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Bathymetry

February 02, 2023


Bathymetry (/bəˈθɪmətri/; from Ancient Greek βαθύς (bathús) 'deep', and μέτρον (métron) 'measure')[1][2] is the study of underwater depth of ocean floors (seabed topography), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water depth measurements are from Ancient Egypt over 3000 years ago.[3] Bathymetric (or 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. 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. The global bathymetry is sometimes combined with topography data to yield a global relief model. Paleobathymetry is the study of past underwater depths.

Bathymetry of the ocean floor showing the continental shelves and oceanic plateaus (red), the mid-ocean ridges (yellow-green) and the abyssal plains (blue to purple)
Methane venting offshore Virginia
A seafloor map captured by NASA

Synonyms include seafloor mapping, seabed mapping, seafloor imaging and seabed imaging. Bathymetric measurements are conducted with various methods, from depth sounding, sonar and Lidar techniques, to buoys and satellite altimetry. 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. Despite modern computer-based research, the ocean seabed in many locations is less measured than the topography of Mars.[4]

Seabed topography

This section is an excerpt from Seabed § Topography.[edit]

Seabed topography (ocean topography or marine topography) refers to the shape of the land (topography) when it interfaces with the ocean. These shapes are obvious along coastlines, but they occur also in significant ways underwater. 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. Tidal networks depend on the balance between sedimentary processes and hydrodynamics however, anthropogenic influences can impact the natural system more than any physical driver.[5]

Marine topographies include coastal and oceanic landforms ranging from coastal estuaries and shorelines to continental shelves and coral reefs. Further out in the open ocean, they include underwater and deep sea features such as ocean rises and seamounts. The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[6] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains.

The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3,682 m, resulting in an estimated volume of 1.332×109 km3.[7]

Measurement

 
First printed map of oceanic bathymetry, produced with data from USS Dolphin (1853)

Originally, bathymetry involved the measurement of ocean depth through depth sounding. Early techniques used pre-measured heavy rope or cable lowered over a ship's side.[8] This technique measures the depth only a singular point at a time, and is therefore inefficient. It is also subject to movements of the ship and currents moving the line out of true and therefore is not accurate.

The data used to make bathymetric maps today typically comes from an echosounder (sonar) mounted beneath or over the side of a boat, "pinging" a beam of sound downward at the seafloor or from remote sensing LIDAR or LADAR systems.[9] The amount of time it takes for the sound or light to travel through the water, bounce off the seafloor, and return to the sounder informs the equipment of the distance to the seafloor. LIDAR/LADAR surveys are usually conducted by airborne systems.

 
The seafloor topography near the Puerto Rico Trench
 
Present-day Earth bathymetry (and altimetry). Data from the National Centers for Environmental Information's TerrainBase Digital Terrain Model.

Starting in the early 1930s, single-beam sounders were used to make bathymetry maps. 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. The tightly packed array of narrow individual beams provides very high angular resolution and accuracy. 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 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. 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. (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. 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. A computer system processes all the data, correcting for all of the above factors as well as for the angle of each individual beam. The resulting sounding measurements are then processed either manually, semi-automatically or automatically (in limited circumstances) to produce a map of the area. As of 2010 a number of different outputs are generated, including a sub-set of the original measurements that satisfy some conditions (e.g., most representative likely soundings, shallowest in a region, etc.) or integrated Digital Terrain Models (DTM) (e.g., a regular or irregular grid of points connected into a surface). Historically, selection of measurements was more common in hydrographic applications while DTM construction was used for engineering surveys, geology, flow modeling, etc. Since ca. 2003–2005, DTMs have become more accepted in hydrographic practice.

Satellites are also used to measure bathymetry. Satellite radar maps deep-sea topography by detecting the subtle variations in sea level caused by the gravitational pull of undersea mountains, ridges, and other masses. On average, sea level is higher over mountains and ridges than over abyssal plains and trenches.[10]

In the United States the United States Army Corps of Engineers performs or commissions most surveys of navigable inland waterways, while the National Oceanic and Atmospheric Administration (NOAA) performs the same role for ocean waterways. Coastal bathymetry data is available from NOAA's National Geophysical Data Center (NGDC),[11] which is now merged into National Centers for Environmental Information. Bathymetric data is usually referenced to tidal vertical datums.[12] For deep-water bathymetry, this is typically Mean Sea Level (MSL), but most data used for nautical charting is referenced to Mean Lower Low Water (MLLW) in American surveys, and Lowest Astronomical Tide (LAT) in other countries. Many other datums are used in practice, depending on the locality and tidal regime.

Occupations or careers related to bathymetry include the study of oceans and rocks and minerals on the ocean floor, and the study of underwater earthquakes or volcanoes. The taking and analysis of bathymetric measurements is one of the core areas of modern hydrography, and a fundamental component in ensuring the safe transport of goods worldwide.[8]

 
STL 3D model of Earth without liquid water with 20× elevation exaggeration

Satellite imagery

Further information: Satellite imagery

Another form of mapping the seafloor is through the use of satellites. The satellites are equipped with hyper-spectral and multi-spectral sensors which are used to provide constant streams of images of coastal areas providing a more feasible method of visualising the bottom of the seabed.[13]

Hyper-spectral sensors

Main article: Hyperspectral imaging

The data-sets produced by Hyper-Spectral (HS) Sensors tend to range between 100 and 200 spectral bands of approximately 5–10 nm bandwidths. Hyper-Spectral Sensing, or imaging spectroscopy, is a combination of continuous remote imaging and spectroscopy producing a single set of data.[13] Two examples of this kind of sensing are AVIRIS (Airborne visible/infrared imaging spectrometer) and HYPERION.

The application of HS sensors in regards to the imaging of the seafloor is the detection and monitoring of chlorophyll, phytoplankton, salinity, water quality, dissolved organic materials, and suspended sediments. However, this does not provide a great visual interpretation of coastal environments.[13][clarification needed]

Multi-spectral sensors

Main article: Multispectral imaging

The other method of satellite imaging, multi-spectral (MS) imaging, tends to divide the EM spectrum into a small number of bands, unlike its partner Hyper-Spectral Sensors which can capture a much larger number of spectral bands.

MS sensing is used more in the mapping of the seabed due to its fewer spectral bands with relatively larger bandwidths. The larger bandwidths allow for a larger spectral coverage, which is crucial in the visual detection of marine features and general spectral resolution of the images acquired.[13][clarification needed]

Airborne laser bathymetry

Main article: Airborne lidar bathymetry

High-density airborne laser bathymetry (ALB) is a modern, highly technical, approach to the mapping the seafloor. First developed in the 1960s and 1970s,[citation needed] ALB is a "light detection and ranging (LiDAR) technique that uses visible, ultraviolet, and near infrared light to optically remote sense a contour target through both an active and passive system." What this means is that airborne laser bathymetry also uses light outwith the visible spectrum to detect the curves in underwater landscape.[13]

LiDAR (Light Detection and Ranging) is, according to the National Oceanic and Atmospheric Administration, "a remote sensing method that uses light in the form of a pulsed laser to measure distances".[14] These light pulses, along with other data, generate a three-dimensional representation of whatever the light pulses reflect off, giving an accurate representation of the surface characteristics. A LiDAR system usually consists of a laser, scanner, and GPS receiver. Airplanes and helicopters are the most commonly used platforms for acquiring LIDAR data over broad areas. One application of LiDAR is bathymetric LiDAR, which uses water-penetrating green light to also measure seafloor and riverbed elevations.[14]

ALB generally operates in the form of a pulse of non-visible light being emitted from a low-flying aircraft and a receiver recording two reflections from the water. The first of which originates from the surface of the water, and the second from the seabed. This method has been used in a number of studies to map segments of the seafloor of various coastal areas.[15][16][17]

Examples of commercial LIDAR bathymetry systems

There are various LIDAR bathymetry systems that are commercially accessible. Two of these systems are the Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) and the Laser Airborne Depth Sounder (LADS). SHOALS was first developed to help the United States Army Corps of Engineers (USACE) in bathymetric surveying by a company called Optech in the 1990s. SHOALS is done through the transmission of a laser, of wavelength between 530 and 532 nm, from a height of approximately 200 m at speed of 60 m/s on average.[18]

High resolution orthoimagery

Further information: Orthophoto

High resolution orthoimagery (HRO) is the process of creating an image that combines the geometric qualities with the characteristics of photographs. The result of this process is an orthoimage, a scale image which includes corrections made for feature displacement such as building tilt. These corrections are made through the use of a mathematical equation, information on sensor calibration, and the application of digital elevation models.[19]

An orthoimage can be created through the combination of a number of photos of the same target. The target is photographed from a number of different angles to allow for the perception of the true elevation and tilting of the object. This gives the viewer an accurate perception of the target area.[19]

High resolution orthoimagery is currently being used in the 'terrestrial mapping program', the aim of which is to 'produce high resolution topography data from Oregon to Mexico'. The orthoimagery will be used to provide the photographic data for these regions.[20]

History

 
A three-dimensional echo sounding map

The earliest known depth measurements were made about 1800 BCE by Egyptians by probing with a pole. Later a weighted line was used, with depths marked off at intervals. This process was known as sounding. Both these methods were limited by being spot depths, taken at a point, and could easily miss significant variations in the immediate vicinity. Accuracy was also affected by water movement–current could swing the weight from the vertical and both depth and position would be affected. This was a laborious and time-consuming process and was strongly affected by weather and sea conditions.[21]

There were significant improvements with the voyage of HMS Challenger in the 1870s, when similar systems using wires and a winch were used for measuring much greater depths than previously possible, but this remained a one depth at a time procedure which required very low speed for accuracy.[22] Greater depths could be measured using weighted wires deployed and recovered by powered winches. The wires had less drag and were less affected by current, did not stretch as much, and were strong enough to support their own weight to considerable depths. The winches allowed faster deployment and recovery, necessary when the depths measured were of several kilometers. Wire drag surveys continued to be used until the 1990s due to reliability and accuracy. This procedure involved towing a cable by two boats, supported by floats and weighted to keep a constant depth The wire would snag on obstacles shallower than the cable depth. This was very useful for finding navigational hazards which could be missed by soundings, but was limited to relatively shallow depths.[21]

Single-beam echo sounders were used from the 1920s-1930s to measure the distance of the seafloor directly below a vessel at relatively close intervals along the line of travel. By running roughly parallel lines, data points could be collected at better resolution, but this method still left gaps between the data points, particularly between the lines.[21] The mapping of the sea floor started by using sound waves, contoured into isobaths and early bathymetric charts of shelf topography. These provided the first insight into seafloor morphology, though mistakes were made due to horizontal positional accuracy and imprecise depths. Sidescan sonar was developed in the 1950s to 1970s and could be used to create an image of the bottom, but the technology lacked the capacity for direct depth measurement across the width of the scan. In 1957, Marie Tharp, working with Bruce Charles Heezen, created the first three-dimensional physiographic map of the world's ocean basins. Tharp's discovery was made at the perfect time. It was one of many discoveries that took place near the same time as the invention of the computer. Computers, with their ability to compute large quantities of data, have made research much easier, include the research of the world's oceans. The development of multibeam systems made it possible to obtain depth information across the width of the sonar swath, to higher resolutions, and with precise position and attitude data for the transducers, made it possible to get multiple high resolution soundings from a single pass.[21]

The US Naval Oceanographic Office developed a classified version of multibeam technology in the 1960s. NOAA obtained an unclassified commercial version in the late 1970s and established protocols and standards. Data acquired with multibeam sonar have vastly increased understanding of the seafloor.[21]

The U.S. Landsat satellites of the 1970s and later the European Sentinel satellites, have provided new ways to find bathymetric information, which can be derived from satellite images. These methods include making use of the different depths to which different frequencies of light penetrate the water. When water is clear and the seafloor is sufficiently reflective, depth can be estimated by measuring the amount of reflectance observed by a satellite and then modeling how far the light should penetrate in the known conditions. The Advanced Topographic Laser Altimeter System (ATLAS) on NASA's Ice, Cloud, and land Elevation Satellite 2 (ICESat-2) is a photon-counting lidar that uses the return time of laser light pulses from the Earth's surface to calculate altitude of the surface. ICESat-2 measurements can be combined with ship-based sonar data to fill in gaps and improve precision of maps of shallow water.[23]

Mapping of continental shelf seafloor topography using remotely sensed data has applied a variety of methods to visualise the bottom topography. Early methods included hachure maps, and were generally based on the cartographer's personal interpretation of limited available data. Acoustic mapping methods developed from military sonar images produced a more vivid picture of the seafloor. Further development of sonar based technology have allowed more detail and greater resolution, and ground penetrating techniques provide information on what lies below the bottom surface. Airborne and satellite data acquisition have made further advances possible in visualisation of underwater surfaces: high-resolution aerial photography and orthoimagery is a powerful tool for mapping shallow clear waters on continental shelves, and airborne laser bathymetry, using reflected light pulses, is also very effective in those conditions, and hyperspectral and multispectral satellite sensors can provide a nearly constant stream of benthic environmental information. Remote sensing techniques have been used to develop new ways of visualizing dynamic benthic environments from general geomorphological features to biological coverage.[24]

See also

References

  1. ^ βαθύς, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  2. ^ μέτρον, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
  3. ^ Wölfl, A.C.; Snaith, H.; Amirebrahimi, S.; et al. (2019). "Seafloor Mapping – The Challenge of a Truly Global Ocean Bathymetry". Frontiers in Marine Science. 6: 283. doi:10.3389/fmars.2019.00283.
  4. ^ Jones, E.J.W. (1999). Marine geophhysics. New York: Wiley.
  5. ^ Giovanni Coco, Z. Zhou, B. van Maanen, M. Olabarrieta, R. Tinoco, I. Townend. Morphodynamics of tidal networks: Advances and challenges. Marine Geology Journal. 1 December 2013.
  6. ^ Sandwell, D. T.; Smith, W. H. F. (2006-07-07). "Exploring the Ocean Basins with Satellite Altimeter Data". NOAA/NGDC. Retrieved 2007-04-21.
  7. ^ Charette, Matthew A.; Smith, Walter H. F. (June 2010). "The Volume of Earth's Ocean". Oceanography. 23 (2): 112–114. doi:10.5670/oceanog.2010.51.
  8. ^ a b Audrey, Furlong (November 7, 2018). "NGA Explains: What is hydrography?". National Geospatial-Intelligence Agency via YouTube.
  9. ^ Olsen, R. C. (2007), Remote Sensing from Air and Space (PDF), SPIE, ISBN 978-0-8194-6235-0
  10. ^ Thurman, H. V. (1997), Introductory Oceanography, New Jersey, USA: Prentice Hall College, ISBN 0-13-262072-3
  11. ^ "Bathymetry and Global Relief". www.ngdc.noaa.gov. NOAA National Centers for Environmental Information. Retrieved 8 July 2022.
  12. ^ "Coastal Elevation Models". www.ngdc.noaa.gov. NOAA National Centers for Environmental Information. 15 September 2020. Retrieved 8 July 2022.
  13. ^ a b c d e Charles W. Finkl, ed., 2016, Seafloor Mapping Along Continental Shelves: Research and Techniques for Visualizing Benthic Environments. Internet resource edition. Volume 13. pp. 31–35
  14. ^ a b National Oceanic and Atmospheric Administration (NOAA) (15 April 2020). "What is LIDAR?". National Ocean Service. Retrieved 21 June 2020.
  15. ^ Brock & Purkis (2009). "The emerging role of Lidar remote sensing in coastal research and resource management". In: Brock J, Purkis S (eds.). "Coastal applications of airborne lidar". Journal of Coastal Research, Special Issue No. 53: pp. 1–5
  16. ^ Bukata et al. (1995) Optical properties and remote sensing of inland and coastal waters. CRC Press, Boca Raton, p. 365
  17. ^ Deronde et al. (2008). "Monitoring of the sediment dynamics along a sandy shoreline by means of airborne hyper-spectral remote sensing and LIDAR: a case study in Belgium". Earth Surface Processes 33: pp. 280–294
  18. ^ Charles W. Finkl, ed., 2016, Seafloor Mapping Along Continental Shelves: Research and Techniques for Visualizing Benthic Environments. Internet resource edition. Volume 13. p. 23
  19. ^ a b USGS, Date Last Edited 2015, High Resolution Orthoimagery (HRO), https://lta.cr.usgs.gov/high_res_ortho
  20. ^ State of California Ocean Protection Council, 2009, Mapping California's Resources,http://www.opc.ca.gov/2009/12/mapping/
  21. ^ a b c d e "Underwater Frontiers: A Brief History of Seafloor Mapping". www.arcgis.com. NCEI: National Centers for Environmental Information. Retrieved 8 July 2022.
  22. ^ Wölfl, Anne-Cathrin; Snaith, Helen; Amirebrahimi, Sam; Devey, Colin W.; Dorschel, Boris; Ferrini, Vicki; Huvenne, Veerle A. I.; Jakobsson, Martin; Jencks, Jennifer; Johnston, Gordon; Lamarche, Geoffroy; Mayer, Larry; Millar, David; Pedersen, Terje Haga; Picard, Kim; Reitz, Anja; Schmitt, Thierry; Visbeck, Martin; Weatherall, Pauline; Wigley, Rochelle (2019). "Seafloor Mapping – The Challenge of a Truly Global Ocean Bathymetry". Frontiers in Marine Science. 6: 283. doi:10.3389/fmars.2019.00283. ISSN 2296-7745.
  23. ^ Carlowicz, Michael (2020). "Sounding the Seafloor with Light". earthobservatory.nasa.gov. NASA. Retrieved 8 July 2022.
  24. ^ Finkl, Charles W.; Makowski, Christopher, eds. (2016). Seafloor Mapping Along Continental Shelves: Research and Techniques for Visualizing Benthic Environments. Coastal Research Library. Vol. 13 (Internet resource ed.). pp. 3–53. doi:10.1007/978-3-319-25121-9. ISBN 978-3-319-25121-9. ISSN 2211-0585. S2CID 132980629.

External links

 
Wikimedia Commons has media related to Bathymetry.
 
Scholia has a topic profile for Bathymetry.
  • Overview for underwater terrain, data formats, etc. (vterrain.org)
  • High resolution bathymetry for the Great Barrier Reef and Coral Sea
  • Bathymetric Data Viewer from NOAA's NCEI
  • WebMapping Application for searching free and open source Bathymetry datasets
  • Interactive Web Map, Set Negative Elevation for Bathymetry
  • NOAA Ocean Explorer
  • Schmidt Ocean Institute: Seafloor Mapping
  • Seafloormapping.co.uk
  • Seabed 2030 2019-06-16 at the Wayback Machine

February 02, 2023
bathymetry, from, ancient, greek, βαθύς, bathús, deep, μέτρον, métron, measure, study, underwater, depth, ocean, floors, seabed, topography, lake, floors, river, floors, other, words, bathymetry, underwater, equivalent, hypsometry, topography, first, recorded,. Bathymetry b e ˈ 8 ɪ m e t r i from Ancient Greek ba8ys bathus deep and metron metron measure 1 2 is the study of underwater depth of ocean floors seabed topography lake floors or river floors In other words bathymetry is the underwater equivalent to hypsometry or topography The first recorded evidence of water depth measurements are from Ancient Egypt over 3000 years ago 3 Bathymetric or 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 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 The global bathymetry is sometimes combined with topography data to yield a global relief model Paleobathymetry is the study of past underwater depths Bathymetry of the ocean floor showing the continental shelves and oceanic plateaus red the mid ocean ridges yellow green and the abyssal plains blue to purple Methane venting offshore Virginia A seafloor map captured by NASA Synonyms include seafloor mapping seabed mapping seafloor imaging and seabed imaging Bathymetric measurements are conducted with various methods from depth sounding sonar and Lidar techniques to buoys and satellite altimetry 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 Despite modern computer based research the ocean seabed in many locations is less measured than the topography of Mars 4 Contents 1 Seabed topography 2 Measurement 2 1 Satellite imagery 2 1 1 Hyper spectral sensors 2 1 2 Multi spectral sensors 2 2 Airborne laser bathymetry 2 2 1 Examples of commercial LIDAR bathymetry systems 2 3 High resolution orthoimagery 3 History 4 See also 5 References 6 External linksSeabed topography EditThis section is an excerpt from Seabed Topography edit Seabed topography ocean topography or marine topography refers to the shape of the land topography when it interfaces with the ocean These shapes are obvious along coastlines but they occur also in significant ways underwater 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 Tidal networks depend on the balance between sedimentary processes and hydrodynamics however anthropogenic influences can impact the natural system more than any physical driver 5 Marine topographies include coastal and oceanic landforms ranging from coastal estuaries and shorelines to continental shelves and coral reefs Further out in the open ocean they include underwater and deep sea features such as ocean rises and seamounts The submerged surface has mountainous features including a globe spanning mid ocean ridge system as well as undersea volcanoes 6 oceanic trenches submarine canyons oceanic plateaus and abyssal plains The mass of the oceans is approximately 1 35 1018 metric tons or about 1 4400 of the total mass of the Earth The oceans cover an area of 3 618 108 km2 with a mean depth of 3 682 m resulting in an estimated volume of 1 332 109 km3 7 Measurement Edit First printed map of oceanic bathymetry produced with data from USS Dolphin 1853 Originally bathymetry involved the measurement of ocean depth through depth sounding Early techniques used pre measured heavy rope or cable lowered over a ship s side 8 This technique measures the depth only a singular point at a time and is therefore inefficient It is also subject to movements of the ship and currents moving the line out of true and therefore is not accurate The data used to make bathymetric maps today typically comes from an echosounder sonar mounted beneath or over the side of a boat pinging a beam of sound downward at the seafloor or from remote sensing LIDAR or LADAR systems 9 The amount of time it takes for the sound or light to travel through the water bounce off the seafloor and return to the sounder informs the equipment of the distance to the seafloor LIDAR LADAR surveys are usually conducted by airborne systems The seafloor topography near the Puerto Rico Trench Present day Earth bathymetry and altimetry Data from the National Centers for Environmental Information s TerrainBase Digital Terrain Model Starting in the early 1930s single beam sounders were used to make bathymetry maps 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 The tightly packed array of narrow individual beams provides very high angular resolution and accuracy 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 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 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 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 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 A computer system processes all the data correcting for all of the above factors as well as for the angle of each individual beam The resulting sounding measurements are then processed either manually semi automatically or automatically in limited circumstances to produce a map of the area As of 2010 update a number of different outputs are generated including a sub set of the original measurements that satisfy some conditions e g most representative likely soundings shallowest in a region etc or integrated Digital Terrain Models DTM e g a regular or irregular grid of points connected into a surface Historically selection of measurements was more common in hydrographic applications while DTM construction was used for engineering surveys geology flow modeling etc Since ca 2003 2005 DTMs have become more accepted in hydrographic practice Satellites are also used to measure bathymetry Satellite radar maps deep sea topography by detecting the subtle variations in sea level caused by the gravitational pull of undersea mountains ridges and other masses On average sea level is higher over mountains and ridges than over abyssal plains and trenches 10 In the United States the United States Army Corps of Engineers performs or commissions most surveys of navigable inland waterways while the National Oceanic and Atmospheric Administration NOAA performs the same role for ocean waterways Coastal bathymetry data is available from NOAA s National Geophysical Data Center NGDC 11 which is now merged into National Centers for Environmental Information Bathymetric data is usually referenced to tidal vertical datums 12 For deep water bathymetry this is typically Mean Sea Level MSL but most data used for nautical charting is referenced to Mean Lower Low Water MLLW in American surveys and Lowest Astronomical Tide LAT in other countries Many other datums are used in practice depending on the locality and tidal regime Occupations or careers related to bathymetry include the study of oceans and rocks and minerals on the ocean floor and the study of underwater earthquakes or volcanoes The taking and analysis of bathymetric measurements is one of the core areas of modern hydrography and a fundamental component in ensuring the safe transport of goods worldwide 8 STL 3D model of Earth without liquid water with 20 elevation exaggeration Satellite imagery Edit Further information Satellite imagery Another form of mapping the seafloor is through the use of satellites The satellites are equipped with hyper spectral and multi spectral sensors which are used to provide constant streams of images of coastal areas providing a more feasible method of visualising the bottom of the seabed 13 Hyper spectral sensors Edit Main article Hyperspectral imaging The data sets produced by Hyper Spectral HS Sensors tend to range between 100 and 200 spectral bands of approximately 5 10 nm bandwidths Hyper Spectral Sensing or imaging spectroscopy is a combination of continuous remote imaging and spectroscopy producing a single set of data 13 Two examples of this kind of sensing are AVIRIS Airborne visible infrared imaging spectrometer and HYPERION The application of HS sensors in regards to the imaging of the seafloor is the detection and monitoring of chlorophyll phytoplankton salinity water quality dissolved organic materials and suspended sediments However this does not provide a great visual interpretation of coastal environments 13 clarification needed Multi spectral sensors Edit Main article Multispectral imaging The other method of satellite imaging multi spectral MS imaging tends to divide the EM spectrum into a small number of bands unlike its partner Hyper Spectral Sensors which can capture a much larger number of spectral bands MS sensing is used more in the mapping of the seabed due to its fewer spectral bands with relatively larger bandwidths The larger bandwidths allow for a larger spectral coverage which is crucial in the visual detection of marine features and general spectral resolution of the images acquired 13 clarification needed Airborne laser bathymetry Edit Main article Airborne lidar bathymetry High density airborne laser bathymetry ALB is a modern highly technical approach to the mapping the seafloor First developed in the 1960s and 1970s citation needed ALB is a light detection and ranging LiDAR technique that uses visible ultraviolet and near infrared light to optically remote sense a contour target through both an active and passive system What this means is that airborne laser bathymetry also uses light outwith the visible spectrum to detect the curves in underwater landscape 13 LiDAR Light Detection and Ranging is according to the National Oceanic and Atmospheric Administration a remote sensing method that uses light in the form of a pulsed laser to measure distances 14 These light pulses along with other data generate a three dimensional representation of whatever the light pulses reflect off giving an accurate representation of the surface characteristics A LiDAR system usually consists of a laser scanner and GPS receiver Airplanes and helicopters are the most commonly used platforms for acquiring LIDAR data over broad areas One application of LiDAR is bathymetric LiDAR which uses water penetrating green light to also measure seafloor and riverbed elevations 14 ALB generally operates in the form of a pulse of non visible light being emitted from a low flying aircraft and a receiver recording two reflections from the water The first of which originates from the surface of the water and the second from the seabed This method has been used in a number of studies to map segments of the seafloor of various coastal areas 15 16 17 Examples of commercial LIDAR bathymetry systems Edit There are various LIDAR bathymetry systems that are commercially accessible Two of these systems are the Scanning Hydrographic Operational Airborne Lidar Survey SHOALS and the Laser Airborne Depth Sounder LADS SHOALS was first developed to help the United States Army Corps of Engineers USACE in bathymetric surveying by a company called Optech in the 1990s SHOALS is done through the transmission of a laser of wavelength between 530 and 532 nm from a height of approximately 200 m at speed of 60 m s on average 18 High resolution orthoimagery Edit Further information Orthophoto High resolution orthoimagery HRO is the process of creating an image that combines the geometric qualities with the characteristics of photographs The result of this process is an orthoimage a scale image which includes corrections made for feature displacement such as building tilt These corrections are made through the use of a mathematical equation information on sensor calibration and the application of digital elevation models 19 An orthoimage can be created through the combination of a number of photos of the same target The target is photographed from a number of different angles to allow for the perception of the true elevation and tilting of the object This gives the viewer an accurate perception of the target area 19 High resolution orthoimagery is currently being used in the terrestrial mapping program the aim of which is to produce high resolution topography data from Oregon to Mexico The orthoimagery will be used to provide the photographic data for these regions 20 History Edit A three dimensional echo sounding map The earliest known depth measurements were made about 1800 BCE by Egyptians by probing with a pole Later a weighted line was used with depths marked off at intervals This process was known as sounding Both these methods were limited by being spot depths taken at a point and could easily miss significant variations in the immediate vicinity Accuracy was also affected by water movement current could swing the weight from the vertical and both depth and position would be affected This was a laborious and time consuming process and was strongly affected by weather and sea conditions 21 There were significant improvements with the voyage of HMS Challenger in the 1870s when similar systems using wires and a winch were used for measuring much greater depths than previously possible but this remained a one depth at a time procedure which required very low speed for accuracy 22 Greater depths could be measured using weighted wires deployed and recovered by powered winches The wires had less drag and were less affected by current did not stretch as much and were strong enough to support their own weight to considerable depths The winches allowed faster deployment and recovery necessary when the depths measured were of several kilometers Wire drag surveys continued to be used until the 1990s due to reliability and accuracy This procedure involved towing a cable by two boats supported by floats and weighted to keep a constant depth The wire would snag on obstacles shallower than the cable depth This was very useful for finding navigational hazards which could be missed by soundings but was limited to relatively shallow depths 21 Single beam echo sounders were used from the 1920s 1930s to measure the distance of the seafloor directly below a vessel at relatively close intervals along the line of travel By running roughly parallel lines data points could be collected at better resolution but this method still left gaps between the data points particularly between the lines 21 The mapping of the sea floor started by using sound waves contoured into isobaths and early bathymetric charts of shelf topography These provided the first insight into seafloor morphology though mistakes were made due to horizontal positional accuracy and imprecise depths Sidescan sonar was developed in the 1950s to 1970s and could be used to create an image of the bottom but the technology lacked the capacity for direct depth measurement across the width of the scan In 1957 Marie Tharp working with Bruce Charles Heezen created the first three dimensional physiographic map of the world s ocean basins Tharp s discovery was made at the perfect time It was one of many discoveries that took place near the same time as the invention of the computer Computers with their ability to compute large quantities of data have made research much easier include the research of the world s oceans The development of multibeam systems made it possible to obtain depth information across the width of the sonar swath to higher resolutions and with precise position and attitude data for the transducers made it possible to get multiple high resolution soundings from a single pass 21 The US Naval Oceanographic Office developed a classified version of multibeam technology in the 1960s NOAA obtained an unclassified commercial version in the late 1970s and established protocols and standards Data acquired with multibeam sonar have vastly increased understanding of the seafloor 21 The U S Landsat satellites of the 1970s and later the European Sentinel satellites have provided new ways to find bathymetric information which can be derived from satellite images These methods include making use of the different depths to which different frequencies of light penetrate the water When water is clear and the seafloor is sufficiently reflective depth can be estimated by measuring the amount of reflectance observed by a satellite and then modeling how far the light should penetrate in the known conditions The Advanced Topographic Laser Altimeter System ATLAS on NASA s Ice Cloud and land Elevation Satellite 2 ICESat 2 is a photon counting lidar that uses the return time of laser light pulses from the Earth s surface to calculate altitude of the surface ICESat 2 measurements can be combined with ship based sonar data to fill in gaps and improve precision of maps of shallow water 23 Mapping of continental shelf seafloor topography using remotely sensed data has applied a variety of methods to visualise the bottom topography Early methods included hachure maps and were generally based on the cartographer s personal interpretation of limited available data Acoustic mapping methods developed from military sonar images produced a more vivid picture of the seafloor Further development of sonar based technology have allowed more detail and greater resolution and ground penetrating techniques provide information on what lies below the bottom surface Airborne and satellite data acquisition have made further advances possible in visualisation of underwater surfaces high resolution aerial photography and orthoimagery is a powerful tool for mapping shallow clear waters on continental shelves and airborne laser bathymetry using reflected light pulses is also very effective in those conditions and hyperspectral and multispectral satellite sensors can provide a nearly constant stream of benthic environmental information Remote sensing techniques have been used to develop new ways of visualizing dynamic benthic environments from general geomorphological features to biological coverage 24 See also Edit Oceans portalAcoustic seabed classification partitioning of a seabed acoustic image into discrete physical entities or classes Bathometer a scientific instrument for measuring water depth Bathymetric chart map depicting the submerged terrain of bodies of water Coastal morphodynamics interaction of shoreline seafloor topography and fluid hydrodynamic processes Depth gauge instrument that indicates depth below a reference surface General Bathymetric Chart of the Oceans publicly available bathymetric chart of the world s oceans Global relief model model of Earth s relief including elevation and depth underwater Hydrographic survey science of measurement and description of features which affect maritime activities Ocean surface topography shape of the ocean surface relative to the geoid Seabed the bottom of the ocean Seabed 2030 Project Terrain vertical and horizontal dimension and shape of land surfaceReferences Edit ba8ys Henry George Liddell Robert Scott A Greek English Lexicon on Perseus metron Henry George Liddell Robert Scott A Greek English Lexicon on Perseus Wolfl A C Snaith H Amirebrahimi S et al 2019 Seafloor Mapping The Challenge of a Truly Global Ocean Bathymetry Frontiers in Marine Science 6 283 doi 10 3389 fmars 2019 00283 Jones E J W 1999 Marine geophhysics New York Wiley Giovanni Coco Z Zhou B van Maanen M Olabarrieta R Tinoco I Townend Morphodynamics of tidal networks Advances and challenges Marine Geology Journal 1 December 2013 Sandwell D T Smith W H F 2006 07 07 Exploring the Ocean Basins with Satellite Altimeter Data NOAA NGDC Retrieved 2007 04 21 Charette Matthew A Smith Walter H F June 2010 The Volume of Earth s Ocean Oceanography 23 2 112 114 doi 10 5670 oceanog 2010 51 a b Audrey Furlong November 7 2018 NGA Explains What is hydrography National Geospatial Intelligence Agency via YouTube Olsen R C 2007 Remote Sensing from Air and Space PDF SPIE ISBN 978 0 8194 6235 0 Thurman H V 1997 Introductory Oceanography New Jersey USA Prentice Hall College ISBN 0 13 262072 3 Bathymetry and Global Relief www ngdc noaa gov NOAA National Centers for Environmental Information Retrieved 8 July 2022 Coastal Elevation Models www ngdc noaa gov NOAA National Centers for Environmental Information 15 September 2020 Retrieved 8 July 2022 a b c d e Charles W Finkl ed 2016 Seafloor Mapping Along Continental Shelves Research and Techniques for Visualizing Benthic Environments Internet resource edition Volume 13 pp 31 35 a b National Oceanic and Atmospheric Administration NOAA 15 April 2020 What is LIDAR National Ocean Service Retrieved 21 June 2020 Brock amp Purkis 2009 The emerging role of Lidar remote sensing in coastal research and resource management In Brock J Purkis S eds Coastal applications of airborne lidar Journal of Coastal Research Special Issue No 53 pp 1 5 Bukata et al 1995 Optical properties and remote sensing of inland and coastal waters CRC Press Boca Raton p 365 Deronde et al 2008 Monitoring of the sediment dynamics along a sandy shoreline by means of airborne hyper spectral remote sensing and LIDAR a case study in Belgium Earth Surface Processes 33 pp 280 294 Charles W Finkl ed 2016 Seafloor Mapping Along Continental Shelves Research and Techniques for Visualizing Benthic Environments Internet resource edition Volume 13 p 23 a b USGS Date Last Edited 2015 High Resolution Orthoimagery HRO https lta cr usgs gov high res ortho State of California Ocean Protection Council 2009 Mapping California s Resources http www opc ca gov 2009 12 mapping a b c d e Underwater Frontiers A Brief History of Seafloor Mapping www arcgis com NCEI National Centers for Environmental Information Retrieved 8 July 2022 Wolfl Anne Cathrin Snaith Helen Amirebrahimi Sam Devey Colin W Dorschel Boris Ferrini Vicki Huvenne Veerle A I Jakobsson Martin Jencks Jennifer Johnston Gordon Lamarche Geoffroy Mayer Larry Millar David Pedersen Terje Haga Picard Kim Reitz Anja Schmitt Thierry Visbeck Martin Weatherall Pauline Wigley Rochelle 2019 Seafloor Mapping The Challenge of a Truly Global Ocean Bathymetry Frontiers in Marine Science 6 283 doi 10 3389 fmars 2019 00283 ISSN 2296 7745 Carlowicz Michael 2020 Sounding the Seafloor with Light earthobservatory nasa gov NASA Retrieved 8 July 2022 Finkl Charles W Makowski Christopher eds 2016 Seafloor Mapping Along Continental Shelves Research and Techniques for Visualizing Benthic Environments Coastal Research Library Vol 13 Internet resource ed pp 3 53 doi 10 1007 978 3 319 25121 9 ISBN 978 3 319 25121 9 ISSN 2211 0585 S2CID 132980629 External links Edit Wikimedia Commons has media related to Bathymetry Scholia has a topic profile for Bathymetry Overview for underwater terrain data formats etc vterrain org High resolution bathymetry for the Great Barrier Reef and Coral Sea A PO MA B Academy of Positioning Marine and Bathymetry Bathymetric Data Viewer from NOAA s NCEI WebMapping Application for searching free and open source Bathymetry datasets Interactive Web Map Set Negative Elevation for Bathymetry NOAA Ocean Explorer Schmidt Ocean Institute Seafloor Mapping Seafloormapping co uk Seabed 2030 Archived 2019 06 16 at the Wayback Machine Retrieved from https en wikipedia org w index php title Bathymetry amp oldid 1136972213, wikipedia, wiki, book, books, library,

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