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Acoustic Mapping of Diffuse flow


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Diffuse flow of hydrothermal solutions commonly occurs in patchy areas up to tens of meters in diameters in seafloor hydrothermal fields. It is recognized as a quantitatively significant component of thermal and chemical fluxes, yet is elusive to map. Our team developed and applied a new acoustic method to detect and map areas of diffuse flow using phase-coherent correlation techniques. The sonar system was modified to phase information and mounted on DSV SEA CLIFF. The submersible occupied a stationary position on the seafloor surrounding Monolith Vent, a sulfide edifice venting black smokers, at a nominal range of 17 m at a depth of 2249 m on the Juan de Fuca Ridge. Patchy areas of uncorrelated returns clearly stood out from a background of returns that exhibited ping to ping correlation. The areas of uncorrelated returns coincided with areas of diffuse flow as mapped by a video survey with the Navy’s Advanced Tethered Vehicle (ATV). Correlated returns were backscattered from invariant seafloor. Uncorrelated returns were distorted by index of refraction inhomogeneities as they passed through diffuse flow between the seafloor and the transducer. The acoustic method presented can synoptically map areas of diffuse flow. When combined with standard in situ measurement and sampling methods the acoustic mapping will facilitate accurate determination of diffuse thermal and chemical fluxes in seafloor hydrothermal fields.
 
 
Acoustic image of a sector of a submarine hydrothermal site, Monolith Vent, on the Juan de Fuca Ridge showing the application of correlation techniques to synoptically map diffuse flow (colored areas) discharging from the seafloor. Diffuse flow has been elusive to map and may be cumulatively more important than flow from individual vents in thermal and chemical fluxes. The image was produced by scanning the seafloor with a sonar transducer mounted on a submersible (DSV Sea Cliff) from a fixed position on the seafloor at the apex of the sector. The gray portion is the ordinary signal strength for the return pings from normal seafloor, which correlate in phase with outgoing pings. The three-colored areas of the image show uncorrelated returns attributed to shimmering of the sonar image of the seafloor due to random changes in sound speed as outgoing pings passed through a variable lens of warm diffuse flow and were backscattered from the invariable seafloor back through that lens and intervening ambient water to the sonar transducer.(See paper by Rona et al. October 1997)
 
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