Monitoring Hydrothermal Plumes at ASHES Vent Field, Axial Seamount through Quantitative Acoustic Imaging

Abstract:

A series of acoustic images shows substantial variations in the sizes, shapes, and orientations of hydrothermal plumes at ASHES vent field. The buoyant plumes issuing from seafloor hydrothermal vents are strong sound reflectors due to the presence of intense temperature fluctuations produced by the turbulent mixing between high-temperature vent fluids and cold seawater. Thus they are prominent targets in acoustic backscatter images, as exemplified in the data recorded by the Cabled Observatory Vent Imaging Sonar (COVIS) that is currently connected to the Ocean Observatories Initiative’s Regional Cabled Array (OOI-RCA) on Axial Seamount. Following its initial deployment in July 2018, COVIS has been used to monitor hydrothermal venting at ASHES vent field located in Axial’s summit caldera. We present a time series of 3D acoustic images created from the backscatter data recorded since July 2019, when COVIS was recovered and redeployed to replace a broken instrument cable. Those images feature the buoyant plumes issuing from the vents on two major sulfide structures, Inferno and Mushroom, within the sonar’s field of view (Figure 1). Variations in the plumes are presumably due to the combined effects of source variations and ambient currents. In addition, we examine an inversion method for quantitative estimation of vent-source intensity from the observed backscatter from water-column plumes based on theories of acoustic scattering from turbulent volume heterogeneities and empirical scaling laws for buoyancy-driven plumes. Lastly, we conduct numerical simulations using an analytical plume model and a three-dimensional convection model to provide theoretical constraints for the inversion method and gain insights into the temporal variations of hydrothermal plumes observed in the acoustic images.