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- Shallow water tomography in a highly variable scenarioPublication . Soares, C.; Jesus, S. M.In October 2000, SiPLAB and the Instituto Hidrográfico (IH - PN) conducted the IN-TIFANTE’00 sea trial in a shallow area off the Peninsula of Tróia, approximately 50 km south from Lisbon, in Portugal. The experiment itself and results obtained in most of the data set have been reported at various occasions in the last two years. This paper focuses on the data acquired during Event 2, where the acoustic propagation path was approximately range independent and the source ship was held on station at a constant range of 5.8 km from the vertical line array. Although these conditions were, in general, relatively benign for matched-field tomography, retrieval of water column and bottom parameters over a 14-hour-long recording revealed to be extremely difficult. This paper analysis in detail the characteristics of this data set and determines the causes for the observed inversion difficulties. Is is shown that the causes for the poor performance of the conventional methods are mainly the tide induced spatially correlated noise and the relative source-receiver motion during time averaging. An eigenvalue-based criterion is proposed for detecting optimal averaging time. It is shown that this data selection procedure together with hydrophone normalization and an appropriate objective function provide a better model fit and consistent inversion results and thus a better understanding of the environmental variability.
- The INTIFANTE'00 sea trial: preliminary source localization and ocean tomography data analysisPublication . Jesus, S. M.; Coelho, E.; Onofre, J.; Picco, P.; Soares, C.; Lopes, C.The INTIFANTE'00 sea trial was a multidisciplinary experiment including testing of an autonomous surface vehicle, underwater communications, source localization and acoustic ocean tomography. The results shown here will concentrate on the source localization and ocean tomography data sets. The data gathered during a 24 hour run along a range independent track shows strong oceanographic features, possibly due to internal tide signature, both on the temperature data, as measured on the thermistor chain collocated with a vertical line array(VLA), and on the acoustic data. A range dependent track between 120 and 60 m water depth, shows a highly variable channel impulse response along time and range when the source was moving outwards from the VLA. In another acoustic track, the source was navigatated across a underwater canyon where the energy was rapidly distributed over a deep acoustic channel with sound trapped well below the thermocline. Good agreement between the modeled and measured channel responses represents the rst step towards matched- eld processinglike methods such as source localization and tracking and ocean tomography.
- Underwater communications using virtual time reversal in a variable geometry channelPublication . Silva, A.; Jesus, S. M.Field experiments using Time-Reversal Mirror( TRM) techniques have shown that the underwater acoustic channel presents a longer stability than it was previously anticipated. Applying such techniques to underwater communications requires that, either the emitted signals are previously ltered by time-reversed replicas of the channel impulse response or that a probe-signal should be transmitted ahead of the data-signal for post channel ltering. In the former case, the time-reversed ltered message is expected to be undone by the actual acoustic channel between the array and the receiver, while in the later, the undoing of the multipath is performed electronically inside the computer and is therefore termed as virtual Time Reversal Mirror(vTRM). The main issues being addressed in recent literature deal with channel stability, focus width and the required array aperture for obtaining reasonable temporal and/or spatial focusing. This paper focus essentially in two practical aspects, generally not addressed: one is to demonstrate the potential application of the vTRM approach to undo the severe intersymbol interference in a real variable geometry channel scenario and, the other, is the importance of optimization of the probe-signal time window length in a real application.
- Geoacoustic seafloor exploration with a towed array in a shallow water area of the Strait of Sicily (2)Publication . Caiti, A.; Jesus, S. M.; Kristensen, AgeAcoustic propagation in shallow water is greatly dependent on the geoacoustic properties of the seabottom. This paper exploits this dependence for estimating geoacoustic sediment properties from the bottom acoustic returns of known signals received on a hydrophone line array. There are two major issues in this approach: one is the feasibility of acoustic inversion with a limited aperture line array, the other is related to the knowledge of the geometry of the experimental configuration. To test the feasibility of this approach, a 40-hydrophone4-m spaced towed array together with a low-frequency acoustic source, was operated at a shallow water site in the Strait of Sicily. In order to estimate the array deformation in real time, it has been equipped with a set of nonacoustic positioning sensors (compasses, tiltmeters, pressure gauges). The acoustic data were inverted using two complementary approaches: a genetic algorithm (GA) like approach and a radial basis functions (RBF) inversion scheme. More traditional methods, based on core sampling, seismic survey and geophone data, together with Hamilton’s regression curves, have also been employed on the same tracks, in order to provide a ground truth reference environment. The results of the experiment, can be summarized as follows: 1) the towed array movement is not negligible for the application considered and the use of positioning sensors are essential for a proper acoustic inversion, 2) the inversion with GA and RBF are in good qualitative agreement with the ground truth model, and 3) the GA scheme tends to have better stability properties. On the other hand, repeated inversion of successive field measurements requires much less computational effort with RBF.
- Single sensor source localization in a range dependent environmentPublication . Jesus, S. M.; Porter, M. B.; Stephan, Y.; Coelho, E.; Rodríguez, O. C.; Demoulin, X.Source localization with a single sensor explores the time spread of the received signal as it travels from the emitter to the receiver. In shallow water, and for ranges larger than a few times the water depth, the received signal typically exhibits a large number of closely spaced arrivals. However, not all the arrivals are equally important for estimating the source position since a number of them convey redundant information. Theoreticaly, identifying the non-redundant arrivals is feasible in a isovelocity range independent waveguide. In previous work, the number of non-redundant arrivals and the dimension of the data sample signal subspace have been related in a range-independent case. This paper addresses the problem of determining the number of significant arrivals for localizing a sound source over a range-dependent environment on the West coast of Portugal during the INTIMATE'96 sea trial.
- Closed-form estimation of normal modes from a partially sampled water columnPublication . Gazzah, Houcem; Jesus, SergioThe output of a vertical linear array is used to infer about the parameters of the normal mode model that describes acoustic propagation in a shallow water. Existing subspace algorithms perform singular vector decomposition of the array data matrix to estimate the sampled model functions. Estimates are exact only if the sensing array is totally covering the water column. We design a new subspace algorithm free from this very restrictive requirement. We use two short hydrophone arrays and activate a monochromatic source at different depths. Estimates of both the modal functions and the wave numbers are obtained in a fully automatic and search-free manner. The algorithm can be qualified as truly high resolution in the sense that, while using short sensing arrays, estimation error becomes arbitrarily low if observation noise is arbitrarily low. This method compares advantageously to existing subspace techniques, as well as transform-domain techniques that require impulsive sources, among other constraints. With two (eigen and singular) vector decompositions, the proposed technique has the complexity of a regular subspace algorithm.
- Single hydrophone source localizationPublication . Jesus, S. M.; Porter, M. B.; Stephan, Y.; Demoulin, X.; Rodríguez, O. C.; Coelho, E.The method presented in this paper assumes that the received signal is a linear combination of delayed and attenuated uncorrelated replicas of the source emitted waveform. The set of delays and attenuations, together with the channel environmental conditions, provide sufficient information for determining the source location. If the transmission channel is assumed known, the source location can be estimated by matching the data with the acoustic field predicted by the model conditioned on the estimated delay set. This paper presents alternative techniques that do not directly attempt to estimate time delays from the data but, instead, estimate the subspace spanned by the delayed source signal paths. Source localization is then done using a family of measures of the distance between that subspace and the subspace spanned by the replicas provided by the model. Results obtained on the INTIMATE’96 data set, in a shallow-water acoustic channel off the coast of Portugal, show that a sound source emitting a 300–800-Hz LFM sweep could effectively be localized in range or depth over an entire day.