A Rotary Sonar for Long-Term Acoustic Monitoring of Deep-Sea Gas Emissions

Natural methane gas release from the seafloor is a widespread phenomenon that occurs at cold seeps along most continental margins. Since their discovery in the early 1980s, seeps have been the focus of intensive research, partly aimed to refine the global carbon budget. However, deep-sea research is challenging and expensive and, to date, few works have successfully monitored the variability of methane gas release over long periods of time (> 1 year). Long-term monitoring is necessary to study the mechanisms that control seabed gas release. In 2017, the University of Bremen initiated the M3 project, which aims to study the temporal and spatial variability of gas emissions at the Southern Hydrate Ridge by acoustically monitoring gas effluxes over several years. Located at 800 m depth on the Cascadia accretionary prism offshore Oregon, the Southern Hydrate Ridge is one of the most studied seep sites where persistent but variable gas release has been observed for more than 20 years. We present the Southern Hydrate Ridge Overview Sonar, a long-range multibeam echosounder mounted on a rotator, which detects every gas bubble stream, or bubble plume, located in the study area. Built to resist to the harsh, corrosive conditions of the deep- sea, the instrument aims to produce data time-series that span over several years. It is powered and controlled from land through the Ocean Observatories Initiative’s Cabled Array observatory. The sonar was deployed and connected to the Cabled Array in June 2018 and started collecting data at fixed time intervals. This paper describes the design of the sonar, the dataflow, the post-processing steps that are required to process the enormous amount of data produced, as well as some preliminary data products.