QUANTITATIVE CHARACTERIZATION OF COAL BY MEANS OF MICROFOCAL X-RAY COMPUTED MICROTOMOGRAPHY (CMT) AND COLOR IMAGE-ANALYSIS (CIA)

Microfocal X-ray computed microtomography (CMT) is a novel technique t hat produces three-dimensional maps of the distribution of the linear attenuation coefficient inside an object. In contrast to the more conv entional 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 phys ical density and the mineralogy of the object to be imaged, and on the energy of the radiation used. Earlier work by Verhelst et al. (Verhel st, F., David, P., Ferment, W.J.J., Jegers, L., Vervoort, A., 1996. Co rrelation of 3D-computerized tomographic scans and 2D-color image anal ysis of Westphalian coal by means of multivariate statistics. Int. J. Coal Geol. 29, 1-21) presented the results of the correlation of the t omodensities obtained from three-dimensional CT scans with two-dimensi onal data on the composition of a coal sample acquired with color imag e analysis (CIA), a camera technique. This analysis assumed the linear proportionality of the tomodensity with the real physical bulk densit y, which is true only for certain energy ranges. In this paper, we use CMT devices for a similar correlation. For sake of comparison, the sa me core sample was used. First, new CIA data on the surface compositio n along two profiles were sampled with greater detail (100 mu m). Thes e data are subjected to a geostatistical analysis to quantify the spat ial dependence between the measurements. Second, CMT tomograms were ma de, yielding spatial resolutions twice as high as medical CT. A multiv ariate 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 c onstituents. We therefore suggest that the linear attenuation coeffici ent be treated as a separate unit. No attempt was made to translate th e linear attenuation coefficient to the physical bulk density of the d ifferent constituents of coal (e.g. macerals). (C) 1997 Elsevier Scien ce B.V.