Sonar evidence for methane ebullition in Eckernfِrde Bay

A bottom-mounted, circularly scanning sonar was used to observe the methane-rich seafloor of Eckernförde Bay during the months of April and May in 1993. Event-like changes in the acoustic signal were observed and are shown to be caused by scatterers in the water column that are interpreted to be gas bubbles rising in columns having transverse dimensions 2–5 m. The events do not correlate with seafloor current stress, temperature, or refraction due to stratification, but a strong correlation is seen with pressure at the seafloor, consistent with gas ebullition due to pressure release. It is not possible to definitively exclude scattering from pelagic animals as the cause of these events, but the observed localization at a few spots on the seafloor appears to be inconsistent with the biological explanation. These data are insufficient to determine the flux of free methane, but bounds are estimated and suggestions are made for future measurements that could determine flux.