Continuous nondestructive measurements of the spatial distributions of soil water content and bulk density are essential prerequisites to the resolution of many problems in the study of soil-plant-water systems. Computerized axial tomography (CAT) was applied to dual-source (Cs-137 and Yb-169) gamma-ray attenuation in soil columns to determine nondestructively the spatial distributions of volumetric water content (theta-v) and bulk density (rho-s) for two soils that exhibit swelling to different degrees. Beam slice thickness was 2 mm and pixel dimensions 2 by 2 mm. The CAT scanning technique for average rho-s and theta-v provided excellent agreement with the corresponding values obtained gravimetrically. Average standard deviations (sigma) for rho-s of pixels for the dry soils, although large, were highly reproducible and provide a measure of the structural status of the soils. Changes in the mean and sigma of pixel rho-s following wetting were related to the texture, structure, mineralogy, and Ca or Na exchange status of the soil. Systematic errors arising from the random nature of radioactive emissions, for count times of 2 s, resulted in large sigma in estimated theta-v of pixels even for dry soils. Very large counting times (some 169 s for an individual ray-sum and, hence, 112 h to complete a dual-source scan) were required to achieve sigma values of pixel theta-v on the order of 0.025 cm3/cm3 for a uniform field of water. Such large count times limit this approach to steady-state or only slowly changing systems. Thus, while the application of CAT to dual-energy-level scanning has the potential to become a major tool for nondestructive studies of soil water content, structural status, and stability, realization of this potential awaits substantial improvements in scanning geometry and counting electronics.
