Richard H. Love

A low‐frequency acoustic scattering model for small schools of fish

A new low‐frequency scattering model for small to moderately sized fish schools has been developed. The model, which uses a mathematical formalism based upon the harmonic solution of sets of coupled differential equations, allows a verified swimbladder scattering ‘‘kernel’’ for the individual fish to be incorporated. It includes all orders of multiple scattering interactions between fish, and calculates the aggregate scattering field by coherent summation. Application to simulated ensembles of closely spaced fish indicates significant deviations from incoherent scattering returns. Peak target strength reductions, and shifts in the resonance frequency, appear due to multiple scattering. The target strength also varies strongly with frequency as a result of interference effects. When applied to widely dispersed ensembles, the model reproduces the results of incoherent scattering. For larger ensembles, at greater depth, the model predicts sharply reduced target strength values around the main resonance. The ...

Abyssopelagic grenadiers: the probable cause of low frequency sound scattering at great depths off the Oregon and California coasts

Abstract Volume reverberation measurements from the slope base and abyssal plain along the coasts of Oregon and northern California show an unexpected scattering layer peaking around 2000 Hz at depths greater than 1000 m. A model of swimbladder resonance applied to published records of bottom-dwelling grenadier size and abundance provided a good fit to the data, suggesting the widespread pelagic occurrence of grenadiers, Coryphaenoides spp., of 20–68 cm length at densities near 0.004 ind. m −2 over the slope base and abyssal plain.

A highly reflective low cost backscattering target

A low cost biplanar reflector target has been developed and used in a system performance test of a near surface omnidirectional source and receiver. The target was constructed from sheets of closed cell plastic bubble sheeting. Theoretically, this material should have very high reflectance characteristics, and this was demonstrated by the experiment. The strongly reflecting target was located at the sea surface, and allowed the interpretation of data as the ship passed through a range of azimuthal angles. The higher than expected backscatter from the target was presumed to be due to the bubble sheeting behaving as a coherent reflector, like a thin layer of air, rather than an ensemble of individually resonating bubbles. This was verified by the data analysis. Lloyds mirror effects were strong, because practically all of the return signals from the 10-ms continuous wave pulses were overlapping. The target strength (TS) of the reflector was strongly reduced at ranges over 100 m. The experiment shows that studies of the statistical distribution of fish school TS must consider the effects of Lloyds mirror.

Changes in volume reverberation from deep to shallow water in the eastern Gulf of Mexico.

Scattering from fish is a primary cause of volume reverberation and, since fish populations change from deep to shallow water, the character of volume reverberation should also change. However, there are few data available to document expected changes. Therefore, an experiment was conducted in the eastern Gulf of Mexico to investigate possible changes in volume reverberation from deep to slope to shelf waters. Results showed that volume reverberation in outer shelf waters varied more rapidly with respect to both time and space than that in deeper waters. Day-time scattering was similar for deep, slope and shelf waters, total scattering strengths generally increased with frequency. Night-time scattering for the deep ocean and slope also increased with increasing frequency. Scattering modeling suggests that swimbladder-bearing fishes smaller than 10 cm were responsible for the observed volume reverberation. Night-time scattering at the outer shelf location was very different, with strong scattering peaks at...

Response to “Comment on ‘Resonant acoustic scattering by swimbladder-bearing fish’ ” [J. Acoust. Soc. Am. 64, 571–580 (1978)]

In the 1970s a model of resonant scattering from a swimbladder-bearing fish was developed. The fish was modeled as an air bubble, representing a swimbladder, encased in a viscous spherical shell, representing the fish flesh. This model has been used successfully to correlate acoustic scattering data with fish information in a number of ocean locations. Recently, questions have arisen about viscous damping of the flesh and the thickness of the shell [K. Baik, J. Acoust. Soc. Am. 133, 5-8 (2013)]. This Letter responds to those questions and provides practical insight into the models use.

Clutter statistics of long-range wideband echoes from fish aggregations off the Oregon coast

Echoes from fish can be the dominant source of reverberation over a range of important sonar frequencies and grazing angles. Moreover, fish echoes from broadband signals often retain coherent structure (generate clutter) after undergoing normalized match-filter processing. Coupled with their inherent spatiotemporal variability, fish can thus be a significant clutter problem for active sonars. Using a towed source and horizontal line-array receiver, measurements of mid-frequency (1.5-11 kHz) backscattering from aggregations of fish were made from the R/V New Horizon in five shallow-water and shelf-break areas off the coast of Oregon (Astoria Canyon to Heceta Bank) during July and August 2012. The experiment and the frequency-dependent echo statistics in relation to the observed distribution and behavior of the two primary resident fish species (Pacific hake and Pacific sardines) are discussed. For example, the short-time echo variability and spatial patchiness were characteristic of the mid-water (hake) an...

Fish schools as potential clutter and false targets: Observations on the New Jersey shelf

Fish schools can appear as clutter or false targets on search sonars and can confuse the interpretation of scattering from the sea floor. During Boundary Characterization 2001, a high‐frequency echosounder was used to quantify fish schools in an effort to provide estimates of false targets and clutter at low frequency. Schools were quantified from the echosounder data using an image processing algorithm designed to provide estimates of school size, acoustic intensity, and a variety of diagnostic features. The number of schools that had the potential to be low‐frequency false targets was estimated using information on fish species obtained from fisheries research trawls and an NRL swimbladder scattering model. A few pelagic fish schools of intermediate size and high intensity, with low‐frequency target strengths estimated at +12 dB, occurred near the sea surface at night in the northern corner of the study site, at a density of about one per km2. These schools were most likely to appear as false targets. Demersal fish, those near the sea floor, although abundant along the 80‐m contour, were not likely to be strong false targets at low frequency. [Work supported by ONR.]

Herring hydroglyphics in littoral waters of the northern Gulf of Mexico

A large shoal of fish occurring in the vicinity of the 220‐m isobath was observed using a standard 38‐kHz fisheries echosounder and a 1.5‐ to 10‐kHz low‐frequency fish sonar (LFFS) for several days in July 2000. The fish behaved like herring, exhibiting a rapid rise to the sea surface at dawn, formation into schools, and a rapid descent to the sea floor. Schools remained at depth through the day and gradually rose to the sea surface at dusk and then rapidly descended and spread out into a diffuse scattering layer at 75‐ to 125‐m depth. Shifts in resonance frequencies during migration, release of gas bubbles during migration, and strong avoidance of the vessel when maneuvering, all suggest the fish were most likely round herring, Etrumeus teres, which are common at these depths in the NMFS historical trawl survey data. An examination is made of some of the scattering characteristics of the schools and layers of these fish and comparisons of the 38‐kHz data to scattering at 500‐Hz bands from 1.5 to 5 kHz an...

Volume reverberation in littoral waters

Until recently, virtually no low‐ and mid‐frequency volume reverberation data were available from littoral waters. However, in the past few years, the Naval Research Laboratory has conducted volume reverberation measurements in deep, slope, and shelf waters in several oceanic regions. Deep ocean volume reverberation is relatively uniform over broad ocean areas at both low‐ and mid‐frequencies. As water depths become shallower, many fishes responsible for deep ocean volume reverberation eventually disappear and the character of the reverberation changes. Measurements in littoral waters demonstrate that low‐ and mid‐frequency volume reverberation levels can be very high and can vary greatly over short distances and from day to night. Results also show that changes in volume reverberation from deep to shallow water can be significantly different in different regions. Variability is the principal feature of volume reverberation in littoral waters.

Characteristic resonance signatures for acoustical scattering from fish

Scattering from fish is an important issue for biologists, aquaculturalists, and the military community. One can track the habits of fish, determine their abundance, and learn to distinguish them from targets of military interest by accurate mathematical formulations of the problem. The aim of this work is to present a mathematical/physical formulation by coupling acoustical signals with fish bladder properties to describe the acoustical scattering from fish bladders. In this work we first develop the theory to describe acoustical scattering from a viscous spherical shell filled with air; and then use it to show how the scattering amplitude and Q of the monopole resonance are modified by the viscosity and thickness of the shell. We then show how the theory may be extended, using a mathematical formulation based on coupled boundary integral equations, to account for the systematics of fish bladder scattering when elongation and viscous effects are combined.