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Marine core and dredge sampling, collecting of samples of sea-floor sediments (infl cluding rocks) by means of special coring and dredging devices, normally attached to steel cables and lowered from ships. Coring tools are long metal cylinders, resembling pipes. One type of corer is the gravity coring tool; it is weighted and allowed to fall free and penetrate muds by force. Another type of coring device is the piston coring tool, which uses a piston inside the metal cylinder, normally immobilized, to help create a suction and thus allow soft sediment to enter the tube. A third type of coring device is the rotary coring system regularly used to drill oil wells on land but occasionally mounted on large ships for coring deep into the ocean floor . Both gravity and piston coring tools are limited by the type of sediment they can readily penetrate (usually muds) and the depth of possible penetration. Gravity coring tools seldom penetrate more than 3 to 5 metres (10 to 15 feet) under ideal conditions, and piston coring tools, although usually superior, seldom exceed 12 metres (40 feet), Under ideal conditions, some piston cores of greater length (up to 24 metres) have been recovered. The purpose of employing coring devices to sample the sea floor is to recover samples of the several layers of mud, ooze, and similar fine-grained deposits in such a way as to preserve the depositional sequence. By studying the contained mineral grains, mucrofossils, and interstitial water (water in the pore spaces), interpretation of the depositional history and past oceanic events is possible.
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Dredging devices are employed to obtain samples of sediments, biological remains, and rocks at the surface of the sea floor. Such devices seldom penetrate more than a few centimetres. Dredges are categorized according to their mode of operation. The simplest types, the true dredges, are either chain-link baskets or metal-walled, bucket-shaped devices, which are towed across the sea floor and thus collect a composite sample of the bottom. Samples at a specific site on the sea floor are obtained with devices fitted with enclosing scoops or half buckets, which come together upon reaching the bottom and hold or grab the sediment until the tool is pulled on board the ship. Since World War II, some grab-type devices have been used that employ spring-wound closing chambers and that actuate automatically upon reaching the sea floor. Although costly, small submarines with external manipulators have been used to sample the sea floor selectively, usually obtaining exposed rocks. In commercial development on the sea floor of sand and mineral deposits, which are usually close to land and in shallow water, special vibratory samplers are used, which involve forcing a metal tube into the bottom by rapid, mechanical vibration, not unlike the principle of pile driving.
