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. (1996) 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 micron). 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).
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