Underwater archaeology - in 3D!
At an ancient shipwreck site computer experts and archaeologists from France, Italy and Portugal have been working together under the banner of the VENUS research project. They are developing a technique to create 3D reconstructions of shipwrecks at sites that have in the past offered little or no accessibility to archaeologists.
The team have been trying out their new system at a site in the cove of Sormiou, near Marseille in southern France. Here lies the wreck of a “corbita”, a Roman cargo ship that is estimated by archaeologists to have sunk 2100 years ago. It is believed to have carried around 3000 amphorae (two-handled ceramic vases with narrow necks) containing Tuscan wine.
The study of this site will help establish trading patterns in the Mediterranean during Roman times. A better understanding of the history of trading in the area will shed light on the cultural exchanges within the Roman Empire.
However, at a depth of 105 metres, the wreck could not be properly surveyed since its discovery ten years ago. Divers carrying out underwater archaeology are limited to a depth of 60 metres out of safety reasons. Up until now analysis of the wreck could only be done with standard 2D photos and videos.
The team of the VENUS project want to achieve a digital model – accurate enough to measure distance between objects, calculate volumes or identify thumb-marks from the maker of an amphora.
15 minutes is required for their remotely operated submarine to reach the wreck from the surface. After the various technical difficulties during the first days of the expedition, the submarine could begin to systematically photograph each area from at least three different angles. The different angles are required for the 3D reconstruction, as well as the positional data of the submarine for each photo, including latitude, longitude, depth below the surface, and the height above the wreckage. This is a task made more difficult by the fact that the submarine is too deep to receive signals from Global Positioning Satellites.
The process is given extra accuracy through the use of sonar. Since light travels so poorly in water, a photograph can not be taken from more than 3 metres away. However, the measurements of rebounding sound waves can provide very accurate dimensional data.
After 3 hours of underwater photography, the computer scientists can use their specially designed software to sort the photos according to their position, after which the sonar data is integrated into the image. The transfer of data and the following calculations require about 2 hours. The result is a 3-dimensional photomosaic; proving the success of the technique.
A key advantage of this technique is its implementation in real-time. This means that, if further photos are required, they can be taken on the same day, rather than having to first wait for results before returning to the site one or two days later. This will save both time and money. The archaeologists can also observe the results and make decisions on how the submarine should continue to work, concentrating on a specific are of the shipwreck, for example.
The next challenge for the computer scientists will be creating better graphics, bringing their virtual underwater environment another step closer to the real thing.