1,720,969 research outputs found
A study of ship wakes and the enhancement of sonar within them
No full textGas bubbles underwater are powerful scatterers of sound, particularly when the frequency of that sound coincides with the resonance frequency of the bubble (which is inversely proportional to bubble size for air bubbles greater than 25 μm in radius, close to the sea surface). Such powerful scattering of sonar by bubbles might hinder sonar detection and identification of other scatterers that could be targets of interest, for which bubbles are clutter. An example would be when submerged cargo that has fallen off a container ship at night (a hazard that would normally be easy for collision avoidance sonar on a following vessel to detect in bubble-free water) is difficult to detect if still in the wake of the vessel because of the clutter provided by the bubbles. Similar problems occur when underwater hazards are hidden from sonar near the shore by the bubbles of breaking coastal waves. Furthermore, the potential of bubbles to act as sonar clutter has raised questions as to whether techniques to mitigate against this are used when echolocating dolphins blow bubble nets whilst hunting fish. In recent years, two new sonar techniques have been proposed, both using two (or potentially more) pulses which are identical in all but one aspect (phase or amplitude), to detect objects hidden in bubble clutter: Twin Inverted Pulse Sonar (TWIPS) and Biased Pulse Summation Sonar (BiaPSS). They do this by exciting nonlinear scatter from the bubbles using high amplitude sonar, and separating it out from the linear scatter from the targets of interest. TWIPS and BiaPSS rely upon bubbles being driven to large nonlinear pulsations, but this is dependent upon the availability of a high-amplitude source. This thesis experimentally validates TWIPS and BiaPSS using a commercially available underwater acoustic source. In former studies, purpose-built sources were used. The aim of the experiments in this thesis was to demonstrate the level of relevance this novel technology has and how/whether existing hardware could be used to adopt these new techniques. Recent access to a broadband high-power source, harvested from an acoustic doppler profiling system, has allowed exploitation of bubble resonances across a large bubble size range (the size range of near-surface ocean bubbles under waves and wakes being broad), therefore allowing bubbles to expand sufficiently, in the rarefaction phase of a driving signal. As such, the contrast between linear and nonlinear scatter is enhanced, improving target detection capability. At some point within the duration of an entire broadband pulse, each bubble in the cloud becomes excited at its resonance frequency, whilst at all other times every bubble is off-resonance (and so likely to scatter only linearly). However it is shown in this thesis that exciting resonances in this way can be enough to enhance sonar target detection capability in bubbly water. Having achieved experimental validation of the broadband potential of TWIPS and BiaPSS, this investigation exploits the capability of using broadband linear sine sweeps to study when TWIPS and BiaPSS processing can work on a simulated Atlantic Bottlenose Dolphin (Tursiops truncatus) echolocation click too. The thesis then proceeds to propose and test new sonar systems: Time-Reversed Pulse Pair Sonar (TRePPS) explores the area of commonality between these two-pulse sonar systems and time-reversal to explore whether a synergy can be found. Notably, TWIPS and BiaPSS required a critical period of no emission between projecting the first and second pulse of the pair, that period being long enough to allow the bubble pulsations excited by the first pulse to damp out before the onset of the second pulse, but short enough to prevent evolution of the entire population of bubbles present in the sonar field-of-view. A new sonar is devised which does away with this off-time, so that the two pulses are presented simultaneously (TRePPS).The sonar systems are tested in simulation, tank tests, and field tests.Testing of these novel sonar systems requires a bubble field, for these simulations, tank tests, and field tests. In keeping with the scenario outlined at the start of this abstract, the bubble field chosen for field tests is a ship wake. A wealth of information exists on bubble populations generated through natural physical processes. However, the interest in bubble populations generated from wakes at sea has yielded relatively fewer publications, in the public domain. Therefore, to understand the composition and evolution of a ship wake, the thesis conducts a review of existing knowledge of ship wakes, and take acoustic absorption and backscatter measurements on real ship wakes. An acoustic model of a ship wake is produced, to simulate the scatter from an insonified wake and has the potential to be used in the testing of the novel sonar systems. In this thesis, a real wake is used to experimentally validate TWIPS. The novel outcomes of the research presented in this thesis comprise of: • an extension of a review of the existing open-source literature on the acoustic properties of the bubble wake behind travelling sea-surface vessels (originally conducted by Leighton & White, 2008),• a numerical model to represent a generalisation of those acoustic qualities and how this might be used in sonar simulations,• the examination of existing two-pulse sonar techniques, using a commercially available underwater acoustic source,• the study of these techniques in a field trial, using boat wakes as the source of bubbles, and• the formation of new techniques, inspired by the former inventions and motivated by the need for such techniques to be employed by existing platforms with ease.<br/
Ocean noise: the human footprint on underwater soundscapes
No full textYou may have heard that our oceans are under threat due to the impacts of climate change, plastic pollution, and ocean acidification. But there is another threat that must be highlighted—noise pollution. Humans are becoming more and more reliant on the ocean for transportation and renewable energy, but these activities introduce noise. Every fishing vessel, cruise ship, ferry, cargo ship, and jet ski leaves a sound “footprint,” meaning our oceans are becoming increasingly noisy places. Many animals, including whales, dolphins, and fish, produce unique sounds—and scientists are looking at how man-made noises are affecting their communication, behavior, and habitats. We hope to discover ways to create harmony between humans and marine wildlife, to reduce the impact of noise pollution on marine ecosystems. In this article, we will introduce how marine species use sound, how noise pollution affects them, and how they are adapting to sharing their environments with humans
Enhanced target detection and classification using two-pulse sonar methods
No full textA number of two-pulse sonar techniques can be employed to separate linear and nonlinear scatterers. Twin Inverted Pulse Sonar (TWIPS) and Biased Pulse Summation Sonar (BiaPSS) are processes that exploit nonlinear bubble dynamics to perform such a classification, with clutter reduction. Consequently, these techniques can be used to enhance target detection in bubbly waters. TWIPS and BiaPSS rely upon bubbles being driven to large nonlinear pulsations, but this is dependent upon availability of a high-amplitude source. Previous studies could only access a narrow-band source and therefore used a low frequency so that, over the rarefaction cycle, most ocean bubbles would expand sufficiently (the size range of near-surface ocean bubbles under waves and wakes being large). However, recent access to a broadband high-power source has allowed exploitation of bubble resonances. This talk presents results of investigations using broadband linear sine sweeps, of which one is based upon the simulation of an Atlantic Bottlenose Dolphin (Tursiops truncatus) echolocation click from Capus et al. (2006). Insonifying a cloud of bubbles with a wide band of frequencies excites as many of those sizes at, or close to, their resonance. As such, the contrast between linear and nonlinear scatter is enhanced, improving the target detection capability
Efficacy of up and down-chirps for two-pulse sonar techniques
No full textTwin-Inverted Pulse Sonar (TWIPS) and Biased Pulse Summation Sonar (BiaPSS) were inspired by a video of dolphins using bubble nets to hunt. There are a number of animals that have been observed to use the chirp (a sweep in sine waves of shifting frequency) in echolocation. This talk will present the difference in efficacy of the sonar techniques, mentioned above, when using upwards and downwards sweeping chirps as sonar signals. These techniques have been simulated and experimentally tested with a number of bubble size distributions (BSD). This includes simulations using the results of BSD measurements by Farmer and Vagle (1989). Further, another two-pulse technique will be outlined, proposed as a means to overcome the limitation on detection range, owing to the inter-pulse delay in TWIPS and BiaPSS
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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