![]() ![]() ![]() The resulting sounding measurements are then processed either manually, semi-automatically or automatically (in limited circumstances) to produce a map of the area. A computer system processes all the data, correcting for all of the above factors as well as for the angle of each individual beam. Sound speed profiles (speed of sound in water as a function of depth) of the water column correct for refraction or "ray-bending" of the sound waves owing to non-uniform water column characteristics such as temperature, conductivity, and pressure. (Most modern MBES systems use an integrated motion-sensor and position system that measures yaw as well as the other dynamics and position.) A boat-mounted Global Positioning System (GPS) (or other Global Navigation Satellite System (GNSS)) positions the soundings with respect to the surface of the earth. Attitude sensors allow for the correction of the boat's roll and pitch on the ocean surface, and a gyrocompass provides accurate heading information to correct for vessel yaw. The beams update many times per second (typically 0.1–50 Hz depending on water depth), allowing faster boat speed while maintaining 100% coverage of the seafloor. In general, a wide swath, which is depth dependent, allows a boat to map more seafloor in less time than a single-beam echosounder by making fewer passes. ![]() The tightly packed array of narrow individual beams provides very high angular resolution and accuracy. Today, multibeam echosounders (MBES) are typically used, which use hundreds of very narrow adjacent beams (typically 256) arranged in a fan-like swath of typically 90 to 170 degrees across. ![]() Starting in the early 1930s, single-beam sounders were used to make bathymetry maps. Data from the National Centers for Environmental Information's TerrainBase Digital Terrain Model. Present-day Earth bathymetry (and altimetry). The oceans cover an area of 3.618 ×10 8 km 2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332 ×10 9 km 3. The mass of the oceans is approximately 1.35 ×10 18 metric tons, or about 1/4400 of the total mass of the Earth. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. Further out in the open ocean, they include underwater and deep sea features such as ocean rises and seamounts. Marine topographies include coastal and oceanic landforms ranging from coastal estuaries and shorelines to continental shelves and coral reefs. Tidal networks depend on the balance between sedimentary processes and hydrodynamics however, anthropogenic influences can impact the natural system more than any physical driver. The effectiveness of marine habitats is partially defined by these shapes, including the way they interact with and shape ocean currents, and the way sunlight diminishes when these landforms occupy increasing depths. These shapes are obvious along coastlines, but they occur also in significant ways underwater. Seabed topography (ocean topography or marine topography) refers to the shape of the land ( topography) when it interfaces with the ocean. Despite modern computer-based research, the ocean seabed in many locations is less measured than the topography of Mars. Various methods have advantages and disadvantages and the specific method used depends upon the scale of the area under study, financial means, desired measurement accuracy, and additional variables. Bathymetric measurements are conducted with various methods, from depth sounding, sonar and Lidar techniques, to buoys and satellite altimetry. Synonyms include seafloor mapping, seabed mapping, seafloor imaging and seabed imaging. Paleobathymetry is the study of past underwater depths. The global bathymetry is sometimes combined with topography data to yield a global relief model. Bathymetric maps (a more general term where navigational safety is not a concern) may also use a Digital Terrain Model and artificial illumination techniques to illustrate the depths being portrayed. Bathymetric charts (not to be confused with hydrographic charts), are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines (called depth contours or isobaths) and selected depths ( soundings), and typically also provide surface navigational information. The first recorded evidence of water depth measurements are from Ancient Egypt over 3000 years ago. In other words, bathymetry is the underwater equivalent to hypsometry or topography. Bathymetry ( / b ə ˈ θ ɪ m ə t r i/ from Ancient Greek βαθύς ( bathús) 'deep', and μέτρον ( métron) 'measure') is the study of underwater depth of ocean floors ( seabed topography), lake floors, or river floors. ![]()
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