Quantitative characterization of coal by means of microfocal X-ray computed microtomography (CMT) and color image analysis (CIA)

Abstract Microfocal X-ray computed microtomography (CMT) is a novel technique that produces three-dimensional maps of the distribution of the linear attenuation coefficient inside an object. In contrast to the more conventional medical computerized tomography (CT) systems, a microfocal X-ray source is used. This enables a far better spatial resolution. The linear attenuation coefficient or tomodensity is dependent on the physical density and the mineralogy of the object to be imaged, and on the energy of the radiation used. Earlier work by Verhelst et al. (Verhelst, F., David, P., Fermont, W.J.J., Jegers, L., Vervoort, A., 1996. Correlation of 3D-computerized tomographic scans and 2D-color image analysis of Westphalian coal by means of multivariate statistics. Int. J. Coal Geol. 29, 1–21) presented the results of the correlation of the tomodensities obtained from three-dimensional CT scans with two-dimensional data on the composition of a coal sample acquired with color image analysis (CIA), a camera technique. This analysis assumed the linear proportionality of the tomodensity with the real physical bulk density, which is true only for certain energy ranges. In this paper, we use CMT devices for a similar correlation. For sake of comparison, the same core sample was used. First, new CIA data on the surface composition along two profiles were sampled with greater detail (100 μm). These data are subjected to a geostatistical analysis to quantify the spatial dependence between the measurements. Second, CMT tomograms were made, yielding spatial resolutions twice as high as medical CT. A multivariate correlation was carried out, and two improved (geo)statistical methods are suggested. The different energy range of the microfocal X-ray source compared to medical CT, however, produces some bias in the correlation of the tomodensities with the surface percentages of the constituents. We therefore suggest that the linear attenuation coefficient be treated as a separate unit. No attempt was made to translate the linear attenuation coefficient to the physical bulk density of the different constituents of coal (e.g. macerals).