Multifractal Analysis of 3D Tomographic Images of Soil Aggregates.

Soil aggregates play an important role in many soil processes, such as transport of water and solutes, storage of various nutrients and organic compounds, and environment for diverse microbial activities. The aggregate interiors are extremely heterogeneous with heterogeneity appearing at multiple spatial scales. Fractal and multifractal methods have been used in a number of studies to describe spatially heterogeneous soil structures in 2D soil images. Advances in X-ray microtomography open up a new way for examining the internal structures of soil aggregates in 3D space with a resolution of only several microns. However, processing of X-ray soil images in order to obtain reliable representations of within aggregate pore structures is still not well established. One of the areas of concern is approaches used to segment the originally gray-scale image into pores and solid material. We compared the information on aggregate heterogeneity that could be obtained with fractal and multifractal methods in original grayscale images and in binary images. In addition, we compared naturally air-dry aggregates with the aggregates from the same soil but pretreated by subjecting them to several wetting and drying (WD) cycles. For that, we hypothesize that swelling and shrinkage caused by WD cycles will greatly alter spatial arrangement of soil pores and particles within the aggregate. Soil aggregate samples were scanned on the bending magnet beam line, station 13-BM-D of the GeoSoilEnvironCARS (GSECARS) at Advanced Photon Source (APS), Argonne National Laboratory (ANL) with 28 keV incident energy. Multifractal spectra calculated for greyscale and binary images demonstrated that there was no significant difference between the two image types, providing we could obtain ‘optimal’ threshold(s) for pore/solid separation. Also the difference between the capacity dimension (D₀) and the information dimension (D₁) indicated that soil aggregates subjected to multiple WD cycles had more heterogeneous pore distributions.