Search

We propose a modal approach developed in the framework of the paraxial approximation to investigate the effects of deterministic surface perturbations in a planar waveguide. In the first part, the sensitivity of the modal amplitudes is theoretically formulated for a three-dimensional perturbation at the air-water interface. When applied to a broadband ultrasonic signal in a laboratory tank experiment, this approach results in travel-time and amplitude fluctuations that are successfully compared to experimental data recorded between two vertical source-receiver arrays that span the ultrasonic waveguide. The nonlinear shape of the modal amplitude fluctuations is of particular interest and is due to the three-dimensional nature of the surface perturbation. In the second part, a time-harmonic inversion method is built in the paraxial single-scattering approximation to image the dynamic surface perturbation from the modal transmission matrix between two source-receiver arrays. Again, the inversion results for capillary-gravity surface perturbations are successfully compared to similar inversions performed from experimental data processed with a complete set of eigenbeams extracted between the two arrays.

An experimental comparison is reported here between two equivalent resonant subwavelength metasurfaces made of long aluminum beams glued closely together on a thin aluminum plate. One metasurface has a random distribution of the resonator beams, and the other has a regular square lattice of pitch 1.5 cm. The random lattice shows the "resonant" behavior of a typical metasurface, with a wide full bandgap for the first A 0 Lamb mode. Instead, the regular square lattice combines Fano resonance with Bragg scattering at the edges of the passband, thus creating anisotropy and a pseudo bandgap. Comparisons with numerical simulations are performed, with good agreement with the experimental data. The multimodal response of the beams is also responsible for double negativity in a narrow frequency band, and the event of a pseudo bandgap around this same flexural resonance. In addition, the scattering regimes for both the random and regular metasurfaces are characterized using coherent and incoherent signal analysis.

Ocean acoustic tomography is traditionally performed using the travel-time variations of an acoustic path between a source and a receiver. In the context of shallow-water tomography and multipath propagation, the different acoustic paths can be correctly identified if the source and the receiver are arrays of transducers. Here, a double-beamforming algorithm can be applied to extract a collection of eigenbeams from the raw acoustic dataset. In this study, four observables can be measured for each eigenbeam: the travel-time, the amplitude, and the emitting and receiving angles. In this study, the sensitivity kernel (SK) formulation is used to establish a quantitative relation between a perturbation of the surface of an ultrasonic waveguide and the emitting and receiving angles of each eigenbeam. This theoretical relation is experimentally demonstrated using a forward model experiment designed to measure the SK. The SK formulation is then used in a second experiment to quantitatively and dynamically image the propagation of a surface wave traveling across the surface of the waveguide. The inversion results show that the quality of the joint inversion of the emitting and receiving angles is higher than previous results based on amplitude or travel-time observables.

Multiple scattering of acoustic waves offers a noninvasive method for density estimation of a dense shoal of fish where traditional techniques such as echo-counting or echo-integration fail. Through acoustic experiments with a multi-beam sonar system in open sea cages, multiple scattering of sound in a fish shoal, and, in particular, the coherent backscattering effect, can be observed and interpreted quantitatively. Furthermore, a volumetric scan of the fish shoal allows isolation of a few individual fish from which target strength estimations are possible. The combination of those two methods allows for fish density estimation in the challenging case of dense shoals.

Dynamic acoustic imaging of a surface wave propagating at an air-water interface is a complex task that is investigated here at the laboratory scale through an ultrasonic experiment in a shallow water waveguide. Using a double beamforming algorithm between two source-receiver arrays, the authors isolate and identify each multi-reverberated eigenbeam that interacts with the air-water and bottom interfaces. The waveguide transfer matrix is recorded 100 times per second while a low-amplitude gravity wave is generated by laser-induced breakdown at the middle of the waveguide, just above the water surface. The controlled, and therefore repeatable, breakdown results in a blast wave that interacts with the air-water interface, which creates ripples at the surface that propagate in both directions. The amplitude perturbations of each ultrasonic eigenbeam are measured during the propagation of the gravity-capillary wave. Inversion of the surface deformation is performed from the amplitude variations of the eigenbeams using a diffraction-based sensitivity kernel approach. The accurate ultrasonic imaging of the displacement of the air-water interface is compared to simultaneous measurements with an optical camera, which provides independent validation.

Groups of coordinated underwater vehicles or sensors are powerful tools for monitoring the ocean. A requirement of many coordinated surveys is to determine a spatial reference between each node in a swarm. This work considers the self-localization of a swarm of independently moving vehicles using acoustic noise from a dominating incoherent source recorded by a single hydrophone onboard each vehicle. This method provides an inexpensive and infrastructure-free spatial reference between vehicles. Movement between the vehicles changes the swarm geometry and a self-localization estimate must be generated from data collected on short time scales. This challenges past self-localization approaches for acoustic arrays. To overcome this challenge, the proposed self-localization algorithm jointly estimates the vehicle geometry and the directionality of the ambient noise field, without prior knowledge of either estimate. To demonstrate this method, experimental results are provided when a boat is the main dominating source. The results demonstrate the ability to both estimate the direction of arrival of the boat and the relative positions of the vehicles in the swarm. The approach in this paper is not limited to moving vessels. Simulations are provided to examine three different factors that affect the proposed solution: inter-vehicle motion, vehicle geometry, and the azimuthal variance of the noise field.

Subspace algorithms based on higher-order cumulants were developed to achieve high-resolution separation in non-Gaussian processes. However, singular value decomposition (SVD) of a huge matrix is an unavoidable step of these algorithms. The memory space and running time required by the decomposition are super-linear with respect to the size of the matrix, which is prohibitive in terms of practical applications. Thus, in this paper, a fast raypath separation algorithm based on low-rank matrix approximation is proposed in a shallow-water waveguide. The experimental results illustrate that the proposed algorithm dramatically reduces the consumption of time and space, with arbitrarily small error, compared to conventional higher-order cumulant-based algorithms.

Identification of unexploded ordinance buried in the sediment in the littoral waters throughout the world is a problem of great concern. When illuminated by low-frequency sonar some of these targets exhibit an elastic response that can be used to identify them. This elastic behavior is embodied and identified by a quantity called the in vacuo structural admittance matrix Y s , a relationship between the sonar-induced forces and resulting vibration on its surface. When it is known it can be combined with surface impedances to predict the three-dimensional bistatic scattering in any fluid-like media and for any burial state (depth and orientation). At the heart of this is the measurement of Y s and it is demonstrated in this paper that this can be accomplished by studying the target in a simple (acoustically unaltered) in-air laboratory environment. The target chosen in this study is a thick spherical shell that was illuminated by a nearly spatially isotropic array of remote loudspeakers. Y s is constructed from ensemble averages of the cross-correlations of eight collocated accelerometers and microphones placed on the surface of the object. The structural admittance determined from the data showed excellent agreement with theory.

In this letter, an experimental and numerical study of the influence of symmetric resonators on Lamb waves that propagate in a thin metallic plate is reported. The resonators consist of a set of small, closely packed rods that pass through the plate from side to side to create a locally resonant metamaterial. Adjusting the symmetry of these rods creates degenerate flexural resonances that can be tuned to couple only with a given Lamb mode that is either symmetric (S0) or anti-symmetric (A0). This approach offers promising perspectives in the development of metamaterials for elastic waves that involve non-scalar interactions.

Multiple raypaths propagation is caused by reflection and refraction at the surface and the bottom of the water column. In this study, an active wideband higher-order separation is proposed, which enables the separation of raypaths interrupted by colored noise (as traditionally found in ocean environments) in the angle-vs-time domain. A comparative study shows that the proposed algorithm achieves a more accurate separation than other algorithms. Moreover, with the proposed approach, it is not necessary to assume that the number of sensors is larger than that of the sources. Furthermore, numerical results validate the noise suppression property of the proposed method.

This paper presents a demonstration of ambient acoustic noise processing on a set of free floating oceanic receivers whose relative positions vary with time. It is shown that it is possible to retrieve information that is relevant to the travel time between the receivers. With thousands of short time cross-correlations (10 s) of varying distance, it is shown that on average, the decrease in amplitude of the noise correlation function with increased separation follows a power law. This suggests that there may be amplitude information that is embedded in the noise correlation function. An incoherent beamformer is developed, which shows that it is possible to determine a source direction using an array with moving elements and large element separation. This incoherent beamformer is used to verify cases when the distribution of noise sources in the ocean allows one to recover travel time information between pairs of mobile receivers.

In this study, it is shown that under very weak dynamic and quasi-static deformation that is orders of magnitude below the yield deformation of the equivalent stress-strain curve (around 10(-3)), the elastic parameters of a civil engineering structure (resonance frequency and damping) exhibit nonlinear softening and recovery. These observations bridge the gap between laboratory and seismic scales where elastic nonlinear behavior has been previously observed. Under weak seismic or atmospheric loading, modal frequencies are modified by around 1% and damping by more than 100% for strain levels between 10(-7) and 10(-4). These observations support the concept of universal behavior of nonlinear elastic behavior in diverse systems, including granular materials and damaged solids that scale from millimeter dimensions to the scale of structures to fault dimensions in the Earth.

Recently wide frequency band gaps were observed in an experimental realization of a multiresonant metamaterial for Lamb waves propagating in thin plates. The band gaps rose from hybridization between the flexural plate (A0 Lamb waves) and longitudinal resonances in rods attached perpendicularly. Shortly thereafter a theory based on considering a one-dimensional periodic array of rods and the scattering matrix for a single rod successfully described the observations. This letter presents an alternative simpler theory, arguably accurate at high rod density, that treats the full two-dimensional array of rods and makes no assumption of periodicity. This theory also fits the measurements.

This paper deals with the numerical design of a directional invisibility cloak for backward scattered elastic waves propagating in a thin plate (A0 Lamb waves). The directional cloak is based on a set of resonating beams that are attached perpendicular to the plate and are arranged at a sub-wavelength scale in ten concentric rings. The exotic effective properties of this locally resonant metamaterial ensure coexistence of bandgaps and directional cloaking for certain beam configurations over a large frequency band. The best directional cloaking was obtained when the resonators' length decreases from the central to the outermost ring. In this case, flexural waves experience a vanishing index of refraction when they cross the outer layers, leading to a frequency bandgap that protects the central part of the cloak. Numerical simulation shows that there is no back-scattering in these configurations. These results might have applications in the design of seismic-wave protection devices.

Variation of mechanical properties in materials can be detected non-destructively using ultrasonic measurements. In particular, changes in elastic wave velocity can occur due to damage, i.e., micro-cracking and particles debonding. Here the challenge of characterizing damage in geomaterials, i.e., rocks and soils, is addressed. Geomaterials are naturally heterogeneous media in which the deformation can localize, so that few measurements of acoustic velocity across the sample are not sufficient to capture the heterogeneities. Therefore, an ultrasonic tomography procedure has been implemented to map the spatial and temporal variations in propagation velocity, which provides information on the damage process. Moreover, double beamforming has been successfully applied to identify and isolate multiple arrivals that are caused by strong heterogeneities (natural or induced by the deformation process). The applicability of the developed experimental technique to laboratory geomechanics testing is illustrated using data acquired on a sample of natural rock before and after being deformed under triaxial compression. The approach is then validated and extended to time-lapse monitoring using data acquired during plane strain compression of a sample including a well defined layer with different mechanical properties than the matrix.

Structural-change detection and characterization, or structural-health monitoring, is generally based on modal analysis, for detection, localization, and quantification of changes in structure. Classical methods combine both variations in frequencies and mode shapes, which require accurate and spatially distributed measurements. In this study, the detection and localization of a local perturbation are assessed by analysis of frequency changes (in the fundamental mode and overtones) that are combined with a perturbation-based linear inverse method and a deconvolution process. This perturbation method is applied first to a bending beam with the change considered as a local perturbation of the Young's modulus, using a one-dimensional finite-element model for modal analysis. Localization is successful, even for extended and multiple changes. In a second step, the method is numerically tested under ambient-noise vibration from the beam support with local changes that are shifted step by step along the beam. The frequency values are revealed using the random decrement technique that is applied to the time-evolving vibrations recorded by one sensor at the free extremity of the beam. Finally, the inversion method is experimentally demonstrated at the laboratory scale with data recorded at the free end of a Plexiglas beam attached to a metallic support.

Deep sub-wavelength focusing has been demonstrated for locally resonant metamaterials using electromagnetic and acoustic waves. The elastic equivalents of such objects are made of sub-wavelength resonating beams fixed to a two-dimensional plate, as presented here. Independent of a random or regular arrangement of the resonators, the metamaterial shows large bandgaps that are independent of the incident wave direction. Numerical simulations demonstrate that the insertion of a defect in the layout, as a shorter resonator, creates strong amplification of the wave-field on the defect. This energy trapping, which is localized on a spatial scale that is much smaller than the wavelength in the two-dimensional plate, leads to a >1 factor in terms of the local density of energy.

A method to isolate the forward scattered field from the incident field on an object in a complex environment is developed for the purpose of localization. The method is based on a finite-frequency perturbation approach, through the measurement of a data-based sensitivity kernel. Experimental confirmation of the method is obtained using a cylindrical tank and an aggregate of ping-pong balls as targets surrounded by acoustic sources and receivers in a multistatic configuration. The spatial structure of the sensitivity kernel is constructed from field data for the target at a sparse set of positions, and compared with the expected theoretical structure. The localization of one or a few targets is demonstrated using the direct-path only. The experimental observations also show that the method benefits from including later arrivals from the tank wall and the bottom/surface reverberation, which indeed enhance the localization.

The dynamic imaging of a deterministic gravity wave propagating at an air-water interface requires continuous sampling of every point at this interface. This sampling can be done acoustically using waves that propagate in the water column but have specular reflection points that fully scan the air-water interface. This study aims to perform this complex task experimentally, with identical ultrasonic source and receiver arrays that face each other in a 1-m-long, 5-cm-deep fluid waveguide, and with frequencies in the MHz range. The waveguide transfer matrix is recorded 100 times per second between the source-receiver arrays, while a gravity wave is generated at the air-water interface. Through the beamforming process, a large set of acoustic multi-reverberated beams are isolated and identified that interact with the air-water interface. The travel-time and amplitude modulations of each eigenbeam are measured when the surface gravity wave travels through the source-receiver plane. Linear inversion of the travel-time and amplitude perturbations is performed from a few thousand eigenbeams using diffraction-based sensitivity kernels. Inversion results using travel-times, amplitudes, or these two observables together, lead to accurate spatial-temporal patterns of the surface deformation. The advantages and limitations of the method are discussed.

Cross-correlations of ambient noise averaged at two receivers lead to the reconstruction of the two-point Green's function, provided that the wave-field is uniform azimuthally, and also temporally and spatially uncorrelated. This condition depends on the spatial distribution of the sources and the presence of heterogeneities that act as uncorrelated secondary sources. This study aims to evaluate the relative contributions of source distribution and medium complexity in the two-point cross-correlations by means of numerical simulations and laboratory experiments in a finite-size reverberant two-dimensional (2D) plate. The experiments show that the fit between the cross-correlation and the 2D Green's function depends strongly on the nature of the source used to excite the plate. A turbulent air-jet produces a spatially uncorrelated acoustic field that rapidly builds up the Green's function. On the other hand, extracting the Green's function from cross-correlations of point-like sources requires more realizations and long recordings to balance the effect of the most energetic first arrivals. When the Green's function involves other arrivals than the direct wave, numerical simulations confirm the better Green's function reconstruction with a spatially uniform source distribution than the typical contour-like source distribution surrounding the receivers that systematically gives rise to spurious phases.

For shallow-water waveguides and mid-frequency broadband acoustic signals, ocean acoustic tomography (OAT) is based on the multi-path aspect of wave propagation. Using arrays in emission and reception and advanced array processing, every acoustic arrival can be isolated and matched to an eigenray that is defined not only by its travel time but also by its launch and reception angles. Classically, OAT uses travel-time variations to retrieve sound-speed perturbations; this assumes very accurate source-to-receiver clock synchronization. This letter uses numerical simulations to demonstrate that launch-and-reception-angle tomography gives similar results to travel-time tomography without the same requirement for high-precision synchronization.

In the Born approximation, the acoustic scattering from a spherical obstacle of a size comparable to the acoustic wavelength can be evaluated in the framework of the sensitivity kernel approach, which describes the relationship between the pressure-field fluctuation and the position of a local change in the propagation medium. The spatial structure of the sensitivity kernel is here investigated through experimental observations made in a water tank at the ultrasonic scale and compared to an analytical model. The pattern of the sensitivity kernel is discussed in the case of a source-to-receiver wave field that includes a direct path and one surface reflection.

Acoustic waves traveling in a shallow-water waveguide produce a set of multiple paths that can be characterized as a geometric approximation by their travel time (TT), direction of arrival (DOA), and direction of departure (DOD). This study introduces the use of the DOA and DOD as additional observables that can be combined to the classical TT to track sound-speed perturbations in an oceanic waveguide. To model the TT, DOA, and DOD variations induced by sound-speed perturbations, the three following steps are used: (1) In the first-order Born approximation, the Fréchet kernel provides a linear link between the signal fluctuations and the sound-speed perturbations; (2) a double-beamforming algorithm is used to transform the signal fluctuations received on two source-receiver arrays in the time, receiver-depth, and source-depth domain into the eigenray equivalent measured in the time, reception-angle and launch angle domain; and finally (3) the TT, DOA, and DOD variations are extracted from the double-beamformed signal variations through a first-order Taylor development. As a result, time-angle sensitivity kernels are defined and used to build a linear relationship between the observable variations and the sound-speed perturbations. This approach is validated with parabolic-equation simulations in a shallow-water ocean context.

Data collected over more than eight consecutive hours between two source-receiver arrays in a shallow water environment are analyzed through the physics of the waveguide invariant. In particular, the use of vertical arrays on both the source and receiver sides provides source and receiver angles in addition to travel-times associated with a set of eigenray paths in the waveguide. From the travel-times and the source-receiver angles, the eigenrays are projected into a group-velocity versus phase-velocity (Vg-Vp) plot for each acquisition. The time evolution of the Vg-Vp representation over the 8.5-h long experiment is discussed. Group speed fluctuations observed for a set of eigenrays with turning points at different depths in the water column are compared to the Brunt-Väisälä frequency.

Ambient noise was recorded on two vertical line arrays (VLAs) separated by 450 m and deployed in shallow water (depth ~150 m) off San Diego, CA continuously for 6 days. Recordings were dominated by non-stationary and non-uniform broadband shipping noise (250 Hz to 1.5 kHz). Stable coherent noise wavefronts were extracted from ambient noise correlations between the VLAs during all 6 days by mitigating the effect of discrete shipping events and using array beamforming with data-derived steering vectors. This procedure allows the tracking of arrival-time variations of these coherent wavefronts during 6 days and may help in developing future passive acoustic tomography systems.

Extracting coherent wavefronts between passive receivers using cross-correlations of ambient noise (CAN) provides a means for monitoring the seismoacoustic environment without using active sources. However, using cross-correlations between single receivers can require a long recording time in order to extract stable coherent arrivals from CAN. This becomes an issue if the propagation medium fluctuates significantly during the recording period. To address this issue, this article presents a general spatio-temporal filtering procedure to enhance the emergence rate for coherent wavefronts extracted from time-averaged ambient noise correlations between two spatially separated arrays. The robustness of this array-based CAN technique is investigated using ambient shipping noise recorded over 24 h in the frequency band [250-850 Hz] on two vertical line arrays deployed 143 m apart in shallow water (depth 20 m). Experimental results confirm that the array-based CAN technique can significantly reduce the recording duration (e.g., from 22 h to 30 min) required for extracting coherent wavefronts of sufficient amplitude (e.g., 20 dB over residual temporal fluctations) when compared to conventional CAN implementations between single pairs of hydrophones. These improvements of the CAN technique could benefit the development of noise-based ocean monitoring applications such as passive acoustic tomography.

Using the Born approximation, a linearized sensitivity kernel is derived to describe the relationship between a local change at the free surface and its effect on the acoustic propagation in the water column. The structure of the surface scattering kernel is investigated numerically and experimentally for the case of a waveguide at the ultrasonic scale. To better demonstrate the sensitivity of the multipath propagation to the introduction of a localized perturbation at the air-water interface, the kernel is formulated both in terms of point-to-point and beam-to-beam representations. Agreement between theory and experiment suggests applications to sensitivity analysis of the wavefield for sea surface perturbations., (© 2012 Acoustical Society of America.)

Passive methods for the recovery of Green's functions from ambient noise require strong hypotheses, including isotropic distribution of the noise sources. Very often, this distribution is nonisotropic, which introduces bias in the Green's function reconstruction. To minimize this bias, a spatiotemporal inverse filter is proposed. The method is tested on a directive noise field computed from an experimental active seismic data set. The results indicate that the passive inverse filter allows the manipulation of the spatiotemporal degrees of freedom of a complex wave field, and it can efficiently compensate for the noise wavefield directivity., (© 2012 Acoustical Society of America.)

Acoustic tomography in a shallow ultrasonic waveguide is demonstrated at the laboratory scale between two source-receiver arrays. At a 1/1,000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. Two coplanar arrays record the transfer matrix in the time domain of the waveguide between each pair of source-receiver transducers. A time-domain, double-beamforming algorithm is simultaneously performed on the source and receiver arrays that projects the multi-reflected acoustic echoes into an equivalent set of eigenrays, which are characterized by their travel times and their launch and arrival angles. Travel-time differences are measured for each eigenray every 0.1 s when a thermal plume is generated at a given location in the waveguide. Travel-time tomography inversion is then performed using two forward models based either on ray theory or on the diffraction-based sensitivity kernel. The spatially resolved range and depth inversion data confirm the feasibility of acoustic tomography in shallow water. Comparisons are made between inversion results at 1 and 3 MHz with the inversion procedure using ray theory or the finite-frequency approach. The influence of surface fluctuations at the air-water interface is shown and discussed in the framework of shallow-water ocean tomography., (© 2011 Acoustical Society of America)

People are familiar with the acoustic feedback phenomenon, which results in a loud sound that is heard when a musician plays an electric instrument directly into a speaker. Acoustic feedback occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier, such that the amplified received signal is continuously re-emitted by the source. The acoustic feedback can be initiated from a continuous sine wave. When the emitter and the receiver are in phase, resonance is obtained, which appears to be highly sensitive to any fluctuation of the propagation medium. Another procedure consists in initiating the acoustic feedback from a continuous loop of ambient noise. It then generates an unstable self-sustained feedback oscillator (SFO) that is tested here as a method for monitoring temperature fluctuations of a shallow-water oceanic environment. The goal of the present study is to reproduce and study the SFO at the laboratory scale in an ultrasonic waveguide. The experimental results demonstrate the potential applications of the SFO for the detection of a target in the framework of the acoustic-barrier problem in shallow-water acoustics., (© 2011 Acoustical Society of America)

Through experiments and simulations, the consequences of symmetry on coherent backscattering enhancement (CBE) are studied in cavities. Three main results are highlighted. First, the CBE outside the source is observed: (a) on a single symmetric point in a one-dimensional (1-D) cavity, in a disk and in a symmetric chaotic plate; (b) on three symmetric points in a two-dimensional (2-D) rectangle; and (c) on seven symmetric points in a three-dimensional (3-D) parallelepiped cavity. Second, the existence of enhanced intensity lines and planes in 2-D and 3-D simple-shape cavities is demonstrated. Third, it is shown how the anti-symmetry caused by the special boundary conditions is responsible for the existence of a coherent backscattering decrement with a dimensional dependence of R = (½)(d), with d = 1,2,3 as the dimensionality of the cavity.

This study demonstrates experimentally at the laboratory scale the detection and localization of a wavelength-sized target in a shallow ultrasonic waveguide between two source-receiver arrays at 3 MHz. In the framework of the acoustic barrier problem, at the 1/1000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. The two coplanar arrays record in the time-domain the transfer matrix of the waveguide between each pair of source-receiver transducers. Invoking the reciprocity principle, a time-domain double-beamforming algorithm is simultaneously performed on the source and receiver arrays. This array processing projects the multireverberated acoustic echoes into an equivalent set of eigenrays, which are defined by their launch and arrival angles. Comparison is made between the intensity of each eigenray without and with a target for detection in the waveguide. Localization is performed through tomography inversion of the acoustic impedance of the target, using all of the eigenrays extracted from double beamforming. The use of the diffraction-based sensitivity kernel for each eigenray provides both the localization and the signature of the target. Experimental results are shown in the presence of surface waves, and methodological issues are discussed for detection and localization.

A geoacoustic inversion scheme based on a double beamforming algorithm in shallow water is proposed and tested. Double beamforming allows identification of multi-reverberated eigenrays propagating between two vertical transducer arrays according to their emission and reception angles and arrival times. Analysis of eigenray intensities yields the bottom reflection coefficient as a function of angle of incidence. By fitting the experimental reflection coefficient with a theoretical prediction, values of the acoustic parameters of the waveguide bottom can be extracted. The procedure was initially tested in a small-scale tank experiment for a waveguide with a Plexiglas bottom. Inversion results for the speed of shear waves in Plexiglas are in good agreement with the table values. A similar analysis was applied to data collected during an at-sea experiment in shallow coastal waters of the Mediterranean. Bottom reflection coefficient was fitted with the theory in which bottom sediments are modeled as a multi-layered system. Retrieved bottom parameters are in quantitative agreement with those determined from a prior inversion scheme performed in the same area. The present study confirms the interest in processing source-receiver array data through the double beamforming algorithm, and indicates the potential for application of eigenray intensity analysis to geoacoustic inversion problems.

Detecting a target by measuring its forward scattered field is of interest for harbor surveillance because target strength levels are generally higher in the forward direction than in the backward direction for simple geometries. An acoustic barrier based on forward scattering was demonstrated in a nearly range-independent shallow water environment. The experimental location was characterized by high reverberation, low temporal signal coherence, and, as a result, few stable multipath arrivals due to the fluctuating sea surface. This high-frequency experiment utilized a vertical source array, broadcasting broadside pulses, and a vertical receiver array spanning the water column. The signal of interest was the aberration (in space and time) caused by the acoustic forward scattering field of crossing targets (2-m-long aluminum cylinder, 1-m-diameter steel sphere and pair of scuba tanks). Hence, the spatial and temporal coherence of the recorded acoustic signals was first investigated to assess the stability of the early acoustic arrivals in this rapidly fluctuating coastal environment. A principal component analysis of the stable portion of the recorded acoustic signals was then used to determine the crossing time of the target and to isolate some of its scattered wavefield components.

For acoustic propagation through a shallow ocean channel or waveguide, the coherence between different transmissions is controlled primarily by the roughness of the ocean surface and to a lesser degree by fluctuations in the volume. In this study, the coherent-to-incoherent intensity ratio (CTIR) is defined as a way to quantify the coherence between multipath transmissions and ocean surface rms wave height and wind speed. A theory that connects the CTIR and the coherent surface reflection coefficient is developed using both Kirchhoff and small-slope approximations as rough surface scattering models. The CTIRs have been evaluated over a period of several days using broad-band experimental results from shallow-water deployment of source and receiver arrays that span most of the water column. Estimates of wind speed and rms wave height obtained using these CTIR calculations are compared with environmental measurements to demonstrate the validity of the theory.

In recent years, the use of sensitivity kernels for tomographic purposes has been frequently discussed in the literature. Sensitivity kernels of different observables (e.g., amplitude, travel-time, and polarization for seismic waves) have been proposed, and relationships between adjoint formulation, time-reversal theory, and sensitivity kernels have been developed. In the present study, travel-time sensitivity kernels (TSKs) are derived for two source-receiver arrays in an acoustic waveguide. More precisely, the TSKs are combined with a double time-delay beam-forming algorithm performed on two source-receiver arrays to isolate and identify each eigenray of the multipath propagation between a source-receiver pair in the acoustic waveguide. A relationship is then obtained between TSKs and diffraction theory. It appears that the spatial shapes of TSKs are equivalent to the gradients of the combined direction patterns of the source and receiver arrays. In the finite-frequency regimes, the combination of TSKs and double beam-forming both simplifies the calculation of TSK and increases the domain of validity for ray theory in shallow-water ocean acoustic tomography.

Previous studies have shown that small changes can be monitored in a scattering medium by observing phase shifts in the coda. Passive monitoring of weak changes through ambient noise correlation has already been applied to seismology, acoustics, and engineering. Usually, this is done under the assumption that a properly reconstructed Green function (GF), as well as stable background noise sources, is necessary. In order to further develop this monitoring technique, a laboratory experiment was performed in the 2.5 MHz range in a gel with scattering inclusions, comparing an active (pulse-echo) form of monitoring to a passive (correlation) one. Present results show that temperature changes in the medium can be observed even if the GF of the medium is not reconstructed. Moreover, this article establishes that the GF reconstruction in the correlations is not a necessary condition: The only condition to monitoring with correlation (passive experiment) is the relative stability of the background noise structure.

Time-reversing the transfer function between a time-reversal mirror (TRM) and a distant probe source location generates an acoustic spatio-temporal focus at the location. It is shown that a TR focus behaves as a "virtual" source (in the far-field limit) in the outbound direction with respect to the TRM. By extension, a collection of TRM-to-probe source transfer functions constitutes a virtual source array (VSA) that can serve as a remote platform for active imaging methods such as beam-steering and other coherent wavefront techniques. As a demonstration, a set of a-priori sampled TRM-to-VSA transfer functions are steered to coherently focus at a selected location beyond the VSA for which the transfer function is not known a-priori. In this case the VSA acts as a lens that refocuses the TRM field to the target location. Under proper conditions, the resolution is comparable to that of standard TR. While the specific application of active focusing is presented as a validation of the concept, the relationship between coherent focusing and the transfer function implies that the virtual array concept may find use in a range of imaging methods, both active and passive. Possible applications are discussed, and simulation and experimental results are presented.

Acoustic remote sensing of the oceans requires a detailed understanding of the acoustic forward problem. The results of a shallow-water transmission experiment between a vertical array of sources and a vertical array of receivers are reported. The source array is used to provide additional degrees of freedom to isolate and track raylike arrivals by beamforming over both source and receiver arrays. The coordinated source-receiver array processing procedure is presented and its effectiveness in an example of tracking raylike arrivals in a fluctuating ocean environment is shown. Many of these arrivals can be tracked over an hour or more and show slowly varying amplitude and phase. The use of a double-beamforming algorithm lays the foundation for shallow-water acoustic remote sensing using travel time and source and receive angles of selected eigenrays.

Using only ocean ambient noise recordings it is possible to approximate the local time domain Green's function (TDGF) and extract the time delays associated with different ray path between the elements of a bottom hydrophone array. Comparing the strength of the noise correlation function taken over increasing time windows with residual fluctuations points to an optimum time window to use in the noise correlation function. Through comparison with computer simulations the resulting time series is shown to accurately approximate noise responses in the environment. Analysis of the TDGF gives accurate environmental detail, specifically the critical angle at the water-sediment interface.

Green's functions can be retrieved between receivers from the correlation of ambient seismic noise or with an appropriate set of randomly distributed sources. This principle is demonstrated in small-scale geophysics using noise sources generated by human steps during a 10-min walk in the alignment of a 14-m-long accelerometer line array. The time-domain correlation of the records yields two surface wave modes extracted from the Green's function between each pair of accelerometers. A frequency-wave-number Fourier analysis yields each mode contribution and their dispersion curve. These dispersion curves are then inverted to provide the one-dimensional shear velocity of the near surface.

The time-domain cross correlation of incoherent and random noise recorded by a series of passive sensors contains the impulse response of the medium between these sensors. By using noise generated by a can of compressed air sprayed on the surface of a plexiglass plate, we are able to reconstruct not only the time of flight but the whole wave forms between the sensors. From the reconstruction of the direct A(0) and S(0) waves, we derive the dispersion curves of the flexural waves, thus estimating the mechanical properties of the material without a conventional electromechanical source. The dense array of receivers employed here allow a precise frequency-wavenumber study of flexural waves, along with a thorough evaluation of the rate of convergence of the correlation with respect to the record length, the frequency, and the distance between the receivers. The reconstruction of the actual amplitude and attenuation of the impulse response is also addressed in this paper.

A Doppler-based method for using a moving narrow-band source to extract the modes of acoustic propagation in a range-independent shallow ocean waveguide over a partial-water-column spanning vertical line array (VLA) is introduced. Because the modal components propagate at distinct frequencies in the case of uniform radial source motion, the modal depth functions may be isolated and extracted from a frequency decomposition of the field. Because Doppler broadening due to radial source accelerations degrades the effectiveness of the extraction method, the method incorporates a technique to compensate for Doppler broadening. As the basis for the compensation technique, a theory is introduced for describing the VLA field from an accelerating cw source. By connecting the range of the source at the time a signal feature is emitted (the retarded time) to the range of the source at the time the signal feature arrives at the receiver (the contemporary time), the theory incorporates the Doppler effects associated with the finite group velocities of the modal components. The mode extraction method and compensation technique are applied to simulation and ocean data.

The spatial resolution of the focused field of a classical time-reversal mirror has a wavelength-order lambda diffraction limit. Previously reported results for subwavelength focus require either the full knowledge of the original source or the evanescent waves in the near field. Here it is shown that subwavelength focusing can be achieved without a priori knowledge of the original probe source. If the field is recorded at a few wavelengths away from the probe source, where the amplitude of the near field is too low for subwavelength focusing, it is shown that the low amplitude near field can be amplified and the spatial resolution improved, using the near-field time reversal (NTR) procedure introduced here. The NTR is performed from the phase of the spatial spectrum of the field recorded on an array around the original probe source using an analytical continuation for the amplitude of the spatial spectrum. Following theory, lambda/20 resolution is experimentally demonstrated with audible acoustic wavefields in the air.

It has recently been shown [Roux et al., "Extracting coherent wave fronts from acoustic ambient noise in the ocean," J. Acoust. Soc. Am. 116, 1995-2003 (2004)] that the time-averaged correlation of ocean noise between two points yields a deterministic waveguide arrival structure embedded in the time-domain Green's function. By performing a set of these correlations between a vertical receive array and a single receiver, transfer functions necessary for time reversal can be extracted from ocean noise. Theory and simulation demonstrate this process and data of opportunity from the North Pacific Acoustic Laboratory experiment confirm the expected performance of a noise-based time reversal mirror.

It has been experimentally demonstrated that the Green's function between two points could be recovered using the cross-correlation function of the ambient noise measured at these two points. This paper investigates the theory behind this result in the simple case of a homogeneous medium with attenuation.

Coherent deterministic arrival times can be extracted from the derivative of the time-averaged ambient noise cross-correlation function between two receivers. These coherent arrival times are related to those of the time-domain Green's function between these two receivers and have been observed experimentally in various environments and frequency range of interest (e.g., in ultrasonics, seismology, or underwater acoustics). This nonintuitive result can be demonstrated based on a simple time-domain image formulation of the noise cross-correlation function, for a uniform distribution of noise sources in a Pekeris waveguide. This image formulation determines the influence of the noise-source distribution (in range and depth) as well as the dependence on the receiver bandwidth for the arrival-time structure of the derivative of the cross-correlation function. These results are compared with previously derived formulations of the ambient noise cross-correlation function. Practical implications of these results for sea experiments are also discussed.

This work provides a new way to measure the Green's function between two points in an acoustic channel without emitting a pulse by any of the two points. The Green's function between A and B is obtained from a set of secondary sources in the guide by averaging either the correlation or the convolution of the signals received in A and B. A theoretical approach based on mode propagation in a monochromatic regime is presented. Results are then extended to the time domain. Estimation of the Green's function is performed numerically in a range-independent and a range-dependent environment. Application to discreet acoustic communications is discussed.

A new method was developed to acoustically measure the density and total scattering cross-section (sigma(t)) or total target strength [TTS = 10log10(sigma(t)/4pi)] of objects in motion in a highly reflective cavity [J. De Rosny and P. Roux, J. Acoust. Soc. Am. 109, 2587-2597 (2001)]. From an ensemble of pulse-echo recordings, the average contribution of the scatterer(s) to the reverberation within the cavity provides a measurement of the scattering mean free path. The latter was shown through theory and experiment to be proportional to the volume of the cavity and inversely proportional the product of the mean sigma(t) and number of scatterers. Here, the TTS measurement uncertainty is characterized using standard metal spheres as references. Theoretical TTS was calculated for multiple copper and tungsten carbide standard spheres (Cu: 60.0 30.05 and 23 mm and WC: 38.1 and 33.4 mm diameters, respectively), using well-described theory for scattering from elastic spheres and the optical theorem. Measurements of TTS were made over a wide bandwidth (30-120 kHz) and compared to their theoretical values. Measurements were made in a corrugated, cylindrical, galvanized-steel tank with 25 or 50 l of fresh water at a temperature of 21 +/- 1 degrees C. The results indicate the method can provide TTS measurements that are accurate to at least 0.4 dB with an average precision of +/-0.7 dB (95% confidence interval). Discussed are the requisite cavity volumes and signal-to-noise ratios for quality measurements of TTS, tank volume, and/or numerical abundance of mobile targets. Also discussed are multiple potential applications of this technique in bioacoustical oceanography.