CAMBRIDGE, Mass.--Shipwrecks of inestimable historical value have lain hidden on the sea floor for hundreds or even thousands of years, often under layers of thick, gooey mud. Hidden, that is, until engineers like MIT's David Mindell and colleagues at the Woods Hole Oceanographic Institution created high-tech vehicles that can plumb the ocean's depths with sophisticated cameras and sonar equipment to locate these ancient vessels and their cargo.
Mindell, an MIT professor who works in both the history of technology and in engineering systems, will use his newest deep-sea archeology equipment, a high-precision navigation system named "Exact," in two collaborative expeditions this summer.
Exact is a wireless sonar system consisting of three acoustic beacons containing custom signal processors. The system is able to navigate and scan to an accuracy of less than a cubic centimeter an archeological site thousands of meters under the ocean's surface -- much deeper than divers can go.
"Systems like these allow us to make maps of archeological quality without direct human presence at the sites, thus enabling archeology in the deep sea," said Mindell, the Dibner Associate Professor of the History of Engineering and Manufacturing in the Program in Science, Technology and Society at MIT.
Exact's first expedition of the summer will be off the coast of Turkey in the Black Sea, where a team of archeologists, engineers and scientists led by Robert Ballard (who discovered the wreck of the Titanic) will study an extraordinarily well-preserved shipwreck. Later, in the Mediterranean Sea, Ballard, Mindell and a different team hope to work with an eighth-century Phoenician shipwreck first discovered in 1999.
Both expeditions will utilize Mindell's navigation and mapping systems. Lawrence Stager of Harvard University, Fredrik Hiebert of the University of Pennsylvania, and Cheryl Ward of Florida State University will oversee the expedition's diverse archaeological program. Dwight Coleman of the Institute for Exploration (IFE) is chief of research, and Jim Newman of IFE is chief engineer. The new remotely operated vehicle (ROV) Hercules, developed by engineers from IFE and the Deep Submergence Laboratory at Woods Hole Oceanographic Institution will carry the navigation and excavation tools that make it possible to do deep-sea archeology.
Exact's three acoustic beacons, called transponders, are sealed in metal containers each about the size of a breadbox. The host transponder, attached to an ROV that hovers just above the investigative site on the ocean floor, signals the other two transponders which have been set on the floor of the sea at the edges of the site.
The host transponder sends out a ping--a sound familiar to anyone who has seen old war movies with submarines--that says "Where am I?" Transponder A says, "I'm transponder A, and I'm at this range," as does Transponder B. Exact measures the time it takes for these pings to travel (sound travels about 1,500 meters per second in water, much faster than through air), and puts these distances together with information gathered from depth-measurers on the ROV, to pinpoint the host transponder in three-dimensional space. The system can cover a volume of more than 300 cubic meters on a side.
As the ROV travels over and around the site -- another system developed by MIT's DeepArch research group for research in technology, archaeology and the deep sea -- the sub-bottom profiler, bounces ultrasonic waves off objects under the mud on the ocean floor. The profiler uses the data to create color images of the buried parts of the site, potentially revealing the identity of the objects that, even with sufficient light, would look to the naked eye like hills and bumps of sea mud, with just a trace of identifiable shape underneath.
"The thing that makes this archeology and not treasure-hunting or salvage is the scientific approach, which includes precision navigation and mapping," said Brendan Foley, who earned a Ph.D. in the history and archeology of technology from MIT in June. Foley, who is a member of the DeepArch group, will be an archeologist and navigator on the Mediterranean leg of the summer's expedition.
"The thing that makes artifacts important is their spatial relationships to each other. If you don't record those spatial relationships carefully, then you're not doing archeology," Foley said. "The minute you remove an artifact from a site and put it somewhere else, then you've lost information. The precision mapping done by the navigation system and the sub-profiler makes what we do archeological science."