The study of soil structure by coupling serial sections and 3D image analysis

During the past 25 years, studies on soil structure have been made using many different methods. Among these methods, the 2D morphological image analysis of undisturbed samples was the most typical technique. Stereological methods have also been largely applied but they require strong assumptions about the shape of pores and particles. X-ray and -ray computed tomography have been successfully used in 3D analysis of pores but they present some serious drawbacks such as the high price, the generally coarse resolution, the high cost of the maintenance and the overall complexity of the instrumentation. In this respect the system based on mechanical tomography applied to optical serial sectioning seems to be promising and in many cases more sustainable because of the low price and the modular resolution that can be achieved. An example of this methodological approach is given in this paper along with the first results. The system is based on fast impregnation of short hardening time and low viscosity epoxy resin; in the resin a fluorescent dye is added. Sequential grinding and image acquisition are produced by means of a digital photo-scanner (3590  2700 pixels) directly connected to the head of the grinding wheel instrument. Large samples (140  140  350 mm) and high-resolution (20 m) observations can be handled by the system. The 2D-images are sent via LAN to a Silicon Graphics workstation for image processing (3D reconstruction and analysis). Reconstruction has been done on 2D sections spaced by the same distance resulting from pixel resolution, in this way no extrapolation is made on voxel values. The system enables one to perform several kinds of analysis such as pore size distribution, exchange surfaces, pore-connectivity, etc.