Explore the Deep Sea

Tools & Techniques

 

Hot facts

  • "Sonar" is short for "SOund Navigation And Ranging."
  • Sound travels through water at about 4800 feet (1400 m) per second, more than four times the speed through air. The exact speed through water depends on temperature: the warmer the water, the faster sound travels.
  • Sonar maps can be used to track volcanic activity over time: lava flows and other traces of recent volcanic activity can be identified by comparing maps produced now with maps produced some time ago.
  • The use of sonar to map the seafloor landscape is a means of bathymetry: from the Greek for "measuring depth".

Hot links

One way to map the deep ocean seafloor.

Echolocation using technology

Researcher Chris German examines a map. Image courtesy of K. Kusek.

Whales, dolphins and bats have long possessed the ability to use sound to detect and map objects. But humans only developed this ability in the last century, when early sonar systems were developed to locate enemy submarines during the Second World War. Today, sonar is used outside the military, for example, to produce high-resolution maps of the seafloor. These maps show the location of landscape features such as cliffs, ridges and cracks in the seafloor.

Sonar basics

Sonar works as follows:

Sidescan sonar

Chief Scientist Fernando Martinez examines bathymetry maps of the Lau Basin. Image courtesy of B. Applegate.

Sidescan sonar gives us information about the nature of the seafloor as well as its depth. An instrument towed behind the ship measures the intensity of reflected sound, as well as the time taken for the sound to travel out and back. A strong signal means the seafloor is relatively hard (e.g. rock, hardened lava, or gravel). A weak signal indicates a soft or finer surface such as silt or sand. Sidescan surveys typically cover the seafloor in overlapping "swaths" or blocks 100-500 m wide. At the end of a survey, the swaths are pieced together to form a comprehensive map of the seafloor.

Multibeam sonar

Rather than sending out single "pings" like sidescan sonar, multibeam sonar equipment emits an array of sound in a fan-like pattern. The reflected sound waves can be used to determine information about sediment type as well as seafloor depth. Multibeam sonar equipment is usually attached to the ship's hull rather than towed behind it.

Combining ("nesting") sonar maps

Map of Lau basin produced using sonar. Image courtesy of F. Martinez.

The closer the sonar equipment is to the seafloor, the more readily it can detect small changes in seafloor contours. So high-resolution maps are made by towing sonar equipment a short distance above the seafloor. But high-resolution maps take longer to make than low-resolution maps, because the shorter the distance between the seafloor and the sonar equipment, the narrower the "swath" of seafloor hit by the emitted soundwaves, and the more "runs" needed to cover a given area of seafloor (just as mowing a lawn with a 12 inch mower takes about twice as long as with a 24 inch mower). Researchers working in the South Pacific's Lau Basin are using sonar at three different resolutions to provide a "nested map" (swath within a swath within a swath):

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