A scaling approach for binary and grey computed tomography imaging of soil: A study in a coastal reclamation area

Abstract

Coastal reclamation areas are considered important reserve land resources. The short formation time of cultivated soil in coastal reclamation areas makes the soil structure prone to considerable changes under the influence of reclamation activities, which affect the soil functions. Based on computed tomography (CT) scan images, different scaling and descriptive statistical analyses enable a thorough study of soil pore structure or architecture. This study compares scaling analyses of grey and binary CT scan images of soil samples from different coastal reclamation areas, a 512 voxel cube. Two methods were applied for the analyses, including multifractal analysis (MFA) and relative entropy ($\Delta E$). The results revealed that the MFA and $\Delta E$ parameters of grey and binary images differed. In MFA, grey images exhibited stronger multifractal characteristics. By contrast, $\Delta E$ better exhibited pore heterogeneity in binary images. Therefore, in analysing the scaling characteristics of the soil pore structure, MFA is recommended for grey images and $\Delta E$ for binary images. When evaluating the scaling characteristics of the pore structure in coastal reclamation soils at different depths and reclamation years, the topsoils (0–20 cm) exhibited higher amplitude ($\Delta \alpha$) values, smaller asymmetry ($\Delta f$) values, greater maximum $\Delta E$ and wider ranges of $\Delta E$ compared to the deep soils (20–60 cm). Reclamation activities significantly improved the pore structure of the topsoils, resulting in enhanced complexity and heterogeneity of pores. The decrease in bulk density and increase in soil organic matter caused by reclamation activities were the main factors that enhanced the complexity and heterogeneity of pores. However, differences in scaling parameters among soil samples with different reclamation years were not significant. In addition, the correlation between pore scaling and pore structure parameters was statistically significant (p < 0.05). The morphology of soil pores can be obtained through MFA and $\Delta E$, thus simplifying the studied pore parameters. However, there was no significant relation between the connectivity density and scaling parameters of the pores, which can complement pore structure information.