Combining multibeam and single beam echosounders for quantifying gas bubble release from the seafloor

Abstract

Marine gas bubble release is a worldwide phenomenon that impacts the local marine ecosystem and contributes to carbon fluxes with respect to global warming. It is still challenging to assess gas flow for large seep areas (tens of km²), but recent advances in calibrating and processing multibeam echosounder (MBES) data make this problem approachable. A recently published method called “echo grid integration” describes the computation of aggregated backscattering cross-sections of bubble stream layers from calibrated MBES data. Here, we use this method combined with methods for bubble stream detection, acoustic inversion, cross-calibration and dissolution modeling to create a full gas flow quantification workflow. A simultaneously recording scientific single beam echosounder (SBES) is used to acoustically (cross-) calibrate the MBES. The final result of the presented workflow is a map showing the distribution and estimated gas flow of 10 bubble seep clusters and > 50 individual seeps in a 5 × 7 km wide area on the Black Sea shelf. It is shown that the MBES gas flow estimates correlate linearly with estimates from the SBES and are consistent over depth and at different beam angles. Multibeam echosounder data thereby provided 10 times the areal coverage per survey line compared to the scientific SBES. Our results highlight a heterogeneous picture of gas release in the area, where 10% of the flares are responsible for ~ 80% of the overall gas flow (~ 260–780 tons of methane per year). According to bubble dissolution modeling, this free methane dissolves in the water before reaching the surface.