, Ryan et al. 2000, McKenzie and Ryan 1999).
Gamma-ray responses have been used to identify variations in soil along a slope or catena. Mapping catenary sequences using gamma-ray imagery illustrates the close relationship between geomorphic process and gamma-ray response. Catenas are recognised over granitic landforms in Cape York Peninsula (see Ebagoola virtual field trip) and on shaley lithologies in the Wagga Wagga area of NSW (Wilford et al. 1997). In both cases the upper slopes have thin soils with deeper soils and regolith on the lower slopes. The gamma-ray response of the upper slopes is dominated by K-rich bedrock whereas the gamma-ray response over the lower slopes has reduced K concentration due to weathering and leaching (Figure 27).
Figure 25. Ternary gamma-ray image (K in red, Th in green and U in blue) draped over a DEM. The high resolution imagery was acquired using a quad bike (from Roberts et al. 2002). The image response is reflecting soil and bedrock geochemistry and mineralogy.
Figure 26. Digital soil map based on the classification of gamma-ray imagery and EM31 (ground based electromagnetics) conductivity grid by Roberts et al. (2002). Drilling and associated regolith descriptions are used to constrain and attribute the soil classes.
Figure 27. Mapping catenas. Airborne gamma-ray spectrometric K concentrations are draped on a DEM. High K values (red) over ridge tops (A - lithosols) and low values in the adjacent valleys (B - colluvial clay) map repeating soil patterns developed over weathered metasediments (Wilford et al. 1997). Click inside the box to see a field photograph.