Browsing by Author "Démoulin, X."
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- 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.
- Shallow-water tracking in the sea NazaréPublication . Porter, M. B.; Stephan, Y.; Démoulin, X.; Jesus, S. M.; Coelho, E.In the summer of 1996, an experiment was conducted off the coast of Portugal to study the effects of internal tides on sound propagation. This experiment—called INTIMATE '96 (Internal Tide Investigation by Means of Acoustic Tomography Experiment)—has provided a great deal of insight about the variability of pulse transmission over space and time. In contrast to a common view of shallow-water propagation as complicated and unpredictable, we find a steady pattern of echoes. The echo-pattern stretches and shrinks in a systematic way with the tides and allows us to infer the components of the first few oceanographic modes. We also used the echopattern to track the source over a period of several days. During this period the isotherms in the ocean wavered by 20 m as a result of the tides, providing a challenge for model-based tracking. We will discuss these acoustic results with emphasis on the source tracking.
- Single-hydrophone source tracking in a variable environmentPublication . Porter, M. B.; Jesus, S. M.; Stephan, Y.; Coelho, E.; Démoulin, X.Internal tides commonly occur along ocean coasts. They are internal waves driven by the usual tidal force and generated by scattering at a sharp bottom feature such as the shelf break. The internal tides are dramatic features with crests typically 10-30 km apart and wave heights of 20 m. On the ocean surface they cause only a gentle ripple about 10 cm high but they affect the shine of the surface. As a result, astronauts often see them as the tides propagate away from the shelf break. In June 1996, a shallow-water tomography experiment (INTIMATE 96) was conducted off the coast of Portugal to observe these internal tides and learn about their acoustic effects. A source was towed around a vertical hydrophone array to produce acoustic sections along several slices. The experiment also provided an ideal opportunity for testing model-based source tracking. Acoustic sections taken parallel to the Portuguese coast allowed us to understand the propagation physics in a range-independent area. With this we have been able to develop a matched-field algorithm suitable for use in the far more complicated downslope direction which, being perpendicular to the crests of the internal tides, also experiences strong ocean-temperature variations. We will discuss both the approach and the source tracking.
- Using the echo pattern to range a sound sourcePublication . Porter, M. B.; Stephan, Y.; Démoulin, X.; Jesus, S. M.; Coelho, E.One of the currently exciting areas in SONAR research is in the use of acoustic channel models to improve SONAR systems. In this paper, we use the echo pattern at a single phone to localise a sound source in both range and depth in the ocean. To make the localisation robust, the signal processing is set-up with particular regard to which acoustic features are reliable and clear encoders of the source position. The resulting algorithm was tested using experimental data collected during INTIMATE’96- a shallow water experiment conducted on the Portuguese continental shelf[1]. The results show successful tracking over a period of several days encompassing two fixed 25-hour stations and an 18-hour period of ship manoeuvres.