On the usage of the particle velocity field for bottom characterization

Vector sensors (VS) are devices that measure the vectorial particle velocity field. Compared with traditional hydrophone arrays that measure the acoustic pressure, systems based on VS present enhanced spatial filtering capabilities. The feasibility of bottom characterization with a 4-element 40cm length vector sensor array (VSA) in a frequency band of 8-14 kHz was recently demonstrated by Santos et al. The study suggests that systems based on VS outperform traditional hydrophone arrays, when considered in geoacoustic parameter estimation. Vector sensor data can improve the resolution of the estimators, moreover the highest resolution of the estimates were achieved with the vertical particle velocity measurements alone. Bearing in mind that actually VS are not widely available, the present work shows through simulations that using a narrow band signal and a vertical array which elements are pairs of hydrophones one can estimate the vertical particle field and attain a resolution for the bottom parameters similar to that obtained by a VSA. Based on a normal mode description of the pressure and particle velocity field, the resolution gain achieved by a linear estimator based on the vertical component only, is compared with similar estimators based on the pressure or on the horizontal component. Using simulations for different shallow water typical scenarios, we point out sensible values for the number of sensors, inter sensor spacing for system design as well as preferred equipment location for best results. This work is a contribution to the design of a compact array of hydrophones that takes advantage of the higher sensitivity of the vertical particle velocity field for geoacoustic parameter estimation

Estimating the multipath structure of an underwater channel using a single vector sensor

This paper aims at estimating the multipath structure of an underwater acoustic channel using a single vector sensor. The multipath structure of an underwater acoustic channel measured by a single hydrophone from broadband signals is used for source localization or environmental monitoring applying model based methods. Generally in a first step the relative delays between the different echoes impinging the hydrophone and their amplitudes are estimated, then those are compared with a propagation model outputs for a given set of input parameters. Usually this requires a number of time consuming propagation model runs and an optimization procedure. A vector sensor measures the pressure and the particle velocity, thus in addition to delays and amplitudes estimated by a hydrophone can potentially estimate the direction (azimuth and elevation) of the different echoes in an impulse response. This additional information can be used to significantly reduce the number of propagation model runs and simplify the optimization procedure. Although, the estimate of the azimuth is relatively straightforward using a single vector sensor, the estimate of the elevation of the different echoes is difficult due to the signal bandwidth, low amplitude of the latter arrivals or sensitivity to phase errors of available devices, among others. This paper discusses the estimates of the direction of individual echoes observed in impulse responses gathered during the Makai'05 sea trial in the context of single vector sensor model based localization (azimuth, range and depth)

Modelação acústica submarina de alta frequência baseada em traçamento de raios: revisão teórica e aplicações actuais

Nos ultimos anos a evolução dos sistemas electrónicos de processamento de sinal, que combinam uma alta potência de cálculo com um tamanho reduzido, tem permitido o desenvolvimento de sistemas acústicos submarinos que operam a altas frequências (acima de 1 kHz); tais sistemas permitem o processamento e ciente do sinal recebido em antenas com sistemas simples, ou compostos, de hidrófonos. Em paralelo com a vertente de engenharia aplicada tem-se assistido ao reviver do interesse de modelação, relacionada com a utilização de modelos de traçamento de raios; tais modelos permitem a modelação a altas frequências com um tempo de cálculo, compatível com as necessidades de um sistema que deve responder em tempo real. O intenso desenvolvimento das aplicações e dos modelos não tem sido por em acompanhado pela uniformização teórica das diferentes aspectos da modelação, o que dificulta o desenvolvimento de aplicações dos modelos em simulações e em dados reais. Os objectivos principais deste documento consistem em apresentar uma revisão teórica dos modelos de traçamento de raios, em abordar alguns dos problemas numéricos envolvidos na aplicação dos modelos, e em mostrar aplicações concretas, baseadas nos modelos TRACE e TRACEO actualmente em desenvolvimento no SiPLAB

Passive acoustics as a tool for global ocean monitoring and exploration

The ocean is vast, rich, fragile and unexplored. Despite a massive investment in the last years, ocean exploration remains localized at the ocean surface and in shallow coastal areas, covering merely 5% of the total volume. It is well known that marine life has perfectionned acoustics as its primarily sense for every day tasks such as locating preys, sensing danger, breeding, etc. So, in the ocean, sound has the same role as light on land. It is well known that sound propagates to long distances in the ocean and hence acoustics can be seen as the ONLY mean to "see" the ocean interior at a global scale, in respect of the environment and at a reasonable cost. There is nowadays scientific evidence that listening to ocean ambient sound allows to retrieve information about i) anthropogenic sound, related to man activities such as shipping, bathymetric sonar, industrial construction, echo-sounding and defence; ii) environmental related sound such as that of earthquakes, ice, rainfall, waves and wind, and iii) biological related sounds as for example, that of marine mammals, fish and the acoustic activity of invertebrates in coastal areas. Ambient sound can also be used for indirectly estimating environmental and biological quantities such as sub-bottom structure or coastal flora health and abundance. This paper presents an overview of recent results for some of those passive acoustic imaging techniques

Medium frequency (800-1600Hz) geoacoustic inversions with drifting sparse arrays during the MREA BP07 experiment

In order to evaluate properly the acoustic propagation characteristics in shallow water environments, it is well established that appropriate knowledge of the acoustic properties of the seabottom is required. In the last decade, full-field geoacoustic inversion techniques have been demonstrated to provide adequate methodologies to assess those properties. However, several of the developed techniques may suffer a lack of adequacy to the design of low-frequency active sonar systems (LFAS) for which the assessment of seabottom characteristics are drawn. For instance most matched-field inversion techniques demonstrated so far use acoustical signals at much lower frequencies than those of the sonar (few tens to hundreds Hertz to be compared to the 1-2 kHz range of standard LFAS). Furthermore, some of the techniques may be difficult to be handled in an â€oeoperationally relevant context― since they are based on relatively complex designed systems such as highly instrumented vertical line arrays spanning the whole water column. In this paper, we investigate the potentialities of medium frequency acoustical signals (800-1600 Hz) received at several ranges (from 1 km to 10 km) along a field of drifting sparse arrays eventually reduced to a couple of hydrophones or even a single one for spatial coherent geoacoustic inversion purposes. The experimental datasets of the Maritime Rapid Environmental Assessment BP’07 seatrials South of Elba Island in the Mediterranean Sea are used to support this study.FC

Geometric and seabed parameter estimation using a vector sensor array: experimental results from Makai experiment 2005

A vector sensor is constituted by one omni directional pressure sensor and three velocity-meters that are sensitive in a specific direction - x, y or z. Since a vector sensor is able to measure the three particle velocity directional components it acts as a spatial filter and therefore is advantageous in three dimensional direction of arrival (DOA) estimation. The potential gain obtained in DOA estimation can be extended to other geometric parameters such as source range and depth, as well as seabed parameters. The objective of this paper is to present experimental results of a four element vertical vector sensor array (VSA) data set collected during MakaiEx'05 experiment for geometric (range and depth) and seabed geoacoustic parameter estimation (sediment compressional speed, density and compressional attenuation). The parameter estimation problem is posed as an inversion method based on an extension of the conventional pressure only Bartlett estimator to particle velocity. The developed VSA based Bartlett estimator is proportional to the pressure only Bartlett estimator response by a directivity factor, providing an improved side lobe reduction or even suppression when compared with the pressure only response. This behavior will be illustrated for geometric and seabed parameters clearly showing the advantages of the use of VSA over hydrophone arrays. In source localization the VSA outperforms an array of hydrophones of same number of sensors. Moreover, when the VSA Bartlett estimator is applied for seabed parameter estimation, it will be shown that the estimation resolution of these parameters increased significantly, even for density and compressional attenuation, parameters difficult to estimate using an array of hydrophones

Comparing the resolution of Bartlett and MVDR estimators for bottom parameter estimation using pressure and vector sensor short array data

This work compares the resolution of a pressure and vector sensor based conventional Bartlett estimator, with their MVDR estimator counterparts, in the context of bottom characterization with a short vertical array. Santos et al. [1]demonstrated the gain of a vector sensor array (VSA) based linear estimator (Bartlett) for generic parameter estimation. Moreover, it was shown that for bottom characterization the highest resolution of the estimates were achieved with the vertical particle velocity measurements alone. The present work highlights the gain in parameter resolution of a VSA based MVDR estimator. It is shown, that also for a MVDR estimator, the vector sensor array data improves the resolution of parameter estimation. But, it is also shown, through simulations, that for bottom parameter estimation, the pressure based MVDR estimator has higher resolution and sidelobe attenuation than the VSA based Bartlett estimator. These results were verified for experimental data acquired by a four element, 30 cm long vertical VSA in the 8–14 kHz band, during the Makai Experiment 2005 sea trial, off Kauai I., Hawaii (USA)

Evolution of the magnetic properties along the RCuBi2 (R=Ce, Pr, Nd, Gd, Sm) series of intermetallic compounds

FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFINANCIADORA DE ESTUDOS E PROJETOS - FINEPIn this paper, the evolution of the magnetic properties along the series of intermetallic compounds RCuBi2 (R = Ce, Pr, Nd, Gd, Sm) is discussed. These compounds crystallize in a tetragonal ZrCuSi2 (P4/nmm) structure, and our single crystals of RCuBi2 grown from Bi-flux show no evidence for Cu-deficiency [Ye et al., Acta Crystallogr. C 52, 1325 (1996)] as previously reported for R = Ce. For R = Ce, Pr, Gd, and Sm, we found an antiferromagnetic ordering at TN ∼ 16 K, 4.2 K, 13.6 K, and 4.9 K, respectively. For R = Nd, we saw no evidence for a magnetic phase transition down to T = 2 K. These values of TN clearly show a dramatic breakdown of the De Gennes factor in this series. We discuss our data taken into account the tetragonal crystalline electrical field and the anisotropic Ruderman-Kittel-Kasuya-Yoshida magnetic interaction between the R-ions in this family of compounds.1151713FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFINANCIADORA DE ESTUDOS E PROJETOS - FINEPFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFINANCIADORA DE ESTUDOS E PROJETOS - FINEPFAPESP [2012/04870-7, 2006/60440-0, 2009/09247-3, 2010/11949-3, 2011/01564-0, 2011/23650-5, 2012/05903-6]2006/60440-02009/09247-32010/11949-32011/01564-02011/23650-52012/04870-72012/05903-6Sem informaçãoSem informaçãoSem informação58. Annual Conference on Magnetism and Magnetic Materials4 a 8 de Novembro de 2013Denver, C