Loading...
54 results
Search Results
Now showing 1 - 10 of 54
- Monitoring bubble production in a seagrass meadow using a source of opportunityPublication . Felisberto, Paulo; Rodríguez, Orlando; Silva, João P.; Jesus, Sergio; Ferreira, Hugo Q.; Ferreira, Pedro P.; Cunha, Maria E.; de los Santos, Carmen B.; Olivé, Irene; Santos, RuiUnder high irradiance, the photosynthetic activity of dense seagrass meadows saturates the water forming oxygen bubbles. The diel cycle of bubble production peaks at mid-day, following light intensity pattern. It is well known that bubbles strongly affect the acoustic propagation, increasing signal attenuation and decreasing the effective water sound speed, noticeable at low frequencies. Thus, the diurnal variability of bubbles may show an interference pattern in the spectrograms of low frequency acoustic signals. In an experiment conducted in July 2016 at the Aquaculture Research Station of the Portuguese Institute for the Sea and Atmosphere in Olhão, Portugal, the spectrograms of low frequency (<20kHz) broadband noise produced by water pumps in a pond of 0.48ha covered by the seagrass Cymodocea nodosa showed interference patterns that can be ascribed to the variability of the sound speed in the water. Preliminary analysis suggests that the daily cycle of bubble concentration can be inferred from these interference patterns.
- Ultra light vertical array remote data acquisition system (ULVA RDAS)Publication . Felisberto, P.; Lopes, C.; Carmo, A.In 1997, CINTAL has requested Co.L.Mar Srl. (Italy) to provide the plans and the execution for a multichannel Ultra Light Vertical Array (ULVA) underwater acoustic passive system to support its research plans under project INTIMATE (FCT contract 2/2.1/MAR/1698/95). The solution proposed by Co.L.Mar was composed of a vertical line array (VLA) with 16 hydrophones, a radio buoy with an RF data link to a base station, interfaced to a DAT recorder. Under the same project, CINTAL has developed a PC based system that interfaced to the RF receiver on the base station would allow to acquire on computer disk and monitor the signals being received on line - this is the Data Acquisition and Monitoring System (DAMS). The ensemble composed of the receiving array, the RF link and the PC monitoring system will referred to in this report as the ULVA/DAMS system. The ULVA/DAMS system was first tested at sea in July 1999 and then sucessfully operated during the INTIFANTE 00 sea trial in October 2000.
- 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).
- The application of a dual accelerometer vector sensor for the discrimination of seismic reflectionsPublication . Mantouka, A.; Felisberto, Paulo; Jesus, Sergio; J. Santos, P.; Sebastiao, Luís; Pascoal, A.This paper describes the application of a Dual Accelerometer Vector Sensor (DAVS) for the discrimination between the bottom reflections, the source direct arrival and the source ghost or multipath in an unconventional seismic acquisition scenario. The realisation of the DAVS device and the seismic acquisition scenario described in this paper, were carried out in the scope of the WiMUST project, an EU project, supported under the Horizon 2020 Framework Programme. The WiMUST project aims to improve the efficiency of the methodologies used to perform geophysical acoustic surveys at sea, using Autonomous Underwater Vehicles (AUVs) equipped with optimum sensors. In a classical reflection seismic survey scenario, the DAVS can contribute to this aim by steering its acoustic beam to the desired direction, therefore reducing the amount of post processing related to deghosting and multipath removal. Moreover, in an unconventional scenario, this steering capability offers the possibility of distinguishing between direct arrivals and multipath. In this paper, using data acquired during a WiMUST experiment, the device's directional estimation capabilities are demonstrated using a conventional beamformer for the determination of the Direction of Arrival (DOA) of seismic waves. The beamformer inputs are pressure and particle velocities in three directions. For the results presented here the pressure was derived from the devices' two accelerometers.