New observations of gas bubble behaviour across the seabed

Abstract

Understanding the movement of free gas across the seabed is important for describing the role of methane and carbon dioxide seeps in the carbon cycle and develop monitoring methodologies for marine carbon capture and storage complexes. We document new observations of gas bubble behaviour across the seabed using laboratory and field data with passive acoustic and high-speed optical imaging. Our observations show four distinct phases of gas release: fracture propagation, sediment displacement, surface breakthrough and dual-bubble oscillation. We show that each individual gas release event at the seabed comprises of a larger bubble that oscillates in the water column and a smaller oscillating bubble that remains within the seabed. Both of these oscillating bubbles contribute to the sound generated by each gas release event. Our results show that this dual-source phenomenon produces a composite acoustic signature, potentially leading to a 5-15% overestimation in gas flux when using current inversion models. Beyond implications for acoustic flux estimation, the recognition of retained oscillating gas bubbles reshapes our understanding of near-seabed gas dynamics. These findings suggest that sediment structure and bubble-sediment interactions play a more active role in controlling release behaviour than previously thought.