Changes in pore characteristics with depth for structural crusts

Micromorphological techniques have frequently been used to describe processes leading to surface crusting. Image analysis can be helpful in quantifying changes in porosity necessary for estimating hydraulic parameters in crust infiltration models. The objective of this study was to quantify changes in pore area and frequency for three pore shapes (round, irregular, and elongate) and four size classes (50–200, 200–350, 350–500, and >500 μm). Porosity decreased exponentially as the surface was approached from a depth of about 10 mm. The number of pores was roughly inversely proportional to pore area as larger pores were fragmented into numerous small pores. Intermediate-sized (350–500 μm) elongate pores were the most sensitive to crusting processes, and these were divided into small (50–200 μm) and small intermediate (200–350 μm) round pores. Changes in pore shape and size distribution make estimating crust hydraulic conductivity and thickness extremely difficult, but a thickness of 5 mm appears to be a reasonable estimate for modelling purposes.