Oceanographers also use drilling and coring techniques for sampling the seabed. The gravity corer replaced the sampling device on the Baillie sounding machine with an open and weighted tube that is triggered to release as soon as sediments are encountered. It then drills into the sea floor to up to about 33 ft (10 m). When the corer is extracted and brought on board ship, the core can be extruded, and layers in the sediments are logged by a geologist specializing in ocean sediments. Some specially equipped ocean drilling rigs are able to retrieve core samples from greater depths (as much as 4,900 ft or 1,500 m), and samples from the drilling of test wells for oil and gas extraction and the foundations for offshore oil platforms are also examined by oceanographers and other specialists.
Other oceanographic instruments include flow meters for measuring the velocity of deep-sea currents, seismographs for detecting earthquake activity far from land-based equipment, pressure meters that measure pressure beneath the ocean with depth, and thermometers. These instruments are usually attached to sounding devices because their measurements with respect to the depth to the sea floor are important. Research vessels carry these instruments, but the instruments can also be tethered to buoys and left at sea. The research ships themselves are precise, highly equipped floating laboratories with sophisticated navigation systems including links to global positioning satellite (GPS) systems and positioning systems that use computers in the ship's controls to keep it in a fixed location at sea. A sonar beacon seated on the ocean floor usually provided the point of orientation for the ship's fixed position. A variety of television, video, and still cameras and audio detection equipment is also standard for research vessels.
Satellite technology has greatly advanced the science of oceanography. One of the techniques, known as satellite altimetry, utilizes radar to measure the distance from an orbiting satellite to the ocean surface. While usually considered smooth and spherical, the surface of Earth's oceans actually exhibits a multitude of broad dimples and bulges that reflect the topography of the ocean floor. The uneven surface of the ocean is due to localized gravity effects from mountains and depression at the bottom of the ocean. Although the relief of these prominences is greatly subdued when compared to the ocean floor, their extent is sufficient to be quantified by means of satellite altimetry which has an astounding vertical resolution of 1 in (0.03m). The altimetry data provided by the US Navy's Geosat and European Space Agency's ERS-1 satellites permit the construction of topographic maps of the world's ocean basins. This is particularly important in deep, remote portions of the basins where little depth information is available.
Modern surface mapping techniques of similar resolution would require approximately 125 years and several hundred million dollars to complete.
The technique has a wide variety of applications. Navigation of ships, submarines, and even aircraft are frequently affected by local gravitational variations. The information provided by satellite altimetry allows the variations to be accounted for and the course corrections applied. The topographic information permits the identification of subsurface controls of ocean currents and favorable fishing locations. Geologists utilize the information to investigate various aspects of plate tectonic theory, identify and study subsurface volcanoes, locate potential petroleum reserves, and even measure the structural characteristics of Earth's oceanic crust.
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