Browsing by Author "Stephan, Y."
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- Broadband source localization with a single hydrophonePublication . Jesus, S. M.; Porter, M. B.; Stephan, Y.; Coelho, E.; Demoulin, X.Source localization with vertical arrays in shallow water has been a topic of intense research in the last 20 years. Although horizontal arrays can (and have) been used, vertical arrays are better suited for extracting signal modal structure and thus provide a source-location estimate in range and depth. It is well known that broadband signals have a localization capability superior to that of narrowband signals. One question that remains largely unresolved is whether frequency extent can compensate for the spatial diversity provided by sensor arrays, i.e., at the limit, can a broadband signal be localized with a single sensor ? This communication presents theoretical, simulated, and practical evidence that a multipath-delay maximum-likelihood estimator does provide enough signal to background discrimination for localizing a 500 Hz bandwidth signal at 5 km range in a 130 m depth shallow water channel with a single hydrophone. The real data used in this study was collected during the INTI-MATE'96 experiment which was conducted off the west of Portugal in June 1996 during an internal tide tomography experiment. Comparison with localization results provided by direct match between the received and the model-generated arrival patterns will be discussed.
- Dynamics of acoustic propagation through a soliton wave packet: Observations from the INTIMATE'96 experimentPublication . Rodríguez, O. C.; Jesus, S. M.; Stephan, Y.; Demoulin, X.; Porter, M. B.; Coelho, E.; SpringerExperimental observations of acoustic propagation through a Soliton Wave Packet (SWP) show an abnormally large attenuation over some frequencies, that was found to be significantly time dependent and anisotropic. Nevertheless, by considering the problem of signal attenuation, the approach used in most of the studies can be considered as "static" since no additional effects were taken into account as a SWP evolves in range and time. Hydrographic and acoustic data from the INTIMATE'96 experiment clearly exhibit traces of the presence of soliton packets, but in contrast with known observations of attenuation, its frequency response also reveals a sudden increase of signal amplitude, which may be due to a focusing effect. This signal increase coincides with a significant peak found in current and temperature records. However, the correlation of both acoustic and hydrographic features is difficult to support due to the different time scales between the rate of hydrographic data sampling and the rate of signal transmissions. To study the possibility that a SWP could be responsible for the observed signal increase, the INTIMATE'96 hydrographic data was used to generate physically consistent distributions of "soliton-like" fields of temperature and sound velocity, which were used as input for a range-dependent normal-mode model it was found that for a particular soliton field, the set of "dynamic" (i.e., range-dependent and time-dependent) acoustic simulations reveals an acoustic signature similar to that observed in the data. These results contribute to a better understanding of underwater propagation in shallow-water coastal environments and therefore provide a potential basis for range-dependent temperature and sound-speed inversions.
- Estimating equivalent bottom geoacoustial parameters from broadband inversionPublication . Demoulin, X.; Pelissero, L.; Stephan, Y.; Jesus, S. M.; Porter, M. B.; Coelho, E.A simple and fast approach to retrieve equivalent geoacoustic parameters is presented in this paper. The method is based upon the processing of 300-800 Hz broadband signals on a single hydrophone.Two stable characteristics of the impulse response of the shallow water waveguide are estimated: the time dispersion and the bottom reflection amplitudes. This two features are analytically linked to the compressional speed and to the attenuation coefficient of the medium. The inversion of the two latter geoacoustic parameters is straightforward since it relies on an analytical expression. The method is tested on INTIMATE96 data. The results show an excellent agreement between the reflection of the true medium and the reflection coefficient of the equivalent medium.
- Exploiting reliable features of the ocean channel responsePublication . Porter, M. B.; Jesus, S. M.; Stephan, Y.; Demoulin, X.; Coelho, E.Modelling sound propagation in shallow water is notoriously difficult. The main difficulties are, of course, ocean variability characteristic of coastal waters and the typpicaly downward refracting profiles that make the acoustic field extremely sensitive to the bottom characteristics. Meanwhile accurate prediction of transimission loss curves at a single frequency requires that the relative phase of each surface and bottom echo is predicted with precision.
- Internal tide acoustic tomography: reliability of the normal modes expansion as a possible basis for solving the inverse problemPublication . Rodríguez, O. C.; Jesus, S. M.; Stephan, Y.; Démoulin, X.; Porter, M. B.; Coelho, E.Using hydrodynamic and thermodynamic equations appropriate for modelling internal tides, one can predict the current and temperature distributions associated with the ocean's dynamic modes. Comparing such predictions with observations from the INTIMATE'96 experiment, we nd a high degree of correlation between the rst 3 theoretically calculated dynamic modes and corresponding empirical orthogonal functions (EOF's) derived from an ensemble of temperature and current pro les. The implications are twofold. First, this implies that the dominant variability in the INTIMATE'96 experiment is indeed associated with internal tides. Secondly, it suggests that in future tidal experiments a theoretically generated basis may be used as e ectively as an EOF basis (which requires more extensive oceanographic measurements). We have also used the set of dynamic modes to simulate the e ect of the tides on acoustic propagation to understand the relative importance of the usual surface tide (barotropic) and the internal (baroclinic) tides.
- Internal tide impact measured by an acoustic tomography experimentPublication . Stephan, Y.; Demoulin, X.; Jesus, S. M.; Coelho, E.; Porter, M. B.The INTIMATE project is devoted to the internal study of tides by the use of acoustic tomography schemes. The first exploratory experiment was carried out in June 1996 on the continental shelf off the coast of Portugal using a towed broadband acoustic source a four-hydrophone vertical array.
- Intimate '96: shallow water tomography in the sea of the condemnedPublication . Coelho, E.; Jesus, S. M.; Stephan, Y.; Demoulin, X.; Porter, M. B.As is well-known, the tidal force of the moon and the sun can cause notable changes in the sea level. Besides this so-called barotropic effect, the tidal force also drives internal waves in a daily rhythm. Thus, the internal wave spectrum is often dominated by a single component with perhaps 10 km from crest to crest. This ‘‘internal tide’’ tends to propagate toward shore and has its greatest height near the shelfbreak.
- INTIMATE'96. A shallow water tomography experiment devoted to the study of internal tidesPublication . Demoulin, X.; Stephan, Y.; Jesus, S. M.; Coelho, E.; Porter, M. B.The INTIMATE (INternal Tide Investigation by Means of Acoustic Tomography Experiment) project is devoted to the study of internal tides by use of acoustic tomography. The first exploratory experiment was carried out in June 1996 on the continental shelf off the west coast of Portugal. A towed broadband acoustic source and a 4-hydrophone vertical array were used. Acoustic data were collected for 5 days, including legs where the source ship was moving and legs with the ship on station. The purpose of this paper is to briefly discuss some effects of the environment on acoustic fluctuations.
- Nonlinear soliton interaction with acoustic signals. Focusing effectsPublication . Rodríguez, O. C.; Jesus, S. M.; Stephan, Y.; Demoulin, X.; Porter, M. B.; Coelho, E.The problem of nonlinear interaction of solitary wave packets with acoustical signals has been intensively studied in recente years. A key goal is to explain the observed transmission loss of shallow-water propagating signals, which has been found to be strongly time-dependent, anisotropic, and sometimes exhibited unexpected attenuation vs. frequency.
- One-way active tracking in the INTIMATE 96 ExperimentPublication . Coelho, E.; Jesus, S. M.; Porter, M. B.; Stephan, Y.The acoustic data shows a clean multipath structure warped in time by the tides. Ray, mode, FFP models all provide a precise prediction of the arrival structure. The source is tracked continuously over several days as it moves to and from the array both cross−slope and down− slope. Tides cause a small wobble in apparent target range. Flat−bottom model leads to a small range error when the source moves down−slope.