Hydrothermal flow at Main Endeavour Field imaged and measured with Cable Operated Vent Imaging Sonar
Abstract
Initial acoustic monitoring of hydrothermal flow in the Main Endeavour Field (MEF) captures the spatial distribution of diffuse and focused discharge and shows potential for flux determinations. Our Cabled Observatory Vent Imaging Sonar (COVIS) was connected to the NEPTUNE Canada Endeavour Observatory in September 2010. Using a customized Reson 7125 multi-beam sonar, COVIS acquired a 29 day time series of black smoker plume and associated diffuse hydrothermal flow from Grotto, a 30 m diameter vent cluster in the MEF, Juan de Fuca Ridge. Detection of the spatial patterns of diffuse flow utilizes phase decorrelation of the acoustic signal (200kHz) by buoyancy-driven turbulence (acoustic scintillation) to produce a time series of maps. Substantial fluctuation in the detected diffuse flow area (0.1 - 18 m^2) was observed over the 29 days of observation, although position remained stable. Acoustic imaging of focused flow (400 kHz) utilizes high volume backscatter (attributed to particles and turbulent sound speed fluctuations) to image in 3D the initial tens of meters of rise of buoyant plumes. Spectral analysis of bending inclination of a strong plume from multiple fast smokers on the NW end of Grotto (north tower) indicates that the dominant modes correspond with the ambient mixed semi-diurnal tide (based on current meter data at a mooring 2.9 km to the north and on a tidal model), with at least one secondary mode attributable to sub-inertial flow related to inflow to the axial valley. A weaker plume from several slower smokers is present on the NE end of Grotto. On first analysis, the bending inclination of the weaker plume appears to be affected by the stronger plume. Quantification of flow velocity and volume flux of plumes begins with measuring the Doppler phase shift through plume cross-sections beginning at 5 m above source vents where discharge merges. The volume flux measurements enable calculation of entrainment coefficients, which prior work on the same strong plume indicated increase with degree of bending. The acoustic data in concert with in situ measurements support inversions to obtain fluxes to elucidate the role of hydrothermal flow in transfer of heat, chemicals and biological material from the crust to the ocean. We are exploring the feasibility of fitting plume models to Doppler velocity data in order to estimate heat flux. Ongoing analysis pursues quantification of fluid flux from diffuse and focused flow. In addition, the time series provide observations of hydrothermal flow response to tidal, tectonic and volcanic forcing on time scales from hours potentially to years. (Work supported by NSF Grants Nos. OCE-0824612 and OCE-0825088)
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V11E2542R
- Keywords:
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- 3017 MARINE GEOLOGY AND GEOPHYSICS / Hydrothermal systems;
- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes