Particle shape characterisation using Fourier descriptor analysis

A novel technique for the objective assessment of particle shape is presented. The technique uses Fourier shape descriptors and image analysis of scanning electron microscope photographs of sand grains to provide an accurate quantification of particle morphology and texture. Three lower order Fourier descriptors, denoted ‘signature descriptors’, provide measures of elongation, triangularity and squareness, while an additional descriptor, denoted ‘asymmetry’, provides a measure of particle irregularity. Together, these four descriptors quantify the overall shape of soil particles (defined as ‘morphology’). A summary of higher-order descriptors provides textural information that is related to local roughness features (defined as ‘texture’). The results of studies on three silica sands (two standard, laboratory-use and one natural, unprocessed) and one carbonate sand are presented. Breakage of particles by crushing is shown to affect the morphological signature differently depending on the type of sand, though it does not significantly alter texture. The study highlights the value of microscopy, combined with image analysis, in revealing sand grain shape and texture, and shows that simple statistical tools may be used to translate the information provided by relatively few grains to that of a larger body of soil.