Absorption: The removal of energy from a sonar beam as it propagates through the water. Absorption is one of the factors in limiting the range of a sonar system underwater. For lower side scan sonar frequencies, the chemical relaxation of magnesium sulfate in sea water is a major factor in absorption. The higher frequencies are more affected by the physics of shear and viscosity.
Acquisition : The process of detecting and recognizing a seabed anomaly using sonar. Detection is a function of sonar design and acoustical physics while recognition is highly dependent upon the eye-brain interface and operator experience.
Active Sonar: A sonar system consisting of both a projector and hydrophone which is capable of transmitting and receiving acoustic signals. Examples of active sonars include echosounders, commercial side scan sonar and many types of sub bottom profilers.
Along-track: The dimension of the seabed or data record in a direction parallel to the track of the towfish (transverse). This is the opposite of the cross-track dimension. These two terms are used to describe sonar phenomena and dimensional corrections.
Altitude : The height of the towfish above the seabed, which is typically measured in feet or meters. Proper towfish altitude is important for acceptable seabed backscattering and, of course, to avoid seabed collisions. An excessively low altitude will reduce the range at which effective backscattering is produced. Excessively high altitudes can leave uninsonified seabed as well as preventing accurate slant range correction.
Ambient Noise : Acoustic signals, sensed by the sonar system, emanating from a variety of sources in the underwater environment. Ambient noise visible in sonar data can result from propeller cavitation, engine noise, and biological sources. Other sources are environmental such as wind, waves and rain.
Anechoic : An object or area characterized by an unusually low degree of reverberation; echo-free. An anechoic target often appears to be missing in data. Rubber tires and soft wood often sound absorbing using certain frequencies. These echo-free targets are often recognized by the "hole" they leave in the normal background sonar returns. Anechoic chambers and pools are used to test acoutiscal systems in the laboratory.
Angle Of Incidence : The angle that a straight line acoustic pulse meeting a surface makes with a normal to the surface. The angle of incidence is important in sonar backscattering. For instance, if the sonar altitude is too low, at long ranges the angle of incidence with the seabed becomes high. Much of the incident energy is not returned to the transducer. Further, if the seabed angles up out at range, the angle of incidence becomes lower and more energy will be returned, making a notable anomaly in the data.
Attenuation : The process of weakening or reducing the amplitude of a sonar signal. It is caused by numerous factors, including material dispersion, beam spreading and absorption. The attenuation of a sonar signal makes its detection more difficult. Reflected signals from far away are sometimes attenuated to such a degree that system noise in the sonar receiver electronics can be a problem.
Backscatter : The deflection of acoustic radiation in a scattering process through an angle greater than 90 degrees. Backscatter is the term commonly used to describe the return of energy from the seabed to the receiver in an active sonar.
Bathymetry : The measurement of the depths of oceans, seas or other large bodies of water, typically using narrow swath acoustic systems. Some modern side scan type of sonar systems use two transducers per side to produce bathymetric, data along with wide area imaging. These systems, although producing both types of data, leave much to be desired when compared with systems dedicated to either one or the other. Co-registered side scan and bathymetric data from a single instrument is a goal of many sonar designers.
Beam Angle: The amount of rotation needed to bring two opposite sides of a sonar beam, which is diverging from a point (transducers), into coincidence with another. The beam angle determines the rate of divergence during propagation. Narrow horizontal beam angles in side scan sonar provide high transverse resolution. In conventional sonar, as the beam propagates into the environment it spreads the beam. Therefore, resolution at longer ranges is lower than those in the near ranges. A focused beam, used on one sonar type in the 1960's, surmounted this problem by curving the transducer array. However it was only usable at one altitude above the seafloor. Wide vertical beam angles allow good sonar coverage from below the transducers out to long ranges but there is a tradeoff for any given transducer power output level between overall range and vertical beam angle.
Beam Forming: The process of shaping an acoustic beam through the control of the geometry of the transducer array. As the shape of the acoustic beam is crucial in imaging sonar systems, careful beam forming is important. The size, shape and arrangement of groups of transducer elements help form the beam. If a string of elements develops a crack or becomes electrically open, the shape of the beam changes radically. This change in beam form can be seen in acoustic data by an experienced data interpreter.
Beam Spreading: The divergence of a sonar beam as a direct function of angle and range. Beam spreading causes a loss of resolution in the far ranges, however where the beam is wider the system insonifies more of the environment per ping. Beam spreading also causes adjacent beams in the far ranges to overlap which may be advantageous in some side scan sonar operations.
Beam Width: The distance between two opposite sides of a beam at a specific range from its source. Beamwidth widens with distance from the transducer array. Horizontal beamwidth determines the transverse resolution (the minimum distance between two objects parallel to the line of travel that will be displayed as two separate objects in the data) of a sonar system.
Blanking: Sonar signal blocking caused by discontinuities in the water resulting in an empty, unprinted area in the sonar data; sometimes, but rarely, caused by local signals or radiation external to the control/display portion of the sonar system. Blanking often occurs when the transducer is towed through another vessel's (or the towing vessel's) wake. Cavitation and air mixing in these cases, block the sonar signal. A vessel hull can also cause blanking if the transducer is towed at very shallow depths.
Boomer: A seismic instrument typically operating in the .5 to 2.5 kHz range, producing a conical beam directed vertically towards the seafloor. Boomers are used for profiling geological features beneath the seabed. The acoustic energy penetrates the seafloor and some energy from the initial output reflects off layers of differing sediment types. This results in sonar data that resembles a "slice" of the seafloor directly beneath the towbody.
Bottom Lock: The method whereby the sonar continuously detects the seabed directly below the towfish and calculates the towfish height. This calculation is important for the slant range correction process in side scan sonar. In computerized sonar systems this process is referred to as "bottom tracking." Bottom lock can be lost if the sediment changes from hard to soft or if the towfish is pulled through a water column filled with discontinuities.
Catanary: The curve(s) assumed by a tow cable moving through the water, typically induced by the forces of water drag on the cable. The catanary is a significant factor in complicating the process of determining the horizontal distance to the towfish from the towing block. Towfish drag coefficients, cable weight and drag and length-of-cable-out determine the shape of cable catenaries.
Cavitation: The rapid formation and collapse of vapor pockets in water, most often caused by a significant and rapid drop in pressure. Some cavitation bubbles do not re-dissolve rapidly and are a major cause of quenching of the sonar signal and blanking in the data. In a busy waterway, cavitation bubbles may persist in the water column for hours and significantly reduce sonar transmission and reception capabilities.
Channel: One of two or more signals in a multi-signal sonar system; the area on the display or sonar record where data from this signal is shown. Modern instrumented sonar systems may utilize separate channels for many kinds of data such as: port and starboard side scan, sub bottom, towbody heading, depth, conductivity, temperature, and magnetometry.
Chirp: A type of sonar technology utilizing a separate projector and hydrophone where the projector transmits digitally produced, linear, swept FM pulses resulting in an increased system bandwidth. Because these systems are multi-frequency, greater bandwidth, rather than the pulse length, results in higher theoretical range resolution. Chirp technology has been successfully used in sub bottom profilers and is being developed for long range side scan applications.
Compression: A single axis reduction in size of a sonar image due to speed or slant range distortions. Compression in the transverse axis can be corrected through changes in ship speed or display update rate. In the case of computerized sonar processors, changes in the ratio of the display can correct this distortion. Range compression is a function of the sonar's acoustic path and can be corrected with algorithms built into most system's displays and processors.
Correction: The process of removing errors caused by speed, slant range or other sonar distortions. Most sonar systems allow the removal of data errors or distortions (see compression) simply and automatically. These corrections, while making a more accurate graphic display, are crucial if the data collection goal is mosaic construction.
Coverage: An area described by the seabed swath width of a side scan sonar and the distance traveled by the survey vessel on its track; also pertains to the repeat surveying of an area, i.e. one pass equals 100% coverage of an area and two passes over the same area equals 200% coverage.
Cross-track : The direction 90 degrees to the path of the vessel or towfish, the range dimension. This is the opposite of the along-track (transverse) dimension. These two terms are used to describe sonar phenomena and dimensional corrections.
Deadweight Depressor: A heavy, inert weight used to increase towfish depth when attached to the tow cable. When using long lengths of in-water cable, such as when towing lightweight towfish in depths greater than 100 meters, drag forces on the cable often prevent the towbody from descending to the required depth. Slower towspeeds or greater downward pull at the towbody are required. Although, deadweight depressors are more straightforward in application than hydrodynamic depressors, they should be streamlined and rigged as to be tangle free during and after deployment. The use of deadweights also requires greater overside lifting capacity than with hydrodynamic depressors.
Decibel: A unit used to express the intensity of a sound wave, equal to 20 times the common logarithm of the ratio of the pressure produced by the sound wave to a reference pressure (typically 1 micropascal at 1 meter).
Deck Cable: A cable used to connect the sonar control and display units to the slip ring on a winch. Although an armored cable is most commonly used from the winch to the towfish, a deck cable is not required to be high strength and can be made of flexible lightweight materials.
Depressor: An attachment to a sonar tow cable that assists in increasing the depths of the towed body; commonly of two types: deadweight and hydrodynamic. The depressor allows the user to bring a sonar towbody within an optimum altitude above the seabed. Although lower towspeeds also assist in lowering a towfish, speeds below 1 - 1.5 knots may produce towbody instability such as kiting and yaw resulting in data distortions.
Detectability : The size, shape and makeup of a seabed anomaly as related to a sonar's ability to discern its existence. As compared to recognition (the interpreter's acknowledgment of an anomaly), detection occurs when excess energy is returned to the sonar transducer from a target or discontinuity. When a target returns this excess energy from only one of many pings, the system will detect the target but the operator may not recognize it in the data. Modern sonar data processors, such as the Isis by Triton Technologies, are assisting the user in narrowing the gap between detection and recognition with separate graphic displays of the amplitude of individual return signals.
Differentiation: The process of using separate but identical navigational instruments where one is fixed at a known location and provides, via radio link, a second mobile instrument with offset calculations. This process is used to increase the accuracy of certain navigational instruments that may be affected by diurnal or atmospheric variations such as Loran, and those with inherent errors such as GPS.
Discontinuity: A change in the make up of a body of water that causes a change in the speed, and/or direction of sound propagation, of an incident sonar pulse. In comparison to an anomaly, which is usually distinct and separate within the environment, discontinuities are often widespread and difficult to discern as distinct. These include haline changes in the water, aerated or cavitated bodies of water and thermoclines.
Drag: The hydrodynamic forces exerted on the components of a tow assembly that tend to reduce its forward motion. It is important for a towbody to be streamlined for drag reduction. However, cable drag quickly becomes a major factor in towbody altitude when working with in-water lengths of hundreds or thousands of meters.
Event : A mark or notation put on a sonar record, or embedded in stored data, representing the moment of a navigational fix or other critical occurrence during a survey. Event marks are important in assessing progress of a survey and in tying navigation logs to sonar data. Modern computer data processors which geocode every sonar ping have simplified the practice of manual log keeping, but event marks are still important as progress references and data related geodetic markers.
First Bottom Return: The component of a side scan sonar record representing the shortest acoustic path between the towfish and the seabed directly below the towfish. The first bottom return is utilized by many sonar systems to determine fish height (altitude) which is important in the algorithms used for range compression correction. In uncorrected data, the operator will use the first bottom return as a measure of fish height for winch in/out commands to maintain proper towing altitudes. This feature of the sonar display will be very strong in hard bottom or over a rocky substrate, but may be difficult to discern over a mud or silt seabed.
First Surface Return: The component of a side scan sonar record representing the shortest acoustic path between the towfish and the surface directly above the towfish. Because the vertical beamwidth is very wide in side scan sonar, some of the acoustic energy propagates upwards from the transducer. Although this energy is very low level, the sea surface can be a good reflector and return enough of the incident pulse to be noticeable in data. As the fish is lowered, the first surface return moves, in the water column portion of the record, away from the centerline. In shallow water operations, as the fish is lowered past the half-depth point, this return becomes lost in the near range data. Beyond this it may not be discernible because of its low signal strength. In shallow water, when the first surface return crosses the first bottom return, the towfish is halfway between the surface and the seabed.
Fish Height: The distance between the towfish and the seabed, usually measured in feet or meters. (See Altitude).
Footprint: The area of seabed affected by the increase in the level of sound from an outgoing sonar pulse during, or after, a specific period of time. A smaller footprint of sonar on the seabed will result in a higher resolution image in the data. The footprint in the range dimension decreases in size as the sonar pulse propagates away from the transducer. The footprint in the transverse dimension is a function of horizontal beam angle and beam spreading.
Frequency: The number of cycles or completed alternations per unit time of a sound wave, most often measured in Hertz. Frequencies commonly used in conventional side scan sonar range from 25 to 450 kHz. Although pulse widths and beam angles vary in different sonars and at different frequencies within the same system, generally higher frequencies provide a higher level of resolution with a sacrifice in range.
Fully-corrected: A speed and slant range corrected sonar record which accurately depicts the seafloor in a 1:1, two dimensional image. Early side scan sonar systems did not have the capability of correcting the displayed data but in the late 1970's and early 80's, designs emerged which automatically provided corrections. These corrections could be made in real time during data gathering, but, for mosaic construction, the data almost always had to be played from storage several times to match it, lane for lane. Modern systems allow for computerized mosaic construction.
Gain: A measure of the increase in signal amplitude produced by an amplifier. In sonar applications, gains are most often applied in two ways. One is time-varied-gain where signal amplification increases as a function of time. This methodology can be applied because there is a constant speed of sound underwater for most side scan applications and the returning signal level from the seabed decreases as the pulse travels across the seabed and away from the transducer. Closer returns have a far higher intensity than distant ones. The other method of increasing gains in sonar is to apply them to the display only. In hard copy recorders, printer gain will darken the record and with computerized sonar processors, increasing video intensity has a similar effect.
GPS: (Global Positioning System) A satellite based navigation system providing accuracy usable for side scan sonar surveys on a worldwide basis. GPS has become a universal, reliable positioning system. Inherent errors in GPS (implemented by the Department of Defense) create inaccuracies of more than 100 meters. Differential base stations can reduce these errors to less than two meters but are time consuming to survey-in and have limited range. Still, GPS is a convenient navigation system for sonar survey operations. Even if the injected errors are removed from the system by the Government, differential corrections have the potential to further increase system accuracy.
Grazing Angle: The angle at which the side scan sonar pulse strikes, and propagates across, the seafloor. The grazing angle of the sonar pulse has an effect on the reverberation or backscatter of the insonified seabed. Grazing angle algorithms attempt to compensate for these changes and produce a uniform image in the sonar data.
Heave: The rise and fall of a surface vessel or towfish in a rhythmic movement; the disjointed, jagged images on a sonar record produced by towfish heave. Towbody heave is a major cause of data distortion, particularly when rough seas affect the stability of the surface vessel. Although, under some conditions, longer towcables will dampen the effect of heave seen in the data, under others it can result in a harmonic effect, increasing the heave. Heave compensators, which increase and decrease the amount of in-water cable, have been applied in the past. The concept is sound, but the systems are typically very complex and, under some conditions, will significantly add to the problem of heave.
Horizontal Beam Width: The angle of the transmitted (and/or received) sonar beam in the along-track (transverse) dimension, often between 0.1 and 2.5 degrees for side scan sonar. Narrow horizontal beamwidths increase transverse resolution. However, a more stable towbody is required to maintain distortion free data. Because beamwidth is related to transducer length and frequency, beamwidth decreases with increasing frequency for a given transducer length.
Hydrodynamic Depressor: A tow assembly depressor designed with vanes or a wing oriented in such a way as to increase negative lift when exposed to an increased water flow. Hydrodynamic depressors work by increasing the downward pull on the towbody end of the cable during towing. The advantage of this kind of depressor is that they are light weight and relatively easy to deploy. Some types can be rigged to invert and increase drag if they come in contact with the seabed, thus rapidly bringing the towfish to altitude and out of collision danger. They must be rigged carefully so as to be tangle free during and after deployment. A disadvantage to the hydrodynamic depressor is that an increase in towspeed will not raise the towfish to avoid an obstruction. It may instead, bring it deeper. Another disadvantage to this type of depressor is that they typically require significant towspeeds to develop effectiveness.
Hydrophone: a sonar receiver functioning by transforming underwater sound signals (pressure waves) into electrical signals. In many side scan sonars, the functions of the hydrophone and projector are performed by one transducer (active sonar). Often a hydrophone is a passive device doing no transmitting on its own. Hydrophones are commonly used to receive seismic echoes from explosive devices or other low frequency acoustic signals. In these applications, hydrophones are placed in "streamers" or long flexible tubes and towed behind survey ships for deep sub bottom imaging.
Hz: A unit of frequency equal to one cycle per second named after the physicist, H.R. Hertz (1857-1894). Side scan sonar frequencies generally output acoustic signals in the kilohertz (kHz), or thousands of cycles per second region. Bathymetric systems typically operate in the 20-200 kHz range, imaging side scan sonar in the 50-650 kHz range and a few special application sonar systems in the megahertz (1,000,000 Hertz) range. Hertz is also interchanged with the term "cycles" with one kilohertz being equal to one kilocycle.
INS: Integrated Navigation System: Most commonly a computer with a number of inputs which displays a coordinated navigation plot. INS systems have one or more displays and take input from a navigational device such as a GPS receiver and displays the ship track, course and heading on a CRT. Modern units allow the user to log targets, filter errant position fixes, log the ship's position history, display user defined lines the ship is to follow and display shorelines.
Insonify (Ensonify, Br.): To expose an area, or portion of seabed, to sonar energy. Seabed that has been covered by imaging sonar, provided acceptable backscatter and interpretable sonar data is said to have been insonified. Also ping rates (numbers of output pulses per second) are often referred to as "insonification rates."
Instability: The behavior of a towfish experiencing the motion of heave, pitch, roll or yaw; the erratic motion of any part of a towing assembly usually resulting from fluctuating drag forces. Towfish instability is a major factor in degrading sonar data. In particular, heave is a common form of instability which can significantly complicate data interpretation. High through-the-water towbody velocities will also cause instability. These velocities may be caused by high towspeeds or high speed water movement.
Interference: The display of erroneous signals from acoustic or electrical sources that conflict with the display of the primary side scan sonar data. Interference can be internal to the sonar system, but are most commonly from other sources. Causes of external interference include generator noise, electromagnetic radiation from other electronics, ship's engines and propellers, flow noise and biological sources.
Interferometric Sonar: A system based on the process by which two or more sonar waves of the same frequency combine to reinforce or cancel each other, the amplitude of the resulting wave being equal to the sum of the amplitude of the combining waves. Because the angle of interference can be determined, these sonar systems provide bathymetric information over a wide swath. Although the data is referred to as side scan, the imagery produced by interferometric sonars is graphically flawed. Because of the interferometry, light and dark banding persists across the record making the depiction of an even seabed reverberation difficult.
In-water-cable: The amount of tow cable exposed to part or all of the water column and potentially affected by drag forces. When towing sonar with small amounts of cable in the water, ship maneuvers are simpler and towfish behavior is easily controlled. As the water depth (and in-water-cable) increase, cable catanary and towfish height become important parameters. Also, the vessel becomes more restricted in its movements. This complicates maneuverability when trying to avoid towfish-seafloor collisions and ship-towcable accidents.
Kiting: A rhythmic, lateral movement experienced by towbodies on long cables and in deep water; most often induced by poor hydrodynamics of a depressor. The proper application of depressors is important to maintain reliable fish stability. One depressor shape and form may work well at only certain speeds while, creating instabilities at others. Kiting is one of a variety of instabilities that can be recognized in sonar data.
Lane: A course or track, down the center of which, a survey vessel travels during a survey. A lane is delineated on either side by half the distance between the current and adjacent tracks. A common method of using sonar to perform search or survey operations is to set up a series of parallel lanes. Represented as imaginary lines on the sea surface, they are depicted on a computer screen from an Integrated Navigation System.
Lane Spacing: The distance between successive vessel tracks in a multi-lane survey. Careful lane spacing is very important in side scan sonar applications. Lane spacing will vary with the swath width, towfish altitude, frequency used and operational goals. Improper lane spacing can result in data ungathered during survey operations and program failure in crucial search operations.
Lateral Offset: The position of the towfish off to the side of the surface vessel's track. This occurs when the surface vessel track is not aligned with a surface or subsea current. It can also be caused by poor hydrodynamics. The effect of the current will pull the towfish away from the vessel trackline. In extreme cases the towfish can also crab into such a current and result in a loss of data.
Layback: The horizontal distance between the survey vessel, or the navigation antenna, and the towfish. In deep water towing, this distance is important for positioning features on the seabed and geocoding sonar data for mosaics. Because most navigation instruments provide the position of their antenna mounted on the vessel and the X,Y origin of the sonar image is at the towfish, this difference in position can be significant. Many methods of subtracting this distance have been tested using acoustics and algorithms. Acoustic fish positioning methods rarely work well and often cause more problems than they solve. Algorithms, while not calculating such parameters as lateral offset, are very accurate in eliminating layback errors.
Loran: A navigation system developed in the 1950's based on the time displacement between signals from two or more fixed shore based antennas. Two types of Loran (an acronym for LOng RAnge Navigation) were developed. A low resolution Loran-A was first. Cumbersome and difficult to use, Loran-A required the user to tune a receiver and align signal peaks on a scope. A more efficient system, Loran-C, was developed for use in the 1960's and provided the user with a readout of numbers representing time differences in microseconds. American charts were produced with these "TD" lines overprinted on them. Although Loran-C was very repeatable (often to within 20 meters) the system was not accurately tied to any datum. Further, since the radio transmissions propagated over land, conversion to latitude and longitude was only approximate, at best. For several decades since the late 1950's, Loran and a similar British system, Decca, were the major worldwide landbased navigation systems. Offshore beyond the 600 mile range of these systems, dead reckoning with occasional fixes from transit satellites was a large part of ocean navigation. The Global Positioning System is expected to completely replace the need for Loran transmitting stations worldwide.
Mapping: Creating sonar records that accurately represent 1:1 scalar plan views of large sections of seabed; also creating high resolution images of complex underwater targets. With the development of digital sonar systems in the 1980's, the ability to create sonar "mosaics" or large maps of the seafloor, was realized. These scalar views give the user a better comprehension of the overall condition of various sections of seafloor. Mapping the seabed is the process of gathering this 1:1 mosaic-able data. Detail mapping, on the other hand, is the process of creating high resolution sonar images using high insonification rates, stable towfish and accurate navigation.
Mensuration: Scaling of the physical dimensions and volume of an object from a sonar record. With a sonar display or update rate matching tow speed and taking into account the effect of slant range distortion, very accurate dimensions of seabed objects can be taken from sonar data. It is this process that assists the interpreter in classifying various targets.
Microbars: A unit of pressure equal to one millionth of a bar, commonly used to indicate acoustic signal strength.
Mosaic: An assembly of sonar records matched in such a way as to show an accurate, continuous, two dimensional representation of an area of seabed. (See Mapping).
Multipath: Sonar signals arriving at a target, or the towfish, from a single source but along different paths. In the sonar sciences, multipath echoes can be problematic. Multipath returns in imaging sonars typically occur in shallow water or around complex structures such as petroleum platforms or near piers and pilings. A classic multipath environment for side scan sonar is in shallow water with a flat sea surface. Acoustic echoes will return to the transducer along three different paths (1. transducer - target - transducer, 2. transducer - target - sea surface - transducer, 3. transducer - sea surface - target - sea surface - transducer). If these three return paths take three different times, the result will be three images of the same target in the data.
Noise: Extraneous signals detected by a sonar that affect the system's efficiency to display, and the operator's efficiency to interpret, the signals of interest. Noise can originate from many different sources both internal and external to the sonar system. (See Interference).
Out-of-range: Target echoes displayed by the sonar resulting from hard reflectors that are beyond the system range setting. When an imaging sonar transmits an acoustic pulse, it propagates into the environment. After the pulse travels beyond the set range of the system, another pulse is transmitted. However, the first pulse is still moving away from the transducer and sending back echoes. Even though those echoes are returning from out-of-range, this is not normally a problem because the sonar system gains are reduced in the near field (for the second out going pulse). In a quiet environment where there is little backscatter in the near field and there are hard targets out of range, these targets will be imaged after the second pulse starts its propagation. In radar this phenomenon is called a "second sweep return."
Overlap: The area of seabed that is covered two or more times, referred to as a percentage of swath width. Overlap in a sonar survey is important for several reasons. Most sonar surveys need to assure at least 100% coverage of the seabed. Because of variations in ship track (micro-corrections in heading) overlap of one swath with a portion of the next will help insure there are no "holes" where the ship track was slightly north on one track and slightly south on the next. For crucial search operations, overlap may be increased to eliminate any possibility of uncovered seabed. Further, large overlap brings seabed imaged at long ranges on one pass into the high resolution region, closer to the towfish on the next.
Over-the-ground: A measurement of speed of the survey vessel or towfish as a true speed over the seabed, independent of movement in relation to wind or water. Over-the-ground speeds are important in adjusting display update rates and particularly when mensurating targets. Water movement caused by currents, wind and waves can complicate the process of towing sonar. Speed adjustments were difficult in the past, but the accurate speed displayed from GPS systems help the operator determine true OTG speeds.
Pass: A single procession by a seabed anomaly during a sonar survey. Multiple passes by a target are important for the process of target classification. Also, interpretation of phenomena can require multiple passes. For example, thermal discontinuities rarely look the same on any two passes. A target that may be a school of fish can be properly identified by several passes because fish schools rarely stay in one place over any length of time.
Passive Sonar: A sonar system having only a hydrophone and capable of receiving signals but not transmitting them. An example of a passive sonar is a streamer array towed from a seismic vessel. Another is an array on a submarine that detects other vessel's engine and propeller noise.
Path-tracking: The ability of a towed body to accurately follow the path along which it is towed by a surface vessel. In the absence of high velocity currents and using short cables, most sonar towfish will track along the ship's path well. However, if the sonar is towed at 90 degrees to a strong current and over long cable, it will tend to be displaced from the vessel's path and heading (see lateral offset).
Ping: A single output pulse of a sonar system; also the returns from a single output resulting in the lateral display of one individual line of side scan data. A side scan sonar transmits many pings into the underwater environment. Each of these outputs has resultant echoes coming from the seafloor. Any individual ping's returning echoes does not provide imagery alone, but together many pings, juxtaposed in a display, provide the imagery. However, in modern computerized sonar processors and displays, individual ping and return signal strength "values" can provide information to the operator about system operation.
Pitch: An instability in the towfish expressed by the alternate rise and fall of the nose and tail about a horizontal axis. Most often, towfish design inhibits pitch during normal operations. When the towfish is lowered rapidly though, it will temporarily pitch in a nose-down attitude.
Plan View: A to-scale side scan sonar display constructed to represent the top view of a section of seabed. (See Mosaic and Mapping).
Post-processing: Sonar data processing after real time data generation and storage. Modern sonar systems allow the user to concentrate on the task at hand of gathering data by recording "raw" data which can be manipulated or "processed" later. Because ship time is expensive and operational crews must be fully attentive to proper data gathering, post processing is most effectively performed in the less time-budgeted environment of the on-shore laboratory. Post processing can include mosaic construction, slant range and speed correction, false colorization and hard copy print generation.
Pre-plots: Specific points, including tracklines and navigation fix points, the positions of which are determined prior to the commencement of a survey. Planning is a crucial component to an effective side scan sonar deployment. Where the survey is to take place, the location of lanes and what "highway markers" or geodetic positions will be logged and checked are often determined before hand as pre-plots.
Profiler: An instrument that records a vertical section, or simple outline, of the seafloor along a survey line. Profilers come in many types and frequencies but generally the term refers to sub bottom profilers that work in the 0.5-5.0 kHz region. These frequencies will penetrate some types of bottom sediments and provide an image that represents a cross section of the seabed. Other types of acoustic profilers are higher frequency and show only the outline of the seabed surface topography.
Projector: A sonar transducer that translates an electrical signal into pressure waves (sound signals) and transmits them through the water. A projector almost always requires some sort of hydrophone to receive the transmitted signals or returning echoes. The hydrophone may be very close to the projector or, as in the case of Rafos and Sofar floats, may be thousands of kilometers away. With a few exceptions, conventional side scan sonar most often uses the same transducer crystals as the projector and hydrophone.
Propagate: The movement of sound waves through the water; also transmit. The manner in which sound propagates underwater is the basis for quality image formation using side scan sonar. Because it is known how sound behaves in the underwater environment, sonar systems are designed and operated based upon these physics. Understanding the propagation of sound also allows rapid interpretation of sonar imagery.
Pulse: A short burst of sonar, typically measured as a function of time, distance or power. Each pulse of sonar is also known as a ping. However, the pulse is a more formal term and we use it to describe the length in time and width in meters of a sonar ping (See below).
Pulse Length: The length of time that an active sonar is transmitting one pulse, typically expressed in milliseconds. Longer pulse lengths allow more power to be put into the water at the expense of cross track resolution. This has the effect of gaining range for large area surveying.
Pulse Width: The distance of the insonified water, in the range dimension, at a given point in time, expressed in meters and determined by multiplying the pulse length by the speed of sound through the water. Pulse widths are measured to determine the maximum theoretical resolution of an imaging sonar. Some side scan sonars have variable pulse widths although most do not.
Quench: The loss of a sonar signal, most often due to water borne discontinuities and resulting in blank sonar display areas. It should be noted that quenching will affect both the outgoing sonar pulse and the returning echoes. Also, water borne acoustic obstructions might absorb or scatter part of an outgoing pulse, but the returning echoes are of a much lower intensity and their blockage by the same obstruction is more often the result of blanking than the loss of the outgoing pulse.
Q-Route: A route of safe passage through a mined waterway. Side scan sonar is often used not only in mine hunting, but also as a mapping tool to gather baseline data for safe harbor routes in the event of conflict. Once baseline data is gathered for a particular section of seabed, it is often a straightforward task to periodically re-examine the area for "new" anomalies which may represent recently laid mines.
Range: A sonar setting which represents a distance, usually measured in meters, that is the maximum distance from the towfish that the sonar will display (the range setting on the sonar also determines the time between outgoing sonar pulses); also synonymous with the cross-track dimension. Once the range setting on the system is set, when returning echoes arrive from that distance away from the transducer, the ping cycle starts again and a new acoustic pulse is transmitted in to the water.
Range Resolution: The ability of the sonar to image, separately and distinctly, objects that lay in a line 90 degrees to the towfish heading. The range resolution is determined, in part, by the pulse width of the sonar. A narrow pulse width will display two targets close together as separate and distinct anomalies. The same two targets, when insonified by a wider pulse width, can be simultaneously enveloped by the pulse. This results in the two objects appearing as one in the sonar display.
Range Data Compression: Sonar image compression resulting from the geometry of slant range side scan sonar displays. The time differences of returning pulses from the seabed in the near ranges below the towfish is very small. As a result, the separation of different portions of seabed is inaccurately displayed in conventional side scan data. As the pulse travels away from the area below the towfish, the time difference between one section of bottom and another begins to accurately represent their separation. Because of this, slant range compression is most severe nearest the centerline of two channel side scan data.
Range Overlap: The area of seabed, lateral to the towfish track, re-insonified on successive tracks during a survey; equal to the range per side less the lane spacing, usually expressed in meters. Overlap can be a crucial component of a side scan survey. See Overlap.
Ray Bending: Changes in the speed and direction of a sonar beam in the water. Ray bending is a major cause of data distortions from thermoclines, inverted thermoclines and haline fronts. Side scan, like many acoustic imaging instruments, rely on the fact that most ray paths are relatively straight. When they are not, as a result of by ray bending, it is important for the sonar operator to recognize it.
Recognition: The acknowledgment by the sonar operator of the existence of a target or anomaly as displayed in the sonar data. Lack of anomaly recognition can be problematic during sonar operations. Catastrophic towfish altitude loss, severe fish instability and search targets all should be recognized by the operator any time they occur. Recognition is distinguished from detection as being operator dependent. (See Detectability).
Refraction: The change of direction of a sound beam when passing obliquely from one medium into another, where its wave velocity is different. Refraction is a type of ray bending that will affect acoustic returns for proper sonar imaging. This occurs when sonar pulses encounter thermal and haline discontinuities. In a normal summer thermocline environment, a side scan beam can be refracted sharply to the seafloor severely limiting range. (See Ray Bending).
Reverberation: The echoing of a sonar signal from a target or targets. Echo and reverberation are often used interchangeably, although targets are more often described as returning an echo whereas large insonified areas such as the seafloor are described as reverberating under the influence of sonar.
Ringing: In a transducer, this is the reception of the transducer output pulse at the time of transmission. In active sonar systems, the projector and the hydrophone are one and the same, so the hydrophone receives its own outgoing pulse. In a target, this is a well documented phenomenon resulting from multiple echoes from certain types of targets due to the acoustical physics of sound pulse wrap-around and reflections internal to the target.
Roll: The rhythmic movement of a ship or towbody about its longitudinal axis. When working from a well designed support vessel, roll will not contribute to towfish instability to the same extent as heave in any given sea state. Roll can affect the ability of crews to perform at sea, as well as destabilize unsecured equipment. However, to preserve data quality, many surveys will use a different vessel heading allowing the vessel to roll instead of pitch or heave.
Rub-Test: The process of manually creating friction on a transducer face in order to test system electrical continuity. Before a sonar towbody is put in the water, it is a common practice to test the system on deck. Because air is a high impedance medium for sonar, the best method of testing system function is to tap or rub the transducer face. In a dual channel side scan sonar, one transducer is rubbed, then the opposite then the first one again. This is to clarify that the transducers are not wired to the wrong display channels.
Sea clutter: The images created in a sonar display by acoustic returns from a rough sea surface. When using side scan sonar, some energy is projected above the horizontal from the wide vertical beam. If the sea surface is rough and within the range setting of the system, formless patches may overlay the normal seabed returns. This display "clutter" may be more noticeable on one sonar channel depending on the direction of sea surface waves relative to the towpath.
Scale Marks: Equidistant, regular marks on a sonar display used to assist in the mensuration of targets and anomalies and to provide information on the range displacement of targets from the towfish path. Most sonar systems allow the user to choose how and where to display scale lines or scale marks. Common settings in side scan are 10, 15, 25 or 50 meters.
Scattering: The diffusion of the sonar signal in many directions through refraction, diffraction and reflection, primarily due to the material properties of the insonified areas. Scattering is one of the causes of attenuation in sonar, resulting in signal weakening. Also see Backscattering.
Shadow: A light area on a normal sonar record that is less insonified than the surrounding region; most often caused by signal blocking from an acoustically opaque object on or above the seafloor. Shadows in side scan data are an important aid to accurate record interpretation. Often, an acoustic shadow will divulge more about a reflector than the actual acoustic returns. Shadows are also used to calculate the height of an object standing proud of the seabed. The calculation uses an algebraic solution of similar triangles formed by the height of the towfish, the range to the target and the length of the shadow.
Slant Range: The straight-path time of arrival of a sonar signal along the hypotenuse of a triangle described by the towfish, the seafloor directly below it, and the seabed point of interest. In side scan sonar, because the imaging source point (transducer) is not on the seafloor but rather above it, slant range does not represent the true range between any two objects. Below the towfish, the data is compressed. Further away from the towfish, the data becomes less compressed with the least error at the outermost ranges. The near range compression can be corrected using algorithms within modern sonar systems.
Slant Range Correction: A computerized repositioning of sonar data on the display to counteract range data compression. See Slant Range.
Slip Ring: An electromechanical component, most often used on a winch, that allows full electrical continuity of a sonar cable during winch drum operation. When a winch is used for deployment, altitude control and recovery of a side scan towbody, it is important to keep the control display unit and towfish operating during these processes. A slip ring has an internal core usually attached to the turning drum, while the outside of the slip ring is attached to a non-turning part of the winch. The towcable conductors are attached to certain parts (rings) of the core while the deck cable conductors are attached to a part (brushes) of the non-turning portion. In this manner the brushes maintain electrical contact with the rings as they turn along with the drum. Poor quality or dirty slip rings will cause noise in sonar data during drum movement and a bad individual ring can cause a blank sonar channel. A quality, well maintained slip ring will have good continuity and be noise free.
Sonar Geometry: The spatial relationship between the sonar transducers and their environment, including the seafloor, targets and the sea surface. Because of acoustic paths in the ocean environment, sonograms may provide puzzling imagery at times. Accurate data interpretation sometimes requires an understanding of the sonar geometry. A good example of this is when sonar signals return to the transducer over several different paths. (See Multipath)
Sonograph: A hard copy display of sonar data generated either in real time or from recorded data. Also known as sonograms, hard copy "records" of sonar data are generated either by a sonar printer, specialized graphics printer or with modern computerized sonar displays, by any drafting printer in color or black and white. Early sonar printers used a wet paper technology creating dark and light zones on the paper through the migration of ions from a consumable print head or "blade." These records were not dimensionally stable and shrank upon drying. Dry paper recorders were developed in the 1970's ultimately resulting in the use of thermal paper recorders in the 1990's. Unfortunately, many thermal recorders, although clean and easy to use, do not have the dynamic range of other dry paper machines and thermal paper is not an archive-able media. As a result sonar manufacturers and users must depend upon digital processing and mass storage of sonar data for archiving.
Specular Reflector: An object, to which incident sonar beams are largely normal to its surface, making it a strong reflector from a variety of angles. Objects in this category include cylindrical objects such as pipes and pilings and spherical objects such as subsurface floats. Specular reflectors may provide very strong sonar returns and will result in hyperbolic shaped lines in side scan data. The hyperbola is formed when a target is reflective enough to return even the low level energy in the side lobes of the sonar's horizontal beam.
Speed Correction: The proportional matching of sonar chart length with the over-the-ground speed of the survey vessel. When towing side scan sonar at a constant speed over the bottom, if the image generation on the recorder or display unit is too slow, objects in the data will appear to be compressed in the transverse dimension. If the image generation is too fast, they will appear to be stretched. See Correction and Compression.
Swath Width: The lateral coverage of side scan sonar on the seabed. Because side scan sonar projects a beam out to the side of the towpath it creates a wide region of insonified seafloor. Both right and left sonar channels make up the swath. Swath width changes with range settings and is a factor in determining coverage and lane spacing.
Termination: The junction of either end of a towcable where it is fitted with a single or multi-pin connector. Lightweight towcables typically have connectors for the towfish at the "wet end" and at the control/display unit on the "dry end." Armored cable is usually terminated with a connector at the towbody end and wired to a block inside the winch drum at the other. This aids in troubleshooting the towcable if necessary. Quality terminations are important to a functioning system and are also a delicate component of the operational assembly. Termination "kits" are part of a spares inventory and include all the parts and tools needed to reterminate a cable should the connectors or the cable become damaged.
Thermocline: A layer of water where the vertical temperature gradient is greater than that in the water above it or in the water below it. Thermoclines affect the ray path of acoustic signals underwater and can result in a range-limiting type of banding visible in side scan sonar data. Similar to the effects of a haline front, this banding is most evident at the outer ranges of sonar data where the beam's angle of incidence to the thermocline is high. Changing the sonar geometry will minimize or eliminate the effects of thermoclines.
Time Varied Gain: (TVG) A process where amplifier gain is changed based on time and matched with the returning signals between outgoing pulses of a side scan sonar. Because of attenuation of a sonar beam, the receiver gain must be increased as the acoustic returns from greater and greater distances arrive at the transducer. Because these returns are received over a predictable and consistent time, the gains can be increased over a time curve. In many sonar systems, this curve can be adjusted by the operator.
Transducer : The electromechanical component of a sonar system that is mounted underwater and converts electrical energy to sound energy and vice versa. The transducer formation determines the beam shape and is the basis for image formation in side scan sonar. Its condition and stability help determine the final image quality. Transducers can be surrounded by various types of acoustically transparent urethanes or epoxies, or can be within an oil-filled assembly. For the purposes of side scan, transducers are almost always mounted on a towbody which also contains firing and amplifying circuitry. They are towed over a cable instead of hull mounted in order to maximize stability by de-coupling them from ship motion.
Transverse Resolution: The ability of the sonar to image, as separate and distinct, objects that lay in a line parallel with the towfish track. Transverse or along-track resolution is determined, in part, by the horizontal beamwidth of the sonar. A narrow pulse width will display two targets close together as separate and distinct anomalies. The same two targets, when insonified by a wider beam, can be simultaneously enveloped by the single outgoing pulse. This can result in the two objects appearing as one in the sonar display. Transverse resolution decreases with range from the towfish because of beam spreading.
Trigger Pulse: The signal provided to sonar transducer firing circuitry to initiate the outgoing pulse; also two parallel lines on the center of a sonar record that represent the position of the fish in relation to the sonar image. Many sonar displays sense the trigger pulse in order to synchronize other subroutines with the trigger. Because the display of the trigger pulse in data is caused by transducer firing, it is useful in system troubleshooting.
Vertical Beam Width: The angle of the transmitted (and/or received) side scan sonar pulse in the vertical dimension, typically between 40 and 70 degrees. The wide vertical beam in side scan sonar allows the acoustic pulse to propagate and insonify the seafloor over a long range. In a sonar with a narrow vertical beam, depending upon the downward tilt of the main lobe, some areas will be poorly uninsonified. If the narrow beam is tilted down significantly , range will be sacrificed, while near-range seabed will not be insonified if the beam is directed more to the horizontal.
Water Column: The vertical section of water from the surface to the bottom in which a sonar is towed; also the center section of an uncorrected sonar record. The white strip in the center of a non-range-corrected sonar record represents this water column above and below the towfish. This portion of the record can be informative to the survey when depicting the first bottom and surface returns as well as schools of fish and targets near the towfish.
Wavelength: The distance, measured in the direction of propagation, between two successive points in a wave that are characterized by the same phase of oscillation. Along with the transmit power of a sonar, the wavelength (directly related to sonar frequency) will determine the ultimate range of the system. Decreases in wavelength (increases in frequency) bring higher resolution with the tradeoff of reduced range.
Yaw: An instability characterized by the side to side movement of a ship or towed body about its vertical axis. Vessel and towfish yaw most often occur at low towspeeds and with quartering seas. Towfish yaw is distinctive in sonar data and has an effect in the sonar data similar to the effect of micro-turns.
Z-Kinking: The failure of cable conductors (characterized by the "z" shape of the damaged portion) resulting from apparent movement between the core and the jacket. At underwater cable terminations, this can occur with conductor extrusion under pressure (pistoning) from a flexible jacket into the dead end of a connector body.