Techniques For Underwater Surveyance, Retrieval, And Analysis
Before objects can be removed from a shipwreck, the diver must map their precise location in relation both to the other objects and to the surrounding ocean terrain. Because the average diver can move through the water at a rate of only 0.5 mi/h (1 km/h), apparatus have been developed to speed up surveyance. A diver may ride in a hydrodynamic cradle, a flat metal one-diver "bobsled" with a window in front, which can be maneuvered to lower depths. A team of two divers may ride in a towvane, a top-shaped vessel with Plexiglas observation windows and hydrodynamic steering, similar to a diving bell. Or they may descend in a cubmarine, a small electrical battery-powered vessel steered by water jets that can submerge to depths of 164-197 ft (50-60 m).
At great ocean depths or in areas with poor visibility other devices may be employed. Sonar may be used to determine the location of large or encrusted objects by calculating the time it takes echoes to bounce off them. Underwater cameras may be towed by the boat to take pictures of a site in water with good visibility.
The method used to remove the objects will depend on their size and fragility. Ordinary objects may be brought up in plastic or net bags. Heavy objects may be lifted with chains and pulleys or with balloon-like air bags. These bags contain a small amount of compressed air that enables them to expand and rise to the surface as the water pressure decreases.
To determine the age of these objects, archaeologists had to develop new strategies. Although pottery phases and stratigraphy (the analysis of the origin, distribution, and succession of layers or strata of earth) can be used to date objects found on land, neither technique works reliably for objects found in the sea, where water may distort the shape of a jar or may carry away rocks and silt from the scene of the wreck. Instead, archaeologists use anchors (and sometimes ship nails) to determine the age of a shipwreck. Like pottery, anchors were produced in different sizes and shapes and made of different kinds of materials (such as stone or iron) that conform to historical phases. Archaeologists use these features to assign the shipwreck a date.
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