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The problem of determining the acoustic field in the sea due to a point harmonic source around a penetrable sea-mount of conical shape is handled in terms of normal-mode theory. The sea-mount is divided in cylindrical segments-rings, in... more
The problem of determining the acoustic field in the sea due to a point harmonic source around a penetrable sea-mount of conical shape is handled in terms of normal-mode theory. The sea-mount is divided in cylindrical segments-rings, in each of which, a series expansion of the acoustic pressure in terms of normal modes and cosine functions is considered. A similar series expansion is considered for the external field and the coefficients of the various expansions are calculated by solving linear systems of equations resulting from the application of the continuity conditions of the pressure field and normal component of the particle velocity at the artificial interfaces of the cylindrical segments-rings. Since Hankel functions of high order are involved in the expansions, numerical problems arise in the numerical implementation of the scheme in cases of low convergence rate. Numerical results are presented for some simple cases of very low frequency propagation around a cylindrical mount.
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The paper summarizes the research carried out at the University of Crete and the Foundation for Research and Technology-HELLAS aiming at the statistical characterization of underwater acoustic signals and their subsequent use for... more
The paper summarizes the research carried out at the University of Crete and the Foundation for Research and Technology-HELLAS aiming at the statistical characterization of underwater acoustic signals and their subsequent use for geoacoustic inversions and applications in ocean acoustic tomography. In these applications, an acoustic signal recorded in the marine environment due to some source is used as the carrier of information on the physical parameters of the environment. Statistical characterization of acoustic signals is a pre-processing technique aiming at the definition of signal observables, to be used as input data of appropriately defined inverse problems aiming at the estimation of critical parameters of the marine environment. The statistical characterization scheme was introduced as a way to define signal observables especially in cases that typical observables such as ray arrivals or modal arrivals cannot be identified in the recorded signals. Moreover, the setting of the associated inverse problem requires just a single recording device, which renders its application practically and relatively cheap in comparison with signal inversion methods requiring reception at an array of hydrophones. The characterization scheme is based on a wavelet transform of the signal at various levels, followed by the statistical description of the wavelet sub-band coefficients. It is shown that A-stable symmetric distributions are capable of defining the statistics of these coefficients, the characteristic parameters of which are the observables of the signal to be exploited for the inversions. As the inverse problems associated with the sought applications are formulated as optimization processes, an objective function to be used as a similarity measure is defined, which in the case of the statistical characterization method is the Kullback–Leibrer Divergence (KLD), capable of comparing probability density functions. The inversion processes are performed by means of neural networks or genetic algorithms, and the performance of the combined signal characterization and inversion method has been tested with simulated and real data. It is shown, that the method works well especially with noise-free or denoised signals.
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A mode identification process that can be applied to broadband acoustic transmissions in the ocean is presented. The process is associated with a modal travel time inversion scheme for geoacoustic or tomographic inversions. The process is... more
A mode identification process that can be applied to broadband acoustic transmissions in the ocean is presented. The process is associated with a modal travel time inversion scheme for geoacoustic or tomographic inversions. The process is based on the assumption that a reference (background) environment is known and the identification process is based on information on the group (modal) velocities of the reference environment only. Using two characteristic examples corresponding to shallow water environments it is shown that the identification process works well at least for the lower order modes.
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The paper summarizes the research carried out at the University of Crete and the Foundation for Research and Technology-HELLAS aiming at the statistical characterization of underwater acoustic signals and their subsequent use for... more
The paper summarizes the research carried out at the University of Crete and the Foundation for Research and Technology-HELLAS aiming at the statistical characterization of underwater acoustic signals and their subsequent use for geoacoustic inversions and applications in ocean acoustic tomography. In these applications, an acoustic signal recorded in the marine environment due to some source is used as the carrier of information on the physical parameters of the environment. Statistical characterization of acoustic signals is a pre-processing technique aiming at the definition of signal observables, to be used as input data of appropriately defined inverse problems aiming at the estimation of critical parameters of the marine environment. The statistical characterization scheme was introduced as a way to define signal observables especially in cases that typical observables such as ray arrivals or modal arrivals cannot be identified in the recorded signals. Moreover, the setting of...
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EAA AWARD for contributions to the promotion of Acoustics in Europe
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The idea of using the sound to monitor the changes in the marine environment motivated the oceanographers to develop the concept of ocean acoustic tomography. Starting from the early 80's when the pioneers started to pave the way... more
The idea of using the sound to monitor the changes in the marine environment motivated the oceanographers to develop the concept of ocean acoustic tomography. Starting from the early 80's when the pioneers started to pave the way leading to the acoustical monitoring of the ocean change and coming to the beginning of the new millennium, several achievements in the field can be reported vindicating the scientists who believed in this new idea. This lecture note is an attempt to summarize the basic concepts of ocean acoustic tomography methods and address the main issues of this new area of underwater acoustics.
A performance study of a statistical characterization of an underwater acoustic signal in relation to geoacoustic inversion or tomography problems is presented. The method of characterization has been presented by Taroudakis et al. [JASA... more
A performance study of a statistical characterization of an underwater acoustic signal in relation to geoacoustic inversion or tomography problems is presented. The method of characterization has been presented by Taroudakis et al. [JASA 119, 1396–1405 (2006)] and is based on the use of an appropriate distribution law which describes the statistics of the sub-band coefficients of the signal wavelet transform. The method has been applied with synthetic data simulating tomographic experiments with low-frequency sources in shallow environments for the estimation of the water column and bottom properties. In this work the inversion procedure incorporating a genetic algorithm is applied in shallow water environments with simulated noisy data to assess the performance of the method and its limitations under realistic conditions.
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A mode identification process that can be applied to broadband acoustic transmissions in the ocean is presented. The process is associated with a modal travel time inversion scheme for geoacoustic or tomographic inversions. The process is... more
A mode identification process that can be applied to broadband acoustic transmissions in the ocean is presented. The process is associated with a modal travel time inversion scheme for geoacoustic or tomographic inversions. The process is based on the assumption that a reference (background) environment is known and the identification process is based on information on the group (modal) velocities of the reference environment only. Using two characteristic examples corresponding to shallow water environments it is shown that the identification process works well at least for the lower order modes.
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The problem of determining the acoustic field in the sea due to a point harmonic source around a penetrable sea-mount of conical shape is handled in terms of normal-mode theory. The sea-mount is divided in cylindrical segments-rings, in... more
The problem of determining the acoustic field in the sea due to a point harmonic source around a penetrable sea-mount of conical shape is handled in terms of normal-mode theory. The sea-mount is divided in cylindrical segments-rings, in each of which, a series expansion of the acoustic pressure in terms of normal modes and cosine functions is considered. A similar series expansion is considered for the external field and the coefficients of the various expansions are calculated by solving linear systems of equations resulting from the application of the continuity conditions of the pressure field and normal component of the particle velocity at the artificial interfaces of the cylindrical segments-rings. Since Hankel functions of high order are involved in the expansions, numerical problems arise in the numerical implementation of the scheme in cases of low convergence rate. Numerical results are presented for some simple cases of very low frequency propagation around a cylindrical ...
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info:eu-repo/semantics/publishe
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The representation of an acoustic signal in terms of the wavelet sub-band coefficients is studied using their statistical features. This type of representation is suggested as an alternative tool for tomographic or geoacoustic inversions.... more
The representation of an acoustic signal in terms of the wavelet sub-band coefficients is studied using their statistical features. This type of representation is suggested as an alternative tool for tomographic or geoacoustic inversions. Using a set of representative shallow water environments, the variation of the statistical behavior of the sub-band coefficients is associated with corresponding variations of the environmental parameters. Low frequency signals suitable for ocean acoustic tomography are simulated. It is shown that the statistics of the sub-band coefficients are best described using non-Gaussian heavy-tailed distributions such as those of the alpha-stable family. The variations of the distribution parameters are measured using special statistical similarity functions such as the Kullback-Leibler divergence which has been adopted in the present study. The distances between the statistical parameters of a given and reference signals determined by these functions, can be used for subsequent non-linear inversions aiming at the recovery of the environmental parameters.
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An inversion scheme based on normal-mode representation of the acoustic field is applied in ocean acoustic tomography for a range-dependent reconstruction of the sound speed variations at a vertical slice. The scheme is based on the... more
An inversion scheme based on normal-mode representation of the acoustic field is applied in ocean acoustic tomography for a range-dependent reconstruction of the sound speed variations at a vertical slice. The scheme is based on the assumption that modal phase can be measured by suitable mode filtering at some range from the sound source. Two cases have been considered. The first of them makes no use of oceanographic information on the variability to be recovered, while the second one makes use of empirical orthogonal functions (EOFs) that describe sound speed variations in the ocean. The data used in the applications presented in this paper are synthetic ones. It is shown that both modal inversion schemes can be used for the recovery of range-dependent sound speed variations of compact support in the ocean, provided that a range dependent background environment is used to describe an initial guess of the variations. The scheme is more powerful when EOFs are used.
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This paper is concerned with the use of the reassigned wavelet transform for mode identification in shallow water acoustic propagation. Mode identification is important for inverse procedures in underwater acoustics. An efficient way to... more
This paper is concerned with the use of the reassigned wavelet transform for mode identification in shallow water acoustic propagation. Mode identification is important for inverse procedures in underwater acoustics. An efficient way to recognize the modal structure of the acoustic field when a single hydrophone is available is to refer to the time frequency analysis of the recorded signal using wavelet transform. However, the standard wavelet transform in some cases may result in an obscure representation of the dispersion curves. Thus, a reassigned process is proposed which brings important improvements in the time frequency representation of the signal. This is achieved by moving the calculation point of the scalogram in the center of gravity of the energy concentration, associated with each one of the propagating modes. This argument is supported by two illustrative examples corresponding to propagation of low frequency tomographic signals, in shallow water.
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The International Year of Sound 2020–2021 includes activities organized by the IYS Steering Committee, ICA member societies, international affiliates and individual organizations. The web page www.sound2020.org, is the primary contact for... more
The International Year of Sound 2020–2021 includes activities organized by the IYS Steering Committee, ICA member societies, international affiliates and individual organizations. The web page www.sound2020.org, is the primary contact for the IYS and receives more than 3500 visitors per month. The opening was held in Paris at the Grand Amphitheatre of the Sorbonne on 31 January 2020. A film “Sounds of our Life” was launched and is also available on Youtube. An international student competition involved over 15 000 students and attracted excellent contributions. The international scientific conferences included outreach and promotions for the IYS. Despite the restrictions due to COVID-19, ICA members maintained their enthusiasm and contributed an amazing range of innovative virtual activities and resources. Additional outreach included various media items and podcasts. The IYS has encouraged further collaboration with institutions including the WHO, the CHC and groups dealing with sound. This will eventually will strengthen the acoustics community into the future. Information on the activities and all the resources are freely available from the website and will remain as a legacy of the IYS. Despite the challenges of 2020 and 2021, the efforts of so many have shown the resilience and innovation of the acoustic community to enhance the understanding worldwide of the importance of sound.
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The International Commission for Acoustics considered that the proclamation of an International Year of Sound (IYS) would create a worldwide awareness of the importance of sound in our world. Sound is an integral part of culture and... more
The International Commission for Acoustics considered that the proclamation of an International Year of Sound (IYS) would create a worldwide awareness of the importance of sound in our world. Sound is an integral part of culture and society from the basic requirements for communication, awareness of our environment, and expression of our culture through to sophisticated scientific and technological research and applications. After a decade of preparation, the ICA decided to organize the International Year of Sound 2020. The moto of the IYS 2020 was “Importance of Sound for Society and the World” and the IYS was associated with the UNESCO Charter of Sound and resolution 39C/59 on the ‘’Importance of sound in today’s world—Promoting best practices.” The French Organization “La Semaine du Son” became partner in this initiative. The ICA planned a number of central activities and the majority of the ICA member organisations embraced the opportunity to organize local and regional events during 2020. Of course the arrival of the Covid 19 pandemic curtailed the majority of the planned events but the enthusiasm remained to adapt to virtual activities and develop more resources. In response to the continued enthusiasm the ICA Board agreed the IYS should become a two years celebration. This resulted to the new reference as International Year of Sound 2020–2021 (IYS 2020–21)
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It is well known that internal waves in the ocean are an important source of environmental variability which has serious effects in the structure of an acoustic field due to a known source. When measurements of the acoustic field form the... more
It is well known that internal waves in the ocean are an important source of environmental variability which has serious effects in the structure of an acoustic field due to a known source. When measurements of the acoustic field form the input data for an inversion procedure aiming at the recovery of the environmental parameters, the information they carry on includes the internal wave effects. It is therefore natural to assume that neglecting the effects of the internal waves in an inversion procedure based on acoustic field measurements, errors are induced in the inversion. The paper deals with this problem and addresses the case of inversion schemes using travel time information of an acoustic signal. Using a statistical 2D model of the internal waves, based on the Garrett and Munk spectrum, the spatial and temporal evolution of the internal waves field as well as the fluctuations of the sound speed profile is estimated for a characteristic shallow-water environment. Considering a sound speed anomaly in the water column as the oceanographic feature to be recovered, the paper studies the influence of the internal waves field on the modal travel time information obtained through the propagation of a tomographic signal through this environment. The sound speed anomaly denoted as "current" is described by a suitable Gaussian function. Using an analytical expression based on a perturbation approach, the difference in the modal arrival times calculated for a background environment and a perturbed one (considering that the sound speed perturbations are due either to a current or to the summation of a current and of the internal waves field) was calculated for each propagating mode of the waveguide. These calculations led to the conclusion that the internal waves have a non-negligible impact on the arrival times and that the maximum amplitude of a current can be under- or overestimated of several meters per second when these waves are not taken into account in the inversion method, whereas they are present in the oceanic medium.
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A comparative study of two wave-theoretic schemes for ocean-tomography inversion is presented. The first scheme is based on the notion of modal arrivals (modal inversion), while the second scheme is based on the recently introduced notion... more
A comparative study of two wave-theoretic schemes for ocean-tomography inversion is presented. The first scheme is based on the notion of modal arrivals (modal inversion), while the second scheme is based on the recently introduced notion of wave arrivals (peak inversion). A case has been chosen for which both schemes are applicable. For modal inversion an iterative scheme has been used, while peak inversion is carried out as a single-step procedure. The retrieval of the actual sound speed profile is satisfactory for both schemes, with peak inversion giving slightly better results.
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The presentation deals with theoretical factors and technical specifications pertinent to the design of an acoustical observatory for the monitoring of the marine environment. Two types of observatories are mentioned, namely active and... more
The presentation deals with theoretical factors and technical specifications pertinent to the design of an acoustical observatory for the monitoring of the marine environment. Two types of observatories are mentioned, namely active and passive. Among the various cases of active observatories, the ones related to ocean acoustic tomography are presented in some detail and the inverse problem of retrieving information from measured acoustic data is explained. Some basic issues related to the type of measurements that should be made for optimal use of the acoustic field are also given with related references. Finally, the basic features of passive observatories are underlined without going into details. 1.
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A method for denoising underwater acoustic signals used in applications of acoustical oceanography is presented. The method has been introduced for imaging denoising and has been modified to be applied with acoustic signals. The method... more
A method for denoising underwater acoustic signals used in applications of acoustical oceanography is presented. The method has been introduced for imaging denoising and has been modified to be applied with acoustic signals. The method keeps the energy significant part of the raw signal and reduces the effects of noise by comparing overlapping signal windows and keeping components which resemble true signal energy. It is shown by means of characteristic experiments in connection with a statistical signal characterization scheme based on wavelet transform, that using the statistical features of the wavelet sub-band coefficients of the denoised signal, tomography or geoacoustic inversions lead to a reliable estimation of the parameters of a marine environment.
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The ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational... more
The ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational ocean observing systems, and for studying large-scale ocean processes such as water-mass formation. Although the properties of the upper layers of the ocean
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The representation of an acoustic signal in terms of the wavelet sub-band coefficients is studied using their statistical features. This type of representation is suggested as an alternative tool for tomographic or geoacoustic inversions.... more
The representation of an acoustic signal in terms of the wavelet sub-band coefficients is studied using their statistical features. This type of representation is suggested as an alternative tool for tomographic or geoacoustic inversions. Using a set of representative shallow water environments, the variation of the statistical behavior of the sub-band coefficients is associated with corresponding variations of the environmental parameters. Low frequency signals suitable for ocean acoustic tomography are simulated. It is shown that the statistics of the sub-band coefficients are best described using non-Gaussian heavy-tailed distributions such as those of the alpha-stable family. The variations of the distribution parameters are measured using special statistical similarity functions such as the Kullback-Leibler divergence which has been adopted in the present study. The distances between the statistical parameters of a given and reference signals determined by these functions, can ...
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The problem of determining the sound field due to a point harmonic source in an ocean environment with an arbitrary three-dimensional local variation of field parameters (including irregular interfaces) is studied with the help of a... more
The problem of determining the sound field due to a point harmonic source in an ocean environment with an arbitrary three-dimensional local variation of field parameters (including irregular interfaces) is studied with the help of a variational principle. The variational principle is established by proving that the Euler–Lagrange equations of a suitable functional coincide with the field equation and the exact boundary and interface conditions of the complete elliptic boundary value problem. The method presented in the paper is a hybrid one, in the sense that the pressure field is represented by modal series expansions in the range-independent region, and a suitable localized expansion in the range-dependent region of the environment. These representations retain full mode coupling and, in conjunction with the variational principle, reduce the problem to a linear system of equations free of any a priori restrictions. It is expected that this system would be efficiently used for the ...
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This paper is concerned with the use of the reassigned wavelet transform for mode identification in shallow water acoustic propagation. Mode identification is important for inverse procedures in underwater acoustics. An efficient way to... more
This paper is concerned with the use of the reassigned wavelet transform for mode identification in shallow water acoustic propagation. Mode identification is important for inverse procedures in underwater acoustics. An efficient way to recognize the modal structure of the acoustic field when a single hydrophone is available is to refer to the time frequency analysis of the recorded signal using wavelet transform. However, the standard wavelet transform in some cases may result in an obscure representation of the dispersion curves. Thus, a reassigned process is proposed which brings important improvements in the time frequency representation of the signal. This is achieved by moving the calculation point of the scalogram in the center of gravity of the energy concentration, associated with each one of the propagating modes. This argument is supported by two illustrative examples corresponding to propagation of low frequency tomographic signals, in shallow water.
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One subject of Underwater Acoustics that is currently receiving much attention, is the solution of inverse problems, in which some (or all) of the ocean-acoustic parameters characterizing a channel (e.g., the sound speed profile in the... more
One subject of Underwater Acoustics that is currently receiving much attention, is the solution of inverse problems, in which some (or all) of the ocean-acoustic parameters characterizing a channel (e.g., the sound speed profile in the water column, and the subbottom structure) are determined from measurements of the sound field at several points in the channel. Matched Field Processing (MFP) is an example of an inverse technique that relies heavily on numerical propagation codes. The matched field problem is not solved by direct inversion, but by exhaustively solving the forward problem (i.e. the wave equation with the appropriate boundary conditions) using a propagation model, and finding the set of parameter values which give the best match to the measured data, that is the complex pressures from a hydrophone array (Figure 1). Thus, MFP can be formulated as an optimization problem that seeks to maximize a correlation coefficient between the observed and predicted fields; this coefficient can be either linear, such as the Bartlett processor, or nonlinear, such as the Capon processor1.
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Various object functions for broadband inversions of noisy data using matched-field processing for bottom recognition are studied. The study is based on the inversion results obtained by processing synthetic ‘‘noisy’’ data that have been... more
Various object functions for broadband inversions of noisy data using matched-field processing for bottom recognition are studied. The study is based on the inversion results obtained by processing synthetic ‘‘noisy’’ data that have been available at a follow-up stage of a benchmark exercise for bottom geoacoustic inversions that was carried out in 1997 (Vancouver, Canada). A genetic algorithm has been used for the implementation of the matched-field processing. The different behavior of the various processors that has already been observed by the authors at the benchmark exercise based on noise-free data is now considered with data that include noise, and their applicability for bottom recognition is analyzed.
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A matched-field processing method for the simultaneous estimation of ocean current velocity structure and sound speed profiles, based on reciprocal transmissions of cw or broadband acoustic signals between two locations in the water... more
A matched-field processing method for the simultaneous estimation of ocean current velocity structure and sound speed profiles, based on reciprocal transmissions of cw or broadband acoustic signals between two locations in the water column, is presented and discussed. For each transmission, a single source and a vertical array of hydrophones are used. The current velocity is introduced as an additional unknown in the inverse problem, through the concept of the ‘‘effective sound speed,’’ being the sum of the actual sound speed and the projection of the current velocity on a vertical plane, defined by the sound source and the receiver. The work is mainly focused on the study of the effect of variations of the current velocity on the various processors proposed for the matched-field inversions and on the inversion procedure itself, under the assumption that neither the actual position of the source receiver system nor the actual sound velocity structure are known. Using synthetic data,...
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Matched-field processing and a hybrid scheme for vertical slice tomography are studied in characteristic cases of range-independent and range-dependent ocean environments using simulated data obtained at discrete points in the sound field... more
Matched-field processing and a hybrid scheme for vertical slice tomography are studied in characteristic cases of range-independent and range-dependent ocean environments using simulated data obtained at discrete points in the sound field along a vertical line, representing a vertical array of hydrophones. The matched-field processing is associated with a Genetic Algorithm, while the basic characteristic of the hybrid scheme is the application of matched-field processing for the definition of a reference environment to be subsequently used in connection with a modal-phase inversion scheme. In all cases studied, the sound-speed profiles are reconstructed with the use of empirical orthogonal functions. It is shown that the reconstruction of either the single sound-speed profile characterizing the range-independent environment, or the set of profiles characterizing an eddy in the range-dependent environment is achieved with good accuracy, despite the fact that a very wide search space ...
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Matched-Field Processing with a Genetic Algorithm is applied to the problem of bottom recognition with synthetic noise-free acoustic data. The data correspond to three classes of benchmark problems. Four alternative objective functions... more
Matched-Field Processing with a Genetic Algorithm is applied to the problem of bottom recognition with synthetic noise-free acoustic data. The data correspond to three classes of benchmark problems. Four alternative objective functions have been tested, all of them defined to be used with broadband data with either coherent or incoherent processing in the frequency domain. It has been concluded that the parameters corresponding to the sediment layer and especially the sound speed, are more accurately recovered by all means. Source localization and water depth estimation are made with good accuracy in all cases. It has also been shown that incoherent processing in the frequency domain has led to better results for the cases studied.
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ABSTRACT The sensitivity of a Bartlett processor used as an objective function in a matched-field approach to bottom recognition in a shallow-water environment is investigated. The study addresses the varying response of the processor to... more
ABSTRACT The sensitivity of a Bartlett processor used as an objective function in a matched-field approach to bottom recognition in a shallow-water environment is investigated. The study addresses the varying response of the processor to changes in the values of geoacoustic parameters, in different frequencies and ranges from the source. It is shown that certain parameters are better estimated in low frequencies and shorter ranges. This observation led to the development of a two-stage algorithm which searches for certain sets of parameters in different frequencies and ranges. Thus, the complexity and dimensionality of the problem are reduced, improving the efficiency of a genetic algorithm search, both in terms of quality of the results and computational cost. The algorithm is applied to synthetic data of a benchmark exercise validating the conclusions of the study.
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This paper presents a study on the applicability of a method for the statistical characterization of seismic signals which is based on the statistics of their wavelet sub-band coefficients using [Formula: see text] stable distributions.... more
This paper presents a study on the applicability of a method for the statistical characterization of seismic signals which is based on the statistics of their wavelet sub-band coefficients using [Formula: see text] stable distributions. The method was originally applied to underwater acoustic signals of the type used in ocean acoustic tomography and seabed classification applications. The same protocol was applied to seismic signals representing the vertical displacement, measured on a traditional seismograph. The study showed that this process can indeed provide a means of characterizing a seismic signal and be used to estimate similarities between seismic signals, which is an important factor in studying seismic activity in an area.
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A hybrid approach for problems of ocean acoustic tomography is presented, based on the statistical characterization (SC) of the acoustic signal followed by a mode identification and linear inversion. The statistical characterization is... more
A hybrid approach for problems of ocean acoustic tomography is presented, based on the statistical characterization (SC) of the acoustic signal followed by a mode identification and linear inversion. The statistical characterization is used for the estimation of a reference solution to the inverse problem of estimating the sound speed profile in the water column. This non-linear inversion problem is solved using a Genetic Algorithm. By applying first order perturbation approach, variations of the sound speed profile are associated with modal travel time variations. This relationship provides the framework for the development of an iterative linear inversion scheme which converges when the reference environment is close to the actual one and provides a fine tuning of the results obtained by the non-linear inversions. The performance of the method is demonstrated by means of a simulated experiment in range-dependent environment representing a cold eddy.
The work presents a method for characterizing underwater acoustic signals using a Markov chain, with hidden variables, based on their wavelet transform. Initially, we assign to the signal a Hidden Markov Model (HMM) for which the... more
The work presents a method for characterizing underwater acoustic signals using a Markov chain, with hidden variables, based on their wavelet transform. Initially, we assign to the signal a Hidden Markov Model (HMM) for which the conditional posterior probability density function seems to be the most representative using an Expectation-Maximization algorithm. Special techniques are applied to avoid over-fitting which in principle is not desirable for the sought applications. The features used for the assignment consist of two dimensional time series obtained by preprocessing of signal’s wavelet packet coefficients. Subsequently, we use an approximation of the Kullback Leibler (KL) divergence as a similarity measure among the HMMs. The approximation is obtained by employing Monte-Carlo (MC) techniques simulating the significant sampling from the HMMs posterior distributions. This technique is used in cases where the similarity of two or more signals is to be exploited. These cases in...
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The signal characterization method suggested by Taroudakis et al. (J.Acoust. Soc. Am. 119, 1396-1405 (2006)) based on the statistics of its 1-D wavelet transform coefficients and successfully applied for inverting acoustic signals in... more
The signal characterization method suggested by Taroudakis et al. (J.Acoust. Soc. Am. 119, 1396-1405 (2006)) based on the statistics of its 1-D wavelet transform coefficients and successfully applied for inverting acoustic signals in applications of acoustical oceanography has been proven to be sensitive to noise contamination of the signal, but still, it provides good inversion results if an appropriate denoising strategy is applied. In this work, the statistical signal characterization is applied to signals which are both blurred and noise contaminated. Deblurring of the signal is achieved by means of a technique introduced by Taroudaki and O’ Leary (SIAM J. Sci. Comput. 37(6), A2947-A2968 (2015)) for image deblurring, and it is based on a statistical near optimal spectral filtering technique that takes advantage of the singular values of the approximated blurring matrix and the Picard Parameter of the signal that allows for estimation of the additive noise properties and estimati...
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Recordings of pulsed sounds (clicks) from Cuvier’s beaked whales are presented. Such recordings have not been reported in the literature before. Spectrogram analysis of data collected off SW Crete (Greece) from 1998 to 2000 revealed... more
Recordings of pulsed sounds (clicks) from Cuvier’s beaked whales are presented. Such recordings have not been reported in the literature before. Spectrogram analysis of data collected off SW Crete (Greece) from 1998 to 2000 revealed numerous sequences of clicks. Click pulses had durations of about 1 ms and their energy content in the audible spectrum presented a narrow peak between 13 and 17 kHz. Sequences of 35–105 clicks, with duration 15–44 s, were separated by short intersequence pauses of 3–10 s. Interclick intervals appeared fairly constant, primarily oscillating between 0.40 and 0.50 s. Characteristics of Cuvier’s beaked whale clicks were consistent with echolocating cetaceans, suggesting that this species do echolocate.
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... Time and frequency measurements using scaled laboratory experiments of shallow water acoustic propagation. P. Papadakis, M. Taroudakis, Patrick Sanchez 1 , Jean-Pierre Sessarego 1. (2006). 1 : Laboratoire de Mécanique et... more
... Time and frequency measurements using scaled laboratory experiments of shallow water acoustic propagation. P. Papadakis, M. Taroudakis, Patrick Sanchez 1 , Jean-Pierre Sessarego 1. (2006). 1 : Laboratoire de Mécanique et d'Acoustique (LMA). CNRS : UPR7051. ...