Magazine article Oceanus

An AB(L)E Bodied Vehicle

Magazine article Oceanus

An AB(L)E Bodied Vehicle

Article excerpt

The Autonomous Benthic Explorer, known to its friends as ABE, is a robotic vehicle designed for deep-ocean exploration and monitoring. ABE is an example of the class of systems known as Autonomous Underwater Vehicles, or AUVs, that are being developed for a variety of missions by military and civilian groups. ABE is different from most of the other AUVs under development in that it is designed for long-term monitoring missions and w 11 spend the majority of its time asleep, attached to a simple hitching post near the area of interest. At regular intervals. ABE will wake up. Let go of the latch, and, using an acoustic navigation system to guide its movements, travel around its survey area taking video snap shots and making a variety of measurements. At the end of the survey, ABE will return to its hitching post, catch on, and, like a mountain climber roped into a hammock on the face of a cliff, simply go to sleep until the next scheduled survey.

The Mission

ABE is designed for a wide variety of missions, but foremost is monitoring geological and biological changes in hydrothermal vent regions. These dynamic Structures occur in seafloor-spreading areas along the mid-ocean ridges where the proximity of the underlying magma drives an associated hydrothermal circulation. (See Oceanus bank issues including Summer 1979, Winter 1988/89, and Winter 1991/92 for descriptions and discussions of this phenomenon.) While hydrothermal vents are better understood than they were a decade ago, further observations are required to answer many basic questions. For example, the chemical flux from the vents may dominate and stabilize the global, long-term composition of seawater, or it may be trivial, depending on whether the high or low value of the best available estimates of total flux is used. We know very little about the temporal variability of the vents: Magma may be delivered to the surface steadily or in batches. It appears that there are both steady emissions and periodic releases of large amounts of water (called megaplumes) that may play a role in the ocean's heat balance.

Ship time is expensive, and a research ship can only remain in one area for a limited time. As a result, months or years usually pass between visits to a particular site. Often, when scientists return to an area of active venting, many of the features have changed dramatically. We have only limited knowledge of how these complex systems evolve. ABE can remain on station near an area of interest for many months, waking up daily to travel around the area and record the evolution of the biology and geology. The data gathered might include several time-lapse movies showing the growth of a sulfide chimney or a clump of tube worms, as well as a host of oceanographic variables.

The figure opposite shows a typical ABE mission. For the first time around, we'll be waiting on the ship above, listening intently to acoustic signals from ABE as it reports on its progress. If all goes well, the ship will then leave the site and ABE will repeat the surveys entirely on its own. Eventually, when its batteries are depleted, it will simply wait at its dock for the ship to return and send an acoustic command for it to return to the surface. ABE can remain quiescent at its dock for several months, and its periodic excursions can be spread out over that entire time.

Why Such a Strange Shape?

ABE's configuration is unlike most other AUVs. Most are torpedo shaped and designed for speed and range; they are optimized to survey oceanographic variables of interest (such as salinity and temperature) over wide areas. Some of these vehicles will eventually be able to travel several thousand miles. For ABE, the ability to maneuver in tight places close to complex bottom topography is more important. ABE's three-body shape gives it several unique advantages. First, it allows reasonably efficient forward travel, yet provides protected locations for the lateral and vertical thrusters. …

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