Influence of pore fluid on P-wave velocity in unsaturated sands at low effective stresses

Wave-based techniques offer significant potential for characterising unsaturated geosystems, detecting gas leakage, and evaluating the integrity of carbon dioxide storage. This study specifically examines the P-wave propagation in partially saturated sandy soils across varying degrees of saturation and pore-fluid compositions – distilled water mixed with either air or carbon dioxide – using a wave-based approach. This approach comprises multiple input frequencies, minimising subjectivity in signal interpretation. An advanced laboratory setup, featuring bender element tests, was utilised to analyse the relationship between P-wave velocity and degree of saturation. The tests were conducted on Toyoura sand and TP-Lisbon sand. Analyses in both time and frequency domains confirmed consistent wave behaviour. The results demonstrated an increase in P-wave velocity with the degree of saturation, which was attributed to changes in the bulk modulus of the pore-fluid. A multi-physical threshold was identified, where small increases in saturation (around 97% for air and 85% for carbon dioxide) resulted in significant changes in P-wave velocity. Furthermore, the gas type in the pore fluid influenced wave propagation, with carbon dioxide showing greater sensitivity to saturation changes than air.