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HYDROTHERMAL VENT PLUMES AS ACOUSTIC LENSES
by Joseph J. Buff, [IMAGE]2002

Photo Courtesy: Walter P. Noonan
[IMAGE] Hydrothermal vents are of potential interest to submariners. They strongly impact local water temperature and chemistry, and hence their plumes can alter underwater sound propagation. Hot vents were first discovered by deep submergence research craft in 1977. These “black smokers” and their associated biology have been studied extensively since then, although a review of NASA, NOAA, and NSF project proposals listed on Internet websites shows that many questions remain.

This article will briefly summarize the properties of hot vents and then will theorize how they might create undersea “acoustic lenses.” These lenses could focus, or obscure, both passive and active sonar, in a manner similar to convergence zones, sound channels and ducts, and shadow zones. We will argue here that such lensing effects could be exploited as “natural telescopes for sound,” enhancing effective hydrophone sensitivity and improving target detection ranges, thus increasing the military power of a naval submarine’s onboard sensor hardware and software suite. At the suggestion of Jim Hay, we will end with a discussion of ways such thermo-chemical acoustic lenses might be created artificially, on command of the SSN’s or SSBN’s skipper, to help achieve “sonar superiority” in critical tactical situations.

Review -- Rules of Underwater Sound Propagation
The refraction behavior of sound is similar to that of detonation waves (shaped charges, implosion-design nuclear warheads) and electromagnetic radiation (light, radio, radar). Specifically, sound wavefronts are refracted (bent) away from areas of higher transmission velocity and toward areas of lower velocity. Temperature, pressure, and water chemistry affect sound velocity in the sea as follows:

  • 1. A negative temperature gradient (i.e., increasingly cold) will cause sound velocity to decrease.
  • 2. A negative density gradient (i.e. lower mass per unit volume) will cause sound velocity to decrease.
  • 3. Increasing pressure (depth) increases water density -- seawater is not entirely incompressible.
  • 4. Dissolved chemicals (e.g., salt) increase sound velocity (because they increase density -- they may also hasten sound attenuation loss).

    These familiar rules, combined with representative ocean bathythermograph traces, lead to the well known effects of surface ducting, convergence zones, thermal layer shadow zones, and deep water sound channels [1,2,3,4]. (Typically, temperature effects in the permanent thermocline predominate over density effects: sound velocity decreases with greater depth until the isothermal zone is reached, due to progressive cooling, at which point sound velocity then increases with depth, once a constant temperature is reached.)

    [Joe Buff / JoeBuff.Com] Review -- Properties of Hydrothermal Vents
    Hydrothermal vents are found at hundreds of places along the sixty-thousand-kilometer-long Mid-Ocean Ridge, a seam of spreading crustal plates which girdles planet Earth. Hot vents are often found in groups (fields), though they may also occur on isolated seamounts [5]. Seafloor fissures and magma upwellings combine to create a water circulation loop, similar to a geyser, in approximate dynamic equilibrium at large time and distance scales. Water seeps down through cracks in the seabed, penetrating perhaps a mile or two. It then comes in contact with molten lava or other hot geological formations. The seawater is heated, becomes acidic, and takes on dissolved chemicals. The hot water rises through a vent, a kind of chimney on the ocean floor [6].

    Vent nozzle diameters can range from less than one inch up to several feet. Water temperature at the opening can reach 400o C [5,7], and hotter vents may yet be discovered. Two phenomena take place in the region at and above the nozzle:

  • 1. Temperature mixing. Pressure at many atmospheres psia prevents the water from boiling. Instead, it quickly cools by several processes: conduction to the solid chimney (which would then itself radiate heat outward), adiabatic cooling (i.e., lower pressure upon release from the constricting vent pipe), and convective mixing with seawater surrounding the vent at an ambient temperature near 2o C [5,6]. Sources indicate that cooling is very rapid -- in some cases ambient near-freezing water temperatures are reached in the plume within inches of the nozzle [5]. Other references indicate that a) water temperature may hover near 15o C in the immediate vicinity of the chimney [8], b) mean temperature anomalies decline to 0.02o C on a scale of kilometers [7], and c) “very warm water” can be found by towed research sleds 100 meters above an especially active (e.g., Pacific Ocean) sea floor vent field [9].
  • 2. Chemical transport. Hydrogen sulfide, metal sulfides, and calcium and barium sulfates dissolved in the rising superheated water precipitate out rapidly as the water cools. This is the cause of the billowing black exudations seen at many thermal vents. Some of this precipitate accretes and forms the chimney itself. Chimneys up to 150 feet tall have been observed [9]. Some of the precipitate falls as “snow” nearby the vent. (These sulfur compounds support complex and unique biology completely independent of photosynthesis, including Archaea microbes which are of interest to submariners because of their corrosive effects [10]). After precipitation the water exuding from the vent remains saturated with numerous chemicals, including helium-3 and oxides of iron, manganese, zinc, and copper. As the water mixes with the surrounding sea, the concentrations gradually dilute. These enhanced concentrations form extended megaplumes above the vent mouth. In some cases research probes detect these chemical tracers many miles from the vent itself [6, 7].

    Lensing Effects
    In general, asymmetries about the vertical axis can be expected in the shape and structure of a hydrothermal megaplume. These may be caused by a) ocean currents (currents of at least 1 cm/sec have been measured near some hot vents [7]) or b) the merging of plumes from vents distributed unevenly within a field (some vent fields achieve the size of sports stadiums [5]). In addition, there is not necessarily a sharp boundary between a plume and the surrounding sea, but rather an area of mixing and thus a gradual alteration of temperature and chemistry.

    For discussion purposes, to establish the potential military significance of hydrothermal vents, a simple model of a megaplume will be considered, an inverted cone with apex lying at the vent mouth.

    What is special about this plume cone? In general it can be expected to be warmer than the surrounding sea, and (adjusting for any temperature difference) also denser (due to dissolved chemicals, the same way salt in seawater raises density some 3 or 4% [11]). Both effects increase sound velocity (as discussed above, on large scales within the megaplume the chemistry is generally more important than the heat).

    This hypothecated cone (just like real vent plumes) will have internal structure: gradients can be expected as to both temperature and chemical density. These gradients will be negative (decreasing) with distance from the vent mouth both horizontally and vertically. The two dimensions must be considered separately because, everything else being equal, water pressure changes vertically but not horizontally.

    The properties of these gradients will show variation from vent to vent and over time at any particular vent [7]. Exact data is necessary in order to perform specific and reliable ray trace calculations with practical utility. Turbulence and “noise” close to the vent mouth can be expected to be substantial and chaotic. However, conditions somewhat further away, at distances tactically useful to submariners and to ASW/USW forces, would be steadier and smoother.

    Clearly, extensive seaborne computer power, sophisticated signal processing algorithms, and good real-time information on conditions in the megaplume (gathered perhaps by expendable probes or by UUVs), would all be needed to effectively harness the acoustic lensing properties of hydrothermal vents. Different considerations and engineering problems would apply to the different modes of sonar listening and analysis: broadband, narrowband, or demodulated. In practice there would be non-trivial expense and time involved to study these feasibility issues and then perfect the equipment and procedures required for operational exploitation by the fleet.

    Technical Discussion
    For brevity, this article will consider a megaplume deep enough to lie within the isothermal zone, i.e. the surrounding ambient water temperature is a uniform 2o C.

    We will analyze two situations: one vent in isolation, and two vents near each other.

    One Vent in Isolation
    Let us consider first horizontal acoustic effects. Because of increased cooling and dissolving by ambient seawater at greater distance from the vertical axis of the megaplume, a horizontal section through the cone will have greatest density and temperature, and hence greatest sound speed, near its center, and lowest density and temperature, hence lowest sound speed, near its periphery. Thus sound rays moving in parallel in a horizontal plane, when passing through the megaplume, will tend to diverge, as if passing through a concave lense. See Figure 1.

    Let us next consider vertical acoustic effects. A vertical section will capture a triangular slice through the megaplume, surrounded by ambient seawater assumed not affected by the plume. Let us imagine moving upward through this section along a vertical line. See Figure 2.

    A line AB rising directly from the vent nozzle will show a steady decrease in sound speed while moving toward the ocean surface. Ignoring hot vents and shallow thermoclines, this trend would occur simply because of decreasing pressure at shallower depth -- note this pressure gradient is linear with respect to depth. The gradient in the hot vent plume will be steeper than this, quadratic (second power), because dilution of the chemical burden occurs in two horizontal dimensions (i.e., across a plane) as one moves toward less-deep water and the cone cross-section diameter expands. This gradient will cause sound rays to bend upward, as they do in general below the axis of the deep sound channel, but because of this non-linearity they will bend upward more sharply with shallower depth, thus acting to diffuse sound rays, again like in a convex lens.

    A line CD rising from the ocean floor at a point displaced from the vent nozzle itself, will show sound speed reduction with shallower depth at first, until it encounters the megaplume “boundary” at X. There will be a jump in water density and temperature near X, and hence a jump in sound speed. Speed will then decline once more along XD, but as on line AB density will fall at a faster rate that in “clear water,” and there will be some temperature decline effect as well. This argument supports two conclusions:

  • 1. In the immediate area of the megaplume boundary at X, sound rays will converge in the vertical plane: they will bend upward from below and downward from above.
  • 2. More broadly, moving upward from the sea floor on a planar section through CXD, a local maximum in sound speed will occur near X -- the opposite of a velocity minimum at the axis of the deep sound channel. This maximum will cause an overall vertical divergence of sound rays.

    In summary, we may conclude that a single hydrothermal vent megaplume acts like a concave lense, dispersing or diffusing sound, although there will be some local focusing of rays vertically at the boundary between the rising megaplume and ambient seawater underneath.

    Two Vents in Proximity
    Let us next consider the case of two adjacent identical hot vents and examine acoustic lensing in the region between them. See Figure 4. First imagine a horizontal section through AB, at a depth below where the megaplumes of hot vents 1 and 2 intersect. Chemical burden density and temperature, and thus sound velocity, reaches a local minimum at X, between two local maxima above vents 1 and 2. In this case sound rays are focused toward point Y -- see Figure 5. Again because horizontal spreading and dilution of the plume occurs in a two-dimensional plane, the increasing density gradient from point X toward either vent nozzle will rise more rapidly than linearly, helping focus the sound rays.

    This focusing will also occur in a horizontal plane through CD on Figure 4, since although there will be less dilution of each plume once they meet, there will still be a density and sound speed minimum at XX, or anywhere on a plane above XX at a point intersecting the vertical line through X.

    What about the vertical plane? Consider a vertical slice intersecting the midpoint X between two “equal” hot vents on Figure 4. (Let this slice also be perpendicular to the line between the vent nozzles.) The discussion above pertaining to Figures 2 and 3, a vertical slice off-axis through a single plume, once more applies. Specifically: at XX, the boundary between the intersection of the megaplumes and the undisturbed seawater lower down, local focusing occurs.

    Therefore, at the point XX where two adjacent hydrothermal vents’ megaplumes intersect, focusing of sound rays occurs in both the horizontal and vertical planes.

    Diffusion, Focusing, Magnifiers and Telescopes
    The above discussion using the simplistic inverted cone model supports two hypotheses:

  • 1. The megaplume of a single isolated hot vent functions overall as a concave lense with respect to sound rays, diffusing and dispersing them except in the vertical plane in proximity to the boundary layer between the megaplume and the ambient sea beneath it.
  • 2. The region between two adjacent hot vents, particularly at the intersection between their two plumes’ outer boundaries, functions as a convex lens, focusing sound rays much as a magnifying glass focuses the sun.

    We can apply these two observations to make a third: With a “supply” of lenses in a hot vent field, we can exploit acoustic optical systems. In particular, certain combinations of lenses can have the effect of a telescope, an acoustic “amplifier.” One combination is a pairing between a convex objective lense and a concave ocular (eyepiece), as in the refracting telescope Galileo constructed. The other is a pairing between a convex objective lens and a convex eyepiece. The convex/convex pairing tends to be more efficient, and is used in modern telescopes and binoculars. A naval submarine could position its bow sphere or wide aperture array at the focal point of this lensing system, to enhance the effective performance of the vessel’s sonar. If test depth limits or other considerations prevent achieving this directly, a towed array or purpose-designed hydrophone complex could be dangled at the acoustic focal point. By shuttling between different lensing systems, the submarine could improve the effective signal-to-noise ratio and also could triangulate on an enemy target, thus helping perform a target motion analysis to derive a useable firing solution, potentially at considerable engagement ranges.

    Creating Artificial Megaplumes
    The exploitation of undersea acoustic lensing with the goal of greater “sonar superiority” need not be dependent on hydrothermal vent fields. The critical component is an area of water with the necessary thermal and/or chemical properties. This might be achieved artificially, on demand and at an arbitrary location, in at least one of two possible ways:

  • 1. Deployment of chemical-dispensing and/or heat-generating canisters by the submarine, similar to noisemakers but with a different purpose.
  • 2. Exploitation of the heated water discharged from the vessel’s main condensor cooling loop, perhaps with the introduction into the discharge of water-soluble chemicals.

    Approach 1 might have two disadvantages: the canisters may be noisy in operation, thus compromising stealth, and each one represents a point source, so that a useful megaplume would be generated slowly. Approach 2 appears at least to offer ease of implementation: by cruising slowly in tight upward or downward spirals, for instance, thus laying trails of warmer water with the desired controlled geometries, the boat could produce non-chaotic lensing areas that require no special equipment and create no inordinate noise. Note that in both these artificial approaches the lenses would gradually dissipate, whereas natural hydrothermal plumes are continuously renewed.

    Conclusion -- Military Applications
    Hydrothermal plume acoustic lensing can have both offensive and defensive applications. A single vent can be used as an acoustic diffuser, to disguise a submarine’s active and passive signatures from observers on the other side of the vent, thus enhancing stealth. Conversely, a correctly spaced pairing between a) “objective” focusing effects between two adjacent hot vents, and b) “ocular” effects using another single hot vent (concave eyepiece) or another adjacent pair (convex eyepiece), can produce an acoustic amplifier, a kind of sonar telescope, boosting first-detection distances and sharpening directional resolution, helping achieve sonar superiority. Even if the “field of view” were narrow and inflexible, say along a single fixed line of bearing, a useful long-range surveillance barrier or “tripwire” could be produced.

    [Joe Buff / JoeBuff.Com] Obviously these effects require that both the target and the observing hydrophone/transducer array be positioned properly with respect to the vent field megaplume geometry. (As discussed in the previous section, this limitation could be avoided by creating temporary acoustic lenses artificially.) Furthermore, good sonar directivity and spectrum filters would be needed to delete noise emissions from the vent nozzle itself and from underlying magma movements, and considerable computer power combined with lengthy integration intervals -- and thorough sonar watchstander training plus experience -- would be needed to make useful sense of the underlying signal. Practical utilization therefore has two requisites: databases, and platform/user capabilities.

    The databases would need detailed maps of hydrothermal vent locations. The databases would also need real-time statistics on the characteristics of specific hot vents, in particular on temperature gradients and chemistry/density structures in three dimensions. Presumably such real-time data could be sampled by XBT probes or SUAVE chemo-sensors [6].

    Two real-world situations come to mind where existing platforms and their commanders might find acoustic lensing useful. The first would be SubForce mission tasks in regions of the globe where hot vent activity affects depths reachable by current SSNs and SSBNs -- this might include the Red Sea and the Persian Gulf [10]. The second would be situations where bottom-moored SOSUS nets -- including temporary/tactical listening systems [12] -- have been or might be deployed near vent fields, or where towed arrays, sleds, or other gear could be trailed at the proper depth from a parent naval submarine.

    Certainly, detailed calculations using actual hot vent data, applied in advanced (possibly classified) sonar propagation models, would be called for to validate this concept, before reliable results could become available having military applications. This is one manner in which oceanographic research can be significant to national defense and to the proactive maintenance of peace. Thermo-chemical acoustic lensing is also an example of how making the most of the ocean medium in which all undersea warriors operate can yield strategic advantage if and when armed intervention does become required.

    REFERENCES

  • 1. Frieden, David R., LCDR, USN, editor. Principles of Naval Weapons Systems. Annapolis, Naval Institute Press, pp. 189-285.
  • 2. Hervey, John B., RADM, RN, Submarines. London, Brassey’s (UK), pp. 91-123.
  • 3. Payne, Roger, “Appendix: A Primer of Ocean Acoustics.” Among Whales. New York, Scribner, pp. 359-402.
  • 4. Urick, Robert J., Principles of Underwater Sound. Los Altos, CA, Peninsula Publishing.
  • 5. Stover, Dawn, “Creatures of the Thermal Vents.” NASA Internet website, http://seawifs.gsfc.nasa.gov/OCEAN_PLANET.
  • 6. “Hydrothermal Vent Geochemistry.” NOAA Internet website, http://www.pmel.noaa.gov/vents.
  • 7. “Processes and Fluxes on a Superfast Spreading Ridge: The Southern East Pacific Rise,” workshop report sponsored by NSF RIDGE Initiative. University of Hawaii Internet website, http://ridge.unh.edu/sepr/report.
  • 8. “Make a Miniature Deep-sea Vent.” New England Aquarium Internet website, http://www.neaq.org/learn/kidspace/vent.
  • 9. Broad, William J., The Universe Below. New York, Simon & Schuster, pp. 112-122.
  • 10. Rosenblatt, Richard, MD, “Persian Gulf and Fulminate Marine Corrosion.” The Submarine Review, April 1998, pp. 33-39.
  • 11. Pernetta, John, editor, Atlas of the Oceans. London, Rand McNally, p. 30.
  • 12. Jones, D. A., RADM, USN, “Submarine Force Plans and Programs: Preparing for the Challenges of the 21st Century.” The Submarine Review, October, 1995, page 31.

    Originally published in the October 1999 issue of THE SUBMARINE REVIEW, a quarterly publicatication of the Naval Submarine League, PO Box 1146, Annandale, VA, 22003. Posted here with permission of the Naval Submarine League.

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