Acoustic Technology

Acoustic technology utilizes the unique quality of sound in the ocean to essentially "see" in much the same way as vision is used on land. Sound travels through water in wave-like form, with different sizes and frequencies being created by different sounds. A large selection of different acoustic tools are used by scientists, generally involving the emission of sound (from a research vessel or other source) and recording how it bounces off of different surfaces (the ocean floor, fish schools, etc.) in its path. This acoustic technology, commonly used to track submarines, has advanced tremendously in recent years and has become a primary tool for oceanographers, both for mapping the physical shape of the ocean floor and for tracking and counting marine animals of all sizes. A number of different techniques are now used; applied in concert with other technology, acoustics are a critical tool for CoML scientists.

1. Acoustic Doppler Current Profiler (ADCP) is an instrument that measures the speed and direction of water currents. This device is generally used in conjunction with other instruments, such as CTD's, and with organism counting and collecting devices such as VPR's and trawls. ADCP's provide important information about water currents at the depths that animals are found, telling researchers critical information about how currents may impact animal distribution and movement.
   
   
2. Side-scan SONAR is used extensively for mapping the ocean floor and for tracking schools of fish. This has become a well-established technique for effective study of marine life. Pulses of sound are projected by a ship or a towed device and as the sound waves bounce off of different objects, whether living things or physical features of the ocean floor, they come back to the ship. This can generate an image of the shapes from the reflected sound. This allows scientists to "see" the ocean much like dolphins view the world around them with their hearing.
   
   
3. Standard & Multi-frequency echo sounders, also called multi-beam SONAR, allow for density estimation of plankton and fish and are even capable of some degree of species identification. This is a remarkable ability for acoustic technology. Multi-frequency echo sounders increase the success of imaging animals because they allow for the fact that all species of fish react or image differently to different sound frequencies. When multi-frequencies are used, at least one or two are likely to work well for a given species.
   
   
4. Bottom-penetrating sounders are used for mapping both the surface of the ocean floor and structures of different density below the floor. Rocks, sunken ships, mud, sand and masses of animals all have different densities and show up as discrete images. This is useful not only for the study of physical attributes of the ocean floor, but can also effectively be used to study the distribution of organisms that live on or below the benthos, literally imaging animals that are burrowed down into the sediment.
   
   
5. Towed arrays are devices that can emit and receive sound for acoustic research. They are towed behind a research vessel and can be anywhere from one to several units long. They provide an advantage over devices attached to a ship in that they are farther away from interfering engine noises and can work in concert with each other to provide an overlapping series of emitted sounds and detection. This creates a higher resolution acoustic image.