Innovative Engineering Solutions
Exploration of shipwrecks in the deep ocean is not new, but exploration is really just seeing a site, “planting the flag,” then going home. Field archaeology, on the other hand, is a precise science. For Mindell and the research team that discovered Skerki D, the fundamental questions are: How do you execute very precise and accurate scientific work thousands of meters below the ocean’s surface in a place you can never touch? And, How do you excavate those sites to exacting archaeological standards?
“One of the things that concerned me early on was there was too much technology driving the archaeology,” says Mindell. “This technology is big; it’s expensive; it’s impressive; it’s dazzling; and it’s very easy for people to get lost in the very momentum of it.” To make sure archaeological needs drive the engineering research, Mindell and his colleagues talk frequently with the archaeologists. “We spend a lot of time getting people in a room together and asking what kinds of questions archaeologists have and how they translate into an engineering requirement,” he says.
Mindell and members of his research lab are working on solutions for two of those requirements. They are developing a navigation system that allows an underwater research robot to take detailed pictures of a site, as well as a subbottom profiler that can penetrate the sediment around a ship to create a 3-D image of the entire wreck, including the portions that are buried.
During that expedition in 1997, Mindell’s navigation system, called “Exact,” was used in deep water to do a precision mapping of Skerki D. The system’s two high-frequency wireless transponders were positioned at either end of the shipwreck by a remotely operated vehicle. The transponders, which operate on the same principal as the Global Positioning System, guided the remotely operated vehicle in precise, parallel lines one meter apart over the ship while also mapping the site with digital and video cameras. About 200 of the still photographs these cameras snapped were later put together in a mosaic that showed the entire exposed surface of the wreck. To compensate for distortions in the photomosaic, the vehicle also produced an acoustic map. Together, the two mapping methods gave scientists a complete record of the site and a database that documented the location and size of each artifact.
This summer the next generation of the navigation system will be tested on expeditions to the Black Sea and to the wreck of the USS Monitor off the North Carolina coast. The new system uses digital signal processing akin to cell phone technology to produce stronger, more accurate signals and to improve the resolution of the maps made of the site.
In 1999 the DeepArch Lab tested its second engineering contribution, the subbottom profiler, on a shipwreck off the coast of Israel. The profiler uses ultrasound to “see” into the mud around the wreck. Subbottom profilers were not new to ocean exploration, but until Mindell came along, their beams were too broad for the precision work of archaeology. Mindell built a profiler that uses higher acoustic frequencies to create narrower beams and used it first to map the Tanit, an eighth-century b.c.e. Phoenician trading ship-the oldest deep-water shipwreck discovered to date-that rests 300 meters below the surface. Guided by Exact, the profiler produced a series of cross-sectional images of the Tanit, revealing artifacts buried in the mud. Eventually, as the navigation system becomes more precise and the distance shrinks between the computer-controlled track lines used for mapping, the profiler will be able to create 3-D images that allow “virtual excavation” of a site.
A photomosaic (top) of Skerki D shows the artifacts in place, and the colors in an acoustic map (bottom) indicate changes in depth across the site. (Images courtesy of woods hole oceanographic institution)