Detection and monitoring of seafloor methane bubbles using hydrophones

Natural marine seeps of methane are important sources of greenhouse gas emissions that enter the global environment. Monitoring marine methane seeps will reveal important information about how they form, their source regions, and how much of the inventory is microbially consumed within the water column before the gas is released to the atmosphere. While active acoustics methods have been extensively used to detect and monitor methane emissions from the seafloor, there are only a few studies showing the use of passive acoustics for bubble sound detection and monitoring. In this presentation, we use passive acoustics data recorded in a methane seep field in Puget Sound to characterize the bubble sound and estimate the bubble radii from their sound frequency. We use the ratio of a short-term average from a bubble burst over the long-term average to automatically detect the bubble sounds. We also show the relationship between the number of bubble sound detections and the hydrostatics seafloor pressure fluctuations due to tidal changes in water height.