27/11/2013' OCEANOGRAPHIC APPLICATIONS. Acoustic Current Meters

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1 egm502 seafloor mapping lecture 17 water column applications OCEANOGRAPHIC APPLICATIONS Acoustic Current Meters An acoustic current meter is a set of transducers fixed in a frame. Acoustic current meters are used to measure the velocity and direction of currents and waves. They do this using a technique called "reciprocal transmission." reading lurton 2010, chapter 7 A typical acoustic current meter will transmit a 1 MHz acoustic signal in pulses or continuous wave bursts along 4 paths. Flow velocity and direction is measured by observing the timeof-arrival difference of sound along the acoustic paths. A sound pulse moving in the same direction as a current travels faster than one moving against the current. A compass is used to measure the earth's magnetic field, and a tilt sensor measures the instrument's angle. Using these measurements, the direction of the instrument can be determined and consequently the flow direction. Additional pressure data can be obtained to determine the depth of the current. Acoustic Doppler Current Profiler (ADCP) The Acoustic Doppler Current Profiler (ADCP) measures the speed and direction of ocean currents using the principle of Doppler shift. Anyone who has ever heard a train whistle is familiar with the Doppler effect. When the train is travelling towards you, the whistle s pitch is higher. When it is moving away from you, the pitch is lower. The change in pitch is proportional to the speed of the train. ADCP sensor head: Typical 4 beam ADCP sensor head. The red circles denote the 4 transducer faces. 1'

2 The ADCP exploits the Doppler effect by emitting a sequence of high frequency pulses of sound that scatter off of moving particles in the water. Depending on whether the particles are moving toward or away from the sound source, the frequency, or pitch, of the return signal bounced back to the ADCP is either higher or lower. Particles moving away from the instrument produce a lower frequency return and vice versa. Since the particles move at the same speed as the water that carries them, the frequency shift is proportional to the speed of the water, or current. The ADCP has 4 acoustic transducers that emit and receive acoustical pulses from 4 different directions. Current direction is computed by using trigonometric relations to convert the return signal from the 4 transducers to earth coordinates (north-south, east-west and up-down). Because the emitted sound extends from the ship down to the bottom of the ocean, the ADCP measures the current at many different depths simultaneously. This way, it is possible to determine the speed and direction of the current from the surface of the ocean to the bottom. BIOMASS ECHOES (FISHERIES ACOUSTICS) Marine biologists study ecological systems, food chains and the dynamics of marine populations. Backscattered sound from fish or zooplankton is a complex function of physical factors that influence the speed of sound through water, and biological factors associated with the orientation and reflective properties of the target. The table on the next slide lists the primary physical and biological sources of backscatter amplitude and variability. Potential Sources of Backscatter Amplitude and Variability Physical Sonar Carrier Frequency: determines acoustic wavelength Water Temperature: influences speed and attenuation of sound Water Salinity: influences speed and attenuation of sound Biological Swimbladder presence: major source of reflected sound Organism behaviour: tilt, roll, and activity influence the amount and direction of reflected sound Organism morphology: size and shape influences the amount of reflected sound Organism physiology: gut fullness and gonad development influences the shape of a swimbladder 2'

3 The presence of a swimbladder is the primary biological factor influencing the amount and variability of backscattered sound from a fish. Swimbladder size and the angle relative to the longitudinal or sagittal axis of the fish body will determine the amount of sound reflected back to a transducer. A 'typical' fish anatomy is pictured below in the radiograph of brook trout (Salvelinus fontinalis). The amount of food in the stomach influences the volume and shape of the swimbladder. Swimbladder volume does not affect the magnitude of a returned echo except at low frequencies. At frequencies around 1 khz the swimbladder resonates like the chamber of a wind or brass musical instrument and will reflect even more sound than at higher frequencies. Horne, 2000 Horne, 2000 Fisheries Acoustic Research, 2006 h)p:// 3'

4 Echo amplitudes or targets measured using any sonar equipment are variable signals. Variability in reflected sound is influenced by physical factors associated with the transmission of sound through a compressible fluid, and by biological factors associated with the location, reflective properties, and behaviour of a target. The current trend in acoustic target identification is to increase the amount of information collected through increases in frequency bandwidth or in the number of acoustic beams. Exclusive use of acoustics to identify aquatic organisms reliably will require a set of statistical metrics that discriminate among a wide range of similar body types at any packing density, and incorporation of these algorithms in routine data processing. 4'

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6 MILITARY APPLICATIONS ASDIC Royal Navy WWII 6'

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