DEVELOPMENT AND APPLICATION OF HYDROTHERMAL PLUME IMAGING SONAR

Principal Investigator: David R. Palmer
Collaborating scientist(s):
Peter A. Rona, Institute of Marine and Coastal Studies, Rutgers University
Darrell R. Jackson, Applied Physics Laboratory, University of Washington
J. George Dworski, Applied Physics Laboratory, University of Washington

M. J. Mottt. Hawaii Institute of Geophysics, University of Hawaii

Objective: The overall objective of this effort is to develop and apply a high-frequency sonar system designed to acoustically image plumes that discharge from black smoker-type, high-temperature, seafloor hydrothermal vents and to measure flow velocities within the plumes. The purpose of this work is to determine the spatial and temporal behavior of hydrothermal plumes as agents of dispersal of heat and chemicals into the ocean. The sonar is being developed for initial operation from research or operational Navy submersibles. It can be used in search mode to detect hydrothermal plumes at ranges up to 100's of meters and in detailed observational mode to determine physical characteristics of plumes and to guide chemical sampling. Possible future applications include incorporating the plume imaging sonar in remotely operated vehicles (ROVs) and to monitor activity of seafloor hydrothermal fields as components of long-term ocean bottom observatories.

Rationale: Characterization of the behavior of hydrothermal plumes is important because they are agents of dispersal of quantitatively significant amounts of chemicals and heat derived from exchange between circulating seawater and the lithosphere in seafloor hydrothermal convection systems at seafloor spreading centers. The plumes rise buoyantly up to hundreds of meters through the overlying water column to a level of neutral buoyancy determined by the initial density deficit, flow momentum, ambient ocean stratification, and entrainment of surrounding seawater. Prior plume investigations have employed video and photo imagery, which is limited to illumination of small volumes adjacent to a vent , asynchronous individual records made with standard sonar instruments, and laboratory tank simulations. These techniques are inadequate to determine the dynamics of plume injection and entrainment into the surrounding water mass or to guide the chemical sampling and temperature measurements in the plume as a whole.

Method: The development and application of the plume imaging sonar is organized into two stages, as follows:

I. Modification of a Mesotech 971 sonar system to image hydrothermal plumes and application to imaging of black smoker plumes. This stage has been completed with support from NOAA, the U.S. Navy, and the National Science Foundation.

II.This stage is divided into two phases, as follows:

IIa. Incorporate a Doppler capability into the sonar. This phase has been completed with funding from NOAA's National Undersea Research Program.

IIb. Carry out a collaborative dive cruise with a U.S. Navy submersible (DSV TURTLE or SEA CLIFF) to test Doppler capability and apply imaging/flow velocity measurement capability to study the hydrothermal venting at the northern cleft segment of the Juan de Fuca Ridge. This stage is in progress.

Accomplishment:
1) A review has been completed of all the research done to acoustically image naturally occurring underwater plumes (Ref. 2 below). The review was undertaken to gain perspectives that might help in efforts to image high-temperature, black smoker, hydrothermal plumes in the deep ocean. The review identifies common themes in hardware development, experimental design , and data analysis and interpretation.
2) A comprehensive framework has been developed for calculating the intensity received by a monostatic sonar system due to backscattering from a cloud of nonspherical particles suspended in the ocean (Ref. 1 below). This study has relevance to studies of the feasibility of imaging plumes in the ocean since in many cases the particulates that comprise the plumes have complex shapes that in no way resemble the spherical shape usually assumed in feasibility studies.
3) In an experiment conducted on the East Pacific Rise using the U. S. Navy Deep Submergence Vehicle TURTLE, the first acoustic images were obtained of large scale structure and time variability of buoyant plumes emanating from black smoker-type seafloor hot springs (Ref. 4 below). For the year in which these results were reported, this work was judged by the American Institute of Physics to be one of the two most important advances in Acoustics (Refs. 3 and 4 below).
4) A feasibility study was undertaken that indicates the backscattered intensity from the particles that define a black smoker plume could be detected at ranges of hundreds of meters using a conventional sonar (Ref. 7 below). The study consisted of two distinct investigations. First, navigational sonar images collected from DSVR ALVIN of a vent complex were examined and second, theoretical estimates of the minimum detectable concentrations of plume particles were compared with observed concentrations. The imaging experiment reported in Ref. 4 below confirmed the results of this study.

Key reference:
1) D. R. Palmer, Rayleigh scattering from nonspherical particles, accepted for publication in the Journal of the Acoustical Society of America (J. Acoust. Soc. Am.)
2) D. R. Palmer, Acoustic Imaging of Underwater Plumes, in Acoustical Imaging, Volume 21, Joie Jones (ed.), Plenum Publishing Co. (1995).
3) R. A. Rona and D. R. Palmer, Imaging plumes beneath the sea, J. Acoust. Soc. Am. 93, 569 (1993).
4) P. A. Rona and D. R. Palmer, Acoustic Imaging Beneath the Sea, in 1992 Yearbook of Advances in Physics, American Institute of Physics, 1992.
5) P. A. Rona, D. R. Palmer, D. A. Chayes, C. Jones, M. Czarnecki, E. W. Carey, and J. C. Guerrero, Acoustic Imaging of Hydrothermal Plumes. East Pacific Rise, 21 deg N, 109 deg W, Geophys. Res. Letters 18, 2233 (1991).
6) D. R. Palmer and P. A. Rona, Comment on "Acoustic Doppler Current Profile Observations of a Mid-Ocean Ridge Hydrothermal Plume," J. Geophys. Res. 95, 5409 (1990).
7) D. R. Palmer, P. A. Rona, and M. J. Mottl, Acoustic imaging of high-termperature hydrothermal plumes at seafloor spreading centers, J. Acoust. Soc. Am. 80, 888 (1986).

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