Underwater Gliders

Ocean gliders are autonomous vehicles that can operate in all weather conditions, for long time periods, and in remote locations. They move slowly, at about 0.50 knots (about 25 cm/s), and with little noise because they do not have a standard engine or propeller. Gliders can be equipped with a variety of oceanographic sensors, and are frequently used by scientists to measure ocean parameters such as temperature, salinity, pressure, as well as the structure and velocity of ocean currents. Gliders can also include acoustic sensors, such as hydrophones, and echosounders. These gliders are an effective tool for passive acoustic monitoring. They are relatively inexpensive compared with traditional ship-based and aerial surveys. Scientists are using gliders to study underwater sound propagation and are developing glider technologies to monitor sound fields of erupting underwater volcanoes (and other natural sounds), estimate wind speeds (to measure weather), acoustically map submerged portions of icebergs, and detect and measure krill biomass.

Diagram of a Seaglider
A Seaglider (shown above) is a type of underwater glider, designed to operate at depths up to 1000 m. The vehicle is 1.8 m (5.9 ft) long and weighs 52 kg (115 lb). It has a cylindrical hull which is surrounded by a fiberglass fairing to give it a hydrodynamic shape; the hull actually compresses as it sinks, matching the compressibility of seawater. After each dive, Seaglider dips its nose to raise its antenna out of the water. It then determines its position via GPS, connects with a satellite, uploads oceanographic data it has collected, then downloads a file complete with any new instructions. Image credit University of Washington (http://www.apl.washington.edu/projects/seaglider/summary.html).

Ocean gliders move by changing their buoyancy using an internal oil-filled bladder. Oil is pumped into or out of the bladder to change its volume, causing the glider to ascend or descend. Changes in water depth are converted into forward movement by the wings of the glider. The pitch and roll of the glider are controlled by shifting the position of the internal battery packs. Periodically, the glider rises to the surface, reports its location, and transmits recorded data and diagnostic information to a satellite. It may also receive commands for its new course.

Acoustic gliders (yellow instrument shown above) use hydraulic buoyancy changes to alter the vehicle’s density in relation to the surrounding water thereby causing the vehicle to either float or sink.r
Underwater gliders (yellow instrument shown above) use hydraulic buoyancy changes to alter the vehicle’s density in relation to the surrounding water thereby causing the vehicle to either float or sink. The glider periodically surfaces, so that it can relay (to a satellite or a ship) a navigational fix, transmit its data, and receive further instructions for command and control. Image courtesy of Teledyne Webb Research.

Gliders collect physical, chemical, acoustic, and biological data. Being able to record one or more environmental condition simultaneously is a major advantage of this tool. For example, scientists use gliders to obtain passive acoustic data on marine animals.

Scientists use gliders to estimate and measure the presence, distribution, behavior, and/or habitat associations of cetaceans including sei whales, fin whales, North Atlantic right whales, humpback whales, beaked whales, sperm whales and a variety of dolphin species. Scientists recently equipped acoustic gliders with a digital acoustic monitoring instrument (DMON), a smartphone sized electronics package inside the glider that is programmed to record whale calls and compute spectrograms, from which a “pitch track” (a lower bandwidth representation of the whale call) is generated. The software also contains a “call library” of whale vocalizations, which aids in species identification. Subsets of pitch tracks are transmitted by the glider every 2 hours, from which estimates of the location of recorded species determined. One application of these data is to determine the location of endangered species that may be impacted by human activities, such as shipping.

Click either choice below to hear the low frequency vocalization of a sei whale, as recorded by an underwater glider:
 
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Sei whale downsweeps recorded by an underwater glider. Please note, these sounds were sped up to make it easier to hear them. The sei whale downsweeps in the original recordings are at frequencies of 30-50 Hz.
Sound credit: Mark Baumgartner, WHOI.

Hydrophone-equipped gliders are also used to study the spatial and temporal distribution of fishes. For example, sounds produced by red grouper, toadfishes (Opsanus spp.), and several unidentified fishes were detected in the Gulf of Mexico. Red grouper are a critical commercial and recreational fish species in the Gulf of Mexico and sustaining their population levels is important. Gliders may offer a new method map their distribution, better understand their spawning habitats, and enhance fisheries management. Gliders can be programmed to actively follow tagged fish, and this will allow scientists to monitor fish migrations on a large scale.

Click either choice below to hear a red grouper vocalization that was recorded by an underwater glider:
 
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Red grouper call recorded by an underwater glider. Note, the vocalization is immediately preceded by noise from the glider's rudder.
Sound credit: Carrie Wall and David Mann. Sound released under Creative Commons Attribution, Non-Commercial.
Additional Links on DOSITS

References

  • Baumgartner M.F. and Fratantoni, D.M. 2008, "Diel periodicity in both sei whale vocalization rates and the vertical migration of their copepod prey observed from ocean gliders." Limnology and Oceanography. 53(5, part 2): 2197-2209. 
  • Baumgartner, M.F., and Mussoline, S.E. 2011, "A generalized baleen whale call detection and classification system." Journal of the Acoustical Society of America. 129(5), 2889-2902. 
  • Baumgartner, M.F., Frantoni, D.M., Hurst, T.P., Brown, M.W., Cole, T.V.N., Van Parijs, S.M., and Johnson, M. 2013, "Real-time reporting of baleen whale passive acoustic detections from ocean gliders." Journal of the Acoustical Society of America. 134(3), 1814-1823. 
  • Baumgartner, M.F., Van Parijs, S.M., Wenzel, F.W., Tremblay, C.J., Esch, H.C., and Warde, A.M. 2008, "Low frequency vocalizations attributed to sei whales (Balaenoptera borealis)" Journal of the Acoustical Society of America. 124(2): 1339-1349. 
  • Klinck, H.l., Mellinger, D.K., Klinck, K., Bowue, N. M., Luby, J.C., Jump, W.A., Shilling, G.B., Litchendorf, T., Wood, A.S., Schoor, G.S., and Baird,R.W. 2012, "Near-Real Time Acoustic Monitoring of Beaked Whales and Other Cetaceans Using a Seaglider" PLoS One. 7(5), 1-8. 
  • Wall, C.C., Lembke, C., and Mann, D. A. 2012, "Shelf-scale mapping of sound production by fishes in the eastern Gulf of Mexico, using autonomous glider technology." Marine Ecology Progress Series. 449, 55-64. 
Additional Resources

  • WHOI, Baumgartner Lab, "Autonomous investigations of baleen whales." (Link)
  • Teledyne Webb Research, "Slocum Glider." (Link)
  • "University of Alaska: Underwater gliders may change how scientists track fish." (Link)