Sonar

Sonar, also written SONAR, is formally an acronym for Sound Navigation and Ranging. It connotes an acoustic system that can be used as an aid to navigation and that can determine the range, or distance, to objects, also called targets. The range r to a single target is determined by (1) measuring the time of flight t of an acoustic signal, or ping, from its transmission to the reception of the echo, (2) multiplying t by the speed of sound c, and (3) dividing the product by two, i.e., r=ct/2. The quantity c is typically in the range from about 1450 to 1550 m/s, depending on the exact conditions of temperature and salinity. The targets might include obstructions and the sea floor. Originally, development of sonar was spurred by the sinking of the “Titanic” through collision with an iceberg. The intention of the development was to devise an underwater collision-avoidance method. As the term is currently used, sonar refers to a system that can determine the range and direction to objects in the water column as well as boundary surfaces such as the sea floor.

The essential elements of sonar are (1) a transducer, (2) electronics to control the excitation of the transducer and reception of echoes, including their amplification and other processing, and (3) a display. Implicit in this definition is control or knowledge of the orientation of the transducer.


Figure 1. Example of a transducer mounted for calibration. This is the Simrad ES38-12, which is resonant at 38 kHz. Note the visible outlines of individual transducer elements. The array pattern is basically square, with side length 20.5 cm. Credit: photo by M. Parmenter ©Woods Hole Oceanographic Institution. Enlarge

Transducer

The transducer mentioned above may refer to a single transducing element or to an array of transducing elements. While a single element is called a transducer, an array of elements is also sometimes called a transducer, in addition to being called a transducing array (Fig. 1).

Transduction is the process by which signal energy is transformed or converted under controlled conditions from one form to another. A transducer is the device that effects this conversion. There are a number of materials with special physical properties that can render this conversion relatively efficient. One popular class of materials that is exploited in transducers is the piezoelectric. The property of piezoelectricity refers to the formation of electrical charge on the surface of the material when squeezed, or subjected to stress, and vice versa. If a voltage is applied to a piezoelectric crystal, the crystal changes its volume. If this is in contact with a fluid medium such as water, the change in volume generates a pressure wave that propagates away from the crystal. If a pressure wave is incident on the crystal, then the voltage developed across the crystal can be registered. If the incident pressure is due to an echo from a pressure wave originally launched by the transducer, then the elapsed time can be translated directly into a measure of range.

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Electronics

While the principle of transduction is easy to state, its harnassing in electronic devices is quite a complicated undertaking. It is merely stated that the electronics associated with a sonar are designed to control the transmission of an acoustic signal, the reception of echoes resulting from the transmission, and the display of the echo signal. Control of the transmission process generally involves specification of the signal type, pulse duration, and pulse repetition frequency. It also involves triggering a high-power electrical source to excite the transducer precisely as specified. Reception involves the registration of echoes through the induced voltage amplitude, often with conversion to digital format. The electronics also control the display of received echo signals.

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Display

Echo signals may be displayed in a variety of ways. At one time, they were typically shown on the screen of an oscilloscope, i.e, that of a cathode ray tube. Now, the corresponding signal would be shown on a digital screen. A single echo signal would be represented as the variation of a voltage amplitude in time.


Figure 2. Echogram of squid spawning in Monterey Bay, CA, derived with the Simrad EK60/200-kHz scientific echo sounder system. The upper frame displays echoes over a 50-m depth range, including the bottom echo; the lower frame displays echoes from 8 m over the bottom to 2 m under the bottom . The interval of sailed distance is about 1 km. Credit: photo by K. Foote ©Woods Hole Oceanographic Institution. Enlarge


Figure 3. Echo image formed by the Simrad SM2000/90-kHz multibeam echo sounder. The image is formed by a fan of 128 beams, shown originating at the apex. The echo from a centered target at 11.7 m range is shown just inside of the 12-m range arc. Three walls of the tank, width 12 m, are shown in bright outline. Credit: photo by K. Foote ©Woods Hole Oceanographic Institution. Enlarge

Successive echo signals may also be concatenated in time or space, depending on their source. The resulting display is called an echogram if derived from an echo sounder and an echo image if derived from a multibeam sonar.

An echogram consists of a series of echo records in which each record represents the receiver output signal in time, visually encoded by intensity or color. The echo records are aligned vertically, thus indicating the whereabouts of potential targets in the water column. If the echoes are derived by pinging at regular intervals while the transducer platform is in motion, a two-dimensional image is obtained. An echogram is shown in Fig. 2.

An echo image consists of a series of echo records from multiple beams that are spatially distinct. The magnitude of the echo on each beam is usually encoded by intensity or color as in an echogram. The echo records are aligned, usually in an angular sector. The echo image from a multibeam sonar is shown in Fig. 3.

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Calibration

The relationship of pressure and voltage is exceedingly important in scientific applications of sonar. Briefly, this relationship is established through calibration. In the case of conventional sonar applications involving quantification of biological resources by backscattering, this relationship is typically established by the standard-target method. For a given set of sonar parameters, e.g., transmit signal type, duration, amplitude, spectrum, and receiver characteristics, the echo is related to the standard target. In particular, a measure of the echo, such as its amplitude, intensity, or energy content, is related to the backscattering cross section of the standard target. The backscattering cross section of the standard target may be known a priori, in which case the calibration is primary, hence not requiring reference to other, secondary standards that must themselves be calibrated. By knowing this relationship, similarly processed echoes from other targets can be expressed in physical scattering units. The method has another major advantage over other methods: it can measure the entire set of transmitting and receiving electronics as a black box or lumped system.

When the results of calibration exercises are tracked over time, deviations can be readily observed. When outside of reasonable bounds, for example, ?0.4 dB or ?10% in intensity or energy, then a more detailed investigation can be made; otherwise, the system can be used as is.

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Sonar Types

There are a number of number of substantially different designs of transducer arrays and methods of processing echoes. These differences are reflected in the particular names of sonars, some of which are described here.

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Echo sounder

An echo sounder is a special form of sonar in which the transducer or transducer array is fixed in its spatial orientation. The echo sounder, like the more general sonar, consists of several parts, including transducer or transducer array, electronics for controlling transmission and reception, and a means of displaying the echo data. Traditionally, the echo data are displayed in an echogram, as explained above. At one time, this would have been done on a special paper chart recorder; now echograms are customarily displayed on an electronic screen. In addition, provision is usually made for storage of the echo signal, but this varies greatly with the kind of echo sounder, as contrasted by fisheries and scientific echo sounders.


Figure 4. The Furuno Marine Electronics model LS-6100 fish finder is a commercial fisheries echo sounder with dual-frequency capability: 50 and 200 kHz. The screen shown here is divided, with the lower frequency on the left. The bottom depth is 67.4 m. Credit: Furuno Marine Electronics. Enlarge


Figure 5. Simrad EK60 scientific echo sounder with transducer in front of screen, but without power supply and transducer cable. Echograms from two different frequencies are displayed. Credit: Simrad, Inc. Enlarge


Figure 6. BioSonics DT-X scientific echo sounder, an example of a portable system. This operates with both single- or split-beam digital transducers at one of these fixed frequencies: 38, 70, 120, 200, 420, and 1000-kHz. Credit: Biosonics, Inc. Enlarge

Figure 7. Echogram of walleye pollock in the water column, bottom depth about 20 m, recorded with the BioSonics DT-X Series Digital Transducer scientific echo sounder at 120 kHz and displayed on the Biosonics Visual Analyzer. Credit: BioSonics, Inc. Enlarge


Figure 8. Hydroacoustic Technology, Inc. (HTI) scientific echo sounder, Model 244 System, showing hardware. Credit: Hydroacoustic Technology, Inc. Enlarge


Figure 9. Received echo beam (left) and echogram (top right) as displayed on the HTI Model 244 split-beam echo sounder system. Credit: Hydroacoustic Technology, Inc. Enlarge


Figure 10. The EdgeTech 272-TD towfish with dual frequency sidescan sonar analog arrays, frequencies 105 and 390-kHz, with attached towing cable. Credit: EdgeTech Marine. Enlarge


Figure 11. Sidescan sonar image showing shellfish beds in a sea area off Chatham, Cape Cod, Massachusetts, measuring 200 m in width and 130 m in length, as measured at nominal 100-kHz frequency. The beds are the dark masses indicated by arrows. Credit: Fish and Carr (1990). Enlarge


Figure 12. Echo image of a ship wreck from the EdgeTech Marine Side Scan 4300-MPX System. The image is color-enhanced. Credit: Image courtesy of BSH and EdgeTech Marine. Enlarge


Figure 13. RESON SeaBat 8101/240-kHz multibeam sonar. Credit: photo by M. Parmenter ©Woods Hole Oceanographic Institution. Enlarge


Figure 15. Simrad SM2000/200-kHz multibeam sonar. Two arrays of elements are shown as indicated by black bands: circular arc (left), which both transmits and receives in the imaging mode, and external transmitting array (right), which acts with the circular-arc array in reception, in the multibeam echo sounder mode. Credit: photo by M. Parmenter ©Woods Hole Oceanographic Institution. Enlarge


Figure 14. Simrad SM2000/90-kHz multibeam sonar. Two arrays of elements are shown as indicated by black bands: circular arc (bottom), which both transmits and receives in the imaging mode, and external transmitting arry (top), which acts with the circular-arc array in reception, in the multibeam echo sounder mode. Credit: photo by M. Parmenter ©Woods Hole Oceanographic Institution. Enlarge


Figure 16. Echo image of underwater hull of a small boat using Dual-Frequency IDentification SONar (DIDSON®) at a megahertz frequency. Credit: Sound Metrics Corp. Enlarge

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Fisheries echo sounder

A fisheries echo sounder (Fig. 4), which is sometimes called a “fish-finder,” is an echo sounder whose primary purpose is imaging the echo content of the water column, especially to detect and image fish of commercial importance. Information in the visualization is of predominant interest. Signal processing in the receiver does not generally follow a fixed gain function, but may employ automatic gain control (AGC) to boost weak, potentially important echo signals from fish. As a result, it is often difficult to use such echo sounders in scientific work. In many cases, the only output signal is a video signal corresponding to the echogram displayed only on an electronic screen, without paper or printed record, although this was once the norm.

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Scientific echo sounder

In contrast to a fisheries echo sounder, a scientific echo sounder performs its processing operations with a high degree of control. In the case of amplification of the received signal, this involves fixed gain functions to compensate for spherical spreading from single fish or from extensive layers of fish. The echo signal is expressed through a so-called calibrated output signal. This signal is quantitative, hence suitable for scientific applications. It is typically stored by the system accompanying the echo sounder, which is properly called a scientific echo sounder system, although often simply called a scientific echo sounder. Examples of scientific echo sounders are shown in Figs. 5, 6, and 8. Displays accompanying the systems in Fig. 6 and 8 are shown in Figs. 7 and 9, respectively.

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Single-beam echo sounder

Thus far, the transducer of a scientific echo sounder has been regarded as having a single beam. This means that only one beam is formed in reception. Usually, for planar transducers, all elements in the array are excited in the same way and with the same phase during transmission. All elements usually contribute to a single echo signal in reception. They may be weighted in amplitude and/or phase, in a process called shading, but the result of this remains a single echo signal.

Transducers are configured in other ways too, for example, to form dual or split beams.

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Dual-beam echo sounder

In this configuration, all transducer elements act equally in transmission. In reception, however, two arrays are used. In one, a central core of elements forms a rather broad beam. In the other, all the elements are joined to form a coaxial but narrow beam. When a single target is detected, comparison of the two output signals enables the beam pattern to be determined. Knowing this, the beam pattern can be removed from a measurement of echo energy, giving a direct measure of backscattering cross section.

Split-beam echo sounder

As with the single- and dual-beam transducers, all elements act in concert in transmission. In reception, however, each of four quadrant beams is formed. From these, half-beams are formed and the phase differences are determined. If the transducer is aligned with the direction of motion of the platform bearing it, the angles in the alongship and athwartship directions are determined. When a single target is detected, these angles specify its angular location, allowing determination of the beam pattern in that direction. The beam pattern can thence be removed from the measurement of echo energy, yielding a direct measure of backscattering cross section.

The difference in dual- and split-beam determinations of backscattering cross section is that the angular location of the target remains partly unknown in the first case, but is completely known in the second. With a dual-beam determination, the polar angle, or angle of the target relative to the acoustic axis, is determined, but the azimuthal angle remains unresolved. For both systems, the assumption is made that the target lies within the main lobe of the transmit beam. This assumption is usually excellent and defensible.

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Sidescan sonar

A sidescan sonar (Fig. 10) is based on a long linear array of elements that operate synchronously in transmission and reception. Together they launch a beam that is very narrow in the longitudinal plane of the transducer axis and very broad in the plane transverse to the transducer axis. Objects in the path of the beam generate echoes that are received. Typically, echoes from the bottom dominate all other scatterers, but such bottom echoes may include echoes from benthic organisms such as mussel beds and squid egg beds.

Sidescan sonar arrays are typically towed rather close to the bottom, with the axis of the transverse beam oriented below the horizontal, say, at 20 deg. Because of the narrowness of the beam in the longitudinal plane, echoes can be exquisitely sensitive to bottom features. It has been found that maps of bottom backscatter from sidescan sonar are very useful in detecting variations in the bottom roughness and in defining the bottom habitat (Figs. 11 and 12).

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Multibeam sonar

A multibeam sonar consists of one or two arrays of transducing elements. These may be common in transmission and reception or separate. A single beam is formed in transmission. Each receiving element is processed separately, enabling a number of receive beams to be formed by combining the outputs of the several elements with different phasing functions. These effectively steer the beam in different directions at the same time. The beams are generally aligned side by side in the same plane, allowing high resolution of targets over a sizeable angular sector.

In many multibeam sonar models, only the bottom echo is sought in the receiving beam. In these cases, the processing is designed to extract and present the bottom echo over the full angular sector.

In a very few but correspondingly important multibeam sonar models, the water-column signal is made available on each of the beams. This is very useful for imaging pelagic organisms.

The gross characteristics of three multibeam sonars that present the water-column signals are briefly described. In the RESON SeaBat 8101 (Fig. 13), the linear array serves as a transmitter and the curved array as a receiver. The operating frequency is 240 kHz. Each of 101 beams is formed with nominal beamwidth 1.5×1.5 deg. over an angular sector of 150 deg. In both of the other sonars, the Simrad SM2000 multibeam sonar at 90 and 200 kHz, respectively Figs. 14 and 15, an external transducer is available. If this is used, the 128 receive beams have beamwidth 1.5×1.5 deg. If the external array is not used, then the normal beamwidth of each receive beam is 1.5×20 deg. The respective angular spans are 95 and 88 deg.

Dual-Frequency IDentification SONar (DIDSON®) is an acoustic-lens-based sonar, designed by Ed Belcher of the Applied Physics Laboratory of the University of Washington. The sonar records video-like images to hard disk (Fig. 16).

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Links

References

Fish, J.P. and H.A. Carr. 1990. Sound underwater images, a guide to the generation and interpretation of side scan sonar data. Second edition. (Lower Cape Publishing, Orleans).

Fish, J.P. and H.A. Carr. 2001. Sound reflections, advanced applications of side scan sonar. (Lower Cape Publishing, Orleans).

Foote, K.G., H.P. Knudsen, G. Vestnes, D.N. MacLennan, and E.J. Simmonds. 1987. Calibration of acoustical instruments for fish density estimation: a practical guide. In: International Council for the Exploration of the Sea, Palaegade 2-4, 1261 Copenhagen K, Denmark Cooperative Research Report, No. 144.

Foote, K.G., D. Chu, T.R. Hammar, K.C. Baldwin, L.A. Mayer, L.C. Hufnagle, Jr., and J.M. Jech. 2005. Protocols for calibrating multibeam sonar. J. Acoust. Soc. Am., 117: 2013-2027.

Mitson, R.B. 1983. Fisheries sonar. Fishing News Books (Farnham, Surrey, England).

Urick, R.J. 1983. Principles of underwater sound. Third Edition. (McGraw-Hill, New York).