Gary D. Melvin

Quantification of a multibeam sonar for fisheries assessment applications.

The acoustic theory is developed for a multibeam fisheries-type sonar employing a circular arc of transducer elements. Specifically, numerical relations for transmit and receive beam patterns are derived and methodologies set forth for the derivation of appropriately scaled acoustic target strength and acoustic volume backscattering strength from an ideally performing multibeam device. Predicted and measured beam characteristics of a realizable multibeam sonar, a Kongsberg Simrad-Mesotech SM 2000, are compared. Practical techniques for the extraction of calibrated acoustic volume backscattering strength from real systems are advanced.

Extraction and comparison of acoustic backscatter from a calibrated multi- and single-beam sonar

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 (S a ) 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 S a 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.

The diel-vertical distribution and characteristics of pre-spawning aggregations of pollock (Pollachius virens) as inferred from hydroacoustic observations: the implications for survey design

The characteristics of pollock (the synonymous European common name is saithe) pre-spawning aggregations were described at two locations with contrasting bathymetric features on the Scotian Shelf, off the Canadian Maritimes. The data were collected using a split-beam echosounder onboard a research vessel, augmented with periodic, bottom-trawl samples. Pollock form aggregations each fall that persist at the same location over time. Such aggregations appeared to be associated with spawning. Hydroacoustic information indicates that pollock become more densely aggregated at night. This could reflect movement away from the study area during the day, or changes in the proportion of pollock in the acoustic dead zone over the 24-h period. The hydroacoustic information indicates that while pollock can occur up to 30 m off bottom, the greatest proportion remains within 1-5 m off bottom during both day and night. The length composition of the pollock aggregations differed between the two sites, with larger fish found at the site further offshore. Within an aggregation, there was spatial heterogeneity with respect to fish size, with larger fish found primarily within the core area of aggregation as shown by the hydroacoustics. An appropriate survey design for obtaining an index of abundance for pollock would reflect both the contagious (patchy) distribution as they prepare to spawn, and the diel differences in the availability of the fish to the hydroacoustic-sampling gear.

Multibeam sonar calibration: target localization in azimuth

Multibeam sonar requires calibration for use in absolute backscattering measurements of water-column targets. In an initial study, a least-squares method has been developed to locate a standard calibration target in the sonar transmit plane from the individual transducer element receive signals. By enabling the target position to be specified in azimuth, the overall transmit and receive sensitivity may be measured simultaneously for a number of beams, considerably rationalizing the amount of work required for calibration by standard target. The localization technique has been applied to data derived from test measurements with the Simrad SM2000 Multibeam Echo Sounder, operating at 200 kHz, with 128 receive beams of nominal beamwidth 1.5/spl times/20 deg. The approximate root-mean-square phase mismatch is about 0.1 rad, and the resultant error of the predicted azimuth angle is less than 0.1 deg.

Multibeam sonar calibration: removal of static surface reverberation by coherent echo subtraction

Calibration of multibeam sonar in confined environments may require treatment of surface reverberation. In the case of tanks, the method of coherent echo subtraction may be employed to remove static surface reverberation, allowing the tanks to be used for standard-target calibration. This is illustrated for the Simrad SM2000 Multibeam Echo Sounder operating at 200 kHz. The signal-to-noise ratio is typically improved by about 25 dB.

Acoustic near field effects for the Simrad–Mesotech SM 2000 multibeam sonar

Targets quantitatively observed with a multibeam sonar based on a truncated circular arc transducer geometry, focused at infinity, display anomalous apparent target strengths at short measurement ranges. Three near field mechanisms, affecting both the sonar transmit and receive responses, are identified: (1) Anomalous inter‐elemental differential phase shifts, (2) inter‐elemental differential spreading losses, (3) shifts in elemental viewing aspect. With reference to a commercial 200 kHz Simrad–Mesotech SM 2000 multibeam sonar, the relative importances of the three mechanisms are numerically examined at a 1‐m target range. Combined effects in both transmit and in receive are further evaluated for target ranges between 0.5 and 20 m. For the instrument considered, near field effects are significant at ranges of 5 m and less and are characterized by systematically reduced target amplitudes after the application of ‘‘normal’’ sonar time variable gain. Systematic broadening of combined transmit–receive respons...

Multibeam sonars: Applications for fisheries research

Multibeam sonars are rapidly becoming a standard tool for seafloor mapping in support of geological, geophysical, and engineering applications. More recently, the ability of multibeam sonars to provide high‐resolution, large areal coverage, and potentially quantitative, coregistered, backscatter has been applied very successfully to problems of defining fisheries habitat. While most multibeam sonars are designed to gate out all midwater returns, newly developed systems now allow access to the full data stream and thus offer the possibility of application to studies of pelagic and demersal fisheries. Traditional acoustic approaches to fisheries issues have used single beam echo sounders that sample a relatively small volume of the water column within a survey area. Multibeam sonars provide a mechanism to greatly enhance both the resolution and the area of coverage. When combined with powerful new 3‐D visualization techniques, they can offer immediate feedback on fish behavior as well as the critical questi...