An Acoustic Tape Measure for Deep-Sea Archaeologists
New York TImes
12 June 2003

CAMBRIDGE, Mass. -- THERE'S the derring-do version of underwater exploration - the sea dog with the power saw, cutting through the hull of a ship to find hidden gold. And then there are the deep-sea archaeologists, who want to explore submerged sites while causing minimal damage, making detailed maps that mark each minute change as artifacts are painstakingly removed.

David Mindell, a professor at the Massachusetts Institute of Technology, is on the side of the archaeologists. Leaning over his workbench in the basement of his condominium a few blocks from Harvard Square, he is fine-tuning his latest invention to help them do their work more precisely: a wireless sonar system that can map the seafloor thousands of feet below the surface, where divers and global positioning system equipment cannot go.

Part of the device is plugged into a laptop in the electronics workshop that Dr. Mindell has set up at his
home to accommodate computing marathons. As data scrolled on the screen, Dr. Mindell declared the system ready for use. "It has a millimeter of jitter, but it's good to the cubic centimeter," he said. "Super, super accurate, far better than anyone else can measure in the ocean."

Dr. Mindell has just returned from a dress rehearsal of the system in the turbulent waters off Cape Hatteras, N.C., where the Monitor sank in 1862 and lay undiscovered for more than a century. The device worked well there, he said.

In a month he will pack up the equipment and head for the Black Sea and then the Mediterranean to explore, among other sites, a spot off the coast of Israel near Ashkelon where two Phoenician vessels sank in 1,300 feet of water around 750 B.C.

Dr. Mindell will be joining a large, collaborative expedition of archaeologists, scientists and engineers, led by Robert Ballard, known for his discovery of the wreck of the Titanic in 1985.

Dr. Mindell's devices will be lowered to the seafloor at the shipwreck sites and survey each site with a remotely operated underwater vehicle bristling with cameras and other equipment.

The wireless sonar system will provide a precise electronic grid for locating parts of the ship and other objects, similar to the physical grid - usually made of string - that is established at archaeological sites on land.

"Archaeology is about measuring accurately," said Brendan Foley, who recently received his doctorate after working with Dr. Mindell since 1997 and will be part of the expedition. "This system allows us to make accurate maps anywhere on the seafloor. It's an acoustic tape measure."

Deep-sea archaeology is a relatively new academic discipline that combines traditional humanities-based archaeology with engineering advances that enable precision work with remotely controlled equipment in places that humans cannot reach.

The interdisciplinary approach seems suited to Dr. Mindell, a historian of technology and an engineer who has pursued a combination of engineering and liberal arts since he majored in both English and electrical engineering at Yale University. "I've always done both kinds of work," he said, "in parallel."

The wireless sonar system is sealed in metal containers that are roughly the size of bread loaves.

Outside each tube are analog microphones and speakers that receive and send pings. Inside are amplifiers to boost the signals, which are attenuated in salty water, and digital signal processing chips. Batteries provide both power and ballast.

To start the communication process, two sonar beacons are lowered and positioned on either side of the wreck. A similar transponder is placed on the remotely operated underwater vehicle that hovers above. The vehicle pings the beacons, they ping back, and the time interval is recorded to calculate the distance.

The remotely operated vehicle patrols back and forth in narrowly spaced parallel lines, capturing images of the objects on the bottom and determining their location on the grid.

Dr. Mindell will also bring along a subbottom profiler, a sonar-based system he has devised that bounces ultrasonic waves off the sediment on the seafloor to reveal what lies just beneath - artifacts, perhaps.

This is the instrument that Sarah Webster, an engineer at the Woods Hole Oceanographic Institution who will be going on the expedition, looks forward to using.

Ms. Webster, a mechanical engineer and former student of Dr. Mindell's, is working on a robot that will partly excavate several of the expedition sites. She is also designing the tools that will pick up and transport artifacts. "Before you dig a million-dollar hole," she said, "it's good to know as much as you can." Dr. Lawrence Stager, a professor of archaeology at Harvard, will also be on board as archaeological director for the wrecks off Ashkelon. The two Iron Age ships submerged there, the oldest ever discovered in the deep sea, are thought to have been part of a convoy carrying 11 tons of wine in ceramic jars called amphorae, Dr. Stager said.

Many of these amphorae lie on the ocean floor, protected by the 1,300-foot depth from both scavenging divers and strong current.

The subbottom profiler, used in prototype form at the site on an earlier trip, suggested that more artifacts may lie beneath the sediment.

The sonar in the device penetrates the sediment with a powerful pulse of ultrasound twice a second; acoustic signals that bounce back are converted to images. The pulses can travel about six feet into the bottom, a useful distance for revealing small objects.

These acoustic images will be compared with the actual findings from the excavation to see how accurate they are, Dr. Stager said.


Deep Water Archaeology Research Group
Massachusetts Institute of Technology
77 Massachusetts Ave. Rm e51-194
Cambridge, MA 02139



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