|message Board||EDUCATION RESOURCES||SUNKEN CITIES||RECENT DISCOVERIES||SHIP WRECKS|
|CONSERVATION METHODS||LOCATING THE WRECK||EXCAVATION TECHNIQUES||SHIPWRECK DATA BASE||About This Site|
Locating The Shipwreck
Nautical archaeologist use both old fashioned and modern methods to locate wrecks. Many times information provided by local sponge divers is more accurate then towed magnetometers and high-tech satellite imaging.
Nautical Archaeologist realize that the most effective way to recognize and "see" objects underwater is with sound. Underwater, light is rapidly diffused and very unproductive. Even the brightest lights give only a couple feet of visibility.
Side scan sonar (Sound Navigation and Ranging) is a precisely engineered sonar designed to look through the water sideways, from both sides of a towed unit, to scan the bottom and the water above the bottom and produce an image of items of interest. This specialized sonar system for searching and identifying things on the sea-bed. Side Scan Sonar throws out sound energy and studies the return signal that springs back off the seafloor or other large submerged items. The force of the return echo is constantly recorded generating a
"of the sea floor where artifacts that stick out from the bottom create a dark image and shadows from these objects are light areas . While the shape of the seafloor and objects on it can be effectively represented, most side scan systems can not supply any real depth information. When a probable shipwreck is sited, a Remote Operated Vehicle is sometimes used to complete a pre-disturbance study of the shipwreck location.
Mapping and Surveying the Underwater shipwreck
Computer generated sea floor map
When nautical archaeologists locate a wreck site they need to establish how big it is, and what it contains. They survey the site and draw a map. They often use a method called triangulation. Triangulation involves drawing grids and taking many measurements from different fixed points around the site. The artifacts they find can later be drawn onto the overall plan of the excavation. Some archaeologist use an approach called photo-mosaic. This method involves taking a series of overlapping photos of the wreck site, putting them altogether and building one big photograph of the site.
Nautical Archaeologist use many tools when they begin to excavate an underwater site. These tools include everything from good old buckets and shovels to high-tech airlifts, water dredges and water jets. The water dredge sucks up debris for later examination and works kind of like an ordinary vacuumed cleaner. The water dredge is used in both deep and shallow water excavations.
The airlift is another important tool used during the excavation process. A open tube descends down to the excavation site. Compressed air is pumped from the surface into the lower end of tube thereby creating a suction. Sand, water, and little objects are later sifted and examined on deck.
According to Jon Adams, Director, Centre for Maritime Archaeology Graduate program coordinator at the University of Southampton "The reason we abandoned the '-it-and-see' technique is because underwater sites often have so much organic material such as wood, leather, textiles, etc. If these go up an airlift they are likely to be damaged or
utterly destroyed. Certainly, we often fit sieves to airlifts and dredges but this is only to catch the small items we inadvertently miss (rather like land excavators sieving their spoil). The other reason we avoid putting objects up the tube is that it is often impossible to know where it came from and as you know, recording is one of the most important obligations an archaeologist has."
Despite all these high-tech gadgets many diver's consider their on their own hands to be their most valuable tool when it comes to locating underwater artifacts.