Multispectral processing of high‐resolution radiometric data for soil mapping

Soil radioactivity is dominantly a product of the local geology and mineralization, physical and chemical weathering processes within the profile, and soil organic content. Recent studies have shown that local soil patterns can be confidently mapped at scales of 1:10 000 using new high spatial resolution airborne and ground radiometric data acquired with standard 256‐channel gamma‐ray spectrometry systems and sodium‐iodide (NaI) crystal packs. In this study, a new multispectral processing methodology was developed to assess the 256‐channel spectral soil response. The study identified seven distinct gamma‐ray energy peaks, all products of the potassium‐40, uranium‐238 or thorium‐232 decay series, whose spatial and spectral relationships could be used to interpret soil properties and/or soil conditions.

In the case studies presented, interpretation of complementary thorium and uranium channels (isolated using the new multispectral processing methodology) reveal additional spatial patterns and spectral information that were unattainable using standard processing methods. In one example, soil conditions attributed to a local, developing clay‐soil unit were resolved by the increased ²²⁸Ac (actinium) to ²⁰⁸Tl (thorium) channel response. In a separate example, a change in ratio between ²¹⁴Bi (uranium) energies 1120 keV and 1764 keV, produced during the same decay emission, was used to differentiate soil conditions in the top 40 cm of an expansive sandy loam on clay duplex soil unit. A decrease in the 1120 keV to 1764 keV ratio was interpreted as an increase in soil density as the lower gamma‐ray energy emissions are preferentially attenuated with increased density. Relative increases in the 1120 keV and 1764 keV channel responses were interpreted as an increase in uranium concentration in the soil profile. With the clay as the dominant uranium contributor, integrated interpretation of the ²¹⁴Bi channels suggested that in these areas there was an increase in clay percentage within the surface sandy loam, which may result in local increased water‐logging susceptibility.

This paper also theorizes that interpretation of the spatial displacement of thorium‐232 decay series ²²⁸Ac and ²⁰⁸Tl peaks, produced from decay events approximately 2 years (half‐life) apart, can be used to identify decay series disequilibrium conditions resulting from local soil–water processes or other recent soil activity. This information contributes to an understanding of the soil processes active in the area. In addition, identification of disequilibrium decay conditions may highlight locations where standard radiometric (radioelement) processing, which relies on equilibrium conditions, may be erroneous.