We report the first observations based on acoustic imaging of large‐scale structure and time variability of buoyant plumes emanating from black smoker‐type seafloor hot springs. Three‐dimensional plume reconstructions were made from a digital data set of acoustic backscattering information recorded on a prototype submersible‐mounted sonar system. The acoustic images of two adjacent black smokers depict volume and show zones of flow organization (meters to tens of meters) in the lower 40 m of the buoyant plumes. The two plumes coalesce, bend in the prevailing current, exhibit short‐term (minutes) variation in cross section, and rapid (seconds) turbulent eddy variations at small scales (< 1 m). The plume imaging system is being developed for determination of plume dynamics, flux determinations when combined with chemical and thermal measurements, and long‐term monitoring of the activity of seafloor hydrothermal fields.
High frequency sonar systems have been used by the Naval Research Laboratory to study nonlinear internal gravity waves and define the fine structure of ocean temperature and salinity layers that are found in coastal waters, usually within 130 meters of the surface. Of particular interest
is the fine structure of these waves, which are being investigated using high sensitivity sonar systems that provide 1 m horizontal resolution and less than 8 cm vertical resolution. This article describes the integration of commercial and custom-designed components, including a recently patented
transmitter-receiver switch. The significance of this T-R switch is that it improves the sensitivity of short-range sonar systems, enables a more refined measurement of nonlinear internal gravity waves, and could have broad industry applications.
During the November 1990 dive series of the US Navy DSV Turtle, plumes emanating from high-temperature black smoker-type vents were imaged using a prototype sonar system mounted on the submersible. The study focused on two plumes from adjacent sources that were scanned horizontally in arcuate sectors at increasing angular increments from the vent orifices up to 90°. Multiple data sets were recorded at horizontal ranges of 3–70 m from the base of the plumes and sonar ranges to 200 m with submersible stationary on the seafloor. Computer graphics were used to reconstruct the plumes in cross section and in 3D, revealing coherent plume images up to 100 m above the seafloor. Multiple plume cross sections recorded at the same level show changes in shape and distribution of suspended particulate matter on a time scale of seconds. Whole images show coalescing of the buoyant plumes and deflection by the prevailing current. The work demonstrates the value of acoustic imaging of plumes for initial detection and for characterization of plume dynamics.
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