An underwater survey is a survey performed in an underwater environment or conducted remotely on an underwater object or region. Surveys can have several meanings. The word originates in Medieval Latin with meanings of looking over and detailed study of a subject. One meaning is the accurate measurement of a geographical region, usually to plot the positions of features as a scale map of the region. This meaning is often used in scientific contexts, and also in civil engineering and mineral extraction. Another meaning, often used in a civil, structural, or marine engineering context, is the inspection of a structure or vessel to compare the actual condition with the specified nominal condition, usually to report on the actual condition and compliance with, or deviations from, the nominal condition, for quality control, damage assessment, valuation, insurance, maintenance, and similar purposes. In other contexts it can mean inspection of a region to establish presence and distribution of specified content, such as living organisms, either to establish a baseline, or to compare with a baseline. These types of surveys may be done in or of the underwater environment, in which case they may be referred to as underwater surveys, which may include bathymetric, hydrographic, and geological surveys, archaeological surveys, ecological surveys, and structural or vessel safety surveys. In some cases, they can be done by remote sensing, using a variety of tools, and sometimes by direct human intervention, usually by a professional diver. Underwater surveys are an essential part of the planning, and often of quality control and monitoring, of underwater construction, dredging, mineral extraction, ecological monitoring, and archaeological investigations. They are often required as part of an ecological impact study.
Types The types of underwater survey include, but are not necessarily restricted to, archeological, bathymetric and hydrographic, ecological, geological, and construction site surveys, and inspection surveys of marine and coastal structures and vessels afloat. A survey of the vessel structural condition and the adjacent site and hydrographic conditions would also be done when assessing proposed marine salvage operations.
Archaeological surveys
Archaeological surveys of underwater sites have traditionally been done by divers, but at sites where the depth is too great, sonar surveys have been done from surface and submersible vehicles, and photomosaic techniques have been done using ROUVs. Traditional methods include direct measurement from a baseline or grid set up at the site, and triangulation by direct measurement from marks of known position installed at the site, in the same way these would be used at a terrestrial site. Accuracy may be compromised by water conditions. This work is usually done by archaeologists who are qualified scientific divers.
Bathymetric and hydrographic surveys
Bathymetric surveys are traditionally done from the surface, by measuring depth (soundings) at measured positions along transect lines and later plotting the data onto a bathymetric chart, on which lines of constant depth (isobaths) may be drawn by interpolation of soundings. It is also conventional to provide a representative set of spot depths on the chart. Originally, soundings were made manually by measuring the length of a weighted line lowered to the bottom, bur after the development of accurate and reliable echo-sounding equipment it became the standard method. Data recording was automated when the equipment became available, and later precise position data was integrated into the data sets. Multibeam sonar with GPS position data corrected for vessel motion and combined in real time is the state of the art in the early 21st century. Bathymetric surveys of some bodies of water have required different procedures, particularly for sinkholes, caverns and caves where a significant portion of the bottom walls, and in some cases ceilings, are not visible to the sounding equipment from the surface, and it has been necessary to use remotely operated underwater vehicles or divers to gather the data. One of the complications of this class of underwater survey is the relative difficulty of establishing a baseline, or an accurate position for the ROUV, as GPS signals do not propagate through water. In some cases a physical line has been used, but sometimes a baseline can be established using sonar transducers set up at accurately surveyed positions, and relative offsets measured.
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