Long-term, quantitative observations of seafloor hydrothermal venting using an imaging sonar

Abstract

While connected to the NEPTUNE observatory operated by Ocean Networks Canada (ONC) at the Endeavour Segment of the Juan de Fuca Ridge (http://www.oceannetworks.ca/observatories/pacific), the Cabled Observatory Vent Imaging Sonar (COVIS) recorded an unprecedented long-term (>4 years) acoustic dataset capturing local hydrothermal venting. Processing of the acoustic backscatter data yields three-dimensional (3-D) images of plumes rising tens of meters from black smoker vents on a sulfide structure named Grotto. More importantly, analysis of the Doppler frequency shift in acoustic backscatter yields estimates of the flow rates of those plumes and their volume fluxes. Subsequent calculations based on the vertical variation in volume flux and a theoretical heat-to-volume-flux relationship for buoyancy-driven plumes give estimates of plume heat flux, which are essential for studying the temporal evolution of a hydrothermal system and its coupling with geological, oceanic, and biological processes. In addition to black-smoker plume observations, the ping-to-ping decorrelation of seafloor backscatter recorded by COVIS provides an acoustic indicator of diffuse-flow (i.e., low- temperature, clear hydrothermal discharge) distribution over Grotto and its surrounding areas. Furthermore, acoustic techniques for quantifying the temperature fluctuations, and ultimately, heat flux of diffuse-flow venting are currently under development. [Work supported by NSF.]