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Echo Sounder
The most basic type of underwater imaging is echo sounding that only uses one transducer. A sonar transducer is the electronic device that transmits and receives sound waves. The transducer receives the sound echo, which amplifies the signal and records it. If the sound waves return to the ship quickly, the water depth is shallow. Longer return times indicate deeper waters.
Multibeam Sonar
In contrast to the limited capability of a single ping echo sounder, a multibeam sonar can map more than one location on the ocean floor with each ping, and is capable of higher resolution. A multibeam sonar can image a strip of points in a direction that is perpendicular to a ship's path. This wide area is called a swath. Because of the device's complex nature, the cost can be many times that of a single-beam sonar. The savings associated with reduced ship operating time offset this cost. Therefore, multibeam sonar is the primary device used for underwater mapping applications.
Sidescan Sonar
Another technique for imaging the ocean is a sidescan sonar. A towed transducer (towfish) sends a pulse of soundwaves. Because the ship tows the transducer, horizontal sonar images are possible that view different angles of ocean objects. This imaging cannot occur with a perpendicular sonar system.
Towfish
A towfish drags under the water behind a vessel that contains compatible sonar equipment. As a vessel travels, the towfish listens for the echoes of a series of sonar pulses. The echoes of each pulse build a sonar plot for each side of the vessel. This plot is also known as a trace. Strength of echoes decrease over time because of signal attenuation. To fix this problem, a time-varying gain equalizes the signal values.