Browsing by Author "Jesus, S. M."
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- A time-frequency approach to blind deconvolution in multipath underwater channelsPublication . Martins, N.; Jesus, S. M.; Gervaise, C.; Quinquis, A.Blind deconvolution is presented in the underwater acoustic channel context, by time-frequency processing. The acoustic propagation environment was modelled as a multipath propagation channel. For noiseless simulated data, source signature estimation was performed by a model-based method. The channel estimate was obtained via a time-frequency formulation of the conventional matched-filter. Simulations used a ray-tracing physical model, initiated with at-sea recorded environmental data, in order to produce realistic underwater channel conditions. The quality of the estimates was 0.793 for the source signal, and close to I for the resolved amplitudes and time-delays of the impulse response. Time-frequency processing has proved to overcome the typical ill-conditioning of single sensor deterministic deconvolution techniques.
- Accounting for source depth variations in time-reversal communicationsPublication . Jesus, S. M.; Ijaz, S.; Silva, AntónioPassive Time Reversal (pTR) is a low complexity receiver scheme that uses multichannel probing for time signal refocusing, thus reducing time spreading and improving inter-symbol interference. Recognizing that signals travelling through different paths are subject to arrival-angle-related Doppler displacements, this letter proposes a further improvement to pTR that applies correcting frequency shifts optimized for beams formed along each path arrival angle. The proposed equalizer is tested with real data and the results show that the proposed approach outperforms both pTR and the modified pTR channel combiners providing an MSE gain of 4.9 dB and 4.2 dB, respectively.
- Acoustic channel frequency response estimation using sources of opportunityPublication . Santos, Ana Bela; Felisberto, P.; Jesus, S. M.This work addresses the usage of ship radiated noise to estimate the ocean acoustic water propagation channel response between two vertical line arrays. We derive an expression for the frequency response channel estimate using a normal mode development based on cross-correlation methods, in a similar way as Roux et al. [1]. Its applicability and limitations in simulated and real conditions is discussed. Simulations are conducted using the normal mode model KRAKEN, based on the experimental setup and environmental parameters gathered during the RADAR' 07 sea trial, off the west coast of Portugal, in July of 2007. In this sea trial two drifting vertical line arrays with 16 and 8 hydrophones were deployed in a range independent bathymetric area, at 300 m and 1.3 km distance from the Research Vessel NRP D. Carlos I, whose track then moved away from the arrays, radiating noise in the frequency band bellow 750 Hz. The wave fronts structure, obtained from actual acoustic data of the above referred sea trial, reveals agreement with the simulations obtained with the proposed approach. These results suggest the feasibility of the method for future application in a passive ocean acoustics tomography framework to the estimation of sound speed perturbations in the water column.
- Acoustic estimation of seafloor parameters: a Radial Basis Functions approachPublication . Caiti, A.; Jesus, S. M.A novel approach to the estimation of seafloor geoacoustic parameters from the measurement of the acoustic field in the water column is introduced. The approach is based on the idea of approximating the inverse function that links the geoacoustic parameters with the measured field through a series expansion of radial basis functions. In particular, Gaussian basis functions are used in order to ensure continuity and smoothness of the approximated inverse. The main advantage of the proposed approach relies on the fact that the series expansion can be computed off-line from simulated data as soon as the experimental configuration is known. Data inversion can then be performed in true real time as soon as the data are acquired. Simulation results are presented in order to show the advantages and limitations of the method. Finally, some inversion results from horizontal towed array data are reported, and are compared with independent estimates of geoacoustic bottom properties.
- Acoustic field calibration for noise prediction: the CALCOM'10 data setPublication . Martins, N.; Felisberto, P.; Jesus, S. M.Wave energy is one of the marine renewable energies that are becoming increasingly explored. One of the concerns about the respective ocean plants is the noise generated by the mechanical energy converters. This noise may affect the fauna surrounding the energy plant, what induces the idea of planning the location of a prospective plant, optimum in terms of noise minimization. Naturally, in such an approach, the plant noise can be predicted, using information concerning the ocean geometric, water column and bottom properties, if available. This information can be fed to an acoustic propagation code, to solve an acoustic forward problem. Inevitably, this knowledge is often incomplete, and the use of guesses or inferences from nautical charts can lead to erroneous noise predictions. This paper presents a noise prediction tool which can be divided into two steps. The first step consists of characterizing the candidate ocean area, in terms of the environmental properties relevant to acoustic propagation. In the second step, the environmental characteristics are fed to a computational acoustic propagation model, which provides estimates of the plant-noise generated in the candidate area. The first step uses at-sea measured acoustic data, during the CALCOM’10 sea trial (in Portugal), to solve an acoustic inverse problem, which gives environmental estimates. This procedure can be seen as a “field model calibration”, in that the estimated environmental properties are tailored to model the acoustic data. The second step uses the estimates in a forward modeling problem, with the same propagation code. In numerical terms, differences greater than 4.4 dB in the median of the modeled transmission loss difference have been observed, upto 1.6 km from the acoustic source. The results show that the field calibration is important to better model the data at hand, and thus act as a noise prediction tool, as compared to a procedure in which only a partial a priori knowledge of the candidate oceanic area is available. The results are promising, in terms of the application of the present method in the project of ocean power plants.
- Acoustic inversion of the cold water filaments off the Southwest coast of PortugalPublication . Felisberto, P.; Jesus, S. M.; Relvas, PauloCold water filaments have important implications in the biological and chemical exchanges between the coastal and offshore ocean. The Cape São Vicente area in the Southwest coast of Portugal is a well know region where such phenomenon is observed. In October 2004, the multidisciplinary project ATOMS, involving oceanographers and acousticians, was conducted with the objective to complement the sea surface temperature (SST) satellite observation with a full water column characterization. Due to weather and technical conditions during the project sea trial, only CTD measurements in upper layers of the water column were performed. These at sea collected data together with archival data from the NODC database, allowed to establish realistic scenario of the 3D temperature distribution in the area, including deeper water layers. Archival data of temperature profiles suggest the occurrence of other important oceanic phenomena such as the subduction of warm Mediterrenean water, that should also influence the acoustic propagation. With the help of forward acoustic modelling the significance and signature of the individual oceanographic phenomena on the acoustic propagation, regarding different sampling strategies of the area by acoustic means is investigated. These investigations allowed to develop strategies to settle the main problem addressed by this work: invert the cold water filament structure by acoustic means in a complex environment where acoustic propagation is affected also by other important oceanic and bathymetric features. Since, the objective of this work, is to evaluate the ability to perform a 3D characterization of vertical structure of the ocean, a minimal transmit-receive acquisition composed of a suspended source from a ship and a drifting vertical array, is assumed. The spatial structure is obtained by a combination of inversions for ”mean” sound speed/temperature perturbations obtained for source-array cross-sections covering the area of interest. Matched-field and ray tracing based tomography techniques are used in the inversion for the ”mean” perturbations. The planned sampling strategies and necessary acoustic equipment to resolve such oceanic features is discussed having in mind future sea trials.
- Acoustic maritime rapid environmental assessment 2004 during the MREA'04 sea trialPublication . Jesus, S. M.; Soares, C.; Felisberto, P.; Silva, A.; Farinha, L.; Martins, C.Environmental inversion of acoustic signals for bottom and water column properties is being proposed in the literature as an interesting concept for complementing direct hydrographic and oceanographic measurements for Rapid Environmental Assessment (REA). The acoustic contribution to REA can be cast as the result of the inversion of ocean acoustic properties to be assimilated into ocean circulation models specifically tailored and calibrated to the scale of the area under observation. Traditional ocean tomography systems and methods for their requirements of long and well populated receiving arrays and precise knowledge of the source/receiver geometries are not well adapted to operational Acoustic REA (AREA). The Acoustic Oceanographic Buoy (AOB) was proposed as an innovative concept that responds to the operational requirements of AREA. That concept includes the development of water column and geo-acoustic inversion methods being able to retrieve environmental true properties from signals received on a drifting network of acoustic-oceanographic sensors - the AOBs. An AOB prototype and a preliminary version of the inversion code, was tested at sea during the Maritime Rapid Environmental Assessment 2003 (MREA’03) sea trial and was reported in [1]. On a separate register it should be noted that the characterization of the environment between the source and the receiver also contributes to the identification of the acoustic channel response and therefore provides a basis for fulfilling the objectives of project NUACE1. The present report describes the data sets and results gathered during the MREA’04 sea trial that took place from 29 March to 19 April 2004 off the west coast of Portugal, south of Lisboa (Portugal), with the objectives of testing an improved version of the individual AOB and its functionality in a simple network. The acoustic part of the experiment lasted for four days between April 7 and April 10, 2004 and involved the transmission and reception of pre-coded signals along range-dependent and range-independent acoustic tracks.
- Acoustic monitoring of O2 production of a seagrass meadowPublication . Felisberto, P.; Jesus, S. M.; Zabel, F.; Santos, Rui; Silva, João; Gobert, S.; Beer, S.; Bjork, M.; Mazzuca, S.; Procaccini, G.; Runcie, J. W.; Champenois, W.; Borges, A. V.Acoustic data were acquired in October 2011 over a Posidonia oceanica meadow in the Bay of la Revellata, Calvi, Corsica. The purpose was to develop an acoustic system for monitoring the oxygen (O2) production of an entire seagrass meadow. In a shallow water area (<38m), densely covered by P. oceanica, a sound source transmitted signals in 3 different bands (400-800Hz, 1.5-3.5kHz and 6.5-8.5kHz) toward three self-recording hydrophones at a distance of 100m, over the period of one week. The data show a high correlation between the diel cycle of the acoustic signals' energy received by the hydrophones and the temporal changes in water column O2 concentration as measured by optodes. The results thus show that a simple acoustic acquisition system can be used to monitor the O2-based productivity of a seagrass meadow at the ecosystem level with high temporal resolution. The finding of a significant production of O2 as bubbles in seagrass ecosystems suggests that net primary production is underestimated by methods that rely on the mass balance of dissolved O2 measurements. © 2015 Elsevier B.V.
- Acoustic monitoring of the oxygen production of a seagrass meadowPublication . Felisberto, P.; Zabel, F.; Martins, C.; Jesus, S. M.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.
- Acoustic oceanographic buoy (version 1)Publication . Silva, A.; Martins, C.; Jesus, S. M.One of the known impairements in the application of ocean acoustic tomography in operational scenarios has been the size, weight and difficulty of operation of actual ocean going equipment, such as hydrophone arrays and acoustic sources. The Acoustic Oceanographic Buoy - AOB is meant to be, at its final stage, an easy to deploy and easy to maintain autonomous vertical array that puts together in a single system acoustic and non-acoustic sensors, self-storing of geotime and GPS referred data, on the buoy preprocessing capabilities and network seamless integrability and data online transfer via a wireless lan high speed link. These capabilities make the AOB a versatile system for a variety of applications such as ocean tomography, underwater communications, rapid environmental assessment (REA), passive and multistatic sonar and underwater target tracking. The AOB version 1 described in this report was developed between 2003 and 2004 and tested at sea during the MREA’03 and MREA’04 sea trials.