Doppler-based Velocimeters

Doppler Effect

A pressure wave in a fluid medium such as sea water consists of a succession of changes in pressure relative to the ambient hydrostatic pressure. These pressure changes are associated with the movement, or propagation, of the wave. A single-frequency, or monochromatic, wave consists of a regular succession of peaks and troughs described by a sinusoidal function. For a source and receiver of sound in a stationary medium, the propagating wave will sweep past the receiver at a rate equal to the sound speed in the medium. If either the source or receiver is moving away from the other, then the time between two successive peaks at the receiver will be greater than if the source and receiver were stationary. This time is called the wave period. Its inverse is called the frequency. An increase in the period corresponds to a decrease in the frequency. If either the source or receiver is moving toward the other, then the time taken for two successive peaks to pass the receiver will be less than if the source and receiver were stationary. The period will be less, corresponding to an increase in the frequency. This perceived effect of a changing frequency due to the motion of source or receiver is named for the scientist Christian Johann Doppler, who first predicted it in 1842.

The consequences of the Doppler effect for acoustic backscattering are illustrated in Fig. 1. In the first case of directed motion of the scattering object toward the source of sound, the wave period of the sound incident on the scatterer is decreased relative to that transmitted. The wave period of the scattered sound is decreased further by movement of the scatterer, as a secondary source of sound, toward the co-located source and receiver. The combined effect is an upwards shift in frequency, denoted upwards shift in frequency. In the second case, motion of the scatterer away from the source of sound and co-located receiver is associated with a downwards shift in frequency, downwards shift in frequency. In the third case, motion of the scatterer perpendicular to the direction of travel of the sound wave has no effect on the frequency at the scatterer or at the receiver.

 Illustration of the Doppler effect
Figure 1. Illustration of the Doppler effect. Credit: Corrected from an image by Sontek/YSI, Inc. Click to enlarge

For a relative speed v of approach of source and receiver in a medium of sound speed c much greater than v, the change in frequency due to the Doppler effect is change in frequency due to the Doppler to an excellent approximation. If the source and receiver are receding relative to one another, then v is negative.

Doppler Sonar

A Doppler sonar is a sonar system with a processing capability that can detect changes in the frequency of echoes from scatterers relative to the transmitted frequency. In a very early application to fish, the Doppler shift was measured by sonars operating at 11 and 30 kHz (Holliday 1974). The mean Doppler shift for individual schools was associated with movement of the school. The spread in values of the measured Doppler shift about the mean was associated with individual fish, especially due to tail-body oscillations of propulsion. Subsequent studies have confirmed this interpretation, as with observation of individual salmon by a 420-kHz sonar (Dahl and Mathisen 1984).

 
Animation of sampling
Figure 2. Animation of sampling scheme for an upward-looking ADPtm. Credit: Sontek/YSI, Inc.
 ADCP™ in housing on the sea floor
Figure 3. An ADCPtm in housing on the sea floor. Credit: RD Instruments. Click to enlarge
 Diagram of a moored buoy sensor
Figure 4. Diagram of a moored buoy sensor in Mass. Bay, one of several in the Gulf of Maine Ocean Observing System Moored Buoy Program at U. Maine. Red object is a downward-looking RDI 300 kHz ADCPtm tethered 3 m from surface. Credit: GoMOOS / U. Maine PhOG. Click to enlarge
 Self contained Acoustic Doppler Current Profiler™ Workhorse Sentinel
Figure 5. Self contained Acoustic Doppler Current Profilertm Workhorse Sentinel. Credit: RD Instruments. Click to enlarge
 ADP™ devices
Figure 6. ADPtm devices. Credit: Sontek/YSI, Inc. Click to enlarge
 Principle of operation of a Nortek Acoustic Doppler Velocimeter (NDV™)
Figure 7. Principle of operation of a Nortek Acoustic Doppler Velocimeter (NDVtm). Credit: NortekUSA.
 Nortek Vector Velocimeter™
Figure 8. Nortek Vector Velocimetertm. Credit: Nortek USA. Click to enlarge

Acoustic Doppler-based Profilers

An acoustic Doppler-based profiler is a sonar consisting of a number of discrete beams oriented in oblique directions relative to the central axis of the device and to one another (Fig. 2). By processing the echoes due to scatterers in the water column as continuous time series from each of the beams simultaneously, the Doppler shift can be determined in orthogonal directions in the plane transverse to the central axis. Except in special circumstances, the echoes emanate from the myriad of small drifting particulate matter such as plankton and marine snow that are nearly ubiquitous in the water column. Thus, if the central axis is oriented along the vertical, the horizontal velocity, consisting of both speed and direction, can be determined as a function of depth. Determination of current velocity profiles is eminently valuable in oceanographic studies, and acoustic Doppler-based profiler is one of the oceanographer’s standard tools.

The acoustic Doppler-based sonar is typically mounted on the hull of a research vessel. It is also remotely deployed, as on the sea floor (Fig. 3), on buoys (Fig. 4), and on observing towers, as that of the Martha’s Vineyard Coastal Ocean Observatory (MVCO). It is often mounted on towed vehicles, remotely operated vehicles (ROVs), and autonomous underwater vehicles (AUVs). Some acoustic Doppler-based sonars can also measure the speed over ground of an underwater vehicle. The usefulness of such devices in navigation is recognized through mountings on AUVs such as REMUS and SeaBED.

The most widely used acoustic Doppler-based sonar is the Acoustic Doppler Current Profiler (ADCPtm), which is manufactured by RD Instruments (Fig. 5). It consists of four beams, each emanating from its own transducer at an angle of 20 deg relative to the central axis. A number of models exist, distinguished principally by their operating frequency, for example, 38, 75, 150, 300, 600, and 1200 kHz. The lower (higher) frequencies are more suitable for work in deeper (shallower) water because of their longer (shorter) maximum detection range.

Acoustic Doppler-based sonar devices are also manufactured by Nortek and SonTek/YSI, Inc. A three-transducer array, again with obliquely oriented beams, is shown among the Sontek/YSI Acoustic Doppler Profilers (ADPtm) in Fig. 6. RD Instruments, Sontek/YSI, Inc., and Nortek all manufacture acoustic Doppler-based sonars in two-, three-, and four-transducer configurations.

The acoustic Doppler-based sonar device has been used to measure the swimming speed of Atlantic herring (Zedel and Knutsen 2000).

Because of the availability of powerful, multiple beams in acoustic Doppler-based sonars, each of these sonars could also be viewed as a set of echo sounders. This possibility has not escaped researchers, who have attempted to exploit the device to measure scatterer concentration throughout the water column (Flagg and Smith 1989, Cochrane et al. 1994, among others). Essential to such applications is sufficient dynamic range, or capacity to register a wide range of echo strengths without being missed because of a high threshold or low saturation level. The dynamic range in most acoustic Doppler-based sonar devices is very limited.

Acoustic Doppler-based Velocimeters

An acoustic Doppler velocimeter (Figs. 7, 8) measures the two- or three-dimensional velocity of water at a single point. It does this by an array of three or four transducers. The central transducer transmits a powerful beam along the axis of the device. By processing the time-gated returns corresponding to echoes from small particles in the intersection volume, the Doppler shift can be observed in two or three non-coincident directions, thus allowing specification of the two- or three-dimensional velocity of the insonified water parcel. An example is the 10-MHz device used by Voulgaris and Trowbridge (1998), with measurement range of about 10 cm and sampling volume of 0.3 ml.

As in the case of the acoustic Doppler-based profiler, use of the acoustic Doppler-based velocimeter to measure backscatter, and oblique scattering too, is possible. In addition to its determination of velocity, a measure of particulate concentration can be obtained. Small scatterers such as microzooplankton (size range 0.02-0.2 mm), for example, can thus be sensed.

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References
Cochrane NA, Sameoto DD, Belliveau DJ (1994) Temporal variability of euphausiid concentrations in a Nova Scotia shelf basin using a bottom-mounted acoustic Doppler current profiler. Mar Ecol Prog Ser 107:55-66
 
Dahl PH, Mathisen OA (1984) Some experiments and considerations for development of Doppler-based riverine sonars. IEEE J Oceanic Eng OE-9(3):214-217
 
Flagg CN, Smith SL (1989) On the use of the acoustic Doppler current profiler to measure zooplankton abundance. Deep-Sea Res 36:455-474
 
Holliday DV (1974) Doppler structure in echoes from schools of pelagic fish. J Acoust Soc Am 55:1313-1322
 
Voulgaris G, Trowbridge JH (1998) Evaluation of the acoustic Doppler velocimeter (ADV) for turbulence measurements. J Atmos Ocean Technol 15:272-289
 
Zedel L, Knutsen T (2000) Measurement of fish velocity using Doppler sonar. Proc MTS/IEEE Oceans 2000 Conf, pp 1951-1956