New broadband methods for resonance classification and high-resolution imagery of fish with swimbladders using a modified commercial broadband echosounder

Abstract

Stanton, T. K., Chu, D., Jech, J. M., and Irish, J. D. 2010. New broadband methods for resonance classification and high-resolution imagery of fish with swimbladders using a modified commercial broadband echosounder. – ICES Journal of Marine Science, 67: 365–378. A commercial acoustic system, originally designed for seafloor applications, has been adapted for studying fish with swimbladders. The towed system contains broadband acoustic channels collectively spanning the frequency range 1.7–100 kHz, with some gaps. Using a pulse-compression technique, the range resolution of the echoes is ∼20 and 3 cm in the lower and upper ranges of the frequencies, respectively, allowing high-resolution imaging of patches and resolving fish near the seafloor. Measuring the swimbladder resonance at the lower frequencies eliminates major ambiguities normally associated with the interpretation of fish echo data: (i) the resonance frequency can be used to estimate the volume of the swimbladder (inferring the size of fish), and (ii) signals at the lower frequencies do not depend strongly on the orientation of the fish. At-sea studies of Atlantic herring demonstrate the potential for routine measurements of fish size and density, with significant improvements in accuracy over traditional high-frequency narrowband echosounders. The system also detected patches of scatterers, presumably zooplankton, at the higher frequencies. New techniques for quantitative use of broadband systems are presented, including broadband calibration and relating target strength and volume-scattering strength to quantities associated with broadband signal processing.

Computer Simulation of Broadband Single-target Echo Waveforms and its Application

“…These single-target echo waveforms cannot be strictly verified. However, the author validated that their target strengths, which were determined by the same signal processing used for actual singletarget echo waveforms, 13) including pulse compression processing, 9,17,21) coincided with predicted target strengths TS pred ( f ) (=10log 10 |F bs ( f )| 2 , where…”

“…t gate is chosen to be much longer than pulse duration after pulse compression processing. 9) In this study, t gate was 1.024 ms (the thickness of the gated volume is approximately 0.77 m at c = 1500 m/s).…”

“…[9][10][11][12] The author previously built a prototype broadband echosounder, 13) and continues to study its potential applications. The advantages of using of broadband signals are: acoustic scattering spectra of marine organisms can be determined, and range resolution can be improved.…”

“…The signal processing procedure that was used, including pulse compression processing, was mostly followed that described in Stanton et al 9) Volume backscattering strength SV( f ) is expressed …”

“…Pulse compression processing 9,17,21) can be applied to the multiple-target echo waveform and to p t (t) before the Fourier The processing is performed for each ping.…”