Effective herding of flatfish by cables with minimal seafloor contact

Otter trawls are very effective at capturing flatfish, but they can affect the seaf loor ecosystems where they are used. Alaska f latf ish trawlers have very long cables (called sweeps) between doors and net to herd fish into the path of the trawl. These sweeps, which ride on and can disturb the seaf loor, account for most of the area affected by these trawls and hence a large proportion of the potential for damage to seaf loor organisms. We examined modifications to otter trawls, such that disk clusters were installed at 9-m intervals to raise trawl sweeps small distances above the seafloor, greatly reducing the area of direct seafloor contact. A critical consideration was whether flatfish would still be herded effectively by these sweeps. We compared conventional and modified sweeps using a twin trawl system and analyzed the volume and composition of the resulting catches. We tested sweeps raised 5, 7.5, and 10 cm and observed no significant losses of flatfish catch until sweeps were raised 10 cm, and those losses were relatively small (5–10%). No size composition changes were detected in the flatfish catches. Alaska pollock (Theragra chalcogramma) were captured at higher rates with two versions of the modified sweeps. Sonar observations of the sweeps in operation and the seaf loor after passage confirmed that the area of direct seafloor contact was greatly reduced by the modified sweep

Measurements of total scattering spectra from bocaccio (Sebastes paucispinis)

Marine sportfishing in southern California is a huge industry with annual revenues totaling many billions of dollars. However, the stocks of lingcod and six rockfish species have been declared overfished by the Pacific Fisheries Management Council. As part of a multifaceted fisheries management plan, marine conservation areas, covering many million square nautical miles, have been mandated. To monitor the recovery of the rockfish stocks in these areas, scientists are faced with the following challenges: 1) multiple species of rockfish exist in these areas; 2) the species reside near or on the bottom at depths of 80 to 300 m; and 3) they are low in numerical density. To meet these challenges, multifrequency echosounders, multibeam sonar, and cameras mounted on remotely operated vehicles are frequently used (Reynolds et al., 2001). The accuracy and precision of these echosounder results are largely dependent upon the accuracy of the species classification and target strength estimation (MacLennan and Simmonds, 1992).

Aperfeiçoamento do algoritmo algébrico sequencial para a identificação de variações abruptas de impedância acústica via otimização

Neste trabalho são utilizados a técnica baseada na propagação de ondas acústicas e o método de otimização estocástica Luus-Jaakola (LJ) para solucionar o problema inverso relacionado à identificação de danos em barras. São apresentados o algoritmo algébrico sequencial (AAS) e o algoritmo algébrico sequencial aperfeiçoado (AASA) que modelam o problema direto de propagação de ondas acústicas em uma barra. O AASA consiste nas modificações introduzidas no AAS. O uso do AASA resolve com vantagens o problema de identificação de danos com variações abruptas de impedância. Neste trabalho são obtidos, usando-se o AAS-LJ e o AASA-LJ, os resultados de identificação de cinco cenários de danos. Três deles com perfil suave de impedância acústica generalizada e os outros dois abruptos. Além disso, com o objetivo de simular sinais reais de um experimento, foram introduzidos variados níveis de ruído. Os resultados alcançados mostram que o uso do AASA-LJ na resolução de problemas de identificação de danos em barras é bastante promissor, superando o AAS-LJ para perfis abruptos de impedância.

Caracterização do reservatório Marlim por inversão acústica

Exploracionistas tem grande interesse em sistemas turbidíticos, pois em geral estes compõem prolíficos plays exploratórios. No entanto, estes potenciais reservatórios se encontram muitas vezes perto ou abaixo de resolução sísmica. Dessa forma, no processo de inversão, é importante integrar os dados sísmicos com qualidade de resolução obtida a partir do seu pré-condicionamento, para que o resultado final possua características detalhadas das camadas. O pré-condicionamento possibilita melhora na resolução dos dados sísmicos, através da atenuação dos ruídos aleatórios. Como objetivo final, foi realizada a inversão acústica em dados sísmicos post stack, migrados em tempo, a um sistema de turbiditos na Bacia de Campos. O principal objetivo da inversão sísmica é transformar o dado de reflexão em propriedades petrofísicas quantitativas. A inversão para impedância acústica é comumente utilizada para predição de porosidade. O fluxo de trabalho proposto foi dividido em cinco estágios principais: pré-condicionamento sísmico do dado 3D, correlação poço-sísmica, construção do modelo de baixa frequência, inversão do dado, e estimativa da porosidade. Os resultados mostraram que o cubo de impedância acústica invertido possui resolução muito superior quando comparado com o dado em amplitude sísmica, possibilitando melhor visualização das feições geológicas do Campo de Marlim. Além de suas limitações, como desconsiderando os efeitos das variações de fluido e variações litológicas complexas sobre a relação porosidade/impedância, o método fornece uma ferramenta confiável para exploração sísmica. Detalhes mais precisos das propriedades petrofísicas podem ser obtidos através de métodos de inversão mais sofisticados, a partir de dados pre stack.

Estudo da precisão e da exatidão da técnica da birrefringência acústica na avaliação de tensões residuais e aplicadas.

Este trabalho apresenta um procedimento numérico para o estudo da precisão e da exatidão da técnica de birrefringência acústica como usada no Instituto de Engenharia Nuclear (IEN) para avaliação de tensões residuais e aplicadas em estruturas metálicas. Esse procedimento deverá ser incorporado posteriormente ao módulo de processamento de sinal do sistema ultrassônico utilizado no Laboratório de Ultrassom do IEN para levar em conta, de forma automática e sistemática, as incertezas nos dados de entrada e as suas propagações ao longo dos cálculos efetuados. A birrefringência acústica é geralmente definida a partir das velocidades de duas ondas ultrassônicas volumétricas de incidência normal e ortogonais entre si. A birrefringência pode ser definida diretamente a partir dos tempos de percurso dessas duas ondas, uma vez que elas percorrem o mesmo espaço físico. Os tempos de percurso das ondas podem ser assim considerados como as variáveis primárias de interesse. Por meio da teoria da acustoelasticidade é possível relacionar a birrefringência acústica com as tensões atuantes no material explorando o fato da velocidade da onda ultrassônica ser afetada pela presença de um campo de tensões. Nesta dissertação elaborou-se um conjunto de planilhas eletrônicas no aplicativo Microsoft Excel para efetuar de forma automática todos os cálculos necessários ao objetivo proposto, levando em conta as incertezas nos dados, sua propagação ao longo dos cálculos e o número de dígitos significativos nos resultados. Como exemplo do procedimento desenvolvido, o estudo da precisão e da exatidão na determinação das tensões em uma viga de aço sob flexão pela técnica da birrefringência acústica é apresentado. Como valor de referência, para fins do cálculo da exatidão alcançada, utilizou-se uma solução analítica derivada da teoria clássica da resistência de materiais. Os resultados encontrados indicaram uma razoável precisão, com um erro relativo menor do que 8%, e uma baixa exatidão, menor do que 57%, nos pontos da viga sujeitos às maiores tensões principais de flexão.

Demersal groundfish densities in trawlable and untrawlable habitats off Washington: implications for the estimation of habitat bias in trawl surveys

Demersal groundfish densities were estimated by conducting a visual strip-transect survey via manned submersible on the continental shelf off Cape Flattery, Washington. The purpose of this study was to evaluate the statistical sampling power of the submersible survey as a tool to discriminate density differences between trawlable and untrawlable habitats. A geophysical map of the study area was prepared with side-scan sonar imagery, multibeam bathymetry data, and known locations of historical NMFS trawl survey events. Submersible transects were completed at randomly selected dive sites located in each habitat type. Significant differences in density between habitats were observed for lingcod (Ophiodon elongatus), yelloweye rockfish (Sebastes ruberrimus), and tiger rockfish (S. nigrocinctus) individually, and for “all rockfish” and “all flatfish” in the aggregate. Flatfish were more than ten times as abundant in the trawlable habitat samples than in the untrawlable samples, whereas rockfish as a group were over three times as abundant in the untrawlable habitat samples. Guidelines for sample sizes and implications for the estimation of the continental shelf trawl-survey habitat-bias are considered. We demonstrate an approach that can be used to establish sample size guidelines for future work by illustrating the interplay between statistical sampling power and 1) habitat specific-density differences, 2) variance of density differences, and 3) the proportion of untrawlable area in a habitat.

Polarisation diversity of conformal arrays based on geometric algebra via convex optimisation

A simple and general design procedure is presented for the polarisation diversity of arbitrary conformal arrays; this procedure is based on the mathematical framework of geometric algebra and can be solved optimally using convex optimisation. Aside from being simpler and more direct than other derivations in the literature, this derivation is also entirely general in that it expresses the transformations in terms of rotors in geometric algebra which can easily be formulated for any arbitrary conformal array geometry. Convex optimisation has a number of advantages; solvers are widespread and freely available, the process generally requires a small number of iterations and a wide variety of constraints can be readily incorporated. The study outlines a two-step approach for addressing polarisation diversity in arbitrary conformal arrays: first, the authors obtain the array polarisation patterns using geometric algebra and secondly use a convex optimisation approach to find the optimal weights for the polarisation diversity problem. The versatility of this approach is illustrated via simulations of a 7×10 cylindrical conformal array. © 2012 The Institution of Engineering and Technology.

Fractal dimension, wavelet shrinkage and anomaly detection for mine hunting

An anomaly detection approach is considered for the mine hunting in sonar imagery problem. The authors exploit previous work that used dual-tree wavelets and fractal dimension to adaptively suppress sand ripples and a matched filter as an initial detector. Here, lacunarity inspired features are extracted from the remaining false positives, again using dual-tree wavelets. A one-class support vector machine is then used to learn a decision boundary, based only on these false positives. The approach exploits the large quantities of 'normal' natural background data available but avoids the difficult requirement of collecting examples of targets in order to train a classifier. © 2012 The Institution of Engineering and Technology.

Estimation of the detection probability for Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) with a passive acoustic method

Yangtze finless porpoises were surveyed by using simultaneous visual and acoustical methods from 6 November to 13 December 2006. Two research vessels towed stereo acoustic data loggers, which were used to store the intensity and sound source direction of the high frequency sonar signals produced by finless porpoises at detection ranges up to 300 m on each side of the vessel. Simple stereo beam forming allowed the separation of distinct biosonar sound source, which enabled us to count the number of vocalizing porpoises. Acoustically, 204 porpoises were detected from one vessel and 199 from the other vessel in the same section of the Yangtze River. Visually, 163 and 162 porpoises were detected from two vessels within 300 m of the vessel track. The calculated detection probability using acoustic method was approximately twice that for visual detection for each vessel. The difference in detection probabilities between the two methods was caused by the large number of single individuals that were missed by visual observers. However, the sizes of large groups were underestimated by using the acoustic methods. Acoustic and visual observations complemented each other in the accurate detection of porpoises. The use of simple, relatively inexpensive acoustic monitoring systems should enhance population surveys of free-ranging, echolocating odontocetes. (C) 2008 Acoustical Society of America.

A passive acoustic monitoring method applied to observation and group size estimation of finless porpoises

The present study aimed at determining the detection capabilities of an acoustic observation system to recognize porpoises under local riverine conditions and compare the results with sighting observations. Arrays of three to five acoustic data loggers were stationed across the main channel of the Tian-e-zhou Oxbow of China's Yangtze River at intervals of 100-150 m to record sonar. signals of free-ranging finless porpoises (Neophocaena phocaenoides). Acoustic observations, concurrent with visual observations, were conducted at two occasions on 20-22 October 2003 and 17-19 October 2004. During a total of 42 h of observation, 316 finless porpoises were sighted and 7041 sonar signals were recorded by loggers. The acoustic data loggers recorded ultrasonic signals of porpoises clearly, and detected the presence of porpoises with a correct detection level of 77.6% and a false alarm level of 5.8% within an effective distance of 150 m. Results indicated that the stationed passive acoustic observation method was effective in detecting the presence of porpoises and showed potential in estimating the group size. A positive linear correlation between the number of recorded signals and the group size of sighted porpoises was indicated, although it is faced with some uncertainty and requires further investigation. (C) 2005 Acoustical Society of America.

Biosonar behaviour of free-ranging porpoises

Detecting objects in their paths is a fundamental perceptional function of moving organisms. Potential risks and rewards, such as prey, predators, conspecifics or non-biological obstacles, must be detected so that an animal can modify its behaviour accordingly. However, to date few studies have considered how animals in the wild focus their attention. Dolphins and porpoises are known to actively use sonar or echolocation. A newly developed miniature data logger attached to a porpoise allows for individual recording of acoustical search efforts and inspection distance based on echolocation. In this study, we analysed the biosonar behaviour of eight free-ranging finless porpoises (Neophocaena phocaenoides) and demonstrated that these animals inspect the area ahead of them before swimming silently into it. The porpoises inspected distances up to 77 in, whereas their swimming distance without using sonar was less than 20 in. The inspection distance was long enough to ensure a wide safety margin before facing real risks or rewards. Once a potential prey item was detected, porpoises adjusted their inspection distance from the remote target throughout their approach.

Comparison between visual and passive acoustic detection of finless porpoises in the Yangtze river, China

Recently, sonar signals and other sounds produced by cetaceans have been used for acoustic detection of individuals and groups in the wild. However, the detection probability ascertained by concomitant visual survey has not been demonstrated extensively. The finless porpoises (Neophocaena phocaenoides) have narrow band and high-frequency sonar signals, which are distinctive from background noises. Underwater sound monitoring with hydrophones (B&K8103) placed along the sides of a research vessel, concurrent with visual observations was conducted in the Yangtze River from Wuhan to Poyang Lake in 1998 in China. The peak to peak detection threshold was set at 133 dB re 1 mu Pa. With this threshold level, porpoises could be detected reliably within 300 m of the hydrophone. In a total of 774-km cruise, 588 finless porpoises were sighted by visual observation and 44 864 ultrasonic pulses were recorded by the acoustical observation system. The acoustic monitoring system could detect the presence of the finless porpoises 82% of the time. A false alarm in the system occurred with a frequency of 0.9%. The high-frequency acoustical observation is suggested as an effective method for field surveys of small cetaceans, which produce high-frequency sonar signals. (C) 2001 Acoustical Society of America.