GEOLOGY AND MARINE BIOLOGY
Side scan sonar
I wanted to introduce the talk on a tool now widely used in marine geology, I'm talking about the side scan sonar.
It was first experimented between 1950 and 1960 since Professor Harold Edgerton in Hudson's oceanography labs. This instrument was first used by the American navy, then transformed into an excellent ally for the identification of wrecks, only after a few years was it used for the study of the seabed.
It is in effect a sonar, but unlike this one, returns a three-dimensional image of the seabed, having the ability to emit lateral impulses. The sound impulses it emits are between 100 and 500 KHz, but the higher the frequency used, therefore the image resolution, the lower the viewing angle. It will therefore be up to the technician, as the analysis of the background proceeds, to decide whether he prefers a broader view or a more defined image.
The side scan sonar is formed by an instrument similar to a small torpedo that proceeds in the water, called "fish", from a cable which carries the data collected on the boat, and from a data control and recording unit, typically a laptop.
The underwater unit is towed by the boat along the previously decided routes, any anomaly due to the speed of the vehicle is automatically corrected. The instrument does not use the reflection of the acoustic waves but their diffraction, the sound pulse is sent by two transducers present on the "fish": if a wave hits a surface that has an angle facing the wave itself, such as a disconnected surface, the wave front bends around the disconnections giving rise to a diffracted wave. Each point of the background reached by an acoustic wave, if it has suitable characteristics, becomes a source of diffracted waves.
The frequency and length of the wave depend on the characteristics of the bottom. The return of the acoustic wave it is therefore recorded by the transducers and the signal is switched into an image consisting of a series of lines made up of single points (pixels), each line is the representation of the echoes produced by a single pulse. Based on the amplitude of the return signal (therefore the morphology of the background) the instrument creates an image in shades of gray, similar to an aerial photo in black and white.
Once the data is recorded, we will have "streaks" of the seabed, which in order to be returned to the user as a final map, are processed by specific software.
There are two types of tools: one for the survey of the coasts, within 400 meters, and another for deep waters over 1000 m.
In case of investigation on hard ground, therefore rock, the use of the side scan sonar is not recommended, due to the illegibility of the data obtained.
Dr. Rossella Stocco
Side scan sonar
The side scan sonar creates an image of the bottom using sound waves. While this can look like a picture, the image depends on the interaction of the sound waves with the bottom. The system uses the time of the return to compute a distance, and then displays the intensity of the return in a shade of gray. The intensity of the return depends upon:
- Hardness of the bottom. Hard, rocky bottoms reflect most of the sound, whereas soft, muddy bottoms absorb most of the energy. Man-made features are generally hard, and things like crab pots have a number of angled surfaces that act the same way as the radar reflectors on the masts of a sailboat to reflect a lot of energy and can appear larger than they really are.
- Smoothness of the bottom. A smooth bottom acts like a mirror, and reflects sound in only one direction - so unless the beam hits the bottom at a 90 ° angle, the reflection will not return to the towfish. A rough surface will scatter the sound, and some will return. What is smooth depends on the wavelength of the energy - for light, a mirror must be incredibly smooth (about 500 nanometers), but for the sonars, smooth or rough is on the scale of a few centimeters, which is the same size as for the radar energy used for some mapping applications on land.
- Slope of the bottom. There will be better returns when the sound hits a bottom sloping up and away from the fish, and few returns when the bottom slopes down.
- Swath width: the distance covered on either side of the towfish. This is generally governed by a fixed depression angle for the emitted sound beams, and actual width depends on the height of the fish above the bottom which can be controlled.
- TWTT (two way travel time): the time it takes for sound to go from the towfish to the target and return. From the TWTT and speed of sound, the slant range to the target can be computed.
- Slant range: distance to the target traveled by the sound. It is the hypotenuse of triangle with the fish height and the true horizontal ground distance as the other legs.
- Cross track and along track: each ping of the side scan collects data in the cross track direction, perpendicular to the ship and towfish path. As the towfish moves in the along track track direction, the next ping will be displaced and image the next line in the along track direction. The portion of the image showing the water column will indicate the along track direction if the image has been geometrically corrected to remove the water column, the distorted pixels will reveal the along track direction.
|Strong returns are now generally shown in white, and no returns in black, denoting the sound shadow. This was not always the case with the first systems which only used a paper recorder and only put black ink on the targets. You should always verify the color convention used in the imagery you are looking at. Colors are sometimes used as well to highlight very strong returns, and keep a watch stander alert as they come in. A gold color scheme is the most common today.|
|Grayscale||Custom gold color scale |
Lower frequencies attenuate less and travel farther whales use low frequencies to increase the range of their communication.
Figure 1 below shows two views of Submarine S5. The image on the left is smaller, indicating the sonar was operating with a longer range. This provides less detail, but covers a large area, and is generally how sonars are used in searching. Side scan sonars are designed to view the seafloor from the side, and provide very poor geometry directly under the towfish (Figure 2). In both Figure 1 and 2 the track of the towfish is shown by the large pixels. For the image of the S5, once the NOAA survey ship located the wreck, they switched to a short range on the sidescan to collect a better image, and they returned for a second pass with the ship track oriented in the same direction as the wreck . In addition, they insured the wreck was in the middle of one channel, and not under the fish. If the survey that acquired Figure 12 was interested in details of the wreck, they would have taken a second pass and insured they passed to the side of the wreck. In addition to not wanting to pass over the wreck, the towfish needs to be close to the bottom to enhance the shadows. Note that in Figure 11 the shadows provide more information than the actual imaged portion of the wreck.
The illuminated area is the reflection and dark is the shadow.
Survey vessels can acquire multibeam bathymetry and side scan sonar imagery at the same time (Figure 7), and combining this results greatly increases what an analyst can see in the data (Figure 13).
Figure 13. Comparison of older side scan and multi-beam systems on ship wreck. The newer higher-resolution systems are to the right of the image.
The sidescan survey has a swath width, which is the area covered. It is a little less than twice the range, since the quoted range is the slant range for each channel. If the fish is at the preferred height (15% of the swath width), the horizontal distance on the bottom is about 99% of the range. If the ship tracks were at exactly twice the range, you would get almost 100% coverage, but the region directly under the towfish would have terrible coverage. If the ship tracks were at half the spacing, you would cover every point on the bottom twice, and the region directly under the fish on one pass would be at the edge of the next pass. In either case, you would want the spacing to be a little closer, to avoid gaps. What spacing you pick depends on time and money available, and the potential cost of missing what you are looking for.
Side scan sonar collects imagery. A related system, multibeam or swath bathymetry, collects depth information.
Side scan sonar
The industry's leading seafloor mapping software allows surveyors to:
- Detect small targets via improved across track resolution.
- Identify differences between old and new surveys through the SonarWiz transparency, swipe, or line shift tools.
- Preserve the full fidelity of sonar data across multiple formats. For example SonarWiz allows for recording in industry standard XTF or vendor-specific formats such as Edgetech JSF or Kongsberg-GeoAcoustics GCF.
- Simplify mission execution through the use of a single, easy-to-learn mapping solution.
- Create superior contact reports thanks to improved editing features. Easily export data to a variety of formats including PDF, OpenOffice, Microsoft Word, and HTML.
- Optimize time on the water through the use of sophisticated planning tools.
- Reduce costs by leveraging real-time data acquisition to confirm quality and coverage prior to leaving coverage area.
- Generate state of the art mosaics, contact, and waterfall imagery with high resolution 64-bit displays.
SonarWiz Feature Details
- Load background maps and charts from a range of formats including DNC, RNC, S57, and GeoTIFF.
- Automatically plan survey lines parallel to a reference line, within a polygon based on either efficiency or conventional patterns.
- Generate planned survey maps as a GeoPDF, GeoTIFF, ECW, JPEG or Google Earth.
- Estimate survey timing.
- Preview files with the SNIFF feature.
- Add and fix navigation data with NavInjectorPro.
- Leverage advanced signal processing and gain control take advantage of features such as beam angle correction, de-striping, non-linear per channel TVG, AGC, Band Pass Filtering and Stacking, Contact (target) capture, annotation, and summary reporting via 3D Viewer .
- Employ flexible layback configurations.
- Easily printable output.
- Grid / contour isopach-type shapefile / grid generation from selected variables (e.g. altitude + depth).
SonarWiz is compatible with the following Sonar Hardware:
- Atlas NA, C-MAX, EdgeTech, Falmouth Scientific, GeoAcoustics, Imagenex, Innomar, Jetasonic, Klein Marine Systems, Knudsen, Kongsberg Hugin AUV, Kongsberg Mesotech, Marine Sonic, PingDSP, R2Sonic, SyQwest, Teledyne Benthos, Teledyne Gavia, Teledyne Odom , and Tritech.
- Please see our Supported Interfaces and File Formats for a list of the many formats we support. If you don't see one you need just ask!
Things to consider while buying depth finder with side imaging
There are some important things to consider while investing on side view depth finder s.
Given below are these:
High power is always good. This is why choosing one using higher watts is good for getting stronger power.
You need to make sure what resolutions you exactly need. Just determine whether you need single, dual or multiple frequencies. Frequencies play important roles on how effectively the sonar scanners perform.
The rule of thumb is selecting the higher the frequency to get more detail on screen. However, many experienced professionals opine that lower frequencies are ideal for deeper waters and higher frequencies are good for shallow water.
Higher screen resolution is always good. It helps you get more details views and determine your target.
Transducers are used to emit sonar waves out through the water for getting into digital representations of nonliving objects, structures and living things like fleshes. Transducers are essential to make any fish-finding tools.
Choosing high-res screens are good for getting a wide variety of colors that helps you distinguish different objects easily. This is why the right screen selection matters here.
What is a side imaging fish finder?
As the name suggests, a side imaging fish finder is basically a sonar device used for finding fishes under the water. You can have the best utilization of your time on the water. It uses sonar technology coming with a particular transducer. You just need to set it on the outside of the boat transoms to have a view of the shoal of fishes.
The utility aspect of side scan fish finder
With the aid of a side imaging fish finder, you will be able to decide rich fishing locations and the existence of shoal of fishes. Consequently, you can take the necessary actions to catch the fishes easily. Thus, you can simplify your fishing experience.
Based on the best fish finder reviews, we have included to the best side imaging scan fish finders (of 2021) using the best technologies to ensure a smooth experience.
Side-scan sonar may be used to conduct surveys for marine archeology in conjunction with seafloor samples it is able to provide an understanding of the differences in material and texture type of the seabed. Side-scan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the seafloor that may be hazardous to shipping or to seafloor installations by the oil and gas industry. In addition, the status of pipelines and cables on the seafloor can be investigated using side-scan sonar. Side-scan data are frequently acquired along with bathymetric soundings and sub-bottom profiler data, thus providing a glimpse of the shallow structure of the seabed. Side-scan sonar is also used for fisheries research, dredging operations and environmental studies. It also has military applications including mine detection.
Side-scan uses a sonar device that emits conical or fan-shaped pulses down toward the seafloor across a wide angle perpendicular to the path of the sensor through the water, which may be towed from a surface vessel or submarine, or mounted on the ship's hull. The intensity of the acoustic reflections from the seafloor of this fan-shaped beam is recorded in a series of cross-track slices. When stitched together along the direction of motion, these slices form an image of the sea bottom within the swath (coverage width) of the beam. The sound frequencies used in side-scan sonar usually range from 100 to 500 kHz higher frequencies yield better resolution but less range.
The earliest side-scan sonars used a single conical-beam transducer. Next, units were made with two transducers to cover both sides. The transducers were either contained in one hull-mounted package or with two packages on either side of the vessel. Next the transducers evolved to fan-shaped beams to produce a better "sonogram" or sonar image. In order to get closer to the bottom in deep water the side-scan transducers were placed in a "tow fish" and pulled by a tow cable.
Up until the mid-1980s, commercial side scan images were produced on paper records. The early paper records were produced with a sweeping plotter that burned the image into a scrolling paper record. Later plotters allowed for the simultaneous plotting of position and ship motion information onto the paper record. In the late 1980s, commercial systems using the newer, cheaper computer systems developed digital scan-converters that could mimic more cheaply the analog scan converters used by the military systems to produce TV and computer displayed images of the scan, and store them on video tape . Currently data is stored on computer hard drives or solid-state media.
Military application Edit
One of the inventors of side-scan sonar was German scientist, Dr. Julius Hagemann, who was brought to the US after World War II and worked at the US Navy Mine Defense Laboratory, Panama City, FL from 1947 until his death in 1964. His work is documented in US Patent 4,197,591  which was first disclosed in Aug 1958, but remained classified by the US Navy until it was finally issued in 1980. Experimental side-scan sonar systems were made during the 1950s in laboratories including Scripps Institution of Oceanography and Hudson Laboratories and by Dr. Harold Edgerton at MIT.
Military side-scan sonars were made in the 1950s by Westinghouse. Advanced systems were later developed and built for special military purposes, such as to find H-Bombs lost at sea or to find a lost Russian submarine, at the Westinghouse facility in Annapolis up through the 1990s. This group also produced the first and only working Angle Look Sonar that could trace objects while looking under the vehicle.
Commercial application Edit
The first commercial side-scan system was the Kelvin Hughes "Transit Sonar", a converted echo-sounder with a single-channel, pole-mounted, fan-beam transducer introduced around 1960. In 1963 Dr. Harold Edgerton, Edward Curley, and John Yules used a conical-beam 12 kHz side-scan sonar to find the sunken Vineyard Lightship in Buzzards Bay, Massachusetts. A team led by Martin Klein at Edgerton, Germeshausen & Grier (later EG & G., Inc.) developed the first successful towed, dual-channel commercial side-scan sonar system from 1963 to 1966. Martin Klein is generally considered to be the "father" of commercial side-scan sonar. In 1967, Edgerton used Klein's sonar to help Alexander McKee find Henry VIII's flagship Mary Rose. That same year Klein used the sonar to help archaeologist George Bass find a 2000-year-old ship off the coast of Turkey. In 1968 Klein founded Klein Associates (now Klein Marine Systems) and continued to work on improvements including the first commercial high frequency (500 kHz) systems and the first dual-frequency side-scan sonars, and the first combined side-scan and sub- bottom profiling sonar. In 1985, Charles Mazel of Klein Associates (now Klein Marine Systems, Inc.) produced the first commercial side-scan sonar training videos and the first Side Scan Sonar Training Manual and two oceanographers found the wreck of the RMS Titanic.
For surveying large areas, the GLORIA sidescan sonar was developed by Marconi Underwater Systems and the Institute of Oceanographic Sciences (IOS) for NERC.GLORIA stands for Geological Long Range Inclined Asdic.  It was used by the US Geological Survey and the IOS in the UK to obtain images of continental shelves worldwide. It operated at relatively low frequencies to obtain long range. Like most side-scan sonars, the GLORIA instrument is towed behind a ship. GLORIA has a ping rate of two per minute, and detects returns from a range of up to 22 km either side of the sonar fish.
- 1 History
- 1.1 ASDIC
- 1.2 SONAR
- 1.3 US Navy Underwater Sound Laboratory
- 1.4 Materials and designs in the US and Japan
- 1.5 Later developments in transducers
- 2 Active sonar
- 2.1 Project Artemis
- 2.2 Transponder
- 2.3 Performance prediction
- 2.4 Hand-held sonar for use by a diver
- 2.5 Upward looking sonar
- 3 Passive sonar
- 3.1 Identifying sound sources
- 3.2 Noise limitations
- 3.3 Performance prediction
- 4 Performance factors
- 4.1 Sound propagation
- 4.2 Scattering
- 4.3 Target characteristics
- 4.4 Countermeasures
- 5 Military applications
- 5.1 Anti-submarine warfare
- 5.2 Torpedoes
- 5.3 Mines
- 5.4 Mine countermeasures
- 5.5 Submarine navigation
- 5.6 Aircraft
- 5.7 Underwater communications
- 5.8 Ocean surveillance
- 5.9 Underwater security
- 5.10 Hand-held sonar
- 5.11 Intercept sonar
- 6 Civilian applications
- 6.1 Fisheries
- 6.2 Echo sounding
- 6.3 Net location
- 6.4 ROV and UUV
- 6.5 Vehicle location
- 6.6 Prosthesis for the visually impaired
- 7 Scientific applications
- 7.1 Biomass estimation
- 7.2 Wave measurement
- 7.3 Water velocity measurement
- 7.4 Bottom type assessment
- 7.5 Bathymetric mapping
- 7.6 Sub-bottom profiling
- 7.7 Gas leak detection from the seabed
- 7.8 Synthetic sonar apertures
- 7.9 Parametric sonar
- 7.10 Sonar in extraterrestrial contexts
- 8 Effect of sonar on marine life
- 8.1 Effect on marine mammals
- 8.2 Effect on fish
- 9 Frequencies and resolutions
- 10 See also
- 11 Explantory notes
- 12 Citations
- 13 General bibliography
- 13.1 Fisheries acoustics references
- 14 Further reading
- 15 External links
Although some animals (dolphins, bats, some shrews, and others) have used sound for communication and object detection for millions of years, use by humans in the water is initially recorded by Leonardo da Vinci in 1490: a tube inserted into the water was said to be used to detect vessels by placing an ear to the tube. 
In the late 19th century an underwater bell was used as an ancillary to lighthouses or lightships to provide warning of hazards. 
The use of sound to "echo-locate" underwater in the same way as bats use sound for aerial navigation seems to have been prompted by the Titanic disaster of 1912.  The world's first patent for an underwater echo-ranging device was filed at the British Patent Office by English meteorologist Lewis Fry Richardson a month after the sinking of Titanic,  and a German physicist Alexander Behm obtained a patent for an echo sounder in 1913. 
The Canadian engineer Reginald Fessenden, while working for the Submarine Signal Company in Boston, Massachusetts, built an experimental system beginning in 1912, a system later tested in Boston Harbor, and finally in 1914 from the U.S. Revenue Cutter You love me on the Grand Banks off Newfoundland.   In that test, Fessenden demonstrated depth sounding, underwater communications (Morse code) and echo ranging (detecting an iceberg at a 2-mile (3.2 km) range).   The "Fessenden oscillator", operated at about 500 Hz frequency, was unable to determine the bearing of the iceberg due to the 3-meter wavelength and the small dimension of the transducer's radiating face (less than 1 ⁄3 wavelength in diameter). The ten Montreal-built British H-class submarines launched in 1915 were equipped with Fessenden oscillators. 
During World War I the need to detect submarines prompted more research into the use of sound. The British made early use of underwater listening devices called hydrophones, while the French physicist Paul Langevin, working with a Russian immigrant electrical engineer Constantin Chilowsky, worked on the development of active sound devices for detecting submarines in 1915. Although piezoelectric and magnetostrictive transducers later superseded the electrostatic transducers they used, this work influenced future designs. Lightweight sound-sensitive plastic film and fiber optics have been used for hydrophones, while Terfenol-D and PMN (lead magnesium niobate) have been developed for projectors.
In 1916, under the British Board of Invention and Research, Canadian physicist Robert William Boyle took on the active sound detection project with A. B. Wood, producing a prototype for testing in mid-1917. This work for the Anti-Submarine Division of the British Naval Staff was undertaken in utmost secrecy, and used quartz piezoelectric crystals to produce the world's first practical underwater active sound detection apparatus. To maintain secrecy, no mention of sound experimentation or quartz was made - the word used to describe the early work ("supersonics") was changed to "ASD" ics, and the quartz material to "ASD" ivite: "ASD" for " Anti-Submarine Division ", hence the British acronym ASDIC. In 1939, in response to a question from the Oxford English Dictionary, the Admiralty made up the story that it stood for "Allied Submarine Detection Investigation Committee", and this is still widely believed,  though no committee bearing this name has been found in the Admiralty archives. 
By 1918, Britain and France had built prototype active systems. The British tested their ASDIC on HMS Antrim in 1920 and started production in 1922. The 6th Destroyer Flotilla had ASDIC-equipped vessels in 1923. An anti-submarine school HMS Osprey and a training flotilla of four vessels were established on Portland in 1924.
By the outbreak of World War II, the Royal Navy had five sets for different surface ship classes, and others for submarines, incorporated into a complete anti-submarine system. The effectiveness of early ASDIC was hampered by the use of the depth charge as an anti-submarine weapon. This required an attacking vessel to pass over a submerged contact before dropping charges over the stern, resulting in a loss of ASDIC contact in the moments leading up to attack. The hunter was effectively firing blind, during which time a submarine commander could take evasive action. This situation was remedied with new tactics and new weapons.
The tactical improvements developed by Frederic John Walker included the creeping attack. Two anti-submarine ships were needed for this (usually sloops or corvettes). The "directing ship" tracked the target submarine on ASDIC from a position about 1500 to 2000 yards behind the submarine. The second ship, with her ASDIC turned off and running at 5 knots, started an attack from a position between the directing ship and the target. This attack was controlled by radio telephone from the directing ship, based on their ASDIC and the range (by rangefinder) and bearing of the attacking ship. As soon as the depth charges had been released, the attacking ship left the immediate area at full speed. The directing ship then entered the target area and also released a pattern of depth charges. The low speed of the approach meant the submarine could not predict when depth charges were going to be released. Any evasive action was detected by the directing ship and steering orders to the attacking ship given accordingly. The low speed of the attack had the advantage that the German acoustic torpedo was not effective against a warship traveling so slowly. A variation of the creeping attack was the "plaster" attack, in which three attacking ships working in a close line abreast were directed over the target by the directing ship. 
The new weapons to deal with the ASDIC blind spot were "ahead-throwing weapons", such as Hedgehogs and later Squids, which projected warheads at a target ahead of the attacker and still in ASDIC contact. These allowed a single escort to make better aimed attacks on submarines. Developments during the war resulted in British ASDIC sets that used several different shapes of beam, continuously covering blind spots. Later, acoustic torpedoes were used.
Early in World War II (September 1940), British ASDIC technology was transferred for free to the United States. Research on ASDIC and underwater sound was expanded in the UK and in the US. Many new types of military sound detection were developed. These included sonobuoys, first developed by the British in 1944 under the codename High Tea, dipping / dunking sonar and mine-detection sonar. This work formed the basis for post-war developments related to countering the nuclear submarine.
During the 1930s American engineers developed their own underwater sound-detection technology, and important discoveries were made, such as the existence of thermoclines and their effects on sound waves.  Americans began to use the term SONAR for their systems, coined by Frederick Hunt to be the equivalent of RADAR. 
US Navy Underwater Sound Laboratory
In 1917, the US Navy acquired J. Warren Horton's services for the first time. On leave from Bell Labs, he served the government as a technical expert, first at the experimental station at Nahant, Massachusetts, and later at US Naval Headquarters, in London, England. At Nahant he applied the newly developed vacuum tube, then associated with the formative stages of the field of applied science now known as electronics, to the detection of underwater signals. As a result, the carbon button microphone, which had been used in earlier detection equipment, was replaced by the precursor of the modern hydrophone. Also during this period, he experimented with methods for towing detection. This was due to the increased sensitivity of his device. The principles are still used in modern towed sonar systems.
To meet the defense needs of Great Britain, he was sent to England to install in the Irish Sea bottom-mounted hydrophones connected to a shore listening post by submarine cable. While this equipment was being loaded on the cable-laying vessel, World War I ended and Horton returned home.
During World War II, he continued to develop sonar systems that could detect submarines, mines, and torpedoes. He published Fundamentals of Sonar in 1957 as chief research consultant at the US Navy Underwater Sound Laboratory. He held this position until 1959 when he became technical director, a position he held until mandatory retirement in 1963.  
Materials and designs in the US and Japan
There was little progress in US sonar from 1915 to 1940. In 1940, US sonars typically consisted of a magnetostrictive transducer and an array of nickel tubes connected to a 1-foot-diameter steel plate attached back-to-back to a Rochelle salt crystal in a spherical housing. This assembly penetrated the ship hull and was manually rotated to the desired angle. The piezoelectric Rochelle salt crystal had better parameters, but the magnetostrictive unit was much more reliable. High losses to US merchant supply shipping early in World War II led to large scale high priority US research in the field, pursuing both improvements in magnetostrictive transducer parameters and Rochelle salt reliability. Ammonium dihydrogen phosphate (ADP), a superior alternative, was found as a replacement for Rochelle salt the first application was a replacement of the 24 kHz Rochelle-salt transducers. Within nine months, Rochelle salt was obsolete. The ADP manufacturing facility grew from few dozen personnel in early 1940 to several thousands in 1942.
One of the earliest application of ADP crystals were hydrophones for acoustic mines the crystals were specified for low-frequency cutoff at 5 Hz, withstanding mechanical shock for deployment from aircraft from 3,000 m (10,000 ft), and ability to survive neighboring mine explosions. One of key features of ADP reliability is its zero aging characteristics the crystal keeps its parameters even over prolonged storage.
Another application was for acoustic homing torpedoes. Two pairs of directional hydrophones were mounted on the torpedo nose, in the horizontal and vertical plane the difference signals from the pairs were used to steer the torpedo left-right and up-down. A countermeasure was developed: the targeted submarine discharged an effervescent chemical, and the torpedo went after the noisier fizzy decoy. The counter-countermeasure was a torpedo with active sonar - a transducer was added to the torpedo nose, and the microphones were listening for its reflected periodic tone bursts. The transducers comprised identical rectangular crystal plates arranged to diamond-shaped areas in staggered rows.
Passive sonar arrays for submarines were developed from ADP crystals. Several crystal assemblies were arranged in a steel tube, vacuum-filled with castor oil, and sealed. The tubes then were mounted in parallel arrays.
The standard US Navy scanning sonar at the end of World War II operated at 18 kHz, using an array of ADP crystals. Desired longer range, however, required use of lower frequencies. The required dimensions were too big for ADP crystals, so in the early 1950s magnetostrictive and barium titanate piezoelectric systems were developed, but these had problems achieving uniform impedance characteristics, and the beam pattern suffered. Barium titanate was then replaced with more stable lead zirconate titanate (PZT), and the frequency was lowered to 5 kHz. The US fleet used this material in the AN / SQS-23 sonar for several decades. The SQS-23 sonar first used magnetostrictive nickel transducers, but these weighed several tons, and nickel was expensive and considered a critical material piezoelectric transducers were therefore substituted. The sonar was a large array of 432 individual transducers. At first, the transducers were unreliable, showing mechanical and electrical failures and deteriorating soon after installation they were also produced by several vendors, had different designs, and their characteristics were different enough to impair the array's performance. The policy to allow repair of individual transducers was then sacrificed, and "expendable modular design", sealed non-repairable modules, was chosen instead, eliminating the problem with seals and other extraneous mechanical parts. 
The Imperial Japanese Navy at the onset of World War II used projectors based on quartz. These were big and heavy, especially if designed for lower frequencies the one for Type 91 set, operating at 9 kHz, had a diameter of 30 inches (760 mm) and was driven by an oscillator with 5 kW power and 7 kV of output amplitude . The Type 93 projectors consisted of solid sandwiches of quartz, assembled into spherical cast iron bodies. The Type 93 sonars were later replaced with Type 3, which followed German design and used magnetostrictive projectors the projectors consisted of two rectangular identical independent units in a cast iron rectangular body about 16 by 9 inches (410 mm × 230 mm). The exposed area was half the wavelength wide and three wavelengths high. The magnetostrictive cores were made from 4 mm stampings of nickel, and later of an iron-aluminum alloy with aluminum content between 12.7% and 12.9%. The power was provided from a 2 kW at 3.8 kV, with polarization from a 20 V, 8 A DC source.
The passive hydrophones of the Imperial Japanese Navy were based on moving-coil design, Rochelle salt piezo transducers, and carbon microphones. 
Later developments in transducers
Magnetostrictive transducers were pursued after World War II as an alternative to piezoelectric ones. Nickel scroll-wound ring transducers were used for high-power low-frequency operations, with size up to 13 feet (4.0 m) in diameter, probably the largest individual sonar transducers ever. The advantage of metals is their high tensile strength and low input electrical impedance, but they have electrical losses and lower coupling coefficient than PZT, whose tensile strength can be increased by prestressing. Other materials were also tried nonmetallic ferrites were promising for their low electrical conductivity resulting in low eddy current losses, Metglas offered high coupling coefficient, but they were inferior to PZT overall. In the 1970s, compounds of rare earths and iron were discovered with superior magnetomechanic properties, namely the Terfenol-D alloy. This made possible new designs, e.g. a hybrid magnetostrictive-piezoelectric transducer. The most recent of these improved magnetostrictive materials is Galfenol.
Other types of transducers include variable-reluctance (or moving-armature, or electromagnetic) transducers, where magnetic force acts on the surfaces of gaps, and moving coil (or electrodynamic) transducers, similar to conventional speakers the latter are used in underwater sound calibration , due to their very low resonance frequencies and flat broadband characteristics above them. 
Active sonar uses a sound transmitter (or projector) and a receiver. When the two are in the same place it is monostatic operation. When the transmitter and receiver are separated it is bistatic operation.  When more transmitters (or more receivers) are used, again spatially separated, it is multistatic operation. Most sonars are used monostatically with the same array often being used for transmission and reception.  Active sonobuoy fields may be operated multistatically.
Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections (echo) of the pulse. This pulse of sound is generally created electronically using a sonar projector consisting of a signal generator, power amplifier and electro-acoustic transducer / array.  A transducer is a device that can transmit and receive acoustic signals ("pings"). A beamformer is usually employed to concentrate the acoustic power into a beam, which may be swept to cover the required search angles. Generally, the electro-acoustic transducers are of the Tonpilz type and their design may be optimized to achieve maximum efficiency over the widest bandwidth, in order to optimise performance of the overall system. Occasionally, the acoustic pulse may be created by other means, e.g. chemically using explosives, airguns or plasma sound sources.
To measure the distance to an object, the time from transmission of a pulse to reception is measured and converted into a range using the known speed of sound.  To measure the bearing, several hydrophones are used, and the set measures the relative arrival time to each, or with an array of hydrophones, by measuring the relative amplitude in beams formed through a process called beamforming. Use of an array reduces the spatial response so that to provide wide cover multibeam systems are used. The target signal (if present) together with noise is then passed through various forms of signal processing,  which for simple sonars may be just energy measurement.It is then presented to some form of decision device that calls the output either the required signal or noise. This decision device may be an operator with headphones or a display, or in more sophisticated sonars this function may be carried out by software. Further processes may be carried out to classify the target and localise it, as well as measuring its velocity.
The pulse may be at constant frequency or a chirp of changing frequency (to allow pulse compression on reception). Simple sonars generally use the former with a filter wide enough to cover possible Doppler changes due to target movement, while more complex ones generally include the latter technique. Since digital processing became available pulse compression has usually been implemented using digital correlation techniques. Military sonars often have multiple beams to provide all-round cover while simple ones only cover a narrow arc, although the beam may be rotated, relatively slowly, by mechanical scanning.
Particularly when single frequency transmissions are used, the Doppler effect can be used to measure the radial speed of a target. The difference in frequency between the transmitted and received signal is measured and converted into a velocity. Since Doppler shifts can be introduced by either receiver or target motion, allowance has to be made for the radial speed of the searching platform.
One useful small sonar is similar in appearance to a waterproof flashlight. The head is pointed into the water, a button is pressed, and the device displays the distance to the target. Another variant is a "fishfinder" that shows a small display with shoals of fish. Some civilian sonars (which are not designed for stealth) approach active military sonars in capability, with three-dimensional displays of the area near the boat.
When active sonar is used to measure the distance from the transducer to the bottom, it is known as echo sounding. Similar methods may be used looking upward for wave measurement.
Active sonar is also used to measure distance through water between two sonar transducers or a combination of a hydrophone (underwater acoustic microphone) and projector (underwater acoustic speaker). When a hydrophone / transducer receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transducer / projector transmits an interrogation signal and measures the time between this transmission and the receipt of the other transducer / hydrophone reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two platforms. This technique, when used with multiple transducers / hydrophones / projectors, can calculate the relative positions of static and moving objects in water.
In combat situations, an active pulse can be detected by an enemy and will reveal a submarine's position at twice the maximum distance that the submarine can itself detect a contact and give clues as to the submarines identity based on the characteristics of the outgoing ping. For these reasons, active sonar is not frequently used by military submarines.
A very directional, but low-efficiency, type of sonar (used by fisheries, military, and for port security) makes use of a complex nonlinear feature of water known as non-linear sonar, the virtual transducer being known as a parametric array.
Project Artemis was an experimental research and development project in the late 1950s to mid 1960s to examine acoustic propagation and signal processing for a low-frequency active sonar system that might be used for ocean surveillance. A secondary objective was examination of engineering problems of fixed active bottom systems.  The receiving array was located on the slope of Plantagnet Bank off Bermuda. The active source array was deployed from the converted World War II tanker USNS Mission Capistrano.  Elements of Artemis were used experimentally after the main experiment was terminated.
This is an active sonar device that receives a specific stimulus and immediately (or with a delay) retransmits the received signal or a predetermined one. Transponders can be used to remotely activate or recover subsea equipment. 
A sonar target is small relative to the sphere, centered around the emitter, on which it is located. Therefore, the power of the reflected signal is very low, several orders of magnitude less than the original signal. Even if the reflected signal was of the same power, the following example (using hypothetical values) shows the problem: Suppose a sonar system is capable of emitting a 10,000 W / m 2 signal at 1 m, and detecting a 0.001 W / m 2 signal. At 100 m the signal will be 1 W / m 2 (due to the inverse-square law). If the entire signal is reflected from a 10 m 2 target, it will be at 0.001 W / m 2 when it reaches the emitter, i.e. just detectable. However, the original signal will remain above 0.001 W / m 2 until 3000 m. Any 10 m 2 target between 100 and 3000 m using a similar or better system would be able to detect the pulse, but would not be detected by the emitter. The detectors must be very sensitive to pick up the echoes. Since the original signal is much more powerful, it can be detected many times further than twice the range of the sonar (as in the example).
Active sonar have two performance limitations: due to noise and reverberation. In general, one or other of these will dominate, so that the two effects can be initially considered separately.
In noise-limited conditions at initial detection: 
where SL is the source level, PL is the propagation loss (sometimes referred to as transmission loss), TS is the target strength, NL is the noise level, AG is the array gain of the receiving array (sometimes approximated by its directivity index) and DT is the detection threshold.
In reverberation-limited conditions at initial detection (neglecting array gain):
where RL is the reverberation level, and the other factors are as before.
Hand-held sonar for use by a diver
- The LIMIS (limpet mine imaging sonar) is a hand-held or ROV-mounted imaging sonar for use by a diver. Its name is because it was designed for patrol divers (combat frogmen or clearance divers) to look for limpet mines in low visibility water.
- The LUIS (lensing underwater imaging system) is another imaging sonar for use by a diver.
- There is or was a small flashlight-shaped handheld sonar for divers, that merely displays range.
- For the INSS (integrated navigation sonar system)
Upward looking sonar
An upward looking sonar (ULS) is a sonar device pointed upwards looking towards the surface of the sea. It is used for similar purposes as downward looking sonar, but has some unique applications such as measuring sea ice thickness, roughness and concentration,   or measuring air entrainment from bubble plumes during rough seas. Often it is moored on the bottom of the ocean or floats on a taut line mooring at a constant depth of perhaps 100 m. They may also be used by submarines, AUVs, and floats such as the Argo float. 
Passive sonar listens without transmitting. It is often employed in military settings, although it is also used in science applications, e.g., detecting fish for presence / absence studies in various aquatic environments - see also passive acoustics and passive radar. In the very broadest usage, this term can encompass virtually any analytical technique involving remotely generated sound, though it is usually restricted to techniques applied in an aquatic environment.
Identifying sound sources
Passive sonar has a wide variety of techniques for identifying the source of a detected sound. For example, U.S. vessels usually operate 60 Hz alternating current power systems. If transformers or generators are mounted without proper vibration insulation from the hull or become flooded, the 60 Hz sound from the windings can be emitted from the submarine or ship. This can help to identify its nationality, as all European submarines and nearly every other nation's submarine have 50 Hz power systems. Intermittent sound sources (such as a wrench being dropped), called "transients," may also be detectable to passive sonar. Until fairly recently, [ when? ] an experienced, trained operator identified signals, but now computers may do this.
Passive sonar systems may have large sonic databases, but the sonar operator usually finally classifies the signals manually. A computer system frequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or the type of weapon released), and even particular ships.
Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle. For this reason, many submarines operate nuclear reactors that can be cooled without pumps, using silent convection, or fuel cells or batteries, which can also run silently. Vehicles' propellers are also designed and precisely machined to emit minimal noise. High-speed propellers often create tiny bubbles in the water, and this cavitation has a distinct sound.
The sonar hydrophones may be towed behind the ship or submarine in order to reduce the effect of noise generated by the watercraft itself. Towed units also combat the thermocline, as the unit may be towed above or below the thermocline.
The display of most passive sonars used to be a two-dimensional waterfall display. The horizontal direction of the display is bearing. The vertical is frequency, or sometimes time. Another display technique is to color-code frequency-time information for bearing. More recent displays are generated by the computers, and mimic radar-type plan position indicator displays.
Unlike active sonar, only one-way propagation is involved. Because of the different signal processing used, the minimal detectable signal-to-noise ratio will be different. The equation for determining the performance of a passive sonar is  
where SL is the source level, PL is the propagation loss, NL is the noise level, AG is the array gain and DT is the detection threshold. The figure of merit of a passive sonar is
The detection, classification and localization performance of a sonar depends on the environment and the receiving equipment, as well as the transmitting equipment in an active sonar or the target radiated noise in a passive sonar.
Sonar operation is affected by variations in sound speed, particularly in the vertical plane. Sound travels more slowly in fresh water than in sea water, though the difference is small. The speed is determined by the water's bulk modulus and mass density. The bulk modulus is affected by temperature, dissolved impurities (usually salinity), and pressure. The density effect is small. The speed of sound (in feet per second) is approximately:
4388 + (11.25 × temperature (in ° F)) + (0.0182 × depth (in feet)) + salinity (in parts-per-thousand).
This empirically derived approximation equation is reasonably accurate for normal temperatures, concentrations of salinity and the range of most ocean depths. Ocean temperature varies with depth, but at between 30 and 100 meters there is often a marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. This can frustrate sonar, because a sound originating on one side of the thermocline tends to be bent, or refracted, through the thermocline. The thermocline may be present in shallower coastal waters. However, wave action will often mix the water column and eliminate the thermocline. Water pressure also affects sound propagation: higher pressure increases the sound speed, which causes the sound waves to refract away from the area of higher sound speed. The mathematical model of refraction is called Snell's law.
If the sound source is deep and the conditions are right, propagation may occur in the 'deep sound channel'. This provides extremely low propagation loss to a receiver in the channel. This is because of sound trapping in the channel with no losses at the boundaries. Similar propagation can occur in the 'surface duct' under suitable conditions. However, in this case there are reflection losses at the surface.
In shallow water propagation is generally by repeated reflection at the surface and bottom, where considerable losses can occur.
Sound propagation is affected by absorption in the water itself as well as at the surface and bottom. This absorption depends upon frequency, with several different mechanisms in sea water. Long-range sonar uses low frequencies to minimize absorption effects.
The sea contains many sources of noise that interfere with the desired target echo or signature. The main noise sources are waves and shipping. The motion of the receiver through the water can also cause speed-dependent low frequency noise.
When active sonar is used, scattering occurs from small objects in the sea as well as from the bottom and surface. This can be a major source of interference. This acoustic scattering is analogous to the scattering of the light from a car's headlights in fog: a high-intensity pencil beam will penetrate the fog to some extent, but broader-beam headlights emit much light in unwanted directions, much of which is scattered back to the observer, overwhelming that reflected from the target ("white-out"). For analogous reasons active sonar needs to transmit in a narrow beam to minimize scattering.
The scattering of sonar from objects (mines, pipelines, zooplankton, geological features, fish etc.) is how active sonar detects them, but this ability can be masked by strong scattering from false targets, or 'clutter'. Where they occur (under breaking waves  in ship wakes in gas emitted from seabed seeps and leaks  etc.), gas bubbles are powerful sources of clutter, and can readily hide targets. TWIPS (Twin Inverted Pulse Sonar)    is currently the only sonar that can overcome this clutter problem.
This is important as many recent conflicts have occurred in coastal waters, and the inability to detect whether mines are present or not present hazards and delays to military vessels, and also to aid convoys and merchant shipping trying to support the region long after the conflict has ceased. 
The sound reflection characteristics of the target of an active sonar, such as a submarine, are known as its target strength. A complication is that echoes are also obtained from other objects in the sea such as whales, wakes, schools of fish and rocks.
Passive sonar detects the target's radiated noise characteristics. The radiated spectrum comprises a continuous spectrum of noise with peaks at certain frequencies which can be used for classification.
Active (powered) countermeasures may be launched by a submarine under attack to raise the noise level, provide a large false target, and obscure the signature of the submarine itself.
Passive (i.e., non-powered) countermeasures includes:
- Mounting noise-generating devices on isolating devices.
- Sound-absorbent coatings on the hulls of submarines, for example anechoic tiles.
Modern naval warfare makes extensive use of both passive and active sonar from water-borne vessels, aircraft and fixed installations. Although active sonar was used by surface craft in World War II, submarines avoided the use of active sonar due to the potential for revealing their presence and position to enemy forces. However, the advent of modern signal-processing enabled the use of passive sonar as a primary means for search and detection operations. In 1987 a division of Japanese company Toshiba reportedly  sold machinery to the Soviet Union that allowed their submarine propeller blades to be milled so that they became radically quieter, making the newer generation of submarines more difficult to detect.
The use of active sonar by a submarine to determine bearing is extremely rare and will not necessarily give high quality bearing or range information to the submarines fire control team. However, use of active sonar on surface ships is very common and is used by submarines when the tactical situation dictates it is more important to determine the position of a hostile submarine than conceal their own position. With surface ships, it might be assumed that the threat is already tracking the ship with satellite data as any vessel around the emitting sonar will detect the emission. Having heard the signal, it is easy to identify the sonar equipment used (usually with its frequency) and its position (with the sound wave's energy). Active sonar is similar to radar in that, while it allows detection of targets at a certain range, it also enables the emitter to be detected at a far greater range, which is undesirable.
Since active sonar reveals the presence and position of the operator, and does not allow exact classification of targets, it is used by fast (planes, helicopters) and by noisy platforms (most surface ships) but rarely by submarines. When active sonar is used by surface ships or submarines, it is typically activated very briefly at intermittent periods to minimize the risk of detection. Consequently, active sonar is normally considered a backup to passive sonar. In aircraft, active sonar is used in the form of disposable sonobuoys that are dropped in the aircraft's patrol area or in the vicinity of possible enemy sonar contacts.
Passive sonar has several advantages, most importantly that it is silent. If the target radiated noise level is high enough, it can have a greater range than active sonar, and allows the target to be identified. Since any motorized object makes some noise, it may in principle be detected, depending on the level of noise emitted and the ambient noise level in the area, as well as the technology used. To simplify, passive sonar "sees" around the ship using it. On a submarine, nose-mounted passive sonar detects in directions of about 270 °, centered on the ship's alignment, the hull-mounted array of about 160 ° on each side, and the towed array of a full 360 °. The invisible areas are due to the ship's own interference. Once a signal is detected in a certain direction (which means that something makes sound in that direction, this is called broadband detection) it is possible to zoom in and analyze the signal received (narrowband analysis). This is generally done using a Fourier transform to show the different frequencies making up the sound. Since every engine makes a specific sound, it is straightforward to identify the object. Databases of unique engine sounds are part of what is known as acoustic intelligence or ACINT.
Another use of passive sonar is to determine the target's trajectory. This process is called target motion analysis (TMA), and the resultant "solution" is the target's range, course, and speed.TMA is done by marking from which direction the sound comes at different times, and comparing the motion with that of the operator's own ship. Changes in relative motion are analyzed using standard geometrical techniques along with some assumptions about limiting cases.
Passive sonar is stealthy and very useful. However, it requires high-tech electronic components and is costly. It is generally deployed on expensive ships in the form of arrays to enhance detection. Surface ships use it to good effect it is even better used by submarines, and it is also used by airplanes and helicopters, mostly to a "surprise effect", since submarines can hide under thermal layers. If a submarine's commander believes he is alone, he may bring his boat closer to the surface and be easier to detect, or go deeper and faster, and thus make more sound.
Examples of sonar applications in military use are given below. Many of the civil uses given in the following section may also be applicable to naval use.
Until recently, ship sonars were usually with hull mounted arrays, either amidships or at the bow. It was soon found after their initial use that a means of reducing flow noise was required. The first were made of canvas on a framework, then steel ones were used. Now domes are usually made of reinforced plastic or pressurized rubber. Such sonars are primarily active in operation. An example of a conventional hull mounted sonar is the SQS-56.
Because of the problems of ship noise, towed sonars are also used. These also have the advantage of being able to be placed deeper in the water. However, there are limitations on their use in shallow water. These are called towed arrays (linear) or variable depth sonars (VDS) with 2 / 3D arrays. A problem is that the winches required to deploy / recover these are large and expensive. VDS sets are primarily active in operation while towed arrays are passive.
An example of a modern active-passive ship towed sonar is Sonar 2087 made by Thales Underwater Systems.
Modern torpedoes are generally fitted with an active / passive sonar. This may be used to home directly on the target, but wake homing torpedoes are also used. An early example of an acoustic homer was the Mark 37 torpedo.
Torpedo countermeasures can be towed or free. An early example was the German Sieglinde device while the Bold was a chemical device. A widely used US device was the towed AN / SLQ-25 Nixie while the mobile submarine simulator (MOSS) was a free device. A modern alternative to the Nixie system is the UK Royal Navy S2170 Surface Ship Torpedo Defense system.
Mines may be fitted with a sonar to detect, localize and recognize the required target. An example is the CAPTOR mine.
Mine countermeasure (MCM) sonar, sometimes called "mine and obstacle avoidance sonar (MOAS)", is a specialized type of sonar used for detecting small objects. Most MCM sonars are hull mounted but a few types are VDS design. An example of a hull mounted MCM sonar is the Type 2193 while the SQQ-32 mine-hunting sonar and Type 2093 systems are VDS designs.
Submarines rely on sonar to a greater extent than surface ships as they cannot use radar at depth. The sonar arrays may be hull mounted or towed. Information fitted on typical fits is given in Oyashio-class submarine and Swiftsure-class submarine.
Helicopters can be used for antisubmarine warfare by deploying fields of active-passive sonobuoys or can operate dipping sonar, such as the AQS-13. Fixed wing aircraft can also deploy sonobuoys and have greater endurance and capacity to deploy them. Processing from the sonobuoys or dipping sonar can be on the aircraft or on ship. Dipping sonar has the advantage of being deployable to depths appropriate to daily conditions. Helicopters have also been used for mine countermeasure missions using towed sonars such as the AQS-20A.
Dedicated sonars can be fitted to ships and submarines for underwater communication.
The United States began a system of passive, fixed ocean surveillance systems in 1950 with the classified name Sound Surveillance System (SOSUS) with American Telephone and Telegraph Company (AT&T), with its Bell Laboratories research and Western Electric manufacturing entities being contracted for development and installation. The systems exploited the deep sound (SOFAR) channel and were based on an AT&T sound spectrograph, which converted sound into a visual spectrogram representing a time – frequency analysis of sound that was developed for speech analysis and modified to analyze low-frequency underwater sounds. That process was Low Frequency Analysis and Recording and the equipment was termed the Low Frequency Analyzer and Recorder, both with the acronym LOFAR. LOFAR research was termed Jezebel and led to usage in air and surface systems, particularly sonobuys using the process and sometimes using "Jezebel" in their name.    The proposed system offered such promise of long-range submarine detection that the Navy ordered immediate moves for implementation.  
Between installation of a test array followed by a full scale, forty element, prototype operational array in 1951 and 1958 systems were installed in the Atlantic and then the Pacific under the unclassified name Project Caesar. The original systems were terminated at classified shore stations designated Naval Facility (NAVFAC) explained as engaging in "ocean research" to cover their classified mission. The system was upgraded multiple times with more advanced cable allowing the arrays to be installed in ocean basins and upgraded processing. The shore stations were eliminated in a process of consolidation and rerouting the arrays to central processing centers into the 1990s. In 1985, with new mobile arrays and other systems becoming operational the collective system name was changed to Integrated Undersea Surveillance System (IUSS). In 1991 the mission of the system was declassified. The year before IUSS insignia were authorized for wear. Access was granted to some systems for scientific research.  
A similar system is believed to have been operated by the Soviet Union.
Sonar can be used to detect frogmen and other scuba divers. This can be applicable around ships or at entrances to ports. Active sonar can also be used as a deterrent and / or disablement mechanism. One such device is the Cerberus system.
Limpet mine imaging sonar (LIMIS) is a hand-held or ROV-mounted imaging sonar designed for patrol divers (combat frogmen or clearance divers) to look for limpet mines in low visibility water.
The LUIS is another imaging sonar for use by a diver.
Integrated navigation sonar system (INSS) is a small flashlight-shaped handheld sonar for divers that displays range.  
This is a sonar designed to detect and locate the transmissions from hostile active sonars. An example of this is the Type 2082 fitted on the British Vanguard-class submarines.
Fishing is an important industry that is seeing growing demand, but world catch tonnage is falling as a result of serious resource problems. The industry faces a future of continuing worldwide consolidation until a point of sustainability can be reached. However, the consolidation of the fishing fleets are driving increased demands for sophisticated fish finding electronics such as sensors, sounders and sonars. Historically, fishermen have used many different techniques to find and harvest fish. However, acoustic technology has been one of the most important driving forces behind the development of the modern commercial fisheries.
Sound waves travel differently through fish than through water because a fish's air-filled swim bladder has a different density than seawater. This density difference allows the detection of schools of fish by using reflected sound. Acoustic technology is especially well suited for underwater applications since sound travels farther and faster underwater than in air. Today, commercial fishing vessels rely almost completely on acoustic sonar and sounders to detect fish. Fishermen also use active sonar and echo sounder technology to determine water depth, bottom contour, and bottom composition.
Companies such as eSonar, Raymarine, Marport Canada, Wesmar, Furuno, Krupp, and Simrad make a variety of sonar and acoustic instruments for the deep sea commercial fishing industry. For example, net sensors take various underwater measurements and transmit the information back to a receiver on board a vessel. Each sensor is equipped with one or more acoustic transducers depending on its specific function. Data is transmitted from the sensors using wireless acoustic telemetry and is received by a hull mounted hydrophone. The analog signals are decoded and converted by a digital acoustic receiver into data which is transmitted to a bridge computer for graphical display on a high resolution monitor.
Echo sounding is a process used to determine the depth of water beneath ships and boats. A type of active sonar, echo sounding is the transmission of an acoustic pulse directly downwards to the seabed, measuring the time between transmission and echo return, after having hit the bottom and bouncing back to its ship of origin. The acoustic pulse is emitted by a transducer which receives the return echo as well. The depth measurement is calculated by multiplying the speed of sound in water (averaging 1,500 meters per second) by the time between emission and echo return.  
The value of underwater acoustics to the fishing industry has led to the development of other acoustic instruments that operate in a similar fashion to echo-sounders but, because their function is slightly different from the initial model of the echo-sounder, have been given different terms.
The net sounder is an echo sounder with a transducer mounted on the headline of the net rather than on the bottom of the vessel. Nevertheless, to accommodate the distance from the transducer to the display unit, which is much greater than in a normal echo-sounder, several refinements have to be made. Two main types are available. The first is the cable type in which the signals are sent along a cable. In this case there has to be the provision of a cable drum on which to haul, shoot and stow the cable during the different phases of the operation. The second type is the cable-less net-sounder - such as Marport's Trawl Explorer - in which the signals are sent acoustically between the net and hull mounted receiver-hydrophone on the vessel. In this case no cable drum is required but sophisticated electronics are needed at the transducer and receiver.
The display on a net sounder shows the distance of the net from the bottom (or the surface), rather than the depth of water as with the echo-sounder's hull-mounted transducer. Fixed to the headline of the net, the footrope can usually be seen which gives an indication of the net performance. Any fish passing into the net can also be seen, allowing fine adjustments to be made to catch the most fish possible. In other fisheries, where the amount of fish in the net is important, catch sensor transducers are mounted at various positions on the cod-end of the net. As the cod-end fills up these catch sensor transducers are triggered one by one and this information is transmitted acoustically to display monitors on the bridge of the vessel. The skipper can then decide when to haul the net.
Modern versions of the net sounder, using multiple element transducers, function more like a sonar than an echo sounder and show slices of the area in front of the net and not merely the vertical view that the initial net sounders used.
The sonar is an echo-sounder with a directional capability that can show fish or other objects around the vessel.
ROV and UUV
Small sonars have been fitted to remotely operated vehicles (ROVs) and unmanned underwater vehicles (UUVs) to allow their operation in murky conditions. These sonars are used for looking ahead of the vehicle. The Long-Term Mine Reconnaissance System is a UUV for MCM purposes.
Sonars which act as beacons are fitted to aircraft to allow their location in the event of a crash in the sea. Short and long baseline sonars may be used for caring out the location, such as LBL.
Prosthesis for the visually impaired
In 2013 an inventor in the United States unveiled a "spider-sense" bodysuit, equipped with ultrasonic sensors and haptic feedback systems, which alerts the wearer of incoming threats allowing them to respond to attackers even when blindfolded. 
Detection of fish, and other marine and aquatic life, and estimation their individual sizes or total biomass using active sonar techniques. As the sound pulse travels through water it encounters objects that are of different density or acoustic characteristics than the surrounding medium, such as fish, that reflect sound back toward the sound source. These echoes provide information on fish size, location, abundance and behavior. Data is usually processed and analyzed using a variety of software such as Echoview.
An upward looking echo sounder mounted on the bottom or on a platform may be used to make measurements of wave height and period. From this statistics of the surface conditions at a location can be derived.
Water velocity measurement
Special short range sonars have been developed to allow measurements of water velocity.
Bottom type assessment
Sonars have been developed that can be used to characterize the sea bottom into, for example, mud, sand, and gravel. Relatively simple sonars such as echo sounders can be promoted to seafloor classification systems via add-on modules, converting echo parameters into sediment type. Different algorithms exist, but they are all based on changes in the energy or shape of the reflected sounder pings. Advanced substrate classification analysis can be achieved using calibrated (scientific) echosounders and parametric or fuzzy-logic analysis of the acoustic data.
Side-scan sonars can be used to derive maps of seafloor topography (bathymetry) by moving the sonar across it just above the bottom. Low frequency sonars such as GLORIA have been used for continental shelf wide surveys while high frequency sonars are used for more detailed surveys of smaller areas.
Powerful low frequency echo-sounders have been developed for providing profiles of the upper layers of the ocean bottom.
Gas leak detection from the seabed
Gas bubbles can leak from the seabed, or close to it, from multiple sources. These can be detected by both passive  and active sonar  (shown in schematic figure  by yellow and red systems respectively).
Natural seeps of methane and carbon dioxide occur.  Gas pipelines can leak, and it is important to be able to detect whether leakage occurs from Carbon Capture and Storage Facilities (CCSFs e.g. depleted oil wells into which extracted atmospheric carbon is stored).     Quantification of the amount of gas leaking is difficult, and although estimates can be made use active and passive sonar, it is important to question their accuracy because of the assumptions inherent in making such estimations from sonar data.  
Synthetic sonar apertures
Various synthetic aperture sonars have been built in the laboratory and some have entered use in mine-hunting and search systems. An explanation of their operation is given in synthetic aperture sonar.
Parametric sources use the non-linearity of water to generate the difference frequency between two high frequencies. A virtual end-fire array is formed. Such a projector has advantages of broad bandwidth, narrow beamwidth, and when fully developed and carefully measured it has no obvious sidelobes: see Parametric array. Its major disadvantage is very low efficiency of only a few percent.  P.J. Westervelt summarizes the trends involved. 
Sonar in extraterrestrial contexts
Use of both passive and active sonar has been proposed for various extraterrestrial uses ,.  An example of the use of active sonar is in determining the depth of hydrocarbon seas on Titan,  An example of the use of passive sonar is in the detection of methanefalls on Titan, 
It has been noted that those proposals which suggest use of sonar without taking proper account of the difference between the Earthly (atmosphere, ocean, mineral) environments and the extraterrestrial ones, can lead to erroneous values    [ 66]  
Effect on marine mammals
Research has shown that use of active sonar can lead to mass strandings of marine mammals.  Beaked whales, the most common casualty of the strandings, have been shown to be highly sensitive to mid-frequency active sonar.  Other marine mammals such as the blue whale also flee away from the source of the sonar,  while naval activity was suggested to be the most probable cause of a mass stranding of dolphins.  The US Navy, which part-funded some of the studies, said that the findings only showed behavioral responses to sonar, not actual harm, but they "will evaluate the effectiveness of [their] marine mammal protective measures in light of new research findings ".  A 2008 US Supreme Court ruling on the use of sonar by the US Navy noted that there had been no cases where sonar had been conclusively shown to have harmed or killed a marine mammal. 
Some marine animals, such as whales and dolphins, use echolocation systems, sometimes called biosonar to locate predators and prey. Research on the effects of sonar on blue whales in the Southern California Bight shows that mid-frequency sonar use disrupts the whales' feeding behavior. This indicates that sonar-induced disruption of feeding and displacement from high-quality prey patches could have significant and previously undocumented impacts on baleen whale foraging ecology, individual fitness and population health. 
A review of evidence on the mass strandings of beaked whale linked to naval exercises where sonar was used was published in 2019. It concluded that the effects of mid-frequency active sonar are strongest on Cuvier's beaked whales but vary among individuals or populations. The review suggested the strength of response of individual animals may depend on whether they had prior exposure to sonar, and that symptoms of decompression sickness have been found in stranded whales that may be a result of such response to sonar.It noted that in the Canary Islands where multiple strandings had been previously reported, no more mass strandings had occurred once naval exercises during which sonar was used were banned in the area, and recommended that the ban be extended to other areas where mass strandings continue to occur.  
Effect on fish
High-intensity sonar sounds can create a small temporary shift in the hearing threshold of some fish.   [a]
The frequencies of sonars range from infrasonic to above a megahertz. Generally, the lower frequencies have longer range, while the higher frequencies offer better resolution, and smaller size for a given directionality.
To achieve reasonable directionality, frequencies below 1 kHz generally require large size, usually achieved as towed arrays. 
Low frequency sonars are loosely defined as 1–5 kHz, albeit some navies regard 5–7 kHz also as low frequency. Medium frequency is defined as 5–15 kHz. Another style of division considers low frequency to be under 1 kHz, and medium frequency at between 1–10 kHz. 
American World War II era sonars operated at a relatively high frequency of 20–30 kHz, to achieve directionality with reasonably small transducers, with typical maximum operational range of 2500 yd. Postwar sonars used lower frequencies to achieve longer range e.g. SQS-4 operated at 10 kHz with range up to 5000 yd. SQS-26 and SQS-53 operated at 3 kHz with range up to 20,000 yd their domes had size of approx. a 60-ft personnel boat, an upper size limit for conventional hull sonars. Achieving larger sizes by conformal sonar array spread over the hull has not been effective so far, for lower frequencies linear or towed arrays are therefore used. 
Japanese WW2 sonars operated at a range of frequencies. The Type 91, with 30 inch quartz projector, worked at 9 kHz. The Type 93, with smaller quartz projectors, operated at 17.5 kHz (model 5 at 16 or 19 kHz magnetostrictive) at powers between 1.7 and 2.5 kilowatts, with range of up to 6 km. The later Type 3, with German-design magnetostrictive transducers, operated at 13, 14.5, 16, or 20 kHz (by model), using twin transducers (except model 1 which had three single ones), at 0.2 to 2.5 kilowatts. The simple type used 14.5 kHz magnetostrictive transducers at 0.25 kW, driven by capacitive discharge instead of oscillators, with range up to 2.5 km. 
The sonar's resolution is angular objects further apart are imaged with lower resolutions than nearby ones.
Another source lists ranges and resolutions vs frequencies for sidescan sonars. 30 kHz provides low resolution with range of 1000–6000 m, 100 kHz gives medium resolution at 500–1000 m, 300 kHz gives high resolution at 150–500 m, and 600 kHz gives high resolution at 75–150 m. Longer range sonars are more adversely affected by nonhomogenities of water. Some environments, typically shallow waters near the coasts, have complicated terrain with many features higher frequencies become necessary there.