Effects of manual threshold setting on image analysis results of a sandstone sample structural characterization by X-ray microtomography

X-ray microtomography is a nondestructive nuclear technique widely applied for samples structural characterization. This methodology permits the investigation of materials porous phase, without special sample preparation, generating bidimensional images of the irradiated sample. The images are generated by the linear attenuation coefficient mapping of the sample. In order to do a quantitative characterization, the images have to be binarized, separating porous phase from the material matrix. The choice of the correct threshold in the grey level histogram is an important and discerning procedure for the binary images creation. Slight variations of the threshold level led to substantial variations in physical parameters determination, like porosity and pore size distribution values. The aim of this work is to evaluate these variations based on some manual threshold setting. Employing Imago image analysis software, four operators determined the porosity and pore size distribution of a sandstone sample by image analysis. The microtomography measurements were accomplished with the following scan conditions: 60 kV, 165 μA, 1 mm Al filter, 0.45 deg step size and 180.0 deg total rotation angle with and 3.8 μm and 11 μm spatial resolution. The global average porosity values, determined by the operators, range from 27.8 to 32.4 % for 3.8 μm spatial resolution and 12.3 to 28.3 % for 11 μm spatial resolution. Percentage differences among the pore size distributions were also found. For the same pore size range, 5.5 % and 17.1 %, for 3.8 μm and 11 μm spatial resolutions respectively, were noted. (author)