A thin vegetation cover generally has little effect on the gamma-ray response (Figures 19a and b). Spectrometric surveys therefore have an advantage over other remote sensing methods such as Landsat and SPOT satellites that can be difficult to interpret due to fire scarring and vegetation masking regolith materials (Wilford 1992). However, where the vegetation is dense partial attenuation of gamma-ray energy can occur. For example, Aspin et al. (1997) found a signal loss of between 13.5 to 21.9% from airborne gamma-ray data over mature pine plantations. In addition, vegetation and biological processes can also affect the concentration of radioelements in the upper part of the soil. Martz and De Jong (1990) showed a preferential increase of K associated with organic rich topsoil due to recycling of K by plants. Verboom and Gallaway (2000) suggested that Th and U responses of some ferruginous soils are due to specialised bacteria that remove phosphates and metals from carboxylate complexes and then precipitate them on the surfaces of clays and within soil micro pores.
Figure 19 (above). Matched Landsat (a, left) and gamma-ray spectrometric images (b, right) of the same area in central Australia. LEFT - Fire scars on Landsat TM imagery can have a confusing effect on the delineation of regolith, bedrock and structural contacts. RIGHT - Ternary gamma-ray image (K in red, Th in green and U in blue) over the same area. Gamma-ray spectrometry 'sees through' moderately dense vegetation and does not show fire scar boundaries which can limit the use of other remote sensed datasets. Scale approximately 60 km across each image.