ACOUSTIC MAPPING OF DIFFUSE FLOW AT A SEA-FLOOR HYDROTHERMAL SITE - MONOLITH VENT, JUAN-DE-FUCA RIDGE

Diffuse flow of hydrothermal solutions commonly occurs in patchy areas up to tens of meters in diameter in seafloor hydrothermal fields. It is recognized as a quantitatively significant component of thermal and chemical fluxes, yet is elusive to map. We report a new acoustic meth od to detect and map areas of diffuse flow using phase-coherent correl ation techniques. The sonar system was modified to record phase inform ation and mounted on DSV SEA CLIFF. The submersible occupied a station ary position on the seafloor and the transducer scanned the seafloor s urrounding 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 backgr ound 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). Corre lated 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 fl ow. When combined with standard in situ measurement and sampling metho ds the acoustic mapping will facilitate accurate determination of diff use thermal and chemical fluxes in seafloor hydrothermal fields.