Impact of windowing and subsampling algorithms on acoustic scattering strength databases

In this study, the distribution of Nemopilema nomurai in the waters of Mijo-myeon, Namhae and Gijang-gun, Busan was analyzed; furthermore, echo counting and echo integration methods were used to compare the distribution density. The acoustic system used in the study was a split beam scientific echosounder operating at 38 and 120 kHz (EK-60, Simrad, Norway). Echo counting and echo integration methods were used to determine the density of N. nomurai distributed in the survey areas. The distribution of N. nomurai by water columns, estimated using an echo counting method, was concentrated at approximately 10 m deep in the waters of Mijo, Namhae and 10–50 m deep in the waters of Gijang, Busan; moreover, the distributed depth varied by the surveyed date and time. It was shown that analyzing the acoustic scattering strength of jellyfish obtained from the echo counting method would be more effective for distributional survey of N. nomurai with two frequency system.

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Comparisons of day-time and night-time hydroacoustic surveys in temperate lakes

Aquatic Living Resources ◽

10.1051/alr/2020011 ◽

2020 ◽

Vol 33 ◽

pp. 9

Author(s):

Michaël Girard ◽

Chloé Goulon ◽

Anne Tessier ◽

Pascal Vonlanthen ◽

Jean Guillard

Keyword(s):

Acoustic Scattering ◽

Target Strength ◽

Fish Stock ◽

Size Estimation ◽

Temperate Lakes ◽

Night Time ◽

Fish Size ◽

Schooling Behaviour ◽

Scattering Strength ◽

Lower Depth

In recent years, due to an increased need for non-intrusive sampling techniques, hydroacoustics has attracted attention in fishery science and management. Efforts to promote standardisation are increasing the accuracy, efficiency, and comparability of this method. The European Water Framework Directive and the Standard Operating Procedures for Fisheries Hydroacoustic Surveys in North American Great Lakes has recommended that surveys be conducted at night. At night, fish usually disperse in the water column, thus allowing for single echo detection and subsequent accurate fish size estimation, while day-time schooling behaviour hampers the estimation of fish size. However, sampling during the day would often be safer and cheaper. This study analyses how fisheries hydroacoustic results differ between day-time and night-time surveys, using data from 14 natural temperate lakes of various size. Data collected during the day and night at two depth layers linked to thermal stratification were compared in terms of acoustic scattering strength, target strength, and biomass estimates. The results showed a significant correlation between day-time and night-time estimates, though biomass in the upper layer was biased for day-time surveys, mainly due to incorrect fish size estimates resulting from rare single echo detections and schooling behaviour. Biomass estimates for the lower depth layer did not significantly differ between the two diel periods. Thus, this study confirms that hydroacoustic sampling in temperate lakes should be performed at night for accurate fish stock biomass estimates.

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Measurements of acoustic volume backscattering in the Indian and Southern Oceans

Marine and Freshwater Research ◽

10.1071/mf9810855 ◽

1981 ◽

Vol 32 (6) ◽

pp. 855 ◽

Cited By ~ 10

Author(s):

M Hall

Keyword(s):

Indian Ocean ◽

Acoustic Scattering ◽

Average Depth ◽

Night Time ◽

Average Mass ◽

Average Abundance ◽

The Indian Ocean ◽

Scattering Strength ◽

Great Australian Bight ◽

Good Agreement

Volume backscattering strengths have been measured at several positions in the Indian Ocean and the Southern Ocean (across the Great Australian Bight). The positions in the Indian Ocean varied from the vicinity of the Equator to a station off the coast of Western Australia near Fremantle. The backscattering strengths have been analysed at frequencies in third-octave steps from 2.5 to 20 kHz. The average daytime scattering strengths at the Equator and in the Bight are similar and range from around - 87 dB re m-1 at 2.5 kHz to between -75 and -70 dB re m-1 at 20 kHz. At night, the average scattering strengths in the Bight increase from about -75 dB re m-1 at 2.5 kHz to about -70 dB re m-1 at 20 kHz, whereas at the Equator the results increase from about -82 dB re m-1 at 2.5 kHz to -64 dB re m-1 at 20 kHz. Deep scattering layers (DSL) were observed both in the Bight and at the Equator. The DSL in the Bight had a resonance frequency of 4 kHz and the average depth of the bottom of the layer was 950 m. From the acoustic scattering strength. it is inferred that the average population density of the fish in the layer is 10-3 m-3, and that the average mass of the fishes is around 40 g. The DSL at the Equator had a flat frequency response at frequencies above 10 kHz (there was no peak in the spectrum) and the average depth of the layer was about 500 m. The average abundance of the scatterers in the layer is inferred to be of the order of 5 × 10-3 m-3. The backscattering strengths measured in the Great Australian Bight have been compared with predictions based on concurrent net hauls that were conducted to depths of 50 and 100 m. Good agreement occurs only at the higher frequencies and at night-time when most of the organisms are near the surface.

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Effects of Weakly Nonlinear Waves on Acoustic Scattering from the Ocean Surface

Journal of Theoretical and Computational Acoustics ◽

10.1142/s2591728521500079 ◽

2021 ◽

pp. 2150007

Author(s):

Runze Xue ◽

Rui Duan ◽

Yuanliang Ma ◽

Kunde Yang

Keyword(s):

Nonlinear Waves ◽

Grazing Angle ◽

Acoustic Scattering ◽

Ocean Waves ◽

Nonlinear Phenomenon ◽

Weakly Nonlinear ◽

Bistatic Scattering ◽

Weakly Nonlinear Waves ◽

Scattering Strength ◽

The Impact

The elevation of ocean waves is always modeled in linear theory as a superposition of the sinusoidal components with crests and troughs of identical heights. However, under some circumstances, the wave amplitude is outside the linear range and presents as a weakly nonlinear asymmetrical waveform with sharper crests and shallower troughs. We studied the impact of the weakly nonlinear effect of ocean waves in deep and intermediate waters on acoustic scattering from the surface of the ocean using two rough surface models with fractal geometry and power law spectral behavior in the equilibrium range. The classic Weierstrass–Mandelbrot function was used to model the linear waves and a new fractal function, the fractional Weierstrass function developed in studies of electromagnetism, was used to model the weakly nonlinear waves. We evaluated these two models using the Pierson–Moskowitz spectrum and the incident wavelength. The bistatic scattering strength was obtained via a numerical method based on the “exact” solution of the integral equation. The weakly nonlinear phenomenon led to a very small reduction in the narrow area around the specular reflection angle and a small increase in the remaining wide area, including the backpropagation area with a scattering angle [Formula: see text]. The differences in backscattering strength between the two models were similar to the bistatic scattering strength in the backpropagation area and did not depend on the incident grazing angle.

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