Gamma-ray responses over erosional landscapes will broadly correlate with bedrock geology. However, variations within major lithological groups will relate to soils and regolith materials. The gamma-ray response for a given bedrock type will largely depend on the denudation balance or the relative rates of erosion verses weathering (Figure 22). Areas in the landscape with relatively high geomorphic process rates are likely to have thin soils and a gamma-ray response that reflects the bedrock geochemistry. Landscapes with stable surfaces that are less geomorphically active will preserve weathered materials. Gamma-ray responses over these typically more subdued landscapes often reflect older regolith materials that are out of equilibrium with present day geomorphic processes. Therefore gamma-ray spectrometry imagery can be used to separate areas of high and low geomorphic activity. Draping the gamma-ray image over a digital elevation model (DEM) as a 2.5D perspective view is an excellent way of understanding geomorphic relationships and separating bedrock and regolith gamma-ray responses.
Figure 22 A (right) - Relationship between gamma-ray response and denudation balance in landscapes. A - In areas where the rates of weathering are higher than the erosion rates the gamma-ray response will reflect regolith and soil geochemistry and mineralogy. B - areas of active erosion (e.g., where erosion rates are higher than rates of weathering) the gamma-ray response will reflect bedrock geochemistry and mineralogy (Wilford et al. 1997).
Figure 22 B (above) - Applying the denudation concept to a ‘real landscape’. Ternary image with K in red, Th in green and U in blue draped over a DEM. In this landscape regolith materials generally have low radioelement values except for Th and correspond to landforms with low relief and associated geomorphic process rates (A). In contrast, bedrock responses generally have higher radioelement values, particularly in K, and correspond to landforms with higher relief and associated geomorphic process rates (B). An erosional scarp separates these contrasting regolith-landforms (yellow line). These relationships may not transfer to other regions due to different bedrock types and/or landscape evolution histories. Scale approximately 32 km from left to right. Local relief is up to 200 m.