Mapping porosity and density changes in soil and regolith from 256-channel radiometric data

Abstract

Gamma ray emissions at 1120 keV and 1764 keV produced from 214Bi (uranium-238 decay series daughter product) are emitted during the same decay reaction with the same probability of emission during decay. Thus, as uranium concentration varies, the ratio of 1120 keV to 1764 keV should remain stable. However, the lower 1120 keV energy is more susceptible to backscattering and normal Compton scatter than the stronger 1764 keV energy, where the probability for scatter to occur is correlated to the density and thickness of the absorber. In natural settings, soil and/or bedrock acts as an absorber. Consequently, as density or thickness of the soil and/or bedrock increases, the probability of scatter increases. Thus changes in the 1120 keV to 1764 keV ratio may indicate changes in soil thickness and/or density. By processing standard 256-channel radiometric data with multispectral processing techniques, 214Bi 1120 keV gamma rays can be isolated in addition to standard 214Bi 1764 keV. This case study illustrates how the spatial variability of 1120 keV to 1764 keV ratios highlight and map changes in soil thickness and/or density.