3,494 research outputs found
A time-reversal suboptimal detector for underwater acoustic barriers
This paper proposes an experimental setup composed
of two interconnected vertical arrays: one transmit only
array (TOA) and a receive only array (ROA). It is shown that
using the time reversal principle, where the ocean is used as
a spatial matched filter, signal energy can be simultaneously
focused on each ROA hydrophone and thus obtain the detector
output by simple summation of the received energy over the
array. This setup effectively configures a multistatic system with
several transmitters and several receivers coherently processed
both in time and space. Simulations using a ray trace propagation
model combined with a naive scattering formulation show that
the obtained empirical detection probability is close to the
theoretical optimal bound derived assuming space - time white
Gaussian noise. This system was tested in a sea trial that
took place during September 2007 in the Hopav°agen Bay near
Trondheim, Norway. The actual setup was composed of a 2
sources TOA on a shallow area of 8 m depth near the shore
and a 16 receivers ROA approximately 100 m apart in a 8 to
25 m depth range dependent duct. The transmitted signals were
200 ms duration LFMs with 3 kHz bandwidths in two frequency
bands centered on 5 and 10 kHz. The results obtained in realistic
conditions show that a 1.7 m2 hard plate could be detected
when placed across the barrier. Several results are presented and
compared with the theoretical values. Although the system can be
significantely improved, in particular by using more populated
source and receiving arrays, it is believed that these results can
be reproduced at sea in harbor like conditions
Range-dependent regularization of travel-time tomography based on theoretical modes
Travel time inversion is a fundamental method of Ocean Acoustic Tomography, for the estimation of perturbations in sound speed. By discretizing the watercolumn into a system of layers, the method allows to introduce a system of linear equations, relating a known vector of perturbations in travel time, to an unknown vector of perturbations in sound speed, through the so-called \observation matrix". Inverting the system allows to determine a solution, which estimates the perturbation in sound speed in each layer of the watercolumn. However, in most problems of practical interest, the number of unknowns (i.e. the perturbations in sound speed) is larger that the number of equations (which correspond to the number of delays in travel time), which implies that inverting the system of linear equations can be viewed as an ill-posed problem. The discussion presented in this paper illustrates an approach to the problem of inversion, which is based on the usage of theoretical modes. Further, it is shown that for a range-dependent perturbation in sound speed, which corresponds to a superposition of plane waves, the inversion problem can be regularized (i.e. the system of linear equations can be rewritten in order to deal
with more equations than unknowns) by estimating only the amplitudes and phases of the linear waves. Particular examples are given for simulated and real data
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
Doppler Domain Decomposition of the Underwater Acoustic Channel Response
Most underwater applications are very sensitive to environmental perturbations like source/receiver motion and surface variations. In real conditions, the transmitted signal reaches the receiver through different paths where each path is affected by different environmental perturbations. Due to the interaction with these environmental variations different Doppler is induced in each path. By observing the Doppler for the whole signal it is not possible to determine the contribution of each environmental variation. The main goal of this paper is to isolate different paths and analyse the effects of the environmental variations on each path in terms of the induced Doppler. A new technique called Time Windowed Doppler Spectrum is proposed which has been found very effective in tracking the Doppler due to each path separately. By using this technique it can be shown that it is possible to distinguish between surface reflected and direct path by observing the Doppler variations associated with each path. The surface induced Doppler was observed using this technique by analysing the temporal evolution of the surface reflected path. The surface variability effects the Doppler in terms of stretching and shortening the path between the transmitter and receiver. The data processed in this paper was acquired during CALCOMM'10 Experiment which took place in June 2010 at the south coast of Portugal
The TV-APM interface: a web service for collaborative modeling
Current development of Internet access, together with available zero-cost Open Source applications (like, for instance, PHP, Python, etc.) can be integrated in order to minimize the constrains induced by the geographical separation of international centers, which collaborate in a given project. The advantage of such approach lies
in the sharing of common analysis methods, without particular constrains to specific directions of analysis. The discussion presented in this paper describes the Time Variable Acoustic Propagation Model (TV-APM) web interface, which was created as a collaborative service of acoustic modeling for the participants of the PHITOM and UAN projects. This paper describes the general architecture of the interface, its current shortcomings and advantages, and presents a set of modeling results for short range acoustic propagation, which accounts for
source–array and sea surface motion
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
Mitos y ritos de paso en la concepción ibérica del poder: los relieves de Pozo Moro (Albacete)
Este trabajo tiene por objeto el análisis iconológico de uno de los
documentos arqueológicos de mayor importancia en la cultura ibérica del siglo
VI a. C.: el conjunto relivario de Pozo Moro (Albacete). A través de la relectura
crítica de su contenido, efectuada de forma unitaria y global, intentamos abordar
que modelo de organización socio-política e ideológica fue artífice de la erección
del monumento funerario en estudio, como expresión de dicha Sociedad.The purpose of the present work is the iconologyc analysis of one of the
most important archaeological documents of the iberian culture of the sixth
century B.C.: the reliefs of Pozo Moro (Albacete). By means of a critical reading,
developed in a overall way, we try an aproach to the socio-politicians and
ideological organization pattern’s wich created the funerary monument abovequoted,
like expression of this society
The TV-APM interface: a web service for collaborative modelling
Current development of Internet access, together with available zero-cost Open Source applications (like, for
instance, PHP, Python, etc.) can be integrated in order to minimize the constrains induced by the geographical
separation of international centers, which collaborate in a given project. The advantage of such approach lies
in the sharing of common analysis methods, without particular constrains to specific directions of analysis. The
discussion presented in this paper describes the Time Variable Acoustic Propagation Model (TV-APM) web
interface, which was created as a collaborative service of acoustic modeling for the participants of the PHITOM
and UAN projects. This paper describes the general architecture of the interface, its current shortcomings and
advantages, and presents a set of modeling results for short range acoustic propagation, which accounts for
source–array and sea surface motion
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