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- Shallow water tomography with a sparse array during the INTIMATE'98 sea trialPublication . Felisberto, P.; Jesus, S. M.; Stephan, Y.; Demoulin, X.Invert acoustic data using sparse arrays - at the limit with a single hydrophone - is a challenging task. The final goal is to obtain a rapid environmental assessment with systems both easier to deploy and less expensive than full vertical arrays. In this paper, it is shown that using a known broadband source signal and an array with few hydrophones, ocean acoustic tomography can be performed, even in a complex internal waves induced highly variable ocean. The inversion approach presented herein is based on an arrival matching processor and a genetic algorithm search procedure. Due to the poor accuracy on the a priori knowledge of the source range, source depth and water depth, the inversion procedure was split in two stages: in the first stage the geometric parameters where estimated and in the second stage sound speed estimates where obtained. This procedure was applied to field data, acquired during the INTIMATE'98 sea trial, in a shallow water area off the coast of France in the Gulf of Biscay. That area is expected to have a relatively high internal wave activity, specially during the summer. A 4 sec long - 700 Hz bandwidth linear frequency modulated signal was transmitted from a ship suspended sound source and received on a 4 element vertical array at a range of approximately 10.5 km, over a relatively range-independent area. The results from the inversion of the acoustic data are in line with those obtained by concurrent non acoustic data like GPS source range, measured source depth, XBT casts and temperature sensors.
- Field calibration a tool for acoustic noise prediction. The CALCOM 10 data setPublication . Felisberto, P.; Jesus, S. M.; Martins, N.It is widely recognized that anthropogenic noise affects the marine fauna, thus it becomes a major concern in ocean management policies. In the other hand there is an increasing demand for wave energy installations that, presumably, are an important source of noise. A noise prediction tool is of crucial importance to assess the impact of a perspective installation. Contribute for the development of such a tool is one of the objectives of the WEAM project. In this context, the CALCOM’10 sea trial took place off the south coast of Portugal, from 22 to 24 June, 2010 with the purpose of field calibration. Field calibration is a concept used to tune the parameters of an acoustic propagation model for a region of interest. The basic idea is that one can significantly reduce the uncertainty of the predictions of acoustic propagation in a region, even with scarce environmental data (bathymetric, geoacoustic), given that relevant acoustic parameters obtained by acoustic inference (i.e. acoustic inversion) are integrated in the prediction scheme. For example, this concept can be applied to the classical problem of transmission loss predictions or, as in our case, the problem of predicting the distribution of acoustic noise due to a wave energy power plant. In such applications the accuracy of bathymetric and geoacoustic parameters estimated by acoustic means is not a concern, but only the uncertainty of the predicted acoustic field. The objective of this approach is to reduce the need for extensive hydrologic and geoacoustic surveys, and reduce the influence of modelling errors, for example due to the bathymetric discretization used. Next, it is presented the experimental setup and data acquired during the sea trial as well as preliminary results of channel characterization and acoustic forward modelling.
- Experimental results of geometric and geoacosutic parameter estimation using a vector sensor arrayPublication . Santos, P.; Felisberto, P.; Jesus, S. M.; João, J.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.
- Using active and passive acoustics to assess O2 production of a Posidonia oceanica meadowPublication . Felisberto, P.; Zabel, F.; Rodríguez, O. C.; Santos, P.; Jesus, S. M.; Champenois, W.; Borges, A. V.; Santos, RuiThis work discusses the data acquired during two experiments conducted in October 2011 and May 2013 in the Bay of la Revellata, Calvi, Corsica for the purpose of developing an acoustic system for monitoring the oxygen production of a seagrass meadow.
- Correlation between the acoustic noise field measured in a Posidonia oceanica bed and the photosynthetic activityPublication . Felisberto, P.; Zabel, F.; Rodríguez, O. C.; Santos, P.; Jesus, S. M.; Champenois, W.; Borges, A. V.; Santos, RuiDuring 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 .
- 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.
- Comparing the resolution of Bartlett and MVDR estimators for bottom parameter estimation using pressure and vector sensor short array dataPublication . Felisberto, P.; Schneiderwind, J.; Santos, P.; Rodríguez, O. C.; Jesus, S. M.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).
- Close range ship noise cross correlations with a vector sensor in view of geoacoustic inversionPublication . Wu Yanqun; Santos, Ana Bela; Felisberto, P.; Jesus, S. M.Distant ship noise has been utilized for geoacoustic inversion and ocean monitoring for many years. In a shallow water experiment, Makai 2005, a 4-element acoustic vector sensor array was deployed at the stern of the research vessel R/V Kilo Moana. The recorded engine noise of R/V Kilo Moana during its dynamic positioning was analyzed by the DEMON (Detection of Envelope Modulation on Noise) method. The strongest modulation frequency band of the ship noise was found by a group of band-pass filters for further data processing. Multipath arrivals in the vertical particle velocity have higher signal-to-noise ratios than those in the horizontal particle velocities because of steep arrival directions. By exploiting this advantage, the cross-correlation of broadband ship noise between the pressure and the vertical particle velocity can be used for multipath information exploration. Since ship noise is often characterized as continuous broadband noise plus strong tonal noise, the cross-correlation of tonal noise would dominate that of broadband noise, and consequently cover the multipath arrival pattern. Therefore, spectral weighting functions are applied in order to reduce the noise contamination and ensure sharp multipath peaks in the cross-correlation. For engine noise emitted by the dynamically positioned ship, a short correlation time of 0.4s was used in order to keep the time delay fluctuation details of multipath arrivals. Clear multiple arrivals are seen in the cross-correlation of different arrivals, and verified by the ray tracing program TRACEO. The results demonstrate the potential of only one acoustic vector sensor in applications of source localization and geoacoustic inversion.
- On the usage of the particle velocity field for bottom characterizationPublication . Felisberto, P.; Rodríguez, O. C.; Santos, P.; Jesus, S. M.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 sensorPublication . Felisberto, P.; Rodríguez, O. C.; Jesus, S. M.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. Usually these methods require 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).