Passive acoustic monitoring for seabed bubble flows: Case of shallow methane seeps at Laspi Bay (Black Sea)

This research quantifies the gas release rate from a natural shallow methane seep site in the Laspi Bay (Black Sea), whose origin is thermocatalytic. An adaptive single bubble identification technique was applied to analyze gas volume and release rates from passive acoustic data. Gas from the seafloor was emitted by single bubbles that occurred in clusters. The frequency of an individual bubble (830 Hz) was proportional to the bubble's radius (0.4 cm), which coincides with the optical method. The active seepage phases lasted longer than the pauses between bubble clusters. Distribution of bubble clusters and pauses duration for all seasons deviated significantly from a normal distribution. The daily and interseasonal periodicities of gas emissions were quite similar. The bubble gas flow rate ranged from 26 to 37 liters per day. Short-term water pressure fluctuations (up to a Δp of 46 mbar) caused by sea swell can impact gas flow variability. Continuous recording of hydrological parameters showed that average dissolved oxygen concentrations and salinity at the background station were slightly higher than those above the seep site. However, the data did not demonstrate significant environmental effects, such as hypoxia or submarine freshwater inflow, which was observed in other seepages areas.