Explore the Deep Sea
Tools & Techniques
One way to map the deep ocean seafloor.
Echolocation using technology
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 works as follows:
- A machine sends out sound waves ("ultrasound," or ultrasonic sound)
- The sound bounces off the seafloor; the reflected sound waves are detected by the machine
- The distance between the machine and the reflecting surface can be calculated from the time the sound takes to travel to the seafloor and back
- By making measurements in different places, the contours of the seafloor can be plotted. As a general rule, the closer you can get the instrument to the seafloor, the greater the resolution of the contour map.
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.
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
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):
- Lowest resolution: a multibeam/sidescan sonar system attached to the research ship; reveals major features of the seafloor landscape over a swath 10-18 km wide.
- Middle resolution: sidescan sonar towed 500 m above the seafloor maps a swath up to 6 km wide; shows lava flows and other features in more detail.
- Highest resolution: sidescan sonar towed 100 m above the seafloor maps a swath 1 km wide in areas of most interest; provides meter-scale resolution of features such as fissures and faults in the seafloor.