TY - JOUR
AU1 - Melvin, Gary, D
AU2 - Cochrane, Norman, A
AU3 - Li,, Yanchao
AB - Abstract Multi-beam sonar is potentially a powerful analytical tool for investigating the acoustic properties and behaviour of fish in relation to quantitative fisheries research. The SIMRAD SM2000 is a 200 kHz multi-beam sonar employing an 80-element array to transmit and synthesize, electronically, 128 receive beams (20°×2.2°) over a 180° arc simultaneously. Once calibrated, such systems enable the extraction of acoustic target strength and volume backscattering from an extended 3D ocean volume. We present an overview of the theoretical framework for the calibration of a multi-beam sonar, and then compare the acoustic backscatter from a calibrated single-beam 50 kHz echosounder with selected beams from a sphere-calibrated multi-beam sonar. Both systems recorded acoustic data from Atlantic herring contained within a weir, as the fish passed beneath the transducers. Specifically, we examine the relationship between the area-backscattering strength (Sa) from the single-beam system with the nadir beam (beam 63) of the SM2000 sonar. In addition, data are presented on the observed variability in Sa with target aspect for off-vertical angles from 15° to 60° in 15° intervals. Non-standard synthesized SM2000 beam widths are explored for both calibration and field datasets. The implications for biomass estimation are also discussed. Introduction Multi-beam sonar explores the acoustic properties and behaviour of fish in relation to fisheries research. Unlike conventional single- or split-beam acoustic systems, multi-beam sonars can ensonify large volumes of water at high spatial resolution and can visually present real-time target distributions in two, as opposed to a single, dimension (Mayer et al., 2002). Unfortunately, most quantitative multi-beam sonar systems are designed for bathymetric purposes and, consequently, gate-out water column information. Conversely, the commercial fishing sector-scanning and omni-directional sonars used to detect and capture pelagic fish schools are not designed to acquire and store acoustic-signal returns digitally. For the past two decades, the application of multi-beam sonars to fisheries research has been restricted to the enhancement and quantification of visual-output displays. Most investigations have concentrated on behavioural observations, such as school dynamics (Squire, 1978; Misund et al., 1992; Pitcher et al., 1996), target tracking (Arnold et al., 1990; Kvamme et al., 2000), distribution (Olsen, 1990; Melvin et al., 2002), movement (Arnold et al., 1990; Nottestad et al., 1996), and vessel avoidance (Diner and Massé, 1987; Misund, 1990; Soria et al., 1996; Gerlotto et al., 1999). The primary intent of these studies has been to explore and quantify the effects of fish behaviour on acoustic-biomass estimates. More recently, multi-beam sonars have been used as 2D and 3D visualization tools in the estimation of fish density and biomass and also in the study of spatial and temporal changes (Gerlotto et al., 1994, 1999, 2000; Mayer et al., 2002). However, quantification of the information has relied upon extensive post-processing of visual displays to determine the number, size (area and volume), and the distribution of fish schools (Reid, 2000). While multi-beam sonar technology is seen by many researchers as the next step in the evolution of hydroacoustics for fisheries/aquatic research (Fernandes et al., 2002), there are several physical and practical constraints to its development. Until recently, for example, hardware for multiple beams capable of collecting and storing digital information from the entire water column was unavailable. In 1997, the release of the SM2000 sonar by Kongsberg Simrad Mesotech met this initial requirement, but provided no post-processing editing or analytical software for treating the voluminous (gigabytes/hour) data sets. Furthermore, to estimate fish density, the system had to be calibrated and multi-beam signal returns quantified. Acoustic calibration theory and procedures from a fisheries perspective have been well described for single-beam systems (Foote et al., 1987; MacLennan and Simmonds, 1992). Analogous theory and procedures have recently been extended to the multi-beam environment (Cochrane, 2002a). This article presents a brief overview of multi-beam calibration theory and procedures and a comparison of the acoustic backscatter of weir-confined Atlantic herring from a multi-beam system with that obtained from a conventional single-beam echosounder. It is important to note that while one can readily compute the area-backscattering strength (Sa), the extension to biomass estimates for beams far from the nadir lies beyond the scope of this article. Theory The extraction of calibrated, volume-backscattering strength (Sv) from a multi-beam sonar requires modification of the standard Sv equation to incorporate multiple beams. Figure 1 illustrates the general sonar-head configuration and standard deployment orientation. For a multi-beam sonar synthesizing a receive beam at an arbitrary angle θb from the horizontal, volume-backscattering strength can be estimated as follows: where TS(t) is time-dependent axial acoustic target-strength (TS) measure, R(t), time-dependent sonar target range, c, acoustic velocity, Δt, sonar-pulse duration, ΨD, integrated-beam pattern (IBP) or integrated beam width factor, ⁠, DS, transmit directivity function, and DR is the receive directivity function. Figure 1 Open in new tabDownload slide Diagram of multi-beam sonar (SM2000) transducer and its standard deployment configuration illustrating beams and associated angles. Figure 1 Open in new tabDownload slide Diagram of multi-beam sonar (SM2000) transducer and its standard deployment configuration illustrating beams and associated angles. Applying the appropriate summation window function (Hamming) and decoupling the θ (equatorial-fan plane) and φ (wide-beam direction) terms, it is possible to estimate the IBP from a combination of theory (Clay and Medwin, 1977; Medwin and Clay, 1997) and empirical measurement (Cochrane, 2002b). Thus, the IBP is described as: where {DSθ(θb,θ)DRθ(θb,θ)} is proportional to the resultant beam-formed amplitude for a target placed at angle θ and a fixed beam-forming direction θb. In practice, significant simplification occurs by introducing a modified IBP, designated ΨDC, differing from ΨD above in that the beam-specific directivity functions are referenced to the central (90°) beam axial responses as opposed to the axial responses of each individual synthesized beam. Once the redefined IBP has been determined for each beam, the calibrated volume-backscattering strength for any given beam can be extracted from field data by: where VBF(θb,t) is the fully normalized beam-formed voltage for a given beam angle and time and CCal(90°) is the central beam calibration factor experimentally given by: TSCal is the TS of a calibration sphere centred in the 90° (or closest) beam and VBF(90°,tCal) is the resultant beam-formed voltage for the sphere echo. Methods Two hydroacoustic systems were used in this study: a FEMTO Model DE9320 digital transceiver, as detailed in Clay and Claytor (1998), connected to an Airmar 50 kHz transducer; and a SIMRAD SM2000 multi-beam sonar with a 200 kHz head. System specifications of each unit are given in Table 1. The SM2000 synthesizes in real-time 128 receive beams (numbered 0–127) over 180° from an 80-element circular array (radius 0.1085 m) with the elements equi-spaced over a 155° arc. All calibration and experimental data files from the multi-beam system were recorded using the raw (i.e. unbeam-formed) format option to permit the formation of standard (2.2°) and non-standard beam widths. The fore/aft beam width of 20° remained constant (Anon., 1998). Fish data files were collected at 1 ping s−1 using a 20 log R + absorption time-varied-gain (TVG). Table 1 Primary system characteristics for the FEMTO DE9320 digital echosounder and the SIMRAD SM2000 multi-beam sonar. Description . FEMTO DE9320 . SIMRAD SM2000 . Number of beams 1 128 Beam pattern 11° Conical 2.2°×20° Ellipticala Output power (kW) 2 1 Pulse duration (ms) 0.1 0.25 Ping rate (s−1) 1 1–5a Bandwidth (kHz) 7.5 6.4 Target distance (calibration) (m) 7.0 8.0 Description . FEMTO DE9320 . SIMRAD SM2000 . Number of beams 1 128 Beam pattern 11° Conical 2.2°×20° Ellipticala Output power (kW) 2 1 Pulse duration (ms) 0.1 0.25 Ping rate (s−1) 1 1–5a Bandwidth (kHz) 7.5 6.4 Target distance (calibration) (m) 7.0 8.0 a SM2000 ping rate set to 1s−1 for data collection and 5s−1 for calibration. Open in new tab Table 1 Primary system characteristics for the FEMTO DE9320 digital echosounder and the SIMRAD SM2000 multi-beam sonar. Description . FEMTO DE9320 . SIMRAD SM2000 . Number of beams 1 128 Beam pattern 11° Conical 2.2°×20° Ellipticala Output power (kW) 2 1 Pulse duration (ms) 0.1 0.25 Ping rate (s−1) 1 1–5a Bandwidth (kHz) 7.5 6.4 Target distance (calibration) (m) 7.0 8.0 Description . FEMTO DE9320 . SIMRAD SM2000 . Number of beams 1 128 Beam pattern 11° Conical 2.2°×20° Ellipticala Output power (kW) 2 1 Pulse duration (ms) 0.1 0.25 Ping rate (s−1) 1 1–5a Bandwidth (kHz) 7.5 6.4 Target distance (calibration) (m) 7.0 8.0 a SM2000 ping rate set to 1s−1 for data collection and 5s−1 for calibration. Open in new tab Calibration of the single-beam system followed standardized procedures using a 45.0 mm copper sphere (TS50 kHz = −36.1 dB) suspended approximately 7 m beneath the transducer (Foote et al., 1987; MacLennan and Simmonds, 1992). For the SM2000, the sonar head was attached to a turntable shaft such that the circular arc of elements was horizontal and the unit was deployed to a depth of 15 m. A 38.1-mm tungsten-carbide (TS200 kHz = −39.4 dB) calibration sphere was then suspended approximately 8 m in front of the centre of the transducer, and the sphere depth was adjusted to maximize the echo amplitude in the vertical plane. A 350 μs pulse duration was selected to prevent excitation of the nearest computed resonance at 209 kHz. The head was then turned approximately 100° from the central beam and slowly rotated (0.12° s−1) through 180° so that multiple returns from the sphere were obtained for each beam. Calibration data were collected using a 40 log RTVG function and a transmission rate of 5 s−1. These data were later beam-formed to compute the IBP and the CCal(90°) calibration. Parameters computed from the calibration file were subsequently used to quantify field data on fish. Herring weirs are large fixed structures that capture and retain up to 100 t of live and free-swimming herring for several days prior to their removal. Water depths in weirs range from 10 to 20 m, with a tidally-induced fluctuation of about 8 m. While in a weir, herring tend to swim in a circular pattern around the perimeter of the enclosure with some congregation in specific areas. On 20 September 2000, an experiment was conducted in the Abnaki weir, located on the east side of White Island, New Brunswick, to collect acoustic-backscatter data from herring using single- and multi-beam systems mounted side-by-side. The weir enclosed 700 m2 and had a water depth of 10.5 m during the sampling period. One-third of the weir was closed off to concentrate the fish in the deepest water to facilitate seining. Approximately 2 h before seining, a small floating deployment platform (50×50 cm2), containing both the 50 kHz transducer and the SM2000 sonar head each mounted to look downward, was fixed in the centre of the closed area (Figure 2). Data collection was delayed for about 10–15 min to diminish any set-up effects on fish behaviour. Thereafter, data were recorded for 10 min while the herring moved about the confined space. The herring were later seined from the weir, pumped into a volumetric fish hold for total-weight determination (12.9 t), and a sample of fish was removed and measured for length (mean, 19.54 cm). Figure 2 Open in new tabDownload slide Schematic diagram of the Abnaki herring weir and experimental set-up. Herring were confined in an area of approximately 240 m2 by the closing net. The transducer platform was orientated such that the entire area between the closing net and the outer weir net was ensonified on each ping. Figure 2 Open in new tabDownload slide Schematic diagram of the Abnaki herring weir and experimental set-up. Herring were confined in an area of approximately 240 m2 by the closing net. The transducer platform was orientated such that the entire area between the closing net and the outer weir net was ensonified on each ping. Analysis for the single-beam system followed standard procedures using FEMTO Hydroacoustic Data Processing System software (HDPS) to edit data and an output Sa. In-house software was used to beam-form both the calibration and field data files to either standard 2.2° or 11° beam widths in order to obtain comparative outputs from the SM2000. Parameter estimates derived from the calibration file (i.e. IBP and calibration factor) were then applied to the data file(s), in accordance with the procedures previously described, to yield Sv(θb,t) for each ping of each data set. The area-backscattering strength (Sa) was computed by integrating sv (Sv in non-logarithmic form) over depth for the pre-selected intervals and for the entire water column. Linear quantities were converted to logarithmic (dB) form using Sa = 10 log(sa) (MacLennan et al., 2002). All data shallower than 3.0 m were excluded to minimize near-field effects. For the 50 kHz transducer, the near field was estimated to extend to approximately 2.4 m (MacLennan and Simmonds, 1992) and up to 6 m for the SM2000 (Chu et al., 2001), although correction for near-field effects of the latter system is small (<1 dB) beyond 3 m (Cochrane, 2002a). Differences in pulse duration and TVG between the calibration and the data files were accounted for in the computations. Corresponding pings from each data set were identified using the computer time and the first occurrence of a common navigational fix. Data for 542 corresponding pings were collected. Results For the typical SM2000 deployment in which the centre of the array points downward, there is no exact nadir beam (Figure 1). Beams 63 and 64 are offset either side of the nadir by 0.7° (i.e. one-half the beam spacing). Ping-by-ping comparison of area backscatter from beams 63 and 64 reveals highly correlated data sets (r=0.826) with almost identical mean Sa. Consequently, inter- and intra-system comparisons are referenced to beam 63 as an approximate nadir. Observations from the multi-beam sonar showed that herring were not distributed uniformly under the platform throughout the recording period, and that the edge of the school sometimes occurred in beams 63 and 64 (Figure 3). Figure 3 Open in new tabDownload slide A single ping from the SM2000 sonar illustrating the distribution of herring under the transducers. In this case, the fish are concentrated on the seaward side of the weir and the edge of the school occurs almost directly under the sonar. The rectangles identify the position of the seaward weir poles (left) and the shore-side closing net (right). Figure 3 Open in new tabDownload slide A single ping from the SM2000 sonar illustrating the distribution of herring under the transducers. In this case, the fish are concentrated on the seaward side of the weir and the edge of the school occurs almost directly under the sonar. The rectangles identify the position of the seaward weir poles (left) and the shore-side closing net (right). Ping-by-ping comparison of the area-backscattering strength from the single- and multi-beam systems (beam 63) indicated a relatively good correspondence or “tracking” of observations for both specific depth intervals (the 3.0–4.5 m interval was chosen for illustration) and for the entire water column (Figure 4). Significant (p<0.01) inter-system correlation coefficients occurred for individual depth intervals and for total water column Sa. Furthermore, the frequency distribution of Sa was similar in form for both systems with a slight positive skewness (Figure 5). However, there are significant offsets in magnitude: 3.5 and 7.8 dB for the total water column and for the 3.0 to 4.5 m layer, respectively (Table 2). Maximum difference in mean Sa occurred for the 3.0–4.5 m layer. The difference continuously decreased from 5.10 dB for the 4.5–6.0 m depth interval to 0.44 dB for the 9.0–10.5 m (bottom layer) interval (Table 2). Differences in mean Sa were reduced substantially when the data were re-beam-formed from the standard 2.2° to approximately an 11° lateral beam width (11°×20°) (Figure 5). Again the maximum difference in mean area-backscattering strength (5.9 dB) occurred in the 3.0–4.5 m depth interval, yet the mean Sas of the two systems differed by only 1.2 dB for the total water column (Table 2). Figure 4 Open in new tabDownload slide Ping-by-ping comparison of area-backscattering strength (Sa) for the 3.0–4.5 m depth interval from the SM2000 (beam 63) and the HDPS echosounder (bottom), and for the total water column from the SM2000 (beam 63) and the HDPS systems (top). Scales are offset slightly to display trends. Figure 4 Open in new tabDownload slide Ping-by-ping comparison of area-backscattering strength (Sa) for the 3.0–4.5 m depth interval from the SM2000 (beam 63) and the HDPS echosounder (bottom), and for the total water column from the SM2000 (beam 63) and the HDPS systems (top). Scales are offset slightly to display trends. Figure 5 Open in new tabDownload slide The frequency distribution of single-ping, area-backscattering strength (Sa) in 0.5 dB intervals for the entire water column from the HDPS single-beam system, and for beam 63 from the SM2000 multi-beam sonar with a lateral beam width of 2.2° and 11°. Figure 5 Open in new tabDownload slide The frequency distribution of single-ping, area-backscattering strength (Sa) in 0.5 dB intervals for the entire water column from the HDPS single-beam system, and for beam 63 from the SM2000 multi-beam sonar with a lateral beam width of 2.2° and 11°. Table 2 Descriptive statistics of area-backscattering strength (Sa in dB) for SM2000 beam 63 (SM63) with standard 2.2° beam angle, an 11° beam angle (SM63-11) and for the single-beam transducer (HDPS). All values were converted to decibels after computation of statistical properties. . . . . . . 95% CI . Source . Depth interval . Mean Sa . Minimum Sa . Maximum Sa . Standard deviation . Low . High . SM63 3.0–10.5 −17.0 −23.4 −13.7 −21.5 −17.2 −16.9 3.0–4.5 −27.7 −43.7 −21.1 −28.4 −28.0 −27.4 4.5–6.0 −25.0 −41.7 −17.0 −26.7 −25.3 −24.8 6.0–7.5 −24.3 −38.9 −18.1 −26.4 −24.5 −24.1 7.5–9.0 −23.4 −35.6 −17.0 −25.5 −23.7 −23.2 9.0–10.5 −21.7 −34.1 −15.5 −23.3 −22.0 −21.5 HDPS 3.0–10.5 −13.5 −22.0 −8.8 −16.5 −13.6 −13.3 3.0–4.5 −19.8 −38.2 −11.3 −20.0 −20.2 −19.5 4.5–6.0 −19.9 −31.4 −12.3 −20.8 −20.2 −19.6 6.0–7.5 −20.5 −31.9 −13.2 −21.9 −20.7 −20.2 7.5–9.0 −20.9 −31.8 −14.5 −22.4 −21.2 −20.7 9.0–10.5 −21.3 −32.6 −14.4 −22.7 −21.6 −21.1 SM63-11 3.0–10.5 −14.7 −20.8 −11.5 −19.8 −14.8 −14.6 3.0–4.5 −25.8 −35.6 −19.7 −27.5 −25.8 −25.3 4.5–6.0 −22.7 −36.9 −17.9 −25.64 −22.9 −22.5 6.0–7.5 −21.6 −30.5 −16.3 −24.5 −21.8 −21.4 7.5–9.0 −20.8 −31.9 −15.1 −23 −21.0 −20.6 9.0–10.5 −19.8 −31.0 −15.02 −22.51 −20.0 −19.6 . . . . . . 95% CI . Source . Depth interval . Mean Sa . Minimum Sa . Maximum Sa . Standard deviation . Low . High . SM63 3.0–10.5 −17.0 −23.4 −13.7 −21.5 −17.2 −16.9 3.0–4.5 −27.7 −43.7 −21.1 −28.4 −28.0 −27.4 4.5–6.0 −25.0 −41.7 −17.0 −26.7 −25.3 −24.8 6.0–7.5 −24.3 −38.9 −18.1 −26.4 −24.5 −24.1 7.5–9.0 −23.4 −35.6 −17.0 −25.5 −23.7 −23.2 9.0–10.5 −21.7 −34.1 −15.5 −23.3 −22.0 −21.5 HDPS 3.0–10.5 −13.5 −22.0 −8.8 −16.5 −13.6 −13.3 3.0–4.5 −19.8 −38.2 −11.3 −20.0 −20.2 −19.5 4.5–6.0 −19.9 −31.4 −12.3 −20.8 −20.2 −19.6 6.0–7.5 −20.5 −31.9 −13.2 −21.9 −20.7 −20.2 7.5–9.0 −20.9 −31.8 −14.5 −22.4 −21.2 −20.7 9.0–10.5 −21.3 −32.6 −14.4 −22.7 −21.6 −21.1 SM63-11 3.0–10.5 −14.7 −20.8 −11.5 −19.8 −14.8 −14.6 3.0–4.5 −25.8 −35.6 −19.7 −27.5 −25.8 −25.3 4.5–6.0 −22.7 −36.9 −17.9 −25.64 −22.9 −22.5 6.0–7.5 −21.6 −30.5 −16.3 −24.5 −21.8 −21.4 7.5–9.0 −20.8 −31.9 −15.1 −23 −21.0 −20.6 9.0–10.5 −19.8 −31.0 −15.02 −22.51 −20.0 −19.6 Open in new tab Table 2 Descriptive statistics of area-backscattering strength (Sa in dB) for SM2000 beam 63 (SM63) with standard 2.2° beam angle, an 11° beam angle (SM63-11) and for the single-beam transducer (HDPS). All values were converted to decibels after computation of statistical properties. . . . . . . 95% CI . Source . Depth interval . Mean Sa . Minimum Sa . Maximum Sa . Standard deviation . Low . High . SM63 3.0–10.5 −17.0 −23.4 −13.7 −21.5 −17.2 −16.9 3.0–4.5 −27.7 −43.7 −21.1 −28.4 −28.0 −27.4 4.5–6.0 −25.0 −41.7 −17.0 −26.7 −25.3 −24.8 6.0–7.5 −24.3 −38.9 −18.1 −26.4 −24.5 −24.1 7.5–9.0 −23.4 −35.6 −17.0 −25.5 −23.7 −23.2 9.0–10.5 −21.7 −34.1 −15.5 −23.3 −22.0 −21.5 HDPS 3.0–10.5 −13.5 −22.0 −8.8 −16.5 −13.6 −13.3 3.0–4.5 −19.8 −38.2 −11.3 −20.0 −20.2 −19.5 4.5–6.0 −19.9 −31.4 −12.3 −20.8 −20.2 −19.6 6.0–7.5 −20.5 −31.9 −13.2 −21.9 −20.7 −20.2 7.5–9.0 −20.9 −31.8 −14.5 −22.4 −21.2 −20.7 9.0–10.5 −21.3 −32.6 −14.4 −22.7 −21.6 −21.1 SM63-11 3.0–10.5 −14.7 −20.8 −11.5 −19.8 −14.8 −14.6 3.0–4.5 −25.8 −35.6 −19.7 −27.5 −25.8 −25.3 4.5–6.0 −22.7 −36.9 −17.9 −25.64 −22.9 −22.5 6.0–7.5 −21.6 −30.5 −16.3 −24.5 −21.8 −21.4 7.5–9.0 −20.8 −31.9 −15.1 −23 −21.0 −20.6 9.0–10.5 −19.8 −31.0 −15.02 −22.51 −20.0 −19.6 . . . . . . 95% CI . Source . Depth interval . Mean Sa . Minimum Sa . Maximum Sa . Standard deviation . Low . High . SM63 3.0–10.5 −17.0 −23.4 −13.7 −21.5 −17.2 −16.9 3.0–4.5 −27.7 −43.7 −21.1 −28.4 −28.0 −27.4 4.5–6.0 −25.0 −41.7 −17.0 −26.7 −25.3 −24.8 6.0–7.5 −24.3 −38.9 −18.1 −26.4 −24.5 −24.1 7.5–9.0 −23.4 −35.6 −17.0 −25.5 −23.7 −23.2 9.0–10.5 −21.7 −34.1 −15.5 −23.3 −22.0 −21.5 HDPS 3.0–10.5 −13.5 −22.0 −8.8 −16.5 −13.6 −13.3 3.0–4.5 −19.8 −38.2 −11.3 −20.0 −20.2 −19.5 4.5–6.0 −19.9 −31.4 −12.3 −20.8 −20.2 −19.6 6.0–7.5 −20.5 −31.9 −13.2 −21.9 −20.7 −20.2 7.5–9.0 −20.9 −31.8 −14.5 −22.4 −21.2 −20.7 9.0–10.5 −21.3 −32.6 −14.4 −22.7 −21.6 −21.1 SM63-11 3.0–10.5 −14.7 −20.8 −11.5 −19.8 −14.8 −14.6 3.0–4.5 −25.8 −35.6 −19.7 −27.5 −25.8 −25.3 4.5–6.0 −22.7 −36.9 −17.9 −25.64 −22.9 −22.5 6.0–7.5 −21.6 −30.5 −16.3 −24.5 −21.8 −21.4 7.5–9.0 −20.8 −31.9 −15.1 −23 −21.0 −20.6 9.0–10.5 −19.8 −31.0 −15.02 −22.51 −20.0 −19.6 Open in new tab The area-backscattering strength was computed, using standard beam widths for four specific beams approximately 15° apart on either side of the nadir (beam 63), to examine the variation in Sa with increasing angle from the vertical. Beam vector length remained constant (∼7.5 m) for each angle and signal returns within 3 m of the sonar face were excluded. The data indicate a general decline in mean Sa in the shoreward direction (beams <63) up to 45° off vertical, thereafter an increase was documented (Figure 6). This is consistent with the visual inference of fewer fish just beneath the sonar head on the closing-net side and even fewer fish at depth in this section of the weir during the data recording (Figure 3). On the seaward side, Sa increased with increasing angle off-vertical and was again consistent with the observed concentration of herring on the outer portion of the weir and away from the recording vessel. Figure 6 Open in new tabDownload slide Mean Sa (dB) for selected SM2000 beams at approximately 15° increments extending from the shoreside of the weir to the seaward side. Figure 6 Open in new tabDownload slide Mean Sa (dB) for selected SM2000 beams at approximately 15° increments extending from the shoreside of the weir to the seaward side. Temporal observations of the 5-ping average Sa for beam 85 (30° seaward) were compared with those from the corresponding shoreward beam (beam 42), from the (near) nadir beam, and from the HDPS single-beam system (Figure 7). While large fluctuations in Sa were observed for both beam 42 and for the HDPS data, Sa values were far more consistent for beams 63 and 85. It was assumed that the former reflects the fluctuating presence or absence of herring on the shoreward side of the weir in contrast to their continuous presence on the seaward side. Observed differences in Sa, while including variation in fish TS related to the angle of ensonification, cannot be assumed to be a direct measure of this effect, but these more strongly reflect variations arising from the ever-changing distribution of fish during the recording period. Figure 7 Open in new tabDownload slide Five-ping average Sa (dB) for SM2000 beam 42 (30° shoreward of the nadir), beam 63 (nadir), beam 85 (30° seaward of the nadir), and the HDPS single-beam system. Figure 7 Open in new tabDownload slide Five-ping average Sa (dB) for SM2000 beam 42 (30° shoreward of the nadir), beam 63 (nadir), beam 85 (30° seaward of the nadir), and the HDPS single-beam system. Discussion In 1997, Kongsberg Simrad Mesotech released one of the first multi-beam sonars providing complete digital-return data over its entire profiling range. Unlike previous systems, which relied on image-processing methodologies that were sometimes non-linear to visualize and quantify return data, the SM2000 provided a complete geo-referenced set of signal amplitudes. Unfortunately, when the system was released, no post-processing tools were available to explore the generated data and even the playback options were limited. In the first years of use, the traditional approach of using 2D and 3D visualization and image-analysis techniques combined with echosounder density estimates was followed to quantify the observations crudely (Mayer et al., 2002). However, it became apparent that precise multi-beam calibration would offer an extremely useful, and potentially powerful, acoustic tool for fisheries research. Over a period of 2–3 years, the necessary software was developed to undertake system calibration, to compute the IBP, and to apply the necessary calibration parameters for data quantification (i.e. Sv determination). The calibration studies were conducted in 2000 and 2001 and the computational approach has been described in Cochrane (2002a). Our principal objective was to employ these developments to compare the observed acoustic backscatter of naturally swimming fish using calibrated single- and multi-beam systems. In general, ping-by-ping estimates of Sa between the SM2000 nadir beam (standard beam width) and the HDPS system track fairly well for specific depth intervals and also exhibit a significant relationship for the entire water column. There is an especially strong correlation for the 3.0–4.5 m layer. Nevertheless, there are also relatively large and variable offsets in Sa between the two systems. Detailed examination of the multi-beam data shows that several times during data collection, the inner boundary of the fish school occurred almost beneath the floating platform and within the 11° conical beam of the HDPS system. Consequently, one might expect the observed differences in Sa to be a function of target distribution relative to the individual beam patterns. To explore this possibility, ping intervals, where major inclines/depressions in single-beam Sa occurred, were identified. These occurrences were found to correspond almost exactly with time periods when the school boundary occurred beneath the transducers. Furthermore, when the receive beam width was increased to 11°, the difference in mean Sa between the two systems was substantially reduced for all depth layers. For the total water column, the difference in mean Sa fell to 1.2 dB. This data set is not amenable to quantification of off-vertical ensonification effects because of the non-random distribution of fish, and probably fish orientations, in the ensonified area. Ping-by-ping comparison of identically inclined beams on opposite sides of the nadir showed very different results. It was evident from the distribution pattern of Sa that beam 85 (seaward beam) was similar to beam 63 (nadir), while the (shoreward) distribution of beam 42 more closely resembled those for the HDPS observations. This further supports the contention that differences between the two systems were largely a function of fish distribution. The derived pattern of Sa for off-vertical beams also corresponded reasonably well with inferences of fish distribution and concentration revealed by the sonar display. This increases confidence that the theoretical formulations and analytical tools presented in this study constitute an effective technique for investigating off-axis variation in acoustic backscatter. Although the data analysis is limited, it illustrates that the two independent acoustic systems track changes in fish concentrations in a similar manner. Overall, there were relatively small differences in mean Sa when using the broader 11° beam width of the SM2000. A number of factors could account for residual differences in the area-backscattering strength. Broadening the SM2000 beam width does not result in identical beam patterns for the two systems. One is comparing an 11° conical beam with a 20°×11° elliptical beam. The 11° conical beam has a sharper angular roll-off since directivity is attained on both transmit and receive, in contrast to receive-only for the multi-beam. There is also an expected frequency-dependent (50 vs 200 kHz) difference in fish TS. Using Love's equation (Urick, 1983) and a mean length of 19.54 cm, a TS difference of 0.54 dB is anticipated. Other studies have suggested that the mean volume-backscattering strength may be species-dependent and could vary by 2 dB or more between commonly used frequencies (Kirkegaard and Lassen, 1990). No precise estimate of the calibration uncertainty of each system has been made, but it is assumed that 0.1–0.2 dB of error for the single-beam system and 0.5–1.0 dB for the multi-beam system would be realistic. Further research in this area is required. Another potential source of error is the periodic physical movement (1°–5°) of the transducer during recording due to wave action. Smaller errors may arise from the physical separation of the transducers. Summary This article is an overview of the theoretical framework for the extraction of quantitative acoustic data over the entire angular range of a multi-beam sonar system. It also illustrates similar quantitative results from multi-beam and conventional single-beam acoustic survey systems where such comparisons are valid. Multi-beam sonar technology has the potential to supplement—if not eventually replace—conventional single- and split-beam systems in acoustic survey, especially with the future use of fully 2D or 3D multi-beam arrays. Even the current multi-beam systems greatly enhance our capability to visualize and to quantify the aquatic environment. The foundation has now been established to utilize this new technology in fisheries research and to address the many questions associated with its application. However, it will probably be some time before the full capability and scope of multi-beam sonar applications are realized. We must again emphasize that even though multi-beam returns are quantifiable, the translation of these data into fish density/biomass estimates for angles far-off the nadir remains to be solved. We thank all fishers who provided access to fishing gear and fish for our ongoing acoustic research. 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TI - Extraction and comparison of acoustic backscatter from a calibrated multi- and single-beam sonar
JF - ICES Journal of Marine Science
DO - 10.1016/S1054-3139(03)00055-9
DA - 2003-01-01
UR - https://www.deepdyve.com/lp/oxford-university-press/extraction-and-comparison-of-acoustic-backscatter-from-a-calibrated-urlYrDpYaU
SP - 669
EP - 677
VL - 60
IS - 3
DP - DeepDyve
ER -