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- Geophysical seafloor exploration with a towed array in shallow waterPublication . Caiti, A.; Jesus, S. M.The quantitative measurement or estimation of the geophysical properties of the seafloor upper sedimentary strata is a requirement in several marine applications, from geotechnical engineering to underwater acoustics. Traditional methods employ in-situ instrumentation (cone penetrometers, coring, etc.) and are able to probe the seafloor at selected points. This project aims at exploring the feasibility of seafloor model identification by acoustic means. In particular, the project has considered the use of an acoustic system, in which a ship tows both an acoustic source and an horizontal array of receivers. From the acoustic field measured at the receiving array, the seafloor parameters are estimated by suitable inversion algorithms. The project focused in particular on the use of such a system in shallow waters, and considered the advantages and limitations of the method from the point of view of system requirementes, signal processing and inversion strategies. Sensitivity studies show that it is indeed possible to recover the seafloor parameters in shallow water with a moderate aperture towed array, and experimental results are shown to demonstrate the feasibility of the concept. Possible improvements and further lines of research are also discussed.
- Linking acoustic communications and network performance. Integration and experimentation of an underwater acoustic networkPublication . Caiti, A.; Grythe, K.; Hovem, J. M.; Jesus, S. M.; Lie, A.; Munafò, Andrea; Reinen, Tor Arne; Silva, A.; Zabel, F.Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.
- UAN: underwater acoustic networkPublication . Caiti, A.; Felisberto, P.; Husoy, T.; Jesus, S. M.; Karasalo, I.; Massimelli, R.; Reinen, Tor Arne; Silva, A.Acoustic networks are for underwater what wifi is for terrestrial networks. The ocean is a nearly perfect media for acoustic waves in which regards long range propagation but poses a number of challenges in terms of available bandwidth, Doppler spread and channel fading. These limitations originate in the physical properties of the ocean, namely its anisotropy and boundary interaction which are particularly relevant in coastal waters where acoustic propagation becomes predominantly de- pendent on seafloor and sea surface properties. The acoustic communication channel is therefore multipath dominated and time and Doppler spread variable. The problem is aggravated when involving moving receivers as for instance when attempting to establish communication with or between moving autonomous underwater vehicles. The EU-funded project UAN - Underwater Acoustic Network aims at conceiving, developing and testing at sea an innovative and operational concept for integrating in a unique communication system submerged, surface and aerial sensors with the objective of protecting off-shore and coastline critical infrastructures. UAN went through various phases, including the development of hardware and software specific components, its testing independently and then in an integrated fashion, both in the lab and at sea. This paper reports on the project concept and vision as well as on the progress of its various development phases and the results obtained herein. At the time of writing, a final project sea trial is being planned and will take place two weeks before the conference so, although here we will concentrate on the progress obtained so far, the presentation at the conference may include additional results depending on the outcome of the sea trial.
- Acoustic sensing techniques for the shallow water environment: inversion methods and experimentsPublication . Caiti, A.; Chapman, Ross; Hermand, J.-P.; Jesus, S. M.This volume contains the collection of papers from the second workshop on Experimental Acoustic Inversion Techniques for Exploration of the Shallow Water Environment. The workshop theme followed the original concept of the first workshop, held in Carvoeiro, Portugal, in 1999, i.e., to focus on experiments and experimental techniques for acoustic sensing in the shallow ocean. More than forty leading international scientists were invited to meet in the picturesque town of St. Angelo on the island of Ischia, in June 2004, to discuss progress in the application of new experimental techniques for exploration and assessment of shallow water environments. Acoustic techniques provide the most effective means for remote sensing of ocean and sea floor processes, and for probing the structure beneath the sea floor. No other energy propagates as efficiently in the ocean: radio waves and visible light are severely limited in range because the ocean is a highly conductive medium. However, sound from breaking waves and coastal shipping can be heard throughout the ocean, and marine mammals communicate acoustically over basin scale distances.
- 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.
- Estimating geoacoustic bottom properties from towed array dataPublication . Jesus, S. M.; Caiti, A.Estimating the seabottom geophysical structure from the analysis of acoustic returns of an explosive source (air-gun, sparker,...) has been used for a longtime as a routine survey technique. Recent work showed the possibility of using well-suited numerical models to invert the acoustic held for estimating detailed geoacoustic sediment properties. Common implementations used long synthetic aperture arrays (up to 2 km and more) in order to resolve potential environmental ambiguities of the acoustic field. Others, used vertical arrays of sensors covering a significant part of the water column to identify the channel normal mode structure and thus gather information for the bottom physical relevant properties. This paper investigates, with simulated data, the concept of using a moderate aperture physical line array and a sound source simultaneously towed by a single ship for inverting the bottom geoacoustic structure from the acoustic returns received on the array. First, bottom parameter estimators are derived and their system sensitivity is investigated. In particular, it is shown that such a system may be used to sense compressional and shear velocities on the bottom first layers. Density and attenuations (both compressional and shear) have in general small influence on the acoustic field structure and are therefore difficult to estimate. Increasing the signal frequency bandwidth by incoherent module averaging has no significant influence on sensitivity Mismatch cases, mainly those related to array/source relative position, showed that deviations of more than lambda/3 in range and lambda/5 in depth may give erroneous extremum location and therefore biased final estimates. Second, two bottom parameter estimators are compared and their performance tested on a typical shallow water environment. In order to solve the underlying multiparameter inverse problem, global search optimization is used. In particular, it is shown that the use of an adaptive genetic algorithm may, in conjunction with a well-suited maximum likelihood based parameter estimator, rapidly converge to the surface extremum. Inversion results are in agreement with the predictions obtained from the sensitivity study. The mean relative error at 10 dB signal-to-noise ratio is within 1% for the compressional velocity, while greater errors are reported for the shear velocity. Comparison with recent results obtained with a radial basis functions (RBF) inversion strategy showed similar performance. Finally, results obtained with a 156 m aperture towed array showed a good agreement between the inverted compressional velocities and the ground truth measurements.
- Acoustic estimation of seafloor parameters: a Radial Basis Functions approachPublication . Caiti, A.; Jesus, S. M.A novel approach to the estimation of seafloor geoacoustic parameters from the measurement of the acoustic field in the water column is introduced. The approach is based on the idea of approximating the inverse function that links the geoacoustic parameters with the measured field through a series expansion of radial basis functions. In particular, Gaussian basis functions are used in order to ensure continuity and smoothness of the approximated inverse. The main advantage of the proposed approach relies on the fact that the series expansion can be computed off-line from simulated data as soon as the experimental configuration is known. Data inversion can then be performed in true real time as soon as the data are acquired. Simulation results are presented in order to show the advantages and limitations of the method. Finally, some inversion results from horizontal towed array data are reported, and are compared with independent estimates of geoacoustic bottom properties.
- Underwater acoustic networks: the FP7 UAN projectPublication . Caiti, A.; Husoy, T.; Jesus, S. M.; Karasalo, I.; Massimelli, R.; Munafo, A.; Reinen, T. A.; Silva, A.The EU-funded project UAN - Underwater Acoustic Network aims at conceiving, developing and testing at sea an innovative and operational concept for integrating in a unique communication system submerged, surface and aerial sensors with the objective of protecting off-shore and coastline critical infrastructures. A crucial aspect of the project consisted in the use of autonomous underwater vehicles (AUVs) as mobile nodes in the underwater acoustic communication network. In particular, AUVs have the role of adapting the network geometry to the variation of the acoustic channel. This paper reports on the project concept and vision as well as on the progress of its various development phases. The recent at-sea successes that have been demonstrated within the UAN framework are detailed and results of the final UAN project demonstration, UAN11, held in the May of 2011, are reported. The UAN network was in operation for five continuous days with up to five nodes, of which three of them were mobile nodes. © IFAC.