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How Acoustic Waves Help Us to See the Sea Floor: An Interview with Gerardo Acosta
- Published on Wednesday, 21 October 2015 22:14
- Elise Mulder Osenga
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Using AUVs allows researchers to collect data in a minimally disruptive way and opens opportunities to observe some of the Earth’s least-studied regions.
Many people picture waves when they think of the ocean, but the ocean waves Gerardo Acosta talks about are not capped with foam. Dr. Acosta is the research director for Electricity, Technology, and Mechatronics Research (INTELYMEC) at the National University of Central Buenos Aires (Universidad Nacional del Centro de la Provincia de Buenos Aires). One of his current projects is using sonar to observe the sea floor, and the waves he describes are acoustic waves.
Advances in sonar technologies over the last decade have improved ways that sound can be used to “see” what is on the ocean floor. Technology does exist to allow researchers to record video or photographic images of the sea floor, but in low visibility conditions, data collected by sonar can produce a more accurate characterization of the floor.
“Here in Argentina, particularly in Buenos Aires Province, we have a sea with a lot of what one may call turbidity. So (sonar) is a smart way to interact with the environment—to use a sonar image rather than video cam images, for instance,” Acosta explains.
Whether low visibility is caused by particulate matter, as in the turbid waters near Argentina’s coast, or as a result of being too deep in the ocean for light to penetrate, sonar can help provide a solution, and robots offer a means to get there. The type of robots Acosta works with are primarily AUVs, or autonomous underwater vehicles.
“The idea of AUVS is that (they are) mainly to be used with minimal human intervention. So you can send to the robot some comments where it is navigating, but you don’t have a cord or an umbilical link with the vessel … the robot should make autonomously some decisions about its navigation, and, in general, about the tasks of its missions.”
The independent nature of AUVs is one of the characteristics that makes these robots well suited for sonar research. The lack of a physical tie to the boat they are launched from allows AUVs greater mobility than other robotic observers. Additionally, it avoids the disruption caused by a cord swaying underwater behind a sonar device.
The AUVs are not without their own set of challenges, however. Acoustic waves, used by sonar devices, travel much more slowly than electromagnetic waves, used in radar observations. Yet, the acoustic waves often still need to be processed swiftly for the AUV to make decisions about its travel and data collection. This is where Acosta and his team come in. Using radar processing of electromagnetic waves for inspiration, they are working to improve the sonar devices to process information more rapidly. One way of doing this is by using different sonar techniques determined by the task or purpose of the observation mission.
“For instance, to have an idea of different objects in…the sea bed, it is very useful for us to use a side-scan sonar,” Acosta says. “This is a very good perspective to find objects in the seabed because … you have the waves that (reflect) the surface with an inclination, a specific angle. So you have information about the object itself and also about the shadow of that object.”
If the researcher desires more precise information about the object’s size or depth, then another type of sonar device, called an echosounder, might be used. These type of technologies became refined enough for practical use in 2002, and since then the information gathered using sonar has been put to a variety of uses and is applied by researchers, corporations, and governments. The main application that Acosta and his team are working on is using sonar to map the sea floor in harbors prone to sediment loading. The data are used to inform maintenance of the harbor where the river mouth meets the sea. A similar project in Majorca, off the eastern coast of Spain, mapped changes in the coastal ecosystems. Sonar has also been used by the offshore petroleum industry in various locations to track underwater pipelines and cables and by fishery industries to follow and track species of interest. Back in Argentina, Acosta is especially intrigued by the latter.
“It is very exciting, for instance, to imagine that a robot can be following a school of fishes, recognizing what kind of fishes you have to follow … We are still developing this application here in Argentina with the National Institute of Fishery, the INIDEP.”
An AUV can follow a school of fish in a way that is less disruptive than a boat and is less restricted to an individual or single species of fish than a tracking device. To avoid disrupting the species they are following, AUVs could be equipped with passive sonar, rather than active sonar. Active sonar releases a pulse of its own to gauge distance from an object, while passive sonar measures acoustic waves that are already in the environment.
“Passive sonars do not emit any acoustic waves, (they) just listen, like with our ears,” Acosta says.
A colleague of Acosta’s, Ariel Cabreira, sees opportunities to apply this type of sonar to conservation, where a robot would be able to follow whales in a non-disruptive way by passively tracking the sounds they make.
As researchers like Acosta continue to refine AUVs and improve data processing, it is likely that other new uses for these robots will emerge, offering numerous opportunities for observations of the underwater world.