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Bathymetry Survey
Hydrographic survey
Hydrographic surveys use sonar to collect highly accurate water depth and bottom contour information. These echo sounding systems are based on the principle that when a sound signal is sent into the water it will be reflected back when it strikes an object. An instrument, called a transducer, sends a sound pulse straight down into the water. The pulse moves down through the water and bounces off the seafloor. The transducer also picks up the reflected sound. The sonar sensor precisely measures the time it takes for the sound pulse to reach the bottom and return. The water depth is calculated by knowing how fast sound travels in the water (approximately 1,500 meters per second). This method of seafloor mapping is called echo sounding.
(i)Single Beam Bathymetry Survey:-
The term “bathymetry” originally referred to the ocean’s depth relative to sea level, although it has come to mean “submarine topography,” or the depths and shapes of underwater terrain.
In the same way that topographic maps represent the three-dimensional features (or relief) of overland terrain, bathymetric maps illustrate the land that lies underwater. Variations in sea-floor relief may be depicted by color and contour lines called depth contours or isobaths.
Bathymetry is the foundation of the science of hydrography, which measures the physical features of a water body. Hydrography includes not only bathymetry, but also the shape and features of the shoreline etc.
Single-beam echo sounders collect discrete data points along survey track lines
(ii) Multibeam bathymetry
Multibeam bathymetry sonar is the relatively recent successor to echosounding. About 30 years ago, the Navy developed a new technology that uses many beams of sound at the same time to cover a large fan-shaped area of the ocean floor rather than just the small patch of seafloor that echosounders cover. These multibeam systems can have more than 100 transducers, arranged in precise geometrical patterns, sending out a swath of sound that covers a distance on either side of the ship that is equal to about two times the water depth. All of the signals that are sent out reach the seafloor and return at slightly different times. These signals are received and converted to water depths by computers, and then automatically plotted as bathymetric maps. The data acquired by multibeam systems are much more complex than single-beam surveys; this means higher resolution is possible, but also that more involved signal processing is necessary in order to interpret the data.
Multibeam systems produce high-resolution bathymetry data throughout the survey area. Since they acquire dense sounding data both along the ship’s track and between the track lines, they can provide 100% coverage of the seafloor. Multibeam bathymetry sonar is used to locate topographical features on the seafloor such as sediment ridges, rock outcrops, shipwrecks, and underwater cables. Ships also use this technology to avoid areas that would endanger their vessels or gear, to find fishing grounds, and to precisely map the seafloor. Objects as small as one meter long can be located with this technology. Multibeam bathymetry sonar is a valuable tool for scientists hoping to learn more about seafloor habitats in the hopes of conserving them.