Assessing XMT-Measurement Variability of Air-Water Interfacial Areas in Natural Porous Media

This study investigates the accuracy and reproducibility of air-water interfacial areas measured with high-resolution synchrotron X-ray microtomography (XMT). Columns packed with one of two relatively coarse-grained monodisperse granular media, glass beads or a well-sorted quartz sand, were imaged over several years, encompassing changes in acquisition equipment, improved image quality, and enhancements to image acquisition and to processing software. For the glass beads, the specific solid surface area (SSSA-XMT) of 31.6 ± 1 cm⁻¹ determined from direct analysis of the segmented solid-phase image data is statistically identical to the independently calculated geometric smooth-sphere specific solid surface area (GSSA, 32 ± 1 cm⁻¹) and to the measured SSSA (28 ± 3 cm⁻¹) obtained with the N₂-Brunauer, Emmett, and Teller method. The maximum specific air-water interfacial area (A_max) is 27.4 (±2) cm⁻¹, which compares very well to the SSSA-XMT, GSSA, and SSSA-N₂-Brunauer, Emmett, and Teller values. For the sand, the SSSA-XMT (111 ± 2 cm⁻¹) and GSSA (113 ± 1 cm⁻¹) are similar. The mean A_max is 96 ± 5 cm⁻¹, which compares well to both the SSSA and the GSSA values. The XMT-SSSA values deviated from the GSSA values by 7–16% for the first four experiments but were essentially identical for the later experiments. This indicates that enhancements in image acquisition and processing improved data accuracy. The A_max values ranged from 74 cm⁻¹ to 101 cm⁻¹, with a coefficient of variation (COV) of 9%. The maximum capillary interfacial area ranged from 12 cm⁻¹ to 19 cm⁻¹, for a COV of 10%. The COVs for both decreased to 5–6% for the latter five experiments. These results demonstrate that XMT imaging provides accurate and reproducible measurements of total and capillary interfacial areas.