Bathymetry

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Bathymetry of the ocean floor showing the continental shelves (red) and the mid-ocean ridges (yellow-green)

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.

Seabed topography[edit]

Map of underwater topography (1995 NOAA)

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.[4]

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,[5] 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.[6]

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.[7] 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.[8] 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

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.[9]

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),[10] which is now merged into National Centers for Environmental Information. Bathymetric data is usually referenced to tidal vertical datums.[11] 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.[7]

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

History[edit]

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.[12] 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 were started in the early 20th century and 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.[12]

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.[12]

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. 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.[12]

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.[12]

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.[13]

See also[edit]

References[edit]

  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. ^ 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.
  5. ^ Sandwell, D. T.; Smith, W. H. F. (2006-07-07). "Exploring the Ocean Basins with Satellite Altimeter Data". NOAA/NGDC. Retrieved 2007-04-21.
  6. ^ 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.
  7. ^ a b Audrey, Furlong (November 7, 2018). "NGA Explains: What is hydrography?". National Geospatial-Intelligence Agency via YouTube.
  8. ^ Olsen, R. C. (2007), Remote Sensing from Air and Space (PDF), SPIE, ISBN 978-0-8194-6235-0
  9. ^ Thurman, H. V. (1997), Introductory Oceanography, New Jersey, USA: Prentice Hall College, ISBN 0-13-262072-3
  10. ^ "Bathymetry and Global Relief". www.ngdc.noaa.gov. NOAA National Centers for Environmental Information. Retrieved 8 July 2022.
  11. ^ "Coastal Elevation Models". www.ngdc.noaa.gov. NOAA National Centers for Environmental Information. Retrieved 8 July 2022.
  12. ^ 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.
  13. ^ Carlowicz, Michael (2020). "Sounding the Seafloor with Light". earthobservatory.nasa.gov. NASA. Retrieved 8 July 2022.

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