Hydroacoustics
Hydroacoustics is a unspecific term for the study and application of sound in water. Hydroacoustics, utilizing SONAR technology, is most generally used for detection, assessment, and monitoring of underwater physical and biological objects. Hydroacoustics can be utilized to detect the depth of a water body (bathymetry), as well as the presence or absence, abundance, distribution, size, and behavior of underwater plants and animals. Active hydroacoustic sensing involves making a sound and listening for the echo, hence the usual name for the device, echo sounder or echosounder.
Active Acoustics is a means of measuring the range to an object and its relative size by producing a pulse of sound and measuring the time it takes for an echo to return from the object and the amplitude of the returned echo. The range is calculated as a function of the speed of sound and the time it takes for the echo to return.
Digital acoustic systems are capable of detecting organisms as small as krill, with no limit on upper sizes. A target’s 3D position inside the ensonified beam can be accurately found in split beam systems, allowing calculation of absolute target strength, velocity, and direction of movement.
An optional integrated orientation sensor installed in the transducer translates the split beam organizes of the target into real world organizes. The integrated orientation sensor output is used to compensate for vessel or tow body motion, thereby maximizing survey accuracy. Seabed classification based on acoustic properties can be spling into two main categories; seabed surface and seabed sub-surface.
Seabed surface classification is concerned first and foremost with distinguishing habitat characteristics (for example hard, soft, mud, sand, clay, cobble) to the sampled area. The seabed class can be added to other acoustically (or other method) derived information about the place, such as fish distribution and abundance or vegetation characteristics, to establish habitat groups based on the associations.
Sub-surface seabed classification is generally alluded to as Sub-bottom profiling and is commonly used for geological assessment of the sub-surface characteristics, down to over a hundred of meters, as needed in the exploration for oil deposits.
Acoustic secluded sensing of the seabed using single-beam echosounders, multibeam echosounders, and sidescan sonars combined and individually are giving technological solutions to marine-habitat mapping initiatives. We believe the science of acoustic seabed classification (ASC) is at its nascence. A extensive review of ASC science was undertaken by an worldwide group of scientists under the auspices of ICES. The review was prompted by the growing need to classify and map marine ecosystems across a plethera of spatial scales in support of ecosystem-based science for ocean management.
A review of the theory of sound-scattering from seabeds emphasizes the variety of theoretical models currently in use and the ongoing evolution of our comprehending. Acoustic-signal conditioning and data quality assurance before classification using objective, repeatable procedures are crucial technical considerations where standardization of techniques is only just beginning.
The issue of temporal and spatial scales is reviewed, with emphasis on matching observational scales to those of the natural world. It is emphasized throughout that the seabed is not static but changes over several time-scales as a consequence of natural physical and biological procedures.
A summary of existing commercial ASC systems offers an introduction to existing capabilities. Verification (ground-truthing) techniques are reviewed, emphasizing the difficulties of matching observational scales with acoustic-backscatter data. Survey designs for ASC explore techniques that extend beyond traditional oceanographic and fisheries survey methods. Finally, future directions for acoustic seabed classification science were identified in the important areas requiring immediate attention by the worldwide scientific community.