Remote Estimations of Seafloor Gas Flux Using Broadband Acoustics

TitleRemote Estimations of Seafloor Gas Flux Using Broadband Acoustics
Publication TypeConference Proceedings
AuthorsWeidner, E
EditorWeber, TC, Ladroit, Y, Lamarche, G, Mayer, LA
Conference NameUnderwater Acoustic Conference and Exhibition 2021
Conference DatesJune 21-24
Conference LocationVirtual

Free gas bubbles escape from the seafloor worldwide and transport greenhouse gases, such as methane and carbon dioxide, into the ocean and potentially the atmosphere. These greenhouse gas emissions influence climate, contribute to upper ocean acidification, and are vital to the development of chemosynthetic biological communities. While direct observation and sampling by remotely operated vehicles (ROVs) or cable-operated camera systems help to generate quantitative estimates of gas flux, the spatial footprint and cost of such measurements prohibits rapid, large-scale, continuous gas flux estimation. Remote sensing of gas bubbles via acoustic systems offers the opportunity for surveying over much larger spatial scales at greatly reduced cost; however, without direct information about bubble parameters (e.g. size, number, composition) quantitative flux measurements cannot be made with acoustic systems alone. Consequently, quantitative gas flux estimates are rare and gas seeps remain a poorly constrained contributor to the global carbon cycle. Here, we describe an acoustic method for the remote quantification of seafloor gas flux that was developed and applied to a hydrothermal seep system in the Bay of Plenty, New Zealand. This method utilizes a calibrated broadband split-beam echo sounder to discriminate individual bubbles using high vertical range resolution and remotely estimate the bubble size distribution (BSD) of seeps. Additionally, split-beam phase differentiation provides phase-angle data, used to compensate for beam-pattern effects and deliver calibrated target strength measurements for individual seeps. Together with gas composition, individual seep BSD and target strength measurements provide a path, through acoustic inversion, for remote quantification of seafloor gas flux. Following this approach, the carbon dioxide release from a seep system in the Bay of Plenty was estimated to be 146 kg/day (standard deviations of 31.5 kg/day) from a seafloor area of approximately 0.25 km2.