Porosity and Pore-Network Controls on Elastic Properties and Permeability in Porous Ignimbrites

In porous ignimbrites, porosity defines the first-order control on elastic trend, but rocks with similar porosity can still behave differently because their pore networks are arranged differently. We analyzed 50 specimens from seven ignimbrite units in Mexico using density and porosity measurements, permeability tests, mercury intrusion porosimetry, image-based pore descriptors, and ultrasonic P- and S-wave velocities. At the unit scale averages, total porosity ranges from 31.4% in Tl to 42.9%, but elastic properties and permeability vary widely, showing that porosity alone does not define a unique physical state. Two end-member pore-network tendencies can be recognized: crack-linked, throat-restricted systems and more equant or intergranular systems. At similar porosity, crack-dominated networks are generally less stiff and less permeable, whereas more equant networks show higher permeability and stiffer behavior under dry conditions. Effective-medium models indicate that most samples are consistent with KT aspect ratios of 0.15–0.20 and a critical-porosity range of 40–60%. Overall, porosity defines the first-order elastic trend, whereas pore-network architecture explains much of the remaining hydraulic variability and part of the residual elastic spread.