What is Hydroacoustics?

Hydroacoustics is a general term for the study and application of sound in water. Hydroacoustics, utilizing SONAR technology, is most commonly 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 common 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 within the ensonified beam can be accurately located 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 coordinates of the target into real world coordinates. The integrated orientation sensor output is used to compensate for vessel or tow body motion, thereby maximizing survey accuracy.

Hydroacoustic assessments have traditionally been conducted via mobile survey deployment, i.e. from a moving boat. Conversely, fixed-location deployment is a hydroacoustic survey technique where a single or multiple transducers are attached to a stationary or “fixed” object to monitor targets of interest as they move toward and pass through the acoustic beam.

How Does it Work?

Hydroacoustics monitoring is based on a few relatively simple principles. The transducer converts the electrical signal from the transmitter within the echosounder into an acoustic pulse, and transmits that energy into the water. In reciprocal fashion, the transducer receives acoustic echoes (from targets in the water column and the bottom) and converts them to electrical signals. DT-X transducers contain both receiver and analog-to-digital conversion circuitry, very different from analog transducers found in traditional or other scientific echosounder systems. The pulse of energy travels through the water at a speed of approximately 1500 m/sec. When the acoustic pulse encounters an object, such as a fish or the bottom, some of the energy (i.e. an echo) is reflected back to the transducer. If the signal level exceeds a user-selected threshold level, a mark appears on the echogram. The distance from the top of the display to the mark is proportional to the travel time for the pulse to travel from the transducer to the target and back. Since the velocity in water is known, range (distance from the transducer) can be calculated from this travel time. By collecting the echoes from many consecutive transmissions, the time in the acoustic beam, the change in range and the direction of travel of targets can be determined. The BioSonics DT-X Echosounder uses a Linux based embedded processor which automatically time stamps, georeferences, and digitally stores data for playback and analysis.

What Transducer Type and Frequency Do I Need?

Recommended Transducer Frequencies (kHz) by Application

Survey Type
Bathymetry
Zooplankton
Fish
Vegetation
Bottom Classification
Fresh- water, mobile
420
200
120
70
420
200
120
420
200
120
70
38
420
200
200
120
70
38
Marine, mobile
200
120
70
38
420
200
120
200 120
70
38
420*
200
120
70
38
Fresh- water, fixed
N/A
420
200
120
420
200
120
70
N/A
N/A
Marine, fixed
N/A
420
200
120
70
200
120
70
38
N/A
N/A

*420kHz is best suited for vegetation applications.

Both single beam and split beam transducers are suitable for collection of bathymetry, zooplankton, fish aggregations, vegetation and bottom classification data. Split beam transducers are best for counting individual targets (e.g, fish or invertebrates), and are required for tracking (in fixed-location applications) individual targets, and for determination of accurate acoustic target strength measurements. Target strength can often be used for size classification of detected targets.

How do I get Results?

Hydroacoustic assessment is a process which begins with data acquisition using the echosounder computer control unit, or User PC, typically a rugged field-ready notebook computer. The echosounder requires values for a set of parameters to establish the operational characteristics specific to each application. These include such settings as pulse frequency (ping rate), pulse duration, power level, target strength (TS) level, and more. The User PC provides the means to define and pass these parameters to the echosounder. BioSonics Visual Acquisition™ software provides an easy to use interface for input of the operational parameters and for transmission of these parameters to embedded processor of the DT-X echosounder surface unit.

In addition to communicating the operational parameters to the echosounder, the notebook computer provides a means for the user to view and monitor the acoustic data by means of the Visual Acquisition echogram and oscilloscope display components. These functions provide the user with a real-time graphical interpretation of the acoustic data being received. Finally, the notebook computer provides a convenient storage device for the raw acoustic data. The data can be written to the hard disk drive (or other media device) of the computer for storage, analysis, and distribution.

Post Processing of Data

The BioSonics data file format is compatible for post processing of data using BioSonics Visual Analyzer for fish, BioSonics EcoSAV for Submerged Aquatic Vegetation (SAV), and BioSonics VBT for Bottom Classification.

BioSonics is also a reseller of SonarData’s Echoview, and Lindem Data Acquisitions Sonar4 and Sonar5 post-processing software tools.

Real-time Results
BioSonics offers new and innovative monitoring solutions, featuring fully automated, unmanned hydroacoustic systems, remotely accessible via the Internet, capable of real-time data analysis and reporting.

BioSonics leads the way in “smart” hydroacoustic systems designed and programmed to automatically trigger an operational event, such as opening a spillway or triggering a deterrent device based on user -specified events such as the presence of a certain number or size of fish. See Automated Monitoring Systems for more information.



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