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ATI's Acoustic, Noise & Sonar Engineering Terms

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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 sulphate in sea water is a major factor in absorption. The higher frequencies are more affected by the physics of shear and viscosity.

Acceleration: The rate of change of velocity.

Accuracy: The extent to which a measured or enumerated value agrees with the assumed or accepted value. See Precision.

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 (single and multibeam), commercial side scan sonar and many types of sub bottom profilers.

ADCP – Acoustic Doppler Current Profiler: A current measuring instrument employing the transmission of high frequency acoustic signals in the water. The current is determined by a Doppler shift in the backscatter echo from plankton, suspended sediment, and bubbles, all assumed to be moving with the mean speed of the water. Time gating circuitry is employed which uses differences in acoustic travel time to divide the water column into range intervals, called bins. The bin determinations allow development of a profile of current speed and direction over the entire water column. The ADCP can be deployed from a moving vessel, tow, buoy, or bottom platform. In the latter configuration, it is non-obtrusive in the water column and thus can be deployed in shipping channels.

ADR gauge: Analogue to Digital Recording tide gauge. A float or pressure actuated tide gauge that records the heights at regular time intervals in digital format.

Aeration: Suspension of air bubbles in water caused by vessel movement. See also quenching.

Afloat: Floating, as opposed to being aground.

Aground: Touching, resting or lodged on the bottom of shallow water. The opposite is afloat.

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 metres. 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 cavitations, 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 acoustical 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.

Angular Distance: The angular difference between two directions, numerically equal to the angle between two lines extending in the given directions. The arc of the Great Circle joining two points, expressed in angular units.

Aphelion: The point in the orbit of the Earth or any other planet that is farthest from the Sun.

Apogee: The point in the orbit of the Moon or man-made satellite farthest from the Earth. The point in the orbit of a satellite farthest from its companion body.

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.

Automatic Tide Gauge: An instrument that automatically registers the rise and fall of the tide. In some instruments, the registration is accomplished by recording the heights at regular time intervals in digital format; in others, by a continuous graph of height against time.

AUV: Acronym for Autonomous Underwater Vehicle. As the name indicates, this is an underwater vehicle that is ‘free running’. It conducts operations from pre-programmed instructions. See also ROV.

Axis: The line, real or imaginary, about which something centres or revolves.

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.

Baud: Unit of speed for the transfer of data. The speed in baud is the number of discrete conditions or events per second.

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 trade off 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.

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. Beam width widens with distance from the transducer array. Horizontal beam width 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.

BIT: (BInary digiT). The smallest unit of information in a binary system: a 1 or 0 condition.

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. Cavitations 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: Any ground covered by water.

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 vapour 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.

Centibar: The unit of pressure equal to 1 ton per metre per second per second. See decibar.

Centrifugal Force: The force with which a body moving under constraint along a curved path reacts to the constraint. Equaland opposite to the Centripetal Force.

Centripetal Force: The force directed towards the centre of curvature, which constrains a body to move in a curved path. Equal and opposite to the Centrifugal Force.

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.

Chart: A special-purpose map generally designed for navigation or other particular purposes. See nautical chart.

Chart: Bathymetric: A topographic chart of the bed of a body of water, or a part of it. Generally, bathymetric charts show depths by Contour Lines and gradient tints.

Chart: Mercator: A chart on the Mercator projection. This is the chart commonly used for marine navigation. On a Mercator chart a Rhumb line is a straight line.

Chart: Nautical: A chart specifically designed to meet the requirements of marine navigation, showing depths of water, nature of the bottom, elevations, configuration and characteristics of the coast, dangers and aids to navigation. Also called marine chart, hydrographic chart, or simply chart.

Chart Datum: The datum to which soundings on a chart are referred. It is usually taken to correspond to a low-water elevation, and its depression below mean sea level is represented by the symbol Z . Since 1989, chart datum has been implemented to mean lower low water for all marine waters of the United States, its territories, Commonwealth of Puerto Rico, and Trust Territory of the Pacific Islands.

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.

Chlorinity: The total amount of grams of chlorine, bromine and iodine contained in one kilogram of sea water, assuming that the bromine and the iodine had been replaced by chlorine. See salinity.

Circle: Fundamental: See Primary Great Circle.

Circle: Great: The intersection of a sphere and a plane through its centre. See also Orthodrome.

Coastline: The low water datum line for purposes of the Submerged Lands Act (Public Law 31). See shoreline.

Compass: An instrument for indicating a horizontal reference direction relative to the Earth.

Compass Direction: Direction as indicated by a compass without any allowances for compass error. The direction indicated by a compass may differ by a considerable amount from the true or magnetic direction.

Compass Error: The angular difference between a compass direction and the corresponding true direction. The compass error combines the effects of deviation and variation.

Compass: Gyroscopic: A compass having one or more gyroscopes as the directive element, and tending to indicate true north. Also called gyro compass.

Compass: Magnetic: A compass depending for its directive force upon the attraction of the magnetism of the Earth for a magnet free to turn in a horizontal direction.

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.

Contour Line: A line connecting points of equal elevation or equal depth.

Coordinates: Linear or angular quantities which designate the position of a point in relation to a given reference system.

Coordinates: Polar: A system of coordinates used to describe the position of a point in space with respect to an arbitrarily chosen Origin by means of two directions and one distance; the vectorial angles and radius vector magnitude.

Coriolis Force: A term in the relative hydrodynamic equations of motion that takes into account the effect of the Earth’s rotation on moving objects (including air and water) when viewed with reference to a coordinate system attached to the rotating Earth. The horizontal component is directed 90° to the right (when looking in the direction of motion) in the Northern Hemisphere and 90° to the left in the Southern. The horizontal component is zero along the Equator; also, when the object is at rest relative to the Earth.

The Coriolis acceleration = 2vU sin ?: where…

v is the speed of the object,

U is the angular velocity of the Earth, and

? is the latitude.

Named after Gaspard Gustave de Coriolis who published his formula in 1835.

Coordinates: Rectangular: Coordinates on any system in which the axes of reference intersect at right angles.

Coordinates: Spherical: A system of polar coordinates in which the Origin is the centre of a sphere and the points all lie on the surface of a sphere. The Polar Axis of such a system cuts the sphere at its two poles.

Correction: Speed: 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 multibeam echo sounder or side scan sonar and the distance travelled by the survey vessel on its track; also pertains to the repeat surveying of an area, i.e. one pass equals 100 percent coverage of an area and two passes over the same area equals 200 percent coverage.

Cross-track: The direction 90° 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.

Current Meter: An instrument for measuring the speed and direction of water or just the speed of a current. The measurements are Eulerian when the metre is fixed or moored at a specific location. Current meters can be mechanical, electric, electromagnetic, acoustic, or any combination of the above.

Cycle: One complete train of events or phenomena that recur regularly in the same sequence. See also Frequency, Hertz, Period.

Data: General term used to denote facts, numbers, letters, and symbols. The basic elements of information; usually but not always expressed in numerical form.

Data Bits: The number of bits in each ‘packet’ of data being transferred from a SeaBat processor.

Data Collection Platform (DCP): A microprocessor based system that collects data from various sensors, processes the data, stores the data in random access memory (RAM), and provides communication links for the retrieval or transmission of the data.

Datum: Something known or assumed. The base value, level, direction, or position from which any quantity is measured.

Datum: Chart: A permanently established surface from which soundings or tide heights are referenced, usually low water. Also called datum, datum level, datum plane, hydrographic datum, reference level, reference plane. See Datum: Tidal.

Datum: Geodetic: A reference surface consisting of five quantities: the Latitude and Longitude of an initial point, the azimuth of a line from this point, and the parameters of the reference spheroid. It forms the basis for the computation of horizontal-control surveys in which the curvature of the Earth is considered.

Datum: Sounding: The horizontal plane or tidal datum to which the soundings on a hydrographic survey are reduced. Also called datum for sounding reduction.

Datum: Tidal: A level of the Sea defined by some phase of the tide, from which water depths and heights of tide are reckoned.

Datum: Vertical: For marine applications, a base elevation used as a reference from which to reckon heights or depths. It is called a tidal datum when defined in terms of a certain phase of the tide. A tidal datum is a local datum and should not be extended into areas which have differing hydrographic characteristics without substantiating measurements. In order that they may be recovered when needed, such a datum is referenced to a fixed point known as a benchmark. See also chart datum.

Datum: Vertical Control: Any level surface (e.g., Mean Sea Level) taken as a surface of reference from which to reckon elevations. Also called datum level, reference level, reference plane, levelling datum for heights. See also datum plane.

Datum for Heights: See also Datum: Vertical Control

Datum for Sounding Reduction: See also Datum: Sounding

Datum Level: See also Datum: Vertical Control and Datum: Chart.

Datum of Tide Predictions: The level from which the heights of tide are referenced in the tide tables. See also Datum: Chart.

Datum Plane: A vertical control datum. Although a level surface is not a plane, the vertical control datum is frequently referred to as the datum plane.

Day: The duration of one rotation of the Earth, or occasionally another celestial body, on its axis. Its is measured by successive transits of a reference point on the celestial sphere over the meridian, and each type takes its name from the reference used. The period of daylight, as distinguished from night.

Day: Apparent Solar: See Day: Solar.

Day: Astronomical: A Mean Solar Day beginning at Noon.

Day: Civil: A Mean Solar Day beginning at Midnight.

Day: Constituent: The duration of one rotation of the Earth on its axis, with respect to the fictitious Star representing one of the periodic elements in the tidal forces. It approximates the length of a Lunar or Solar Day. The expression is not applicable to a long-period constituent.

Day: The duration of one rotation of the Earth on its axis, with respect to the Moon. Its average length is about 24h 50m of Mean Solar Time. Also called Tidal Day.

Day: Mean Solar: See Day: Solar.

Day: Sidereal: The duration of one rotation of the Earth on its axis, with respect to the Vernal Equinox. It is measured by successive transits of the Vernal Equinox over the upper branch of a meridian. Because of the precession of the equinoxes, the sidereal day thus defined is slightly less than the period of rotation with respect to the Stars, but the difference is less than 0.01 Second. The length of the sidereal day is 24 Hours of Sidereal Time or 23h 56m 04.091s of Mean Solar Time.

Day: The duration of one rotation of the Earth on its axis, with respect to the Sun. This may be either mean solar day or an apparent solar day, as the reference is the Mean or Apparent Sun, respectively.

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 metres, 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 dead weights also requires greater overside lifting capacity than with hydrodynamic depressors.

Decibar: The practical unit for pressure in the ocean, equal to 10 centibars.

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 one micropascal at one metre).

Deck Cable: A cable used to connect the sonar control and display units to the slip ring on a winch. Although an armoured 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.

Density: Water: Mass per unit volume. The reciprocal of specific volume. In oceanography, the density of sea water is numerically equivalent to specific gravity and is a function of salinity, temperature, and pressure. See specific volume anomaly, thermosteric anomaly, sigma-t, and sigma-zero.

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.

Depth: The vertical distance from a given water level to the bottom.

Depth: Charted: The vertical distance from the chart datum to the bottom.

Deviation: of compass: The deflection of the needle of a magnetic compass due to masses of magnetic metal within a ship on which the compass is located. This deflection varies with different headings of the ship. The deviation is called easterly and marked plus (+) if the deflection is to the right of magnetic north, and is called westerly and marked minus (-) if it is to the left of magnetic north. A deviation table is a tabular arrangement showing the amount of deviation for different headings of the ship. Each compass requires a separate deviation table.

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 Technology Inc., 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.

Dip: The vertical angle, at the eye of the observer between the horizontal and the line of sight to the apparent horizon. Also called dip of the horizon or depression of the horizon.

The angle between the horizontal and the lines of forces of the Earth’s Magnetic Field at any point. Also called magnetic dip or magnetic inclination.

Direction: In Surveying and Mapping, the angle between a line or plane and an arbitrarily chosen reference line or plane.

Direction: Grid: Horizontal direction expressed as angular distance from grid north.

Direction of Gravity: See gravity.

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.

Downlink: Provides operator commands from the processor to the sonar head.

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 metres.

Earth: The planet which we inhabit. The solid matter of the Globe in distinction from water and air. The ground.

Earth’s Magnetic Field: See geomagnetic field.

Ebb Current: The movement of a tidal current away from shore or down a tidal river or estuary. In the mixed type of reversing tidal current, the terms greater ebb and lesser ebb and are applied respectively to ebb tidal currents of greater and lesser speed each day. The terms maximum ebb and minimum ebb are applied to the maximum and minimum speeds of a current running continuously ebb, the speed alternately increasing and decreasing with coming to a slack or reversing. The expression maximum ebb is also applicable to any ebb current at the time of greatest speed.

EEPROM: Electrically Erasable Programmable Read Only Memory. Semi-conductor memory that is erasable via electronic pulses.

EPROM: Erasable Programmable Read Only Memory. Semi-conductor memory that is erasable via ultra-violet light and re-programmable.

Equator: The Primary Great Circle of a sphere or spheroid such as the Earth, perpendicular to the polar axis.

Equinox: One of the two points of intersection of the Ecliptic and the Celestial Equator, occupied by the Sun when its declination is zero. That point occupied on or about March 21st, when the Sun’s Declination changes from south to north, is called vernal equinox or first point of Aries; that point occupied on or about September 23rd, when the Declination changes from north to south, is called autumnal equinox or first point of Libra. Also called equinoctial point.

Error: The difference between an observed or computed value of a quantity and an ideal or true value of that quantity.

Error: Absolute: Absolute deviation (the value taken without regard to its sign) from the corresponding true value.

Error: Accidental: An error whose occurrence depends on the law of chance only. Also called random error or irregular error.

Error: Constant: A systematic error which is the same in both magnitude and sign through a given series of observations.

Error: Gaussin: Deviation of a magnetic compass due to transient magnetism which remains in a vessel’s structure for short periods after the inducing force has been removed. This error usually appears after the vessel has been on the same heading for a considerable time.

Error: Gross: The result of carelessness or a mistake; may be detected through repetition of the measurement. Also called blunder.

Error: Heeling: The change in the deviation of a magnetic compass when a vessel heels (lists), due to the change in position of the magnetic influences of the vessel relative to the Earth’s magnetic field and to the compass.

Error: Mean Square: The quantity whose square is equal to the sum of the squares of the individual errors divided by the number of those errors.

Error: Probable: An error (or deviation from the mean) of such magnitude that the likelihood of its being exceeded in a set of observations is equal to the likelihood of its not being exceeded; its value is that of the standard error multiplied by 0.6745. The use of standard error is sometimes preferred in statistical studies.

Error: Random: See accidental error.

Error: Regular: See systematic error.

Error: Residual: The difference between any value of a quantity in a series of observations, corrected for known systematic errors, and the value of the quantity obtained from the adjustment of that series. Sometimes termed as residual.

Error: Root Mean Square: See standard error.

Error: Standard: The square root of the arithmetic mean of squared deviations from the mean. Also called standard deviation, when the deviations do not represent errors, or root mean square error.

Error: Swirl: The additional error in the reading of a magnetic compass during a turn, due to friction in the compass liquid.

Error: Systematic: An error whose magnitude changes in proportion to known changes in observational conditions. Also called regular error.

Error: Theoretical: A systematic error arising from natural physical conditions, beyond the control of the observer. Also called external error.

Error of Magnetic Compass: The angle by which a compass direction differs from the true direction; the algebraic sum of the variation and deviation.

Estuary: An embayment of the coast in which fresh river water entering at its head mixes with the relatively saline ocean water. When tidal action is the dominant mixing agent it is usually termed a tidal estuary. Also, the lower reaches and mouth of a river emptying directly into the sea where tidal mixing takes place. The latter is sometimes called a river estuary.

Eularian Measurement: Observation of the waters current with a device fixed relative to the flow.

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.

p>Fall: Decrease in a value, especially as a fall in temperature. Sinking, subsidence, such as the rise and fall of the Sea due to tidal action. The opposite is rise.

Figure of the Earth: The defining elements of the mathematical surface which approximates the surface of the Geoid. The figure of the Earth has been proved to be approximately an oblate spheroid. See also Geoid and Spheroid.

Firmware: The SeaBat’s software program stored permanently in the EPROM). There are two firmware locations, each having their own version number. (1) Processor and (2) Sonar Head.

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 beam width 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 centreline. 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 metres. See also 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.

Fourier Series: A series proposed by the French mathematician Fourier about the year 1807. The series involves the sines and cosines of whole multiples of a varying angle and is usually written in the following form:

y=Ho + A1 sin x + A2 sin 2x + A3 sin 3x

+ … B1 cos x + B2 cos 2x + B3 cos 3x + …

By taking a sufficient number of terms the series may be made to represent any periodic function of x.

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 sonar 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.

Geoid: The figure of the earth considered as a mean sea level surface extended continously through the Continents. The actual geoid is an equipotential surface to which, at every point, the Plumb Line (direction in which gravity acts) is perpendicular. It is the geoid which is obtained from observed deflections of the vertical and is the surface of reference for astronomical observations and for geodetic levelling. See spheroid.

Geomagnetic Field: The magnetic field of the Earth. Also called terrestrial magnetic field or earth’s magnetic field.

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 Defence) create inaccuracies of more than 100 metres. Differential base stations can reduce these errors to less than two metres 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.

Gravitation: In general, the mutual attraction between masses of matter (bodies). Gravitation is the component of gravity which acts towards the Earth.

Gravity: The force that tends to pull bodies towards the Earth; it is, to give bodies weight. Gravity is the resultant of two opposite forces: Gravitation and Centrifugal Force due to the rotation of the Earth.

Grazing Angle: The angle at which a 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 in side scan sonar systems attempt to compensate for these changes and produce a uniform image in the sonar data.

Great Circle: See Circle.

Great Circle Track: See Track.

Groin (or Groyne): A low artificial wall-like structure of durable material extending from the land to seaward for a particular purpose, such as to protect the coast or to force a current to scour a channel. Sometimes called rip-rap in New England, USA, waters.

Ground: The bottom of the sea. The solid surface of the Earth.

Gyro: See gyroscope.

Gyroscope: A rapidly rotating mass free to move about one or both axes perpendicular to the axes of rotation and to each other. It is characterized by gyroscopic inertia and precession. Sometimes shortened to gyro.

Gyroscope: Directional: A gyroscopic used to indicate a selected horizontal direction for a limited time. It requires periodic resetting dependent upon the gyro precession rate or change in the desired heading or horizontal direction.

Gyroscope Compass: See compass.

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 tow cables 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.

Horizon (adj.): In general, the apparent or visible junction of the Earth and sky, as seen from any specific position.

Horizontal (adj.): Parallel to the plane of the horizon; perpendicular to the direction of gravity.

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 beam widths increase transverse resolution. However, a more stable towbody is required to maintain distortion free data. Because beam width is related to transducer length and frequency, beam width 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 tow speed 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 tow speeds to develop effectiveness.

Hydrographer: A person who studies and practices the science of hydrography; the term is often applied to the person in charge of a hydrographic department or office of a country.

Hydrographic Survey: See survey.

Hydrographic Survey Sheet: An inclusive term used to designate both boat sheets and smooth sheets. Also called survey sheets.

Hydrography: The branch of applied science which deals with the measurement and description of the physical features of the navigable portion of the Earth’s surface and adjoining coastal areas, with special reference to their use for the purpose of navigation.

Hydrophone: A sonar receiver functioning by transforming underwater sound signals (pressure waves) into electrical signals. In side scan sonar, 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 behaviour 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 manoeuvres are simpler and towfish behaviour is easily controlled. As the water depth (and in-water-cable) increase, cable catanary and towfish height become important parametres. Also, the vessel becomes more restricted in its movements. This complicates manoeuvrability when trying to avoid towfish-seafloor collisions and ship-towcable accidents.

KHz: Unit of frequency equal to 1,000 Hz.

Kiting: A rhythmic, lateral movement experienced by tow-bodies 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.

Knot: The unit of speed in the nautical system; one nautical mile per hour. It is equal to 1.1508 statute miles per hour or 0.5144 metres per second.

Lane: A course or track, down the centre 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 that is not gathered 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 track line. In extreme cases the towfish can also crab into such a current and result in a loss of data.

Latitude: Angular distance from a Primary Great Circle or plane.

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.

Librations of Moon: Motions of the Moon due to which certain margins round the Moon’s Limb occasionally pass out of view while a corresponding margin on the other side comes into view. The librations enable 59 percent of the Moon’s surface to be seen at one time or another.

Line-of-site: The area in which acoustic underwater signals are reflected by objects (typically, sound waves travel in direct paths).

Line Spacing: The distance between parallel survey lines that are traversed to build a grid to cover an intended survey area.

Longitude: Angular distance, along a Primary Great Circle, from the adopted Reference Point.

Loran: A navigation system developed in the 1950s 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 1960s 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 metres) 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 1950s, Loran and a similar British system, Decca, were the major worldwide land-based 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.

Magnet: A body that produces a magnetic field around itself. It has the property of attracting certain materials capable of being magnetised.

Magnetic Annual Change: The amount of secular change in the Earth’s magnetic field which occurs in one year. Also referred to as annual change.

Magnetic Field: The space in which a magnetic field exists.

Magnetic North: See North.

Magnetic Pole: Either of the two places on the surface of the Earth where the magnetic Dip is 90 degrees, that in the northern hemisphere being designated north magnetic pole, and that in the southern hemisphere being designated south magnetic pole. The magnetic poles are not fixed and do not coincide with the geographical poles.

Magnetism: The ability to attract magnetic material, notably iron and steel. The branch of physics dealing with magnets and magnetic phenomena.

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 1980s, 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.

Mercator Projection: See projection.

Microbars: A unit of pressure equal to one millionth of a bar, commonly used to indicate acoustic signal strength.

Moon: The Earth’s only naturally occurring satellite. See Librations of Moon and Phases of the Moon.

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.

Motion Reference Unit (MRU): Measures the motion of the vessel or platform, usually in three (3) axes; Heave, Pitch and Roll. Yaw, a fourth axis, is also measured by some newer model sensors.

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 sonar 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.

North: Compass: The direction north as indicated by a magnetic compass.

North: Grid: An arbitrary reference direction used with grid navigation.

North: Magnetic: The direction indicated by the north seeking pole of a freely suspended magnetic needle influenced only by the Earth’s magnetic field.

North: True: The direction of the north geographic pole.

NTSC: Format of the video signal transmitted from the SeaBat (operator selected). This format is used throughout the United States and Japan.

Ocean: The vast body of water on the surface of the Earth, which surrounds the land. One of the main areas into which this body of water is divided geographically.

Oceanography: The study of the sea, embracing and integrating all knowledge pertaining to the sea’s physical boundaries, the chemistry and physics of sea water, Marine Biology, and Submarine Geology. In strict usage oceanography is the description of the marine environment, whereas oceanology is the study of the Oceans and related sciences.

Oceanology: See Oceanography.

Origin: In surveying, the reference position from which angles or distances can be worked out.

Orthodrome: Any line on a chart representing a Great Circle Track between two points.

Orthogonal: At right angles; rectangular, crossing, or lying at right angles.

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 percent 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.

PAL: Format of the video signal transmitted from the SeaBat (operator selected). This format is used throughout Europe and other countries, excluding the United States and Japan.

Parity BIT: A data bit, set at “0” or “1”, is a character advising you that the total number of bits in the data field is even or odd.

Parallel: A circle (or approximation of a circle) on the surface of the Earth, parallel to the Equator and connecting points of equal Latitude.

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: A line of movement; course taken, as the path of a Meteor. A line connecting a series of points in space and constituting a proposed or travelled route.

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.

Phase Measurement: The method by which the phase is measured.

Phase(s) of the Moon: The various appearances of the Moon during different parts of the Synodical Month. The Cycle begins with new moon or change of the Moon at Conjunction. The visible part of the waxing moon increases in size during the first half of the Cycle until full moon appears at Opposition, after which the visible part of the waning moon decreases for the remainder of the Cycle. First quarter occurs when the waxing moon is at east Quadrature, last quarter when the waning moon is at west Quadrature. From last quarter to new and from new to first quarter the Moon is crescent; from first quarter to full and from full to last quarter it is gibbous.

Phase Velocity: Velocity, measured over a short time period, at which a particular wave crest is propagated through water or rock media.

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).

Polar Coordinates: See coordinates.

Pole: Either of the two points of intersection of the surface of a sphere or spheroid and its axis. The origin of system of Polar Coordinates. Either of the two magnetic poles of a magnet.

Position: Data which define the location of a point with respect to a reference system. The coordinates which define such a location. The place occupied by a point on the surface of the Earth, or in space.

Position: Dead Reckoning: The position of a craft determined by Dead Reckoning, or by advancing a previous position for courses and distances.

Position: Estimated: The most probable position of a craft determined by incomplete Data or Data of questionable accuracy. Such a position might be determined by applying a correction to the Dead Reckoning Position.

Position: Geographical: The position of a point on the surface of the Earth expressed in terms of Latitude and Longitude, either Astronomical or Geodetic.

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 track lines 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.

Precision: The degree of refinement of a value not to be confused with accuracy, which is the degree of conformance with the correct value.

Primary Great Circle: A Great Circle used as the origin of measurement of a Coordinate; particularly such a circle 90 degrees from the poles of a system of spherical coordinates, as the Equator. Also called primary circle, fundamental circle.

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 0.5-5.0kHz 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.

Projection: The presentation of a figure on a surface, either plane or curved, according to a definite plan. In a perspective projection this is done by means of projecting lines emanating from a single point, which may be infinity.

In Cartography, any systematic arrangement of Meridians and Parallels portraying the curved surface of the sphere or spheroid upon a plane. Also called map projection or chart projection.

Projection: Mercator: A Conformal Projection of the cylindrical type. The Equator is represented by a straight line true to scale; the Geographic Meridians are represented by parallel straight lines perpendicular to the line representing the Equator; they are spaced according to their distance apart at the Equator. The Geographic Parallels are represented by a second system of straight lines perpendicular to the family of lines representing the Meridians, and therefore parallel with the Equator. Conformality is achieved by mathematical analysis, the spacing of the parallels being increased with the increasing distance from the Equator to conform with the expanding scale along the parallels resulting from the Meridians being represented by parallel lines. Since Rhumb Lines appear as straight lines and directions can be measured directly, this projection is widely used in navigation.

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 kilometres 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.

Protocol: A formal set of conventions governing the formatting and timing of message exchange between the SeaBat processor and an external device. Protocol consists of: baud rate, parity, data bits and stop bits.

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 metres of a sonar ping. (See pulse length).

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 metres 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 sonar systems have variable pulse widths although most do.

Quay: A wharf approximately parallel to the shoreline and accommodating ships on one side only, the other side being attached to the shore. It is usually of solid construction, as contrasted with the open pile construction usually users for piers.

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.

Quenching: The great reduction in underwater sound transmission or reception resulting from absorption and scattering of sound energy by air bubbles entrapped around the sonar dome. See aeration and attenuation.

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 harbour routes in the event of conflict. Once baseline data is gathered for a particular section

Range: A sonar setting which represents a distance, usually measured in metres, 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 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 centreline 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 metres. Overlap can be a crucial component of a side scan survey. See overlap.

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.

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 recognise 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.

Reconnaissance Survey: See survey.

Rectangular Coordinates: See coordinates.

Reference Direction: A direction used as a basis for comparison of other direction.

Reference Line: Any line which can serve as a reference or base for the measurement of other quantities. Also called datum line.

Reference Spheroid (or Ellipsoid): A theoretical figure whose dimensions closely approach the dimensions of the Geoid. The exact dimensions are determined by various considerations of the section of the Earth’s surface considered. The Spheroids of Bessel, Clarke, Delambre, Everest, Hayford, Helmett and others have been adopted as reference spheroids in geodetic work by different countries. Also called spheroid of reference, or ellipsoid of reference. See also Spheroid: Oblate

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 also 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.

Rhumb Line: A line on the surface of the Earth making the same oblique angle with all Meridians; a Loxodrome spiralling toward the poles in a constant true direction. Parallels and Meridians, which also maintain constant true directions, may be considered special cases of the rhumb line. A rhumb line is a straight line on a Mercator Projection. Sometimes shortened to rhumb.

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.

Rip-Rap: See Groin.

Rise: A long, broad elevation that rises gently and generally smoothly from the sea floor.

The increase in the height or value of something, as the rise of the Sea due to tidal action, or a rise of temperature. The opposite is fall.

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.

Rotation: Turing of a body about an axes within the body, as the daily rotation of the Earth.

ROV: Acronym for Remotely Operated Vehicle (submersible). This is an underwater vehicle that is connected to an operator via power and/or communication tether. See also AUV.

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.

Salinity: A measure of the quantity of dissolved salts in sea water. It is normally defind as the total amount of dissolved solids in sea water in parts per thousand (0/00) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized. These qualifications result from the chemical difficulty in drying the salts in sea water. In practice, salinity is not determined directly but is computed from chlorinity, electrical conductivity, refractive index, or some other property whose relationship to salinity is well established. Because of the Law of Constancy of Proportions, the amount of chlorinity in a sea water sample is used to establish the sample’s salinity. The relationship between chlorinity (Cl) and salinity (S) as set forth in Knudsen’s Tables is:

S=0.03 + 1.805 Cl

A joint committee of IAPSO, UNESCO, ICES, and SCOR proposed the universal adoption of the following equation for determining salinity from chlorinity:

S=1.80655 Cl

It was adopted by IAPSO in 1963 and ICES in 1964.

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 metres.

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. See also Backscatter.

Sea: The great body of salt water in general, as opposed to Land. One of the smaller divisions of the Oceans. The state of the surface of the Ocean with regard to wave or swell, as a calm sea. See Cross Sea, Head Sea, Beam Sea, Following Sea, Quartering Sea, Sugar Loaf Sea.

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.

Sea Level: The Height of the surface of the sea at any time. The surface of the sea used as a reference for elevation. The expression is often used as a short expression for Mean Sea Level.

Sea Level Datum: A determination of Mean Sea Level that has been adopted as a standard datum of Heights although it may differ from a later determination over a longer period of time.

Sea Water: The water of the seas, distinguished from fresh water by its appreciable salinity. The degree of the salinity greatly affects the water’s physical characteristics.

Seafloor: The bottom of the Ocean when there is a generally smooth gentle gradient. Also referred to as a sea bed, sea bottom.

Secular Change: An increase or decrease of intensity and/or change of direction of the total magnetic field over a period of many years.

Sediment: Particulate organic and inorganic matter which accumulates in a loose unconsolidated form. It may be chemically precipitated from solution, secreted by organisms, or transported by air, ice, wind, or water and deposited.

Sediment: Bottom: In general all sedimentary material regardless of origin found on or in the submarine bottom, including ballast or other material dumped into the sea by man. More specifically it is limited to unconsolidated mineral and organic material forming the sea bottom, not including coral reefs or bedrock.

Sedimentary: Formed by the deposition of sediment.

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.

Side Scan: a category of sonar system used to map large areas of the sea floor for a wide variety of purposes. These include making nautical charts, detecting and identifying underwater objects and bathymetric features, conducting surveys for maritime archaeology. Side-scan sonar imagery is also used to detect debris and other obstructions on the seafloor that may be hazardous to shipping, rigs and platforms.

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 also 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 also 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 colour 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 1970s ultimately resulting in the use of thermal paper recorders in the 1990s. 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.

SOS: See speed of sound.

Sound Velocity: The rate of motion at which sound energy moves through a medium. The velocity of sound in sea water is a function of temperature, salinity, and the changes in pressure associated with changes in depth. An increase in any of these factors tends to increase the velocity. Also called speed (or velocity) of sound (SOS).

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: Rate of motion. The terms speed and velocity are often used interchangeably, but speed is a scalar, having magnitude only, while velocity is a vector quantity, having both magnitude and direction.

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.

Speed of Sound (SOS): The speed at which acoustic pulse travels through water. See Sound velocity.

Spherical Coordinates: See >Coordinates.

Spheroid: An ellipsoid; a figure resembling a sphere. Also called ellipsoid or ellipsoid of revolution, from the fact that it can be formed by revolving an ellipse about one of its axes. In geodesy this term is frequently used to mean reference spheroid. See also Spheroid: oblate and Spheroid: prolate.

Spheroid: Oblate: An ellipsoid of revolution, the minor axis of which is the axis of revolution. The Earth is approximately and oblate spheroid.

Spheroid: Prolate: An ellipsoid of revolution, the major axis of which is the axis of revolution.

Spheroid of Reference: See spheroid.

Stop Bit: In asynchronous transmission, the last bit, used to indicate the end of a character which serves to return the data line to its original state.

Sun: The luminous celestial body at the centre of the Solar System, around which the planets, planetoids, and comets revolve. It is an average star.

Sun: Apparent: The actual Sun as it appears in the sky. Also called true sun.

Sun: Mean: A fictitious Sun perceived to move eastward along the Celestial Equator at a rate that provides a uniform measure of Time equal to the average Apparent Time. It is used as a reference for reckoning Mean Time, Zone Time, and so on.

Survey: The orderly process of determining data relating to the physical or chemical characteristics of the Earth. The act or operation of making measurements for determining the relative position of points on, above or beneath the Earth surface. The result of such operations.

Survey: Aerial: A survey using aerial photographs as part of the surveying operation; also the taking of aerial photographs for surveying purposes.

Survey: Cadastral: A survey relating to land boundaries and subdivisions, made to create units suitable for transfer or to define the limitations of title. Also called land survey.

Survey: Coastal: A Hydrographic Survey of coastal area including coast-lining.

Survey: Exploratory: A survey executed for the purpose of obtaining general information concerning areas about which such information was not, previously, a matter of record.

Survey: Geodetic: A survey in which the figure and size of the Earth is considered. It is applicable for large areas and long lines and is used for the precise location of basic points suitable for controlling other surveys.

Survey: Geologic(al): A survey of investigation of the character and structure of the Earth, of the physical changes which the Earth’s Crust has undergone or is undergoing, and of the causes producing those changes.

Survey: Gravimetric: A survey made to determine the acceleration of gravity at various places on the Earth’s surface.

Survey: Ground: A survey made by ground methods, as distinguished from an aerial survey. A ground survey may or may not include the use of photographs.

Survey: Hydrographic: A survey having for its principal purpose the determination of data relating to bodies of water. A hydrographic survey may consist of the determination of one or several of the following classes of data; Depth of water; configuration and nature of the bottom; directions and force of currents; Heights and Times of Tides and water stages; and location of fixed objects for survey and navigation purposes.

Survey: Land: See Survey: Cadastral.

Survey: Large Scale: A hydrographic survey at a large scale. Large scale surveys are usually intended to furnish detailed information for dredging, or other types of harbour improvement.

Survey: Magnetic: A survey conducted to measure the strength and direction of the Earth’s magnetic field at specific points on or near the surface of the Earth.

Survey: Magnetometer: A survey wherein the Earth’s magnetic field is mapped by the use of a magnetometer.

Survey: Oceanographic: A study or examination of any physical, chemical, biological, geological of geophysical condition in the Ocean, or any part of it. An expedition to gather data, samples or information to conduct such studies or examination.

Survey: Photogrammetric: A survey using either terrestrial or aerial Photographs.

Survey: Preliminary: See Survey: Reconnaissance.

Survey: Reconnaissance: A hasty preliminary survey of a region made to provide some advance information regarding the area which may be useful, pending the execution of more complete surveys. Also called preliminary survey.

Survey: Running: A hydrographic survey in which the greater part of the work is done from the ship steaming along the coast. fixing chiefly by dead reckoning and astronomical observations, and at the same time observing angles and bearings to selected and prominent coastal features. A line of sounding is run on the course made good and any additional information of a hydrographic nature which can be obtained whilst under way, is collected.

Survey: Wire-Drag: A hydrographic survey made utilizing a wire drag. Mainly used prior to the availability of Multibeam Echo Sounder Systems, and used in areas of rocky bottom or where submerged obstacles such as wrecks are present, a wire-drag survey represents the most practical way of making sure that all obstructions or dangers have been found and least depths over them obtained. Also called wire drag sweep.

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.” Armoured 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 re-terminate 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.

Tide: The periodic rise and fall of the surface of the Oceans, Bays, etc., due principally to the gravitational attraction of the Moon and Sun for the rotating Earth.

Time Synchronisation: The synchronisation, based on time, of the recorded data to produce maps or reports that require the merger of successive SeaBat profiles.

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.

Track: The actual path or route of a craft over the ground or sea bottom, or its graphic representation. In Air Navigation also called track made good.

Track: Great Circle: The track of a craft following a Great Circle, or a Great Circle which a craft intends to follow approximately.

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.

Transducer Face: The rounded rubber end of the sonar head.

Transmitter Sync: An option that provides the means to synchronize external devices with the SeaBat transmitter ping.

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 beam 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 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 centre 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

Uplink: Provides all the sonar data from the sonar head to the processor.

UUV: Acronym for Unmanned Underwater Vehicle. There are two categories of UUVs: AUVs and ROVs.

Variation: Grid: Angular difference in direction between grid north and magnetic north. It is measured east or west from grid north. Also called grivation, or grid magnetic angle.

Variation: Magnetic: The angle between the magnetic and geographical meridians at any place, expressed in degrees east or west to indicate the direction of magnetic north from true north. Also called magnetic variation, or magnetic declination.

Velocimeter: See velocity profiler.

Velocity: A vector quantity equal to speed in a given direction.

Velocity of Sound: See sound velocity.

Velocity Profiler: An instrument used for the ‘in situ’ measurement of the sound velocity in the sea and other natural waters.

Vernal Equinox: See Equinox.

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.

Vessel Reference Unit (VRU): See motion reference unit.

Water Column: The vertical section of water from the surface to the bottom in which a sonar is towed; also the centre section of an uncorrected sonar record. The white strip in the centre 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 trade off of reduced range.

Wharf: A structure serving as a berthing place for vessels.

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.

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