Application of Computer Assisted Tomography to Soil-Plant-Water Studies: An Overview
Application of the technique known as computer assisted tomography (CAT) to x- and gamma-ray attenuation measurements has provided an exciting new method for nondestructive imaging within a solid matrix with considerable potential for studying soil behaviour in space and time. The information provided, however, is currently limited by the capabilities of the instrumentation available. Commercially available medical CAT scanners have proven useful for visual studies of soil structure, the advancement and stability of wetting fronts and the structural changes following wetting and drying. The usefulness of these systems and of single source gamma CAT scanning systems in studying soil-plant-water systems, however, is invariably restricted by their inability to distinguish between changes in water content and bulk density in swelling and shrinking soils, and by the associated physical relocation of soil elements that can occur. Thus their quantitative applications have been limited to the measurement of water drawdowns in proximity to plant roots in nonswelling soils or statistical assessments of macroporosity distributions before and after complete wetting and drying cycles. Though fast in operation, the quantitative usefulness of x-ray scanners is limited by the polychromatic nature of the beam and the process known as beam hardening. Furthermore the proprietary nature of these commercial systems also makes software modification or extensions impossible. Simultaneous measurement of the spatial distributions of water content and bulk density in soils that exhibit swelling and dispersion, has been shown to be feasible using CAT applied to dual source (Cs-137 and Yb-169) gamma-ray attenuation. The relatively low photon emission from gamma sources and the propagation of statistical errors, however, necessitates large counting times to provide acceptable accuracy and restricts the use of present gamma systems to the study of steady state or only slowly changing systems. Realization of the full potential of this technique will require substantial improvements in scanning geometry and counting electronics to improve the speed and precision of measurements. With further development, however, dual source gamma-ray systems should ultimately prove most effective for quantitative soil studies. It is now more than a decade that CAT (Hounsfield, 1972) (also termed computed tomography [CT]) in various forms has been applied to a number of different energy beams including: electrons, protons, positrons, alpha particles, lasers, radar, ultra-sound, and nuclear magnetic resonance (NMR), to provide three dimensional imaging of the internal structure of solid objects. Detailed reviews of various aspects of CAT scanning have been presented by Newton and Potts (1981) and by Kak and Slaney (1988) and of its use in studying water movement around plant roots by Aylmore (1993). From the point of view of soil and plant scientists interested in understanding the processes of soil structure development, water movement in soils and its availability for plant growth, application of CAT to measurements of the attenuation of energy beams undoubtedly provides one of the most exciting new techniques ever developed. Its promise of the ability to see inside soil columns and monitor the processes occurring in a continuous, nondestructive manner, clearly has the potential to resolve the major controversies in soil physics and soil-plant-water relations. This special publication provides the opportunity to review and evaluate the progress that has and has not been made in the application of CAT to soil-plant-water studies and to define the steps necessary to obtain the maximum benefit from this technique.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
Copyright © 1994. . Copyright © 1994 by the Soil Science Society of Agronomy, Inc., 5585 Guilford Rd., Madison, WI 53711 USA