David R. Palmer

Acoustics advances study of sea floor hydrothermal flow

Sub-sea floor hydrothermal convection systems discharge as plumes from point sources and as seepage from the ocean bottom. The plumes originate as clear, 150–400°C solutions that vent from mineralized chimneys; precipitate dissolved metals as particles to form black or white smokers as they turbulently mix with ambient seawater; and buoyantly rise hundreds of meters to a level of neutral density where they spread laterally. The seepage discharges from networks of fractures at the rock-water interface as clear, diffuse flow, with lower temperatures, metal contents, and buoyancy than the smokers. The diffuse flow may be entrained upward into plumes, or laterally by prevailing currents in discrete layers within tens of meters of the sea floor. The role of these flow regimes in dispersing heat, chemicals, and biological material into the ocean from sub-sea floor hydrothermal convection systems is being studied on a global scale.

Features of the Heard Island signals received at Ascension

The Heard Island transmissions were received 9140 km away at Ascension Island by an irregular array of bottom‐mounted hydrophones. The single‐hydrophone signal‐to‐noise ratio sometimes exceeded 30 dB in a 1‐Hz band, confirming the detectability of 57‐Hz underwater sound at global distances. The arrival‐time pattern consists of a single broad pulse about 10 s long, whose fine structure decorrelates in about 12 min, in sharp contrast with the stable, discrete sequences observed over shorter, midlatitude paths. The amplitude fluctuations of both the fine arrival structure and the unmodulated receptions are uncorrelated between hydrophones as little as 3.4 km apart. Phase varies less than one cycle during a 1‐h transmission after correcting for source motion, and the rms phase difference between hydrophones is about 3 rad averaged over the array. Phasor diagrams suggest that the effects of both source motion and ocean dynamics vary over the array. The probability density functions of the real and imaginary pa...

The acoustics of “black smoker” hydrothermal plumes

High‐temperature “black smoker” hydrothermal plumes occur when seawater that has penetrated into the oceanic crust and assimilated heat from magma is discharged from vents located at the axis of a mid‐ocean ridge. The acidic, metal‐rich, discharge mixes with alkaline, oxidizing seawater, and a fine suspension of sulfide particles is precipitated and convected by the flow. Vents fields have now been found at both fast and slow seafloor spreading centers and may be a ubiquitous feature of mid‐ocean ridges. A review of the progress made in using underwater acoustics to study black smoker plumes is presented. Both active and passive techniques are being investigated. Active techniques involve a high‐frequency monostatic sonar mounted on a submersible. Analysis of the amplitude and phase of the signal backscattered from the plume provides information about the three‐dimensional shape of the plume as well as estimates of the flow‐velocity field of the discharging fluid. Passive techniques use bottom‐mounted hyd...

INVESTIGATION OF NEAR-AXIAL INTERFERENCE EFFECTS FOR PROPAGATION IN A DUCTED WAVEGUIDE

The observed time-of-arrival patterns from a number of long-range ocean acoustic propagation experiments show early geometrical-like arrivals followed by a crescendo of energy that propagates along the sound channel axis and is not resolved into individual arrivals. To describe in a simple model case the interference of near-axial waves which results in forming the so-called axial wave and propose formulas for the axial wave in more general cases, the two-dimensional reference point source problem for the parabolic index of refraction squared is investigated. Using the method proposed by V. Buldyrev, the integral representation for the exact solution is transformed in such a way to extract ray summands corresponding to rays radiated from the source at angles less than a certain angle, the axial wave, and a term corresponding to the sum of all the rays having launch angles greater than the indicated angle. Numerical results for the axial wave and the last term are obtained for parameters corresponding to long-range ocean acoustic propagation experiments.

Coherent and incoherent scattering by a plume of particles advected by turbulent velocity flow.

Studies of acoustic remote sensing of the plumes that result from the injection of particulate matter in the ocean, either naturally or by dumping or dredging activities, have assumed the scattering is incoherent. These plumes are always turbulent, however. The particle density is a passive scalar that is advected by the turbulent velocity flow. The possibility exists, therefore, that the scattered waves from a significant number of particles add coherently as a result of Bragg scattering. In this paper, we investigate this possibility. We derive an expression for the ratio of the coherent intensity to the incoherent one in terms of the turbulent spectrum and the properties of the particles that make up the plume. The sonar is modeled as a high-Q, monostatic, pulsed sonar with arbitrary pulse envelope and arbitrary, but narrow, beam pattern. We apply the formalism to acoustic remote sensing of black smoker hydrothermal plumes. We find that, at most, the coherent intensity is less than 1% of the incoherent one. The implications are that Bragg scattering does not lead to a significant coherent component and in analyses of scattering from this type of plume, one can ignore the complications of turbulence altogether.

Development and application of high‐frequency sonars for imaging naturally occurring plumes in the ocean

Hydrothermal plumes are formed when hydrothermal solutions are discharged from chimneylike vents on the ocean floor where adjacent tectonic platesare separating. Their study provides information on the transfer of heatand chemicals from the solid earth to the ocean. When high‐frequency acousticswas proposed for imaging and studying hydrothermal plumes, Joe Blue contributedin a general way to the development of the effort. In this presentation the history of this effort is reviewed. After an introduction to hydrothermal venting, some of the problems addressed in the development and application of the sonar are discussed. These include sonar calibration, identification of the dominant scattering mechanism, estimating cross sections, and the intrinsic statistical variability associated with incoherent scattering. The results obtained from four sea trials are recalled. These trials involved either three‐person, deep‐diving submersibles or the JASON/MEDEA ROV system. The opportunities provided by the somewhat ...

Revisiting Buldyrev’s theory of the ‘‘axial wave’’ after 30 years

We review and summarize some of the more interesting aspects of Buldyrev’s theory of the axial wave as it relates to long‐range propagation. Just as with the experimental observations, the theory describes the near‐axial acoustic signal in terms of early, geometrical‐like arrivals followed by an incoherent arrival that cannot be described using geometrical acoustics. This incoherent arrival, or axial wave, results from a large number of waves which interfere with one another because of the presence of caustic structures along the waveguide axis. It is this interference which prevents the waves from being described in terms of geometrical acoustics. As the propagation range increases, the nature of the axial wave changes because the number of caustics increases and the region of interference associated with each one also increases. For a normal waveguide (axial energy arrives last) the axial wave ‘‘eats’’ a geometrical arrival as each new caustic is encountered along the propagation path. A range is ultima...

Acoustic remote sensing from aboard the Royal Caribbean Cruise Lines passenger ship ExploreroftheSeas

The NOAA/Atlantic Oceanographic and Meteorological Laboratory, Royal Caribbean International, and the University of Miami, with support from the National Science Foundation and the Office of Naval Research, have established modern, well‐equipped oceanic and atmospheric research laboratories on the cruise ship Explorer of the Seas. This novel relationship between commercial, academic, and federal maritime communities is enabling scientists to collect a number of unique data sets. The instrumentation in the oceanic laboratory includes two RD Instruments Inc. Ocean Surveyor Acoustic Doppler Current Profiler’s (ADCP’s) for monitoring ocean currents and biomass, and seawater sensors for measuring temperature, salinity, chlorophyll‐a, dissolved organic material, and dissolved oxygen. Data are collected weekly along the ship’s two repetitive cruise tracks through the Bahamas and the Caribbean. We summarize the acoustic observations made with the ADCP’s during the first year of operation of the laboratories. Thes...

A self‐navigating, unmanned sailboat for studies in acoustical oceanography

Based on a patent by one of us (E.W.S., Patent No. 3, 556, 035) RCA developed during the 1970’s the concept of a self‐navigating, unmanned, sailboat. It was designed to be deployed from a ship or from shore, to sail for thousands of miles on a predetermined course, to collect upper ocean and meteorological data while station keeping, to telemeter those data back to shore by satellite, and then return to the point of departure, or some other point, up to a year later. It could carry any equipment ordinarily placed on buoys and could function in hostile weather and climatic conditions. This vessel seems ideally suited for the measurement of a number of ocean parameters using acoustical methods. We discuss the characteristics of the prototype that was built and tested with U.S. Navy support and compare its properties with Unmanned Air Vehicles (UAVs) and Autonomous Underwater Vehicles (UAVs) that are presently being used or are under development. We describe our plans for resurrecting the RCA effort an...

Rayleigh scattering from a plume of suspended particulates located near an ocean boundary

A number of experiments have been conducted to acoustically image plumes of particulates suspended in the ocean. In many cases the scattering is in the long‐wavelength or Rayleigh region. A comprehensive framework was recently developed for calculating the intensity received by a monostatic sonar system due to backscattering in the Rayleigh region from a plume of suspended, nonspherical particulates [D. R. Palmer, J. Acoust. Soc. Am. 99, 1901–1912 (1996)]. This framework is extended to include the possibility that the plume is close to an ocean boundary. The distances of the particulates from the boundary are assumed to be small compared to the wavelength and the boundary surface is assumed to be planar. The scattering amplitude in the presence of a rigid or a pressure‐release boundary surface is related to the amplitude in the absence of the surface. As a result the intensity can be averaged over the random orientations of the individual particulates and this averaged intensity can be bounded in terms of...