Charles H. Thompson

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...

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.

Identifying individual clicking whales acoustically amid the clutter of other clicking whales.

Exceptionally clear recordings of sperm whale (Physeter macrocephalus) codas reveal time and frequency properties, which show that clicks within a coda are remarkably similar to each other but that they can differ from clicks in other codas. This is consistent with the hypothesis that individual whales can be identified by the characteristics within their codas. Research has centered on the cluster analysis of these codas to help establish whether acoustic identification of individuals is possible. Recently the cluster analysis has been made more robust. This increases the confidence in the applicability of the approach. Data are now available to couple visual sightings and acoustic tracking with recordings for acoustic identification to give an independent verification of the analysis. Acoustic identification of individuals has also been attempted using isolated echolocation clicks of sperm and beaked whales (Mesoplodon densirostris and Ziphius cavirostris). Although this is a more difficult and problematic undertaking, there has been some promising cluster analysis. Again data for acoustic tracking and visual observations are now available with the recordings for acoustic identification using echolocation clicks to test and perhaps validate the method. [Research supported in part by SPAWAR.]

Comparison of surface radar traffic density contact records to receive levels from separate acoustic data sets

The Office of Naval Research is planning an underwater system of acoustic and environmental sensors, termed the Acoustic Observatory, at a site located off of Dania, Florida. It will enable researchers to make acoustic measurements and evaluate signal processing techniques in an ocean environment with downward refracting propagation, low loss bottom, and high levels of vessel traffic noise. As part of the site selection and evaluation process, vessel traffic was recorded using a state-of-the-art logging radar for a period of one week concurrent with recording of omnidirectional noise on hydrophones at two locations in water depths of 145 and 262 meters. Data were processed to provide a characterization of noise and traffic statistics and patterns relative to the Acoustic Observatory site. Differences in noise between the two sites, and correlations between vessel traffic and noise are presented.

In situ acoustic estimates of the swimbladder volume of Atlantic herring, Clupea harengus

Most marine fish maintain swimbladder volumes equivalent to 4%–5% of their body weight in order to maintain neutral buoyancy. In many fish the addition or removal of gas from the swimbladder is accomplished with the gas gland, a blood invested portion of the swimbladder wall. However, several families, including the herring family, Clupeidae, lack a gas gland. Instead, these fish possess a pneumatic duct between the esophagus and the swimbladder by which they are believed to inflate their swimbladders by ‘‘gulping’’ atmospheric air at the sea surface. Acoustic measurements at 1.5–5 kHz on fish in the Gulf of Maine showed a swimbladder resonance peak near 2.3 kHz at 180 m depth. Midwater trawls confirmed that the fish were Atlantic herring (Clupea harengus) of 19–28 cm length. Calculations using a model of swimbladder resonance gives swimbladder volumes of 1.3% at 180 m. Extrapolation using Boyle’s law suggests that at the sea surface, these herring would need to inflate their swimbladders by up to four ti...

Clustering classification methods for acoustic applications including the analysis of clicking whale data.

Several clustering methods are available to group like objects into classes. The K means approach is well established. Both standard and homegrown algorithms have been tested in the following applications. One neural network technique is self‐organizing map clustering [T. Kohonen, Self‐Organizing Maps, 2nd ed. Springer, New York (1997)]. Although it has some similarities to K means, it has notable differences which can be quite helpful in classification. An advantage is that it has the ability to limit the number of clusters automatically. This can be an important aspect when the number of classes is unknown. After a review of the basics of these techniques, the power of the methods is illustrated by applications to the study of clicking whales. The Littoral Acoustic Demonstration Center has acquired underwater acoustic data from clicking whales in the Gulf of Mexico, and it has access to data from the third and fourth International Workshops on Detection, Classification and Localization of Marine Mammals Using Passive Acoustics. Application of K means and SOM clustering to these data is an important component of identifying individual whales acoustically. Additional details on these results are being presented in Animal Bioacoustics sessions at this meeting. [Research supported by ONR and SPAWAR.]