Acoustic Technology -- Glossary

## Glossary of Terms

Active Acoustics--A subdiscipline of the branch of physics that deals with the man-made generation, propagation, and scattering of sound projected into a medium (e.g., air, water or the earth) by an investigator for the purpose of remotely determining some characteristic of the transmission medium along the propagation path.

Acoustic Doppler Current Profiler (ADCP)--An acoustic sensor that measures the Doppler shift of acoustic scatterers in the water column and estimates the magnitude and direction of the 3-D motions of the "water" versus depth using the assumption that the scatterers are passive tracers of the water mass.

Acoustic echo--A pressure (or its representation in voltage) signal that results from the scattering of sound from an acoustic impedance discontinuity (target) in the medium in which the sound is propagating (e.g., an "echo" from a zooplankter or a school of fish). (Also see "Target".)

Acoustic Resolution, Resolved--In space, the minimum distance between two objects for which an active acoustic system can determine that there are two, rather than one, objects present. In frequency, the minimum separation between two frequency components (e.g., tones or lines) for which an analysis system can distinguish the presence of both frequency components. Objects or frequency components are said to be resolved, when there is sufficient precision in the measurement to separate the objects or components into distinguishable entities. The criteria for when this is the case may vary, depending on the characteristics of the system and the intended use of the information. The Rayleigh resolution criteria from geometric optics is often, but not uniformly, applied.

Acoustic scattering--The diversion of sound energy from its original direction of propagation.

Acoustic scattering models--A mathematical expression used to describe the sound scattering process from "targets" (e.g., marine organisms). Models range in complexity from empirically based linear regressions of acoustic scattering on size for a particular acoustic frequency, to complex expressions based on first principles of physics, the acoustic frequency, and the organism's shape, size, morphology, physical structure, compressibility, and density contrasts with the surrounding medium and the target's relative orientation with respect to the acoustic sensor.

Acoustic signatures--Any set of characteristics used to describe a sound signal, including echoes from "targets", radiated, and ambient noise. For an echo, the signature might include target strength, spectral reflectivity versus frequency, doppler shift, doppler spread, or target range extent (size).

Acoustic size classes--A term that loosely refers to acoustic estimates of the relative or absolute abundance of "targets" of different sizes. The degree with which the "acoustic size" matches the physical size depends on the accuracy of the mathematical model one uses to transform from the acoustic measurements to the estimated physical dimensions of the scatterers.

Acoustic tag--These devices, which are typically attached to fish, have also been used to detect and track whales and crabs. They come in at least two types. One is an instrument that periodically transmits a sound, allowing one to detect that sound at a remote location, determining the presence of the animal and its direction. The transponding acoustic tag makes a sound only when interrogated, thereby saving battery power in the tag and extending the tag life. By using the time delay between the interrogating signal and the reception of the acoustic response, the range to the animal can be estimated as well as its direction. Information about the animal, such as its depth or heart rate, can be encoded into the signal on either the simple or transponding tag.

Amphipod--Any of the relatively common and numerous small crustaceans of the order Amphipoda. Often found in assemblages of marine zooplankton.

Anthropogenic--Related to man's activities; man-made or caused by man.

Attenuation--Reduction in acoustic intensity experienced by a signal in transit between two spatially separated points. Parameters that contribute to attenuation include absorption, scattering, refraction (multiple paths), and geometric spreading.

Automatic gain control (AGC)--A form of signal processing used to maintain the amplitude of electrical signals within preset bounds. Signals are amplified or attenuated according to some measure of their amplitude or intensity, often by the mean square, or root mean square amplitude.

Backscattering cross-section--The ratio of the acoustic power scattered at an angle of 180 degrees from the incident acoustic wave, referenced to a stated unit distance, e.g., 1 m, to the acoustic intensity incident on a unit volume or unit area. This measure is the ratio of the reflected acoustic power to incident acoustic power/area, giving rise to units of area (m2) for backscattering cross-section.

Bandwidth--The frequency range spanned by an acoustic or electronic signal of interest to the investigator (or used by an acoustic instrument). For some common types of acoustic signals (CW pulses), the acoustic bandwidth is inversely related to the pulse length of the pulse (ping) length.

Batfish--A towed body, used as an instrument platform, which can be actively controlled in depth by manipulation of its control surface. Often used to make measurements in a "tow-yo" mode along a transect. (See, e.g., Herman and Dauphinee, 1980)

Beam--Many acoustic systems either transmit or receive sound preferentially in some direction, either vertically or horizontally. The intensity of sound transmitted or the response to sound arriving at a sensor, as a function of angle around a preferred direction, defines an acoustic "beam". Roughly analogous to the light "beam" transmitted from a flashlight or the angle of acceptance of light in a telescope.

Beamforming--Measures taken to focus sound in a particular direction. Three typical means of beamforming include placement of a reflector (e.g., a cone or parabola) behind an acoustic source; phasing of narrowband signals emitted or received by an array of transducers or transducer elements; and shifting replicas of signals emitted or received by individual elements in such a way as to maximize the acoustic response in some direction. Can also refer to processing of data from the elements of an array to reject information from a particular direction.

Beamwidth--The angular extent around a maximum response axis within which the signals are some percent of the response in the direction of the maximum. The half (maximum) power points are often taken as the limit for the purpose of defining the beamwidth.

Bent cylinder models--Mathematical algorithms that describe the acoustic scattering from various classes of cylinders which have been characterized as having a radius of curvature along their length. (See, e.g., Stanton, 1989a, b.) Their proposed use has been to characterize and understand the magnitude, directionality, and mechanisms that give rise to acoustic scattering from zooplankton such as euphausiids and shrimp.

Bioacoustic, bio-acoustics--Refers to the use of acoustic technology to study plants or animals. Bioacoustics, as used in this document, refers to applications involving animals in the marine environment. It may employ either active or passive acoustic technology. Bioacoustics is also employed in the terrestrial environment, for example, in the detection of insects in grain and especially with the vocalizations of numerous species, such as apes, birds, and insects.

Biomass--A measure of the quantity of living material, usually in units of weight per unit volume.

BIONESS--A multiple net system with the capability of opening and closing nets on command from the surface in order to sample different depth strata on a single tow. (See Sameoto, et al., 1980.)

Calibration--In this document, calibration refers to the process of establishing the sensitivity of an acoustic sensor or system to an acoustic stimulus and to the quantitative relationship between the electrical and acoustical parameters of an acoustic system. In acoustics, calibrations are expressed in absolute terms, with units that are traceable to the National Institute for Standards and Technology (e.g., meters, seconds, kg, etc.) or some comparable source of standards. In the strict sense, an experiment that compared a net haul with echo integrator outputs from an acoustic system would not be a calibration, rather it would be a comparison. A calibration on a stable acoustic system is deterministic, repeatable, and does not depend in any way on the population under study. A valid calibration allows absolute comparisons between different equipments and different investigators based on widely accepted physical or electronic standards (e.g., source level 212 dB // 1µPa or beamwidth = 14.22 degrees). The word calibration is often misused in bioacoustics when substituted for the word comparison. (See Comparison.)

Classification--In acoustic remote sensing, one can often quantify some set of characteristics associated with a "target" or sound scattering organism, school or aggregation. Examples are quantitative measurement of single frequency reflectivity or target strengths, reflectivity spectra (reflectivity versus frequency), location in the water column, geographic location, time of the year, target size, motion characteristics, etc. Other measures may be qualitative, e.g., compact, dispersed, weak, strong, layer-like, etc. From the entire set of remote measurements ("classification clues"), one can sort the detected acoustic contacts (targets) into "classes" with similar or identical characteristics. If enough is known about the possible set of targets present, this "classification" may lead to an "identification" with some probability and confidence level.

Coherence--A mathematical algorithm expressing a quantitative measure of the spatial or temporal relationships between two or more parameters. Simple coherence is the ratio of the square of the absolute magnitude of the cross spectral density function between two parameters and the product of the power spectral densities for each individual parameter. (For a generalized definition of coherence, including multiple and partial coherence, see Goodman, 1965.)

Comparison--The process of relating acoustic measurements (e.g., volume backscattering or echo integrator outputs) to the biomass or numerical abundance of some organism or assemblage of species. A regression between a series of biomass measurements from a net haul and the values of a comparable series of echo integrator values (or derived quantities) is a comparison -- not a calibration (See Calibration).

Confocal Imaging--An optical technique that uses a source point and a confocal point to image an object. The source and image point are scanned to produce a multi-dimensional image. Advantages of this technique for oceanographic optical imaging are a large reduction in backscattered light.

Copepod--An important component of the zooplankton consisting of at least 4,500 pelagic species, order Calanoida and Cyclopoida. Minute "shrimplike" organisms which range between about 0.05 and 10 mm in length. A major food for fish, these animals often dominate the marine zooplankton biomass.

Crustacean--Primarily marine, this class of zooplankton consists of about 26,000 species. It is characterized by a thin, chitinous exoskeleton.

CritterCam [R]--A camera system using an IR diode laser that was developed by J.R. Strickler

CTD--An instrument that measures conductivity, temperature, and depth.

Cyclosonde--A device that can be used as a platform for a variety of instruments, it profiles the water column by alternately rising to the surface and sinking. The device rises and sinks by adjusting the package buoyancy in a programmed manner (See Van Leer, et al., 1974).

Density--In underwater acoustics and in bioacoustics, when discussing target strengths or acoustic characteristics of animals, the term usually refers to the mass per unit volume of an animal or some part thereof, given in kg/m3. When discussing volume scattering strengths, the usage may refer to abundance, i.e. numbers/m3.

Deconvolution--Mathematically a procedure or calculation that is used to remove the influence produced in a data set by a known system response function. For applications involving the removal of the effects of beam patterns from measurements of target strengths of fish, see Clay, 1983 and Stanton and Clay, 1986.

Detected, detection--This term and its variants have at least two meanings, one involving the calculation of the envelope of an acoustic signal or its analog or digital electronic waveform. In the context of the current report the word is used to mean identified as separate from a noise field in which it is usually embedded. Helstrom, 1968 is a good reference text on detection theory.

Doppler sonar--An acoustic instrument that measures the change in the acoustic frequency of the scattered sound or echo from that of the transmitted pulse. The magnitude and direction of the shift in frequency is related to the relative motion of the sensor and the scatterer.

Doppler shift/spread, spectrum width--The Doppler shift of an echo is the change in the mean frequency of an echo from the mean frequency of the acoustic signal originally transmitted into the water by an acoustic system. When the scattering is from acoustic reflectors (e.g., fish in a school) that are moving with different speeds or directions in relation to the location of the sensor, then each echo will have a different Doppler shift. If different parts of an individual are moving with different velocity components in the direction of the sensor or the relative velocity changes during a measurement (pulse length), then the signal spectrum will be distorted or spread. When the echoes are added together, the result will be a distribution of energy around some mean frequency. The width of the energy distribution is termed the Doppler spread or spectrum width of the echo. Not all of the spectral spread or width is necessarily due to motion, however, since each finite length waveform has a characteristic shape, independent of the Doppler effect.

Drifters--Originally developed to measure currents, this term refers to oceanographic instruments whose path is determined by the sum total of the forces imparted by wind, waves, and currents, i.e., they drift. Physical oceanographers have collectively put significant energy into minimizing the effects of wind and waves in attempting to design instrument platforms to follow particular "water parcels" in Lagrangian studies.

Dual-beam method--A technique for comparing echo voltage differences received on coaxial narrow- and wide- beams of an echo sounding or sonar system to determine whether the target is near the principal axis of the coaxial beams, assuring that the measurement of the target strength of an organism is near the point of maximum (known) response of the beam (e.g., see Traynor and Ehrenberg, 1979). This allows one to place approximate bounds on the acoustic system related measurement errors associated with estimates of the ratio of incident to reflected acoustic intensity from the target of interest, i.e., its target strength.

Echo--A distinct acoustic signal resulting from the reflection of sound from an object.

Echogram--A form of display used to present acoustic data from an echo sounder. Originally, a strip of treated paper moved by a vertically rotating stylus which marked signals detected by the echo sounder on the paper electrically. The rate of the movement of the stylus down the paper was proportional to the speed of sound in the water. Echoes from near the surface appeared near the top of the paper and echoes from deeper in the water column appeared nearer the bottom of the paper record. More modern versions present the same basic kind of display electronically on a computer display or CRT. Colors are sometimes used in this case to encode echo amplitude or some other acoustic characteristic of the echo (e.g., doppler shift).

Echo integrator, echo integration--An electronic instrument or software package which sums echo intensities over a time interval to estimate echo energy. Used in quantifying the scattering from schools of fish or plankton in bioacoustics. (For example, see Forbes and Nakken, 1972 or Mitson, 1983)

Echo sounder--An acoustic system which produces (usually) short acoustic pulses, transmits them into the water column vertically and then detects echoes from impedance discontinuities (e.g., fish, the bottom, or plankton) and displays the result to an operator. Originally used to "sound" for the bottom for navigation purposes, the technique was adapted for use by fishery biologists by Balls (1948) and refined by successive investigators. It is extensively used by fishermen to locate fish and by fisheries scientists to assess fish populations when the data are quantitatively process by techniques such as echo integration.

ESD--The Equivalent Spherical Diameter is the diameter of a sphere of the same volume as the particle or animal being described.

ESR--The Equivalent Spherical Radius is the radius of a sphere of the same volume as the particle or animal being described.

Eulerian--For the purposes of this report, Eulerian methods measure processes and/or water properties at points in a coordinate system fixed to the earth.

Euphausiid--Common "shrimplike" marine crustaceans which grow to as much as 8 cm in length, many species of which migrate vertically in the water column. These animals are known to occur in dense shoals and patches. Euphausiids, or krill, are an important component of many marine food chains. Adult euphausiids are sufficiently good swimmers that they are considered by some investigators to be micronekton.

Expendable Acoustic Profiler (EAP)--A low cost active acoustic sensor concept for assessing zooplankton that could be employed (especially from the air) in remote areas such as the Antarctic, where it is costly to send surface research vessels. The device would be expendable, telemeter data to an aircraft and be somewhat analogous to an AXBT.

Fine-scale, fine structure--Structure in any ocean feature or parameter with energy in the spatial range from about 1 meter to about 100 meters.

Fish larvae--Post-egg, but pre-juvenile fish.

Fluid sphere model--A mathematical description of the spatial scattered acoustic field around a spherical object whose density, compressibility, or both contrast with that of the surrounding medium. This model, first developed by Anderson (1950) and several variations have been widely used to describe scattering from zooplankton in the sea. Its popularity, while having some basis in a proven utility for describing some zooplankters and assemblages of small plankton, is also partly due to its relative analytical tractability and simplicity.

Fluorescence--In the context of this document, the emission of red light by chlorophyll (and phaeopigments) in phytoplankton when stimulated by ultraviolet light. The effect is used to obtain an index of abundance of phytoplankton.

Food web--The interrelated food relationships in an ecosystem.

Frequency--In acoustics, the rate at which an periodic event, e.g., the upward "zero crossing" of a pressure waveform, occurs in time. Frequency is the inverse of the period of a signal. The period is the time interval between two identical points on a repetitive waveform.

Gelatinous zooplankton--Examples include salps, larvaceans, medusae, and ctenophores. These "jellylike" organisms have neither an exoskeleton nor an endoskeleton. Occasionally found in extremely high concentrations.

Geoacoustics--A sub-discipline of geophysics which uses sound reflection and propagation to study the subsurface structure of the terrestrial and marine environments.

Geometric scattering--Acoustic scattering in which the wavelength of the sound used is much smaller than the size of object causing the scattering.

Georges Bank--A relatively shallow, biologically productive, oceanographic area located off the northeast coast of the United States.

GLOBEC--Global Ocean Ecosystems Dynamics

Holographic Imaging--A method which uses interferometric techniques to record the interference pattern between an object in 3-D and a reference wave. By shining a laser through the recording medium, usually a film, a virtual image of the object can be seen. By focusing the reconstructed image at different planes, a three dimensional volume can be scanned.

Hydrophone--An underwater microphone. This term describes an acoustic transducer that receives sound and converts acoustic pressure to an analogous electrical signal.

Identification--In applications of acoustics to biological issues, usually reserved for association of a remotely sensed organism with a genera or species. For example, the vocalization of a marine mammal or bird may lead to a unique determination of the species.

Inversion method, algorithm, technique--Any mathematical process which, in the context of this workshop report, estimates some property or combination of properties of the ocean environment from observed characteristics acoustic scattering or propagation in the sea. This is in contrast to "forward" calculations, which assume or measure the ocean properties and calculate the characteristics of the acoustic parameters one can sense. In most instances, in this document, reference to inverse methods, means the calculation of the abundance and/or sizes of acoustically sensed organisms. The mathematics of many of the inverse methods used in bioacoustics were adapted from some other field, e.g., geophysics, space technology, or medical research.

Imaging, 3-D--Remote sensing that produces a three dimensional "picture" or analogous display of an object or some property of an object within a volume, e.g., the acoustic impedance map of an object or group of objects. One technique uses a sound beam, which may be focused or scanned to interrogate different look directions as a function of time (or simultaneously with multiple beams). The analysis of the backscattered information can be used to discern the position as well as other features of animals in the three dimensional field of view.

IR diode laser--A solid state electronic device that is a source of collimated, narrowband light in the infrared part of the optical spectrum.

JGOFS--Joint Global Ocean Flux Study.

JOI--Joint Oceanographic Institutions Inc.

kHz--The abbreviation for kilohertz, this refers to the units of electronic or acoustic frequency (one thousand cycles per second).

Krill--Norwegian term for euphausiids, originally referring to the North Atlantic species Meganyctiphanes norvegica, Thysanoessa inermis, and occasionally T. raschii. The term is often used to refer to other species of euphausiids that aggregate in dense swarms or patches.

Lagrangian--Measurements in a Lagrangian coordinate system imply that water properties or biological processes are measured while following the mean flow of a water parcel, e.g., from a drifter.

Laser rangegated imaging--An optical technique which synchronizes the firing of a laser and the opening of a camera shutter after a precise time delay. The method can be used to circumvent optical backscatter limitations of traditional imaging as well as to collect information about only a thin slab of targets at a specific range from the camera and light source.

Macrozooplankton--Large (e.g., cm size) plankton that have significant swimming capabilities. An example might be Euphausia superba. Usage varies among different communities.

MAPS--Multifrequency Acoustic Profiling System - Holliday et al., 1989; Pieper and Holliday, 1984.

MBARI--Monterey Bay Aquarium Research Institute

Mesopelagic--Referring to mid-depths in the open ocean.

Mesoscale--Oceanic features with scales on the order of 100 to 300 km.

MHz--The abbreviation for megahertz, this is the units of frequency (one million cycles per second).

Micronekton--A transition term commonly used to describe those animals in the "fish" that are small in size, but can effectively swim in the presence of at least moderate currents. Usage varies among different communities. (Also see macroplankton.)

Micropascal (µPa)--A unit of pressure equal to one millionth of a newton per square meter.

Micro-scale, microstructure--Spatial structure in any ocean feature or parameter with energy at dimensions of less than about 1 meter.

MOCNESS--A multiple net system for sequentially sampling zooplankton at different depths or collecting serial samples at the same depth (see Wiebe, et al., 1976.)

Mysid--Pelagic or demersal crustaceans of the order Mysidaceae. These organisms are known to swarm and occur both in freshwater lakes and in the marine environment.

Nekton--Aquatic organisms that can effectively swim against relatively strong horizontal currents, e.g., adult fish of many species.

NMFS--National Marine Fisheries Service

NOAA--National Oceanographic and Atmospheric Administration

NSF--National Science Foundation

ONR--Office of Naval Research

Open architecture--A form of instrument or "smart" sensor in which there is planned access to hardware, firmware, or software which allows the "user" to modify or adapt the operating characteristics to fit special circumstances and needs.

Ping--A pulse or other acoustic signal of finite temporal duration introduced into the water by an acoustic system, usually for the purpose of echo ranging on an object (target), detecting its presence, determining its location, and classifying or identifying it.

Planar sonar array--A group of acoustic transducers, arranged in some pattern on a plane for the purpose of converting sound into electrical signals or vice versa.

Plankton Image Analyzer--A device developed at URI/NMFS-Narragansett to enumerate samples of zooplankton from their recorded images and classify individuals into taxonomic groups.

Rayleigh scattering--Scattering of a propagating wave, in our case acoustic, when the dimension of the region or object which is causing the scattering is much less than the wavelength of the ensonifying sound.

Reverberation--Acoustic energy reflected from all of the distributed, and often randomly located, scatterers in the path of an acoustic wave. Consists of returns from the surface, inhomogeneities in the volume and the bottom. A dominant characteristic of reverberation is the stochastic nature of the signal, thus multiple samples (pings) must be averaged in order to obtain estimates of the strength of the process.

ROV--Remotely Operated Vehicle

Sea Beam--An acoustic system usually used by the geophysics community to acquire multibeam bathymetry. Has also been used to a limited extent to examine volume scattering.

Shadowgraph--A side-scan sonar with exceptionally high spatial resolution.

SIO/MPL--Scripps Institution of Oceanography, Marine Physical Laboratory

Size-frequency distribution--Histograms, tables, plots or other displays of the abundance of organisms versus size. Sometimes called "size-abundance distributions" in papers on bioacoustics to minimize confusion with alternate uses of the word "frequency" (See the definition of Frequency, above.)

Small zooplankton--For the purposes of this workshop and this document, the term "small zooplankton" was taken to mean zooplankton for which some variant of the fluid sphere scattering model is an appropriate (or at least approximate) mathematical description of the scattering process. This is in contrast to, for example, adult euphausiids, for which there is evidence that alternate models are better descriptors of the scattering process.

Sonar--Originally an acronym that stands for Sound Navigation and Ranging. In hydroacoustics, it has come to mean an active acoustic sensor which uses a propagation path to the target organisms, school or aggregation that is dominantly horizontal as opposed to an echo sounder, where the propagation path for the sound is principally vertical.

Split-beam method--A multibeam target strength measurement technique that uses the phase relationships of a target in the various beams to estimate the location of target and therefore beam pattern correction in the composite beam (see Foote, et al., 1986.)

Target--A target is an inhomogeneity in the surrounding medium (in underwater acoustics, the water), which reflects sound and has finite bounds in relation to the physical volume sampled by the acoustic system with which the entity was detected. The manifestation of the reflection process is an "echo". This is in contrast to "reverberation", which is the result the reflection of sound from a zone or volume with indefinite size, at least insofar as the measurement system can distinguish.

Target strength--A measure of the reflectivity of an acoustically detected entity with defined physical bounds (fish, plankter, fish school, etc.). Technically, it is ten times the common logarithm of the ratio of the incident intensity of a sound wave of distant origin to the intensity reflected in some specified direction, referred to a fixed reference distance, usually 1 meter from the target's "acoustic center". The target strength usually depends on the size, shape and internal structure of the sound scatterer, the frequency used in the acoustic system, and the contrast of the physical properties of the target material with the surrounding medium.

Tilt angle--For fish, in particular, the angle at which the animal is ensonified (usually near dorsal aspect for echo sounders) will affect the target strength one would wish to use in estimating the biomass in a layer or school. If a species swims with other than a horizontal orientation (on the average), the tilt angle, or deviation from the horizontal, should be known in order to make an accurate biomass estimate.

Thermistor--An electronic resistor which has a known dependence on temperature. The change in resistance is used to measure the temperature of the surrounding medium.

Tomographic methods--Mathematical techniques for reconstructing three dimensional volumes from the integrated projections along rays. In medical imaging tomography, x-rays are used to determine the x-ray attenuation coefficient inside of the body. This can then be related to anatomy. Potential uses in oceanography encompass both light and acoustic tomography.

Transducer--In acoustics, a device that is used to convert acoustic energy to electrical energy or vice versa. There are several distinct technologies that are commonly used to accomplish this task.

Triangulation--In general, the unique location of the source of a sound (or echo) from some combination of at least three ranges and/or bearings in three dimensional space.

Trophic levels--Successive stages of nourishment as represented by links of the food chain.

Underwater Acoustics, Hydroacoustics, Fisheries Acoustics--A branch of physics involving the generation, propagation, scattering and reception of sound in the marine environment. The term hydroacoustics is sometimes used in place of underwater acoustics, especially in the fisheries acoustics community.

Video Plankton Recorder (VPR)--A towed video camera system under development at WHOI which is intended to sample on centimeter scales over many kilometer transects.

V-fin--A specially shaped towed body whose hydrodynamics produce a net force downward. Shaped in "end-on" cross section like an inverted "V" or "U", it can be used as a depressor for another body or net system, or as a platform for an oceanographic instrument or sensor such as an acoustic transducer.

Volume reverberation--Acoustic scattering from randomly positioned particles, organisms or other random acoustic impedance discontinuities in the water column. Characterized by the stochastic nature of the acoustic signal, most volume reverberation is of biological origin.

Volume scattering strength--Ten times the common logarithm of the ratio of the acoustic intensity scattered through 180° from a unit volume (e.g., 1 m3) at a specified reference distance (e.g., 1 m) to the incident plane wave intensity. Usually written SV

WHOI--Woods Hole Oceanographic Institution

Zooplankton--Aquatic animals whose horizontal movements are largely subject to local water currents. While the word plankton originated from the Greek work "drifter", many zooplankters are good swimmers. They do not have the option of changing geographic locations over large distances, however, without resorting to such tactics as vertical migration to position themselves in favorable currents.