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KEYNOTE POINTS 1. The main anthropogenic noise sources in the ocean include vessels, industrial activity, including seismic exploration and renewable energy development, and sonar. 2. Anthropogenic noise levels vary across space and time, the primary drivers being levels of human activity and propagation characteristics in the region. Noise does not persist once the sound source has been removed from the environment, although impacts can potentially persist. 3. Areas with the highest levels of anthropogenic noise are those characterized by heavy industrial use, such as the Gulf of Mexico, the North Sea and the North Atlantic Ocean. 4. Areas where anthropogenic noise is expected to increase include the Arctic, as the area opens up to shipping, and Africa, as investment in the region expands. 5. Understanding of the impacts of anthropogenic noise on marine biodiversity is increasing, in parallel with a growing recognition of the need to monitor and possibly reduce the noise entering the marine environment Versión del editor

Seismic inversion with an AUV-based sensor array system is an appealing concept that opens up a number of interesting possibilities but faces also a number of technological and scientific challenges. Among the technological challenges there is the fact that sensor arrays are no longer hardwired to the tow ship and therefore on the fly data monitoring imposes stringent restrictions on the amount of data that can be sent to the support ship. One of the scientific challenges is to determine the optimal sensor array configuration by exploring AUV mobility for inverting the bottom geophysical structure of interest. In fact, the industry standard long planar array and the associated acoustic data processing may not be the setup with the highest performance for each scenario at hand. Generic optimization of sensor distribution through space has been a long standing problem to which there are no closed form solutions. Generically speaking, field diversity maximization is often referred to as a criteria for sensor positioning. This work explores data incoherence as a possible criteria to derive performance of distributed sensor arrays. Additional technological limitations such as array aperture, number of sensors and distances between vehicles impose additional constraints leading to suboptimal configurations. Compressed sensing array processing is used both to explore data incoherence and to offer data reduction for alleviating on the fly monitoring.

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.

During the period of one week, from May 8 to 15, 2013, acoustic data was gathered at three locations over a Posidonia oceanica bed in the Bay of Revellata, Corsica. Preliminary analysis of the acoustic data shows that the environmental noise field in the band 2-7kHz was dominant during the period. The noise in this band is generally associated with wind and surface agitation. However, the noise power was not significantly correlated with wind speed. On the contrary, the diel cycle of the noise power at three locations was highly correlated with the water column concentration of O2, as measured by optodes. These measurements of environmental noise have confirmed the correlation between active acoustic signals transmitted along a seagrass meadow and the photosynthetic activity of the plants observed in a previous experiment conducted in the same area .The results suggest that acoustic noise can be used as a proxy for the photosynthetic oxygen production of a Posidonia oceanica meadow. Therefore, this work is a contribution for the development of a low cost passive acoustic system to assess the primary production of coastal ecosystems .

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). The authors would like to thank Michael Porter, chief scientist for the Makai Experiment, Jerry Tarasek for the loan of the vector sensor array used in the experiment, Paul Hursky, Martin Siderius, and Bruce Abraham for providing assistance with the data acquisition. The Makai Experiment was supported by the U.S. Office of Naval Research. This work was funded by National Funds through FCT- Foundation for Science and Technology under project SENSOCEAN (PTDC/EEA-ELC/104561/2008).

This is a review presentation that addresses recent developments in underwater acoustic telemetry as a tool for ocean observation, monitoring and protection. Distributed sensing is a paradigm with important reflections in oceanic technology where bottom installed structures can not always be connected to a central hub through cabled networks. Moreover, recent developments in ocean robotics lead to the off-the-shelf availability of autonomous underwater vehicles that rely on wireless communications for sending information and receiving commands. Unlike its aerial counterpart wireless underwater communications, are strongly affected and limited by the propagation media: the low speed of propagation, highly limited bandwidth, spatial and time variability of environmental properties and randomness, all together contribute to the slow adoption of standardized and reliable underwater communications networks. This paper addresses the main issues regarding and characterizing the underwater acoustic communication channel as well as the proposed techniques to overcome those issues for, in a first stage, point-to-point (P2P) communications and then for the set up of full underwater networks comprising both fixed and mobile nodes. The presentation is illustrated by real data based examples drawn from experiments carried out at sea by the Signal Processing Laboratory, University of Algarve, Portugal (SiPLAB, www.siplab.fct.ualg.pt) in numerous national and European research projects in the last 15 years. Work supported by FCT (Portugal), ONR (USA), European Commission (EU), CMRE (former NURC, NATO) and numerous other agencies and institutions that will be properly acknowledged and cited in the presentation.

This paper discusses the data acquired in the Bay of la Revellata, Calvi, Corsica during October 2011 for the purpose of developing an acoustic system for monitoring the oxygen production of a seagrass meadow. In a range dependent very shallow water area, densely covered by Posidonia Oceanica seagrass , a sound source transmitted chirp signals in 3 different bands (400-800Hz, 1.5-3.5kHz and 6.5-8.5kHz) to 100m distant 3 self-recording hydrophones during a period of one week. Preliminary data analysis shown a possible correlation between the oxygen production and the instantaneous impulse responses observed. Using a simple procedure to estimate the received power allows to obtain figures of variability along time that seem to be highly correlated with oxygen production and show similar trend with oxygen measurements by optodes conducted in the same area. Those preliminary results show that a light acoustic system can potentially be used in a monitoring system to quantify the oxygen production of a seagrass meadow. The calibration of the acoustic method with other oxygen measurements is an ongoing work.

One of the challenges in the present underwater acoustic communication systems is to combat the underwater channel effects which results in time and frequency spreading of the transmitted signal. The time spreading is caused by the multipath effect while the frequency spreading is due to the time variability of the underwater channel. The passive Time Reversal (pTR) equalizer has been used in underwater communications because of its time focusing property which minimizes the time spreading effect of the underwater channel. In order to compensate for the frequency spreading effect, an improved version of pTR was proposed, called Frequency shift passive time reversal (FSpTR). FSpTR tries to compensate for the frequency spreading by applying a frequency shift in the estimated channel impulse response (IR). In the multipath environment, multiple replicas of the transmitted signal reaches the receiver through different paths where each path is affected differently by environmental variations. In such cases, a single frequency shift fails to compensate for the environmental variations on each path, resulting in degradation in the performance. In this paper, an arrival-based equalizer is proposed to compensate for the environmental variations on each path. The concept of beamforming is integrated with FSpTR equalizer, in this paper, to compensate each arrival separately for the environmental variations. The proposed equalizer is tested with the real data and the results showed that the proposed approach outperforms pTR and FSpTR equalizers and provides a mean MSE gain of 4.9 dB and 4.2 dB respectively. This work is supported by Portuguese Foundation for Science Technology under PHITOM (PTDC/EEATEL/71263/2006) and COGNAT (PTDC/MAR/112446/2009) projects. This work was also supported by European Community’s Sixth Framework Program through the grant to the budget of the Integrated Infrastructure Initiative HYDRALAB III within the Transnational Access Activities, Contract no. 022441.

Um dos principais tópicos atuais de pesquisa é ́ a concepçãoo de técnicas de processamento de sinal eficazes para comunicações submarinas. Este interesse é reforç ̧ado pelos desafios inerentes às condições incontroláveis, como a batimetria, a temperatura, e as variações temporais (geométricas) das profundidades da fonte e da antena de receptores. A obtenção de taxas de transmissão modestas é ainda um desafio, devido a estas variações. O presente trabalho aborda o uso potencial do formador de feixes (beamformer) no sistema de comunicações submarinas. O propósito de usar um beamformer neste trabalho é o de separar os diferentes caminhos que chegam do transmissor para o recetor, o que é motivado pelo fato de que, num ambiente real, não é possível distinguir todos os caminhos de propagação, no domínio do tempo. Neste trabalho, é usado o beamformer de atraso-e-soma, que aplica atrasos diferentes para cada hidrofone da antena vertical, e adiciona as saídas resultantes. Cada um dos atrasos é função do ângulo de chegada e da profundidade do hidrofone. O objetivo do beamformer é aplicar os atrasos opostos a cada elemento da antena vertical, para garantir a soma coerente da frente de onda numa direção particular. Ambos resultados com dados simulados e dados reais são apresentados neste trabalho, para mostrar o desempenho do beamformer. Os resultados do beamformer são comparados com os resultados da análise Doppler, para caracterizar chegadas diferentes e fazer corresponder essas chegadas às variações Doppler, mostrando que cada chegada é afetada pelas variações ambientais de uma maneira diferente, resultando numa quantidade diferente de Doppler.

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.

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.

Acoustic networks are for underwater what wifi is for terrestrial networks. The ocean is a nearly perfect media for acoustic waves in which regards long range propagation but poses a number of challenges in terms of available bandwidth, Doppler spread and channel fading. These limitations originate in the physical properties of the ocean, namely its anisotropy and boundary interaction which are particularly relevant in coastal waters where acoustic propagation becomes predominantly de- pendent on seafloor and sea surface properties. The acoustic communication channel is therefore multipath dominated and time and Doppler spread variable. The problem is aggravated when involving moving receivers as for instance when attempting to establish communication with or between moving autonomous underwater vehicles. The EU-funded project UAN - Underwater Acoustic Network aims at conceiving, developing and testing at sea an innovative and operational concept for integrating in a unique communication system submerged, surface and aerial sensors with the objective of protecting off-shore and coastline critical infrastructures. UAN went through various phases, including the development of hardware and software specific components, its testing independently and then in an integrated fashion, both in the lab and at sea. This paper reports on the project concept and vision as well as on the progress of its various development phases and the results obtained herein. At the time of writing, a final project sea trial is being planned and will take place two weeks before the conference so, although here we will concentrate on the progress obtained so far, the presentation at the conference may include additional results depending on the outcome of the sea trial.

Sonar performance prediction relies heavily on acoustic propagation models and environmental representations of the oceanic area in which the sonar is to operate. The performance estimate is derived from a predicted acoustic eld, which is the output of a propagation model. Though well developed nowadays, acoustic propagation modeling is limited in practice by simpli cations in the numerical methods, in the environmental structure to consider (for computational reasons), and even in the knowledge of some environmental properties. This is complicated by the fact that, in sonar performance prediction, the environmental properties need to be predicted for a far future, in the order of hours or days. These limitations imply that the acoustic eld at the output of the acoustic predictor is biased, in current methods. In mathematical terms, the prediction of the acoustic eld can be seen as a model parametrization problem, in which the model is a numerical propagation model, and the parameters are environmental descriptors which, when fed to the propagation model, best model the future acoustic field. Since the 1980's, signi cant research has been done in the development of propagation model parametrization, using techniques of the so-called \acoustic inversion" family. These techniques, having as objective the estimation of environmental properties of an oceanic area, use observed acoustic elds at the area, to be matched with candidate elds corresponding to candidate environmental pictures. At the end, the best acoustic match gives the estimated environment, in other words, the best model parameters to closely reproduce the measured acoustic eld. In the current work, the technique of acoustic inversion is used in the design of an acoustic predictor, together with oceanographic forecasts and measures. Synthetic acoustic data generated with oceanographic measures taken in the MREA'03 sea trial, is used to illustrate the proposed method. The results show that a collection of environments estimated by past acoustic inversions, can ameliorate the acoustic estimates for future time, as compared to a conventional method.

The objective of this paper is to present an overview of the work developed at SiPLAB, University of Algarve, with vector sensor data collected during Makai experiment 2005, in geometric and geoacoustic parameter estimation. During this experiment devoted to high frequency initiative, acoustic data were acquired by a four element vertical vector sensor array (VSA). A vector sensor is a directional sensor constituted by one omni directional pressure sensor and three velocity-meters, where both the acoustic pressure and the three particle velocity components are measured. The spatial filtering capabilities of the vector sensors are used to estimate the direction of arrival (DOA) of low and high frequency acoustic sources considering a single and a multiple sensor VSA. An inversion method based on Bartlett estimator is used for three dimensional localization of ship’s noise where the noise source is estimated in range and depth taking into accounts the azimuth given by DOA. Moreover, this method is applied to seabed parameters estimation like sediment compressional speed, density and compressional attenuation, contributing to improve the resolution of these parameters.

This work presents a combined geometry-adapted passive Time Reversal (pTR) and Decision Feedback Equalizer (DFE) technique for time-variant underwater communications. We consider sustainable high data rate communications between a moving source and/or a moving receiver array, i.e. there is the presence of geometry changes such as range and depth changes. Such geometry changes can be partially compensated by employing a proper frequency shift on the probe impulse response in the pTR processing. We then refer to the geometry-adapted pTR as Frequency Shift pTR (FSpTR). With dense and long receiver array, a pTR-based technique possesses pulse compression property and can eliminate Inter-Symbol Interference (ISI) problem in multipath static channels. However, with a practical-size array and time-varying channels, a residual ISI always exists. Hence, in this work, we apply an adaptive DFE to further mitigate the residual ISI from the FSpTR, and call the technique as FSpTR-DFE. Performance of the FSpTR-DFE is evaluated using both experimental and simulated data, where an information rate of 2000 bps and BPSK signaling are considered. The RADAR’07 experimental data and the simulated data of the south Elba site are considered. In both data sets, a fast moving source with speed of 1.5 m/s is considered. The results show that the FSpTR-DFE technique outperforms the FSpTR as well as the technique combining the conventional pTR with DFE. Project (contract no. 225669)

Signal scattering due to surface roughness constitutes one of the most important modeling problems in underwater acoustics when dealing with signal processing at large frequencies (i.e. above 1 kHz). Such modeling requires, in particular, realistic predictions of the ocean surface, which is usually perturbed by the propagation of swells and wind driven surface waves. Most of experimental studies had been dealing with short-range propagation, although it still remains unclear the separated impact of periodic vs. stochastic surface roughness on the arrival structure of received signals. The main purpose of the discussion presented in this paper is to clarify such issues through the calculation of impulse responses at high frequencies, at short and large ranges, using a ray tracing acustic propagation model. The results of the simulations are expected to improve strategies of signal processing when scattering affects the received signal. Project (contract no. 225669)

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. Project (contract no. 225669)

Acoustic tomography is now a well known method for remote estimation of water column properties. The problem is ill-conditioned and computationally intensive, if each spatial point varies freely in the inversion. Empirical orhogonal functions (EOFs) efficiently regularize the inversion, leading to a few (2, 3) coefficients to be estimated, giving a coherent estimate of the field. At small scales, EOFs are typically depth-dependent basis functions. The extension of the concept to larger-scale anisotropic fields requires horizontal discretization into cells, with corresponding coefficients. This becomes unstable and computationally intensive, having been overcome by two-dimensional depth-range EOFs, in the past. The present work extends the empirical orthogonal function concept to three dimensions, assessing the performance of the inversion for an instantaneous sound speed field constructed from dynamical predictions for Cabo Frio, Brazil. The results show that the large-scale features of the field are correctly estimated, though with strong ambiguity, using an acoustic source tens of km from an acoustic hydrophone array. Work is under progress, to remove the ambiguity and estimate finer details of the three-dimensional field, via the addition of multiple acoustic arrays.

It is known that small changes in source and receiver locations can cause significant changes in underwater acoustic channel impulse responses. At HYDRALAB III an underwater acoustic experiment was conducted to show that a source depth-shift causes a frequency-shift in the channel impulse response and that such behavior can be used to implement an environmental-based equalizer for underwater communications that compensates for the performance loss due to the source depth-shift.

The Time Variable Acoustic Propagation Model (TV-APM) was developed to simulate underwater acoustic propagation in time-variable environments. Such environment variability induces a strong Doppler channel spread, which is an important factor to test and evaluate the performance of equalization algorithms. In current simulations, Doppler spread is usually included a posteriori in a stationary Acoustic Propagation Model (APM), and is designed for specific environmental parameters such as source-receiver range variability or surface motion. However, environmental variations affect Doppler spread in a complex manner, and an accurate TV-APM simulation for time varying channels, being performed at the same sampling rate as the transmitted signal, would require a large number of runs at high frequencies. A strategy in the current implementation of the TV-APM was developed to reduce the number of runs, while preserving the variable-channel Doppler spread. Simulations were done to draw a performance map for a given equalizer in a given environment and the results revealed that the TV-APM is a useful prediction tool of communication equalizers performance.

Rep 04/10 - SiPLAB November/2010 Submitted by Pedro Sousa (a19661@ualg.pt) on 2014-10-10T10:35:36Z No. of bitstreams: 1 CALCOM'10 Sea Trial - Field calibration data report.pdf: 5105174 bytes, checksum: fd2b6cba59d830ad71bf6131ef423df0 (MD5) Approved for entry into archive by Merja Muzavor (mmuzavor@ualg.pt) on 2014-10-15T13:28:31Z (GMT) No. of bitstreams: 1 CALCOM'10 Sea Trial - Field calibration data report.pdf: 5105174 bytes, checksum: fd2b6cba59d830ad71bf6131ef423df0 (MD5) Made available in DSpace on 2014-10-15T13:28:31Z (GMT). No. of bitstreams: 1 CALCOM'10 Sea Trial - Field calibration data report.pdf: 5105174 bytes, checksum: fd2b6cba59d830ad71bf6131ef423df0 (MD5) Previous issue date: 2010

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.

Vector sensors measure the acoustic pressure and the particle velocity components. This type of sensor has the ability to provide information in both vertical and azimuthal direction allowing increased directivity. These characteristics have been explored by many authors and most of the studies on vector sensors found in literature are related to direction of arrival (DOA) estimation. However, assembled into an array, a Vector Sensor Array (VSA) improves spatial filtering capabilities and can be used with advantage in other applications such as geoacoustic inversion. In this paper it will be shown that a reliable estimation of ocean bottom parameters, such as sediment compressional speed, density and compressional attenuation, can be obtained using high-frequency signals and a small aperture vertical VSA. The introduction of particle velocity on matched-field processing (MFP) techniques is going to be presented. It will be seen how MFP, usually done with acoustic pressure, can be adapted in order to incorporate the three components of the particle velocity. Comparisons between several processors based either in individual particle velocity components or using all the VSA outputs, are made for simulated and experimental data. The quaternion model, which is founded on hypercomplex algebra, thus more appropriate to represent the 4 dimensional VSA data, is also presented in the MFP context. A novel ray tracing model is used to generate field replicas that include both the acoustic pressure and the particle velocity outputs. The data considered herein was acquired by a four element vertical VSA in the 8-14 kHz band, during the Makai Experiment 2005 sea trial, off Kauai I., Hawaii (USA). The results shows that, when the particle velocity is included it can significantly increase the resolution of bottom properties estimation and in some cases a similar result is obtained using only the vertical component of the particle velocity.

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.

Nowadays, vector sensors which measure both acoustic pressure and particle velocity begin to be available in underwater acoustic systems, normally configured as vector sensor arrays (VSA). The spatial filtering capabilities of a VSA can be used, with advantage over traditional pressure only hydrophone arrays, for estimating acoustic field directionality as well as arrival times and spectral content, which could open up the possibility for its use in bottom properties' estimation. An additional motivation for this work is to test the possibility of using high frequency probe signals (say above 2 kHz) for reducing size and cost of actual sub bottom profilers and current geoacoustic inversion methods. This work studies the bottom related structure of the VSA acquired signals, regarding the emitted signal waveform, frequency band and source-receiver geometry in order to estimate bottom properties, specially bottom reflection coefficient characteristics. Such a system was used during the Makai 2005 experiment, off Kauai I., Hawai (USA) to receive precoded signals in a broad frequency band from 8 up to 14 kHz. The agreement between the observed and the modelled acoustic data is discussed and preliminary results on the bottom reflection estimation are presented.

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.

This work addresses the problem of OFDM transmission in dispersive underwater channels where impulse responses lasting tens of miliseconds cannot be reliably handled by recently proposed methods due to limitations of channel estimation algorithms. The proposed approach relies on passive time reversal for multichannel combining of observed waveforms at an array of sensors prior to OFDM processing, which produces an equivalent channel with a shorter impulse response that can be handled much more easily. A method for tracking the narrowband residual phase variations of the channel after Doppler preprocessing is proposed. This is a variation of an existing technique that can improve the spectral efficiency of OFDM by reducing the need for pilot symbols. This work also examines techniques to handle sparse impulse responses and proposes a channel estimation method where an l1 norm is added to the standard least-squares cost function to transparently induce sparseness in the vector of channel coefficients. Algorithms are assessed using data collected during the UAB’07 experiment, which was conducted in Trondheim fjord, Norway, in September 2007. Data were transmitted with bandwidths of 1.5 and 4.5 kHz, and recorded at a range of about 800 m in a 16-hydrophone array. Significant multipath was observed over a period of at least 30 ms. FCT

Um dos maiores obstáculos à utilização da acústica submarina para exploração e observação do oceano (tomografia, geo-acústica, etc) são as dimensões, peso, consumo de energia e custo dos equipamentos normalmente utilizados para transmissão e recepção de sinais acústicos. A maior parte dos métodos desenvolvidos e testados nos últimos 20/30 anos foram dedicados à banda de frequências baixas, arbitrariamente colocada abaixo dos 2 kHz. No outro extremo, frequências acima dos 50 kHz foram utilizadas no mesmo período de tempo, para actividades de observação oceânica a curta distância como por exemplo sonar lateral e multifeixe, perfiladores de corrente acústicos por efeito Doppler (ADCP - Acoustic Doppler Current Profilers), com grande sucesso científico e comercial. Em 2004 foi lançada, por um grupo informal de investigadores de vários países, a “High-Frequency Initiative” (HFi) que tem como objectivo o estudo teórico, em simulação através de modelos de propagação numéricos e experimentalmente, a utilização de acústica submarina a alta frequência para observaçâo oceánica, comunicações submarinas, deteção de alvos e vigilância. HFi tem como objectivo a banda de frequência compreendida entre 2 e 50 kHz. Esta comunicação apresenta os principais problemas e desafios encontrados na HFi, assim como alguns resultados experimentais e de modelação conseguidos até à data. FCT - Portugal POSI; POCTI; POCI

SiPLAB Report 03/07, FCT, University of Algarve,2007. Submitted by Pedro Sousa (a19661@ualg.pt) on 2014-10-10T10:54:19Z No. of bitstreams: 1 Maritime rapid environmental assessment - Blue Planet'2007. Acoustic oceanographic buoys data report.pdf: 91160 bytes, checksum: 8ec5544fd0c9a37761b645f12417ed7c (MD5) Approved for entry into archive by Carla Janeira (cjaneira@ualg.pt) on 2014-10-10T13:35:30Z (GMT) No. of bitstreams: 1 Maritime rapid environmental assessment - Blue Planet'2007. Acoustic oceanographic buoys data report.pdf: 91160 bytes, checksum: 8ec5544fd0c9a37761b645f12417ed7c (MD5) Made available in DSpace on 2014-10-10T13:35:30Z (GMT). No. of bitstreams: 1 Maritime rapid environmental assessment - Blue Planet'2007. Acoustic oceanographic buoys data report.pdf: 91160 bytes, checksum: 8ec5544fd0c9a37761b645f12417ed7c (MD5) Previous issue date: 2007 SiPLAB participation in this sea trial was financed by ULB. Equipment was partially supported by project RADAR (POCTI/CTA/47719/2002) from FCT, Portugal.

Rep 05/07 - SiPLAB 15/December/2007 Submitted by Pedro Sousa (a19661@ualg.pt) on 2014-10-13T09:00:27Z No. of bitstreams: 1 Underwater acoustic barrier 2007 UAB'07 Part A. The hopavagen bay.pdf: 2640356 bytes, checksum: 14d76c9ffb551d12a0b70fbf7ce0aa63 (MD5) Approved for entry into archive by Filipa Custódio (fccustodio@ualg.pt) on 2014-10-13T15:50:05Z (GMT) No. of bitstreams: 1 Underwater acoustic barrier 2007 UAB'07 Part A. The hopavagen bay.pdf: 2640356 bytes, checksum: 14d76c9ffb551d12a0b70fbf7ce0aa63 (MD5) Made available in DSpace on 2014-10-13T15:50:05Z (GMT). No. of bitstreams: 1 Underwater acoustic barrier 2007 UAB'07 Part A. The hopavagen bay.pdf: 2640356 bytes, checksum: 14d76c9ffb551d12a0b70fbf7ce0aa63 (MD5) Previous issue date: 2007

This paper presents an environmental-based equalization algorithm for underwater communications. This algorithm is based on the passive time-reversal (pTR) and waveguide invariant properties of ocean channels. Passive time-reversal allows for the implementation of a simple communications system, but it loses performance in the presence of geometric mismatch between the probe-signal and the actual data symbols transmission. The waveguide invariant properties state that geometric mismatches, both in depth and range, can be partially compensated by applying an appropriate frequency shift in the passive time-reversal operator. Results with binary PSK data at a carrier frequency of 3.6 kHz, collected during the MREA’04 sea trial, show that the Mean Square Error (MSE) between the transmitted and the received data symbols remains stable at least to a range mismatch of about 37.5 m in the presence of source depth and an array depth oscillations of approximately 0.7 m. In such conditions, when comparing the proposed pTR equalizer with plain pTR, an overall gain of approximately 4.11 dB in output MSE is achieved. This work was supported by Fundac¸ ˜ao para a Ciˆencia e a Tecnologia under programs POCI, POCTI and POSI. The authors would like to thank the NATO Undersea Research Centre for the organization of MREA’04 sea trial, NRV Alliance master and personnel and the scientist in charge Emanuel F. Coelho.

This paper reports preliminary environmental inversion results of acoustic data collected simultaneously at two receiving systems during the RADAR’07 sea trial. These receiving systems have communication capabilities that allow for transfering acoustic and telemetric data to a base station with processing capabilities in order to produce environmental estimates during the acoustic experiment. During a large part of the experiment estimates on the temperature field appear to agree with concurrent ground truth data.

Continuous evolution of both methodologies and required technologies for REA; support of sea-based operations as demonstrated through MREA-BP-RADAR acoustic/oceanic model integration; spatially coherent processed buoys/sparse arraysinclude adaptive geoacoustic estimation POSI; POCTI; FCT Portugal; Royal Netherlands Navy.

Nowadays, vector sensors which measure both acoustic pressure and particle velocity begin to be available in underwater acoustic systems, normally configured as vector sensor arrays (VSA). The spatial filtering capabilities of a VSA can be used, with advantage over traditional pressure only hydrophone arrays, for estimating acoustic field directionality as well as arrival times and spectral content, which could open up the possibility for its use in bottom properties' estimation. An additional motivation for this work is to test the possibility of using high frequency probe signals (say above 2 kHz) for reducing size and cost of actual sub bottom profilers and current geoacoustic inversion methods. This work studies the bottom related structure of the VSA acquired signals, regarding the emitted signal waveform, frequency band and source-receiver geometry in order to estimate bottom properties, specially bottom reflection coefficient characteristics. Such a system was used during the Makai 2005 experiment, off Kauai I., Hawai (USA) to receive precoded signals in a broad frequency band from 8 up to 14 kHz. The agreement between the observed and the modelled acoustic data is discussed and preliminary results on the bottom reflection estimation are presented. The authors would like to thank Paul Hursky at HLS Research for their help with the data used in this analysis, Bruce Abraham at Applied Physical Sciences for providing assistance with the data acquisition and Michael Porter, chief scientist for the Makai Experiment. The authors also thank Jerry Tarasek at Naval Surface Weapons Centre for the use of the vector sensor array used in this work. This work was supported by Fundação para a Ciência e a Tecnologia under programs POCI, POCTI and POSI.