Traditional sonar-based systems, such as side-scan sonar and echosounders, operate at long ranges and great depths, but they do not have sufficient resolution to provide the detail required for some applications. Optical imaging systems are effective over short distances where there is sufficient light and clarity. Acoustic cameras use acoustic lenses and a high-frequency transducer array to focus sound energy and produce fast-frame, video-like imagery. They operate at higher frequencies and shorter ranges than conventional sonar systems. Acoustic camera lenses transmit multiple, parallel, narrow beams and receive the return signals. This system generates sharper images than conventional sonars, but the higher frequencies required limit their effective range.
Current-day acoustic camera technologies evolved from medical ultrasound devices developed in the 1960s as well as port surveillance systems developed in the 1990’s. Modern cameras can be configured for operations with remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), or human divers. The Limpet Mine Imaging Sonar (LIMIS) is an efficient method used by U.S. Navy divers to check for mines or improvised explosive devices (IED) on ship hulls and piers.
One acoustic camera, the Dual-Frequency Identification Sonar (DIDSON), has a greater range than LIMIS and was originally developed to effectively identify underwater intruders detected by harbor surveillance systems. It has two operating modes: detection mode and identification mode. In detection mode, the DIDSON transmits 48 beams in one working cycle with a frequency of 1.1 MHz and detection range up to 40 m. In identification mode, it can transmit 96 beams in one working cycle with a frequency of 1.8 MHz and a detection range up to 12 m. Acoustic cameras are also used to inspect structures such as bridge pilings and oil pipelines, to detect debris and aquatic plants and animals that could clog cooling water intakes at power plants, in fisheries management, and research.
Dual-frequency Identification Sonar (DIDSON) imagery of adult Pacific salmonids near the entrain to a saltwater fish drain at the Hiram M. Chittenden Locks, Seattle, WA. (Courtesy of Johnson Fisheries Science, www.johnsonfishsci.com)
Acoustic cameras require less training than traditional sonar systems and their video-like footage is easier to interpret. This enables observations of swimming behavior, directional movements, animal size, and species identification. Data can be used to determine migration timing and spawning behavior and conduct fish counts. It is possible to use acoustic camera outputs for species identification for fish with unique body shapes, fin positions, or characteristic swimming behavior (e.g. eels). However, for other fishes, it can be challenging to identify to species, as a clear image of fish shape is required, but not always produced. Shadows produced when fish move through the acoustic beam can also cause false detections and higher automated counts.
Underwater acoustic camera imagery of a large shark and its acoustic shadow seen off the Dry Tortugas (Florida, USA). Video credit: Sound Metrics (http://www.soundmetrics.com).
Acoustic cameras are used also for other research applications. Systems have been deployed to count and measure animals in the deep sea scattering layers and to characterize deep ocean prey. DIDSON cameras have captured near-continuous data for shark species that transit an important habitat boundary at Palmyra Atoll in the Pacific. Acoustic cameras have also been used to investigate the depredation behavior of harbor seals around salmon setnets.
Additional Links on DOSITS
- Science of Sound > Sound Movement > Sound Absorption
- Science of Sound > Sound Movement > Reflection
- Science of Sound > Sound Movement > Refraction
- Animals and Sound > Effects > Acoustic issues related to diadromous fishes
- People and Sound > How is active acoustics used for fisheries management?
- People and Sound > How is sound used to monitor and defend harbors?
- Technology Gallery > Hydrophone/Receiver
- Technology Gallery > Sonar
- Technology Gallery > Echosounder
- Technology Gallery > Multibeam Echosounder
- Technology Gallery > Side Scan Sonar
- Technology Gallery > Acoustic Fish Tags
- Technology Gallery > Fish Finding Sonar
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