Assessing the reliability of firn microstructural measurements from micro-CT data

Abstract. X-ray micro-computed tomography (micro-CT) has become a common technique used to characterize the microstructure of snow and firn, yet the sensitivity of micro-CT-derived microstructural parameters to image processing choices remains poorly understood. In particular, the selection of a binarization threshold can influence the reconstructed representation of the microstructure and, thus, any quantitative properties computed from it. Here, we systematically evaluate the sensitivity of six firn microstructural parameters to binarization threshold choice using micro-CT data from three samples of the NEEM 2009 S2 firn core that were extracted from shallow (7 m), intermediate (26 m), and deep (70 m) depths of the core. We generated reconstructions of the microstructure of each sample at every threshold value across the grayscale threshold range, and compare three thresholding approaches representing subjective, statistical, and topological strategies. Microstructural parameters describing bulk volume-fraction properties (Percent Object Volume, Percent Open Porosity, and Surface Area to Volume ratio) and microstructural complexity (Structural Model Index, Surface Convexity, and Euler Number) were computed across the full grayscale threshold range and evaluated using a normalized sensitivity metric. We find that bulk volume-fraction parameters are robust to threshold choice across all firn depths, while parameters describing microstructural complexity and connectivity exhibit strong threshold and depth-dependent sensitivity. Modeled estimates of the intrinsic permeability of the reconstructed microstructures generated at threshold values between 60–120 for each sample underscore the impact of threshold choice on the microstructural complexity and connectivity. These results demonstrate that binarization threshold choice can substantially influence interpretations of firn microstructural complexity and, therefore, transport properties, highlighting the need for careful selection of image processing steps, including binarization, in firn micro-CT studies.