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Hydroacoustics (Using Sound to See Nekton & Plankton)

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Hydroacoustics (Using Sound to See Nekton & Plankton) Bangarang February 2014 Backgrounder1 Hydroacoustics (Using sound to see nekton & plankton) Eric Keen Abstract This Backgrounder reviews the basic principles of hydroacoustics – using the echoes of transmitted high- frequency sounds to generate acoustic maps of the water column and the biota swimming within it. This branch of oceanography is increasingly sophisticated and rigorous, something I do not do justice to here. I limit my focus to the basic concept, design considerations for hydroacoustic surveys and how echograms can be interpreted. Contents Principles Time-Varied Gain Beam Choices Transducer Depth Data Reduction Survey Design Other Design Concerns Observation Range Scatterers Frequency-dependence Orientation Buncha littles or a single big? Avoidance Swim Speed Density Internal Waves Taxa Differentiation Δ MVBS Multi-Beam Systems Consideration Examples Literature Cited 1 Bangarang Backgrounders are imperfect but rigorous reviews – written in haste, not peer-reviewed – in an effort to organize and 1 Principles Hydoacoustics have been used to survey fish at sea since 1935 (Sund 1935) 2. “Hydroacoustics is the use of transmitted sound to detect fish. Sound is transmitted as a pulse and travels quickly and efficiently through water. As the sound pulse travels through water it encounters objects that are of different density than the surrounding medium, such as fish, that reflect sound back toward the sound source. These echoes provide information on fish size, location, and abundance. The basic components of the acoustic hardware function to transmit the sound, receive, filter and amplify, record, and analyze the echoes. All quantitative hydroacoustic analyses require that measurements that are made with a scientific-quality echo sounders, having high signal to noise ratios, and ability for easy calibration”3 “Hydroacoustics provides a repeatable, non-invasive method of collecting high-resolution (sub-meter scale), continuous data along transects in three dimensions4. MacLennan and Simmonds (1992) as well as Brandt (1996) give a thorough introduction in the use of hydroacoustics for measuring fish abundances and distributions.” 5 Hydroacoutic transducers convert electric energy into pressure waves, and conversely, incoming pressure waves to voltages6. They both send and receive the pings used in hydroacoustics. The pulse emitted from an underwater transducer propagates down the water column in an expanding cone. As it does its sound intensity decreases due to geometrical spreading and absorption 7 . Density differences in the water (caused by physiography, bubbles or biota) absorb some of this sound energy and scatters another portion of it a variety of directions. Part of the signal is reflected back to the surface. As it travels back up the water column, its signal strength continues to be attenuated by absorption and spreading loss. The echoed signal eventually reaches the transducer in a much reduced form. The transducer measures the pressure of the returned echo, translates it to voltage and sends the signal up the transducer cable. The echo sounder processer converts this voltage to a digital signal8 that is sent to a computer. The time-delay between transmission and the return of echoes indicates how deep the reflective object was. The strength of the echo depends on how far it travelled in the water, how absorptive the water was, and how much of the signal was reflected perfectly back to the transducer (which has to do with both the reflectivity of the struck object and the beam patterns it induces in reflected signals at a specific orientation at a given frequency). Time-Varied Gain To compensate for the signal attenuation that comes with propagation, the returning intensity is amplified digitally9 by the computer with a time-varied gain (TVG) function10 according to when the echo was received. The gain applied increases with time delay (which is indicative of how deep the signal travelled before being reflected). A 20 log R will compensate for a one-way spreading loss, or the loss from transducer to target, while a 40 log R compensates for a two-way spreading loss, or the entire loss from transducer to target and back11. Ten years ago, TVG was only a feature on expensive sounders. 20log TVG is more commonplace now, but 40logR (which compensates for spreading lost in both directions) is usually only found in large expensive instrumentation12. Some studies have not applied TVG to shallow surveys with 38kHz, since attenuation is relatively negligible13. Beam Choices Some frequencies are absorbed better than others. In general higher-frequency sounds are attenuated more quickly than lower-frequency sounds. The width of the beam of sound emitted from the transducer will determine 2 Madureira et al. 1993. 3 Maxwell et al. Year Unknown. 4 MacLennan and Simmonds 1992 5 Maxwell et al. Year Unknown. 6 Mathiesen 2003. 7 Misund 1997 8 Mathiesen 2003. 9 Cochrane et al. 1991. 10 Misund 1997 11 Mathiesen 2003. 12 Mathiesen 2003. 13 Madureira et al. 1993. 2 the rate of geometric spreading loss. Therefore, “sonars constructed for detecting targets at long-range operate at low frequency (from around 20 kHz up to about 50 kHz), and have a beam width of around 10 degrees” 14. High-frequency sonars operate from about 150 kHz to about 200 kHz and have a narrower beam width of about 5 degrees”15. Beam width influences spreading loss and the horizontal area ensonified and (along with ping rate) the horizontal resolution of the returned echoes. A wider beam covers a greater horizontal area but the returned echoes are very coarse. A narrow beam combats geometric spreading loss in well-attenuated high-frequency signals and provides much better horizontal resolution, but at the cost of reduced horizontal coverage. Trade offs! In my brief review of the literature I found the following published beam widths: 4 degree (200kHz)16, 10 degree (200kHz)17, 3 degree (104kHz)r18, 7 degree (200kHz) 19, 5 degrees (200 kHz), 15 degrees (200kHz)20, and 7 degrees (200kHz) 21. Another factor is pulse duration. Shorter pulse duration can provide better vertical resolution of backscatter in the water column. In my cursory look at the literature, I found reported pulse durations of .13ms22, .3ms23 and 1ms24. See this figure: 25 To get the best of both worlds, some echosounder employ two transducers at once, each with a different frequency (usually a low one around 50 kHz and a high one around 200 kHz26). Experienced fishers and scientist use this feature to distinguish between species, because the backscattering properties of some species are frequency dependent.” (See below for much more on this) 27. Many transducers are now a split-beam design, allowing the determination of fish locations in three-dimensional space28. (See figure below.) 14 Misund 1997 15 Misund 1997 16 Rowe 1993. 17 Benoit-Bird et al. 2001. 18 Romaine et al. 2002. 19 Trevorrow 2005. 20 Yule 2000. 21 Gomez-Guitierrez et al. 1999. 22 Benoit-Bird et al. 2001. 23 Gomez-Guitierrez et al. 1999. 24 Romaine et al. 2002. 25 Mathiesen 2003. 26 Maxwell et al. Year Unknown. 27 Misund 1997 28 Maxwell et al. Year Unknown. 3 “The need for a more quantitative method led to the invention of the echo integrator29, in which the voltages generated by the returned echo signals are squared and summed over intervals of depth and distance sailed30.” “By calibrating the echo integrator unit using metal spheres with known backscattering strength31, the recording properties of the instruments can be measured. If the backscattering strength of the recorded fish is known, the echo integrator output can be converted to units of fish density3233. Calibration procedures are precise, difficult and often expensive34, but working alternatives have been developed – some involving Dunlop long-life ping pong balls.3536 High frequency transducers have been calibrated using the reflection off the floor of an enclosed pool37. 38 Transducer depth “The quality of recordings from hull-mounted transducers is weather dependent because wind-induced air- bubbles may attenuate and even block the echosounder transmissions39. Transducers need to be mounted sufficiently below the surface to avoid boundary-layer effects from the sea surface and the boat. Depending on 29 Dragesund and Olsen, 1965 30 Misund 1997 31 Foote et al., 1987 32 MacLennan and Simmonds, 1992 33 Misund 1997 34 Foote et al. 1987. 35 Welsby and Hudson 1972, Cochrane et al. 1991. 36 Yule 2000. 37 Cochrane et al. 1991. 38 Macaulay 1994. 39 Dalen and Løvik, 1981; Novarini and Bruno, 1982 4 the nature of your study area, this tow depth may need to be fairly deep. The large oceanographic vessels use a “fish” towed behind and to the side of the vessel, well below the vessel’s wave and the bubble layer caused by rough open seas. Reported depths have included 6-7m40, 5m41, 10m42, 4m43, 7m44, 6m45, and 3–4 m4647. Small boats working in protected waters are able to keep their transducer face nearer the surface. Such published studies report mounting their transducer “directly to the transom”48, “20cm below the water line on a stanchion, fixed amidships, but away from the side of the boat… and echograms were only recorded in relatively calm conditions (Beaufort 2 or less) 49”, by pole “to a depth of 0.5m on the port side of a 6m boat50”. All studies tend to cull the top 1m or so of their echograms51, as well as the 1-3m above the seafloor, where boundary conditions can muddle echo-integration. Survey Design “Critical to the success of any fisheries assessment program, an efficient survey design must incorporate all available knowledge of the stock in question. Increased survey effort is no substitute for a properly designed survey based on a thorough understanding of the biology of the target species to answer clearly defined objectives”52 “For any stock assessment survey, there is a trade-off between the amount of data collected and the precision of the estimate.
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