April 9, 2004

Seeking Sunken Treasures and Signals in the Aegean

Taken any nifty trips lately?

On its most recent mission, the MIT Sea Grant Autonomous Underwater Vehicle (AUV) Lab traveled back in time. Well, not exactly, but the goal of the expedition in Greece was to inspect archaeological sites in the Aegean Sea for clues to ancient history. The other MITSG research on the trip concerned testing acoustic communications for use in future acoustic modems.

In a collaboration with Greece's Hellenic Center for Marine Research [ http://www.ncmr.gr ] and the Ministry of Culture’s underwater archaeologists, AUV Lab members focused on an area off the coast of the island of Kythira, which had been deemed to be of archaeological interest. Deploying the OII Xanthos–a completely rebuilt and refurbished vehicle, the AUV Lab followed up on an earlier 2001 Greek expedition. In this trip, Xanthos was equipped with a digital camera system, developed in house, as well as an underwater sonar system. "The goal," says Rob Damus, a research engineer with the lab, "was to see if we could have some sort of coherent plan that follows from these two sensing modalities. And we did a really good job actually accomplishing that."

That success was attained by sending the vehicle down to various altitudes above the seafloor, such as 12 meters and 8 meters off the bottom, where the average visibility on either side of the sonar peaked at around 30 meters. "The sonar was a really useful tool for identifying targets that were not directly underneath us. It's like having feelers on either side," explains Damus.

After the sonar identified what appeared to be a target of interest, the AUV performed more surveys over the site. While the team had been looking for relics from the Byzantine era, what it found was an anchor from World War II. That discovery was "not like Odysseus down there waiving a chalice," says Damus, but it was a powerful way of demonstrating that AUVs are superior tools for underwater searches.

Other MITSG staff at work on the AUV mission were research engineers Jim Morash, Samuel Desset, and Victor Polidoro; GIS Specialist Christiaan Adams; Michael Triantafyllou, a professor in MIT's Dept. of Ocean Engineering; and MITSG director Chrys Chryssostomidis.

Also in the Aegean, MITSG principal research scientist Milica Stojanovic conducted acoustic communications experiments. "My experiments are concerned with testing methods for faster (more bits/sec) acoustic communications underwater, and also methods for communication between several simultaneously active users (such as AUVs)," explains Stojanovic. Her group consisted of Costas Pelekanakis, a PhD candidate in MIT's Dept. of Ocean Engineering, Ethem Mutlu Sözer, an MITSG postdoctoral associate and Lee Frietag, a WHOI engineer.

Before the mission, the group generated test signals and then stored them on a laptop computer. In Greece, the researchers conducted experiments by transmitting signals from one boat and receiving them on another. Those received signals were then recorded on computers for later processing.

"The experimental signals include a variety of cases meant to
test special modulation formats (such as multi-carrier modulation, and
multi-user communications) as well as signals that are meant to test the
reconfigurable modem that we are developing in the lab," says Stojanovic. "All in all, we had a very successful expedition and we have collected enough data to last the entire winter. What we need to do now is to actually process the data and demonstrate which of our concepts work, which do not work, and which are promising candidates for actual future implementations in real-time acoustic modems."

Closer to home the AUV Lab is working this fall in Mystic Lake (Medford, Mass.) to attempt transmission of pictures underwater using an underwater acoustic modem that radiates sound on a given frequency band and allows underwater communications–a technological feat not yet achieved with AUVs.

In collaboration with the Northeast Consortium, the lab recently conducted a fisheries survey in Nantucket Sound, off the coast of Hyannis. That work is aimed at an improved understanding of how fisheries that employ dragging nets impact the seafloor. Researchers are not processing the data collected by the AUV, which compares sites that have been fished with those that have not.

The AUV Lab is also collaborating with the Parsons Laboratory on a chemical sensing network in Mystic Lake. In that project, an AUV equipped with an underwater mass spectrometer will communicate acoustically with a network of buoys containing a variety of environmental sensors. The AUV will swim around the lake for up to six hours at a stretch, receiving data collected by the buoys. The AUV would then relay that data to a base station, where it would become available on-line for the ocean community, providing real time monitoring of the lake's chemical properties.

The lab's other major project, funded by the Office of Naval Research [ http://www.onr.navy.mil ], is a joint venture with Bluefin Robotics Corp. [ http://www.bluefinrobotics.com ], a 1997 commercial spin-off from the AUV Lab. The researchers are currently developing a hovering AUV that would be used for scanning ship hulls for attached mines. Currently the state of the art for seeking such explosives involves using a remotely operated vehicle (ROV). An ROV, however, leaves something to be desired as it can have difficulty returning to a point of interest on the ship it has located previously.

In contrast, the hovering AUV will use a Doppler sensor to sense its velocity along the hull of the ship to ascertain its constant location. And that's a lot more attractive than the more traditional, risky method in which divers hunt for mines. "Those guys describe it as walking around two football fields in the dark and trying to find what you are looking for with your hands," says Damus.

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R/V Aegaeo in Kapsali

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