IAS Geograpy Oceanography: Zoning of the Ocean Part 2

ZONING OF THE OCEANS

2. Bathymetric

The term bathymetry is defined as the depth of water relative to sea level. Thus bathymetric measurements can determine the topography of the ocean floor, and have shown that the sea floor is varied, complex, and ever-changing, containing plains, canyons, active and extinct volcanoes, mountain ranges, and hot springs. Some features, such as mid-ocean ridges (where oceanic crust is constantly produced) and subduction zones, also called deep-sea trenches (where it is constantly destroyed), are unique to the ocean floor. 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.

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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; the characteristics of tides, currents, and waves; and the physical and chemical properties of the water itself.

Bathymetric mapping involves the production of ocean and sea maps based upon bathymetric data (see historic map). Bathymetric maps represent the ocean depth as a function of geographical coordinates in the same way topographic maps represent the altitude of Earth's surface at different geographic points. The most popular type of bathymetric maps are ones on which lines of equal depths (called isobaths) are represented.

Bathymetric techniques For hundreds of years, the only way to measure ocean depth was the sounding line, a weighted rope or wire that was lowered overboard until it touched the ocean floor. Not only was this method time-consuming, it was inaccurate; ship drift or water currents could drag the line off at an angle, which would exaggerate the depth reading. It was also difficult to tell when the sounding line had actually touched bottom.

England's Sir John Murray compiled this bathymetric (depth) chart of the North Atlantic in 1911. Murray's chart went far beyond American naval officer Matthew Maury's first attempt at bathymetric mapping in 1855. In addition, Murray's map gave birth to the idea of the Telegraphic Plateau, a submarine land formation from Canada to the British Isles, across which the first transatlantic cable was laid.

In the twentieth century, sounding lines were entirely replaced by sonar systems. Sonar ( sound navigation ranging), invented during World War II (1939–1945) measures distances by emitting a short pulse of high-frequency sound and measuring the time until an echo is heard. After the war, ships with sonar units attached to their hulls crisscrossed the world's oceans systematically, measuring depth. The data collected made possible complete bathymetric maps of the world's oceans. For the first time, scientists knew what 70 percent of Earth's surface really looked like (radar, which produces images by bouncing radio waves rather than sound waves off distant objects, cannot be used for bathymetry because water absorbs radio waves).

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Many sonar techniques have been developed for bathymetry. When high-resolution images are desired, an underwater unit may be towed behind a ship, scanning to the left and right with multiple sonar beams (sidescan sonar). Furthermore, orbiting visible-light cameras image the bottoms of some shallow waters, while 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.

Bathymetric chart

A bathymetric chart is the submerged equivalent of an above-water topographic map. Bathymetric charts are designed to present accurate, measurable description and visual presentation of the submerged terrain.

In an ideal case, the joining of a bathymetric chart and topographic map of the same scale and projection of the same geographic area would be seamless. The only difference would be that the values begin increasing after crossing the zero at the designated sea level datum. Thus the topographic map's mountains have the greatest values while the bathymetric chart's greatest depths have the greatest values. Simply put, the bathymetric chart is intended to show the land if overlying waters were removed in exactly the same manner as the topographic map.

Excerpts from:

http://www.waterencyclopedia.com/Oc-Po/Ocean-Floor-Bathymetry.html

http://oceanservice.noaa.gov/facts/bathymetry.html

http://oceanservice.noaa.gov/facts/light_travel.html

http://en.wikipedia.org/wiki/Oceanic_zone

http://en.wikipedia.org/wiki/Bathymetric_chart

http://biophysics.sbg.ac.at/rovigno/rovigno3.htm