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ABSTRACT: Understanding the physics governing the interaction of sound with targets in an underwater environment is essential to improving upon existing target detection and classification algorithms. Simple models are viable tools for meaningful interpretation of scattering results. To illustrate this, two modeling techniques are employed to study the acoustic scattering from a water-filled cylindrical shell. The first model is a hybrid 2-D/3-D finite element (FE) model, whereby the scattering in close proximity to the target is handled via a 2-D axisymmetric FE model, and the subsequent 3-D propagation to the farfield is determined via a Helmholtz integral. This model is characterized by the decomposition of the fluid pressure and its derivative in a series of azimuthal Fourier modes, a technique that has previously facilitated mode identification [Espana et al., J. Acoust. Soc. Am. 130, 2332 (2011)]. The second is an analytical solution for an infinitely long cylindrical shell, coupled with a simple approximation that converts the results to an analogous finite length form function. These two model results, when examined together on a mode-by-mode basis, offer visualization of the mode dynamics and the ability to distinguish the different physics driving the target response (i.e., structural modes versus water-waveguide modes). [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2013; 133(5):3537. · 1.55 Impact Factor
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ABSTRACT: A basic research reverberation experiment, supported by the US Office of Naval Research, is planned for 2013. Measurement issues that arise when planning such an experiment are discussed. The fundamental requirement for this basic research experiment is that the environment is characterized in sufficient detail to allow accurate numerical modeling of the acoustical results based on the environmental description. The main goal is to measure mid-frequency shallow water reverberation with full companion environmental measurements so that model/data can be compared without ambiguity. Included in the goal is to make statistical estimates of the uncertainties associated with all the environmental conditions. The frequency range of interest is 1-10 kHz with emphasis at 3 kHz. A pilot field experiment was conducted off the coast of Panama City, Florida. Data from the pilot experiment will be discussed in light of the forthcoming main experiment, including simulations on both propagation/forward-scatter and reverberation for given noise background on both vertical and horizontal arrays which will be deployed in the main experiment.
The Journal of the Acoustical Society of America 09/2012; 132(3):1908. · 1.55 Impact Factor
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ABSTRACT: A fast ray model for propagation in a homogenous water column tracks time-of-flight wavepackets from sources to targets and then to receivers. The model uses image sources and receivers to account for interactions with the water column boundaries, where the layer of water lies between an upper semi-infinite halfspace of air and a lower semi-infinite halfspace of a homogenous sediment. The sediment can be either an attenuating fluid with a frequency-independent loss parameter or a fluid consistent with an effective density fluid model (i.e., a fluid limit to Biot's model for a fluid-saturated poroelastic medium). The target scattering process is computed via convolution of a free-field scattering form function with the spectrum of an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicate target can be reduced to a scattering form function, and this form function then can be used within model via interpolation. The fast ray-based model permits the generation of sets of realistic pings suitable for synthetic aperture sonar processing for proud and partially buried target. Results from simulations are compared to measurements where the targets are an inert unexploded ordnance and aluminum cylinder. [Research supported by SERDP and ONR.].
The Journal of the Acoustical Society of America 09/2012; 132(3):1909. · 1.55 Impact Factor
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ABSTRACT: The scattering from elastic targets in the low- to mid-frequency regime is affected by the environment surrounding the target. For axisymmetric targets with the axis of symmetry parallel to the water-sediment boundary, previous work has dealt with the change in the target strength as a function of frequency and aspect angle in relation to the burial depth in the sediment. The present work deals with the extension of a finite element model, based on the decomposition of the acoustic and elastic fields into azimuthal Fourier modes, to the case of a target buried at a tilt angle. The interaction between the target and the sediment is represented by the model up to the first order of the scattering series, which means that the scattering of the incident field and of the target reflected field is taken into account, but the rescattering of the boundary reflected echo from the target is neglected. Model results up to 30 kHz are compared to experimental data for a 2 foot long aluminum cylinder of 1 foot diameter buried in sand at a tilt angle.
The Journal of the Acoustical Society of America 04/2012; 131(4):3393. · 1.55 Impact Factor
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ABSTRACT: Understanding how sound couples to, and radiates from targets, aids in finding frequencies and angles advantageous for target detection and classification. Finite element models, coupled with physical acoustics models, have been used to identify resonant modes of truncated, solid cylinders [D. B. Thiessen et al., J. Acoust. Soc. Am. 129, 2686 (2011)]. This technique is extended here to an aluminum pipe. Scattering from the pipe in the freefield is studied with a finite element model based on the decomposition of the elastic displacements and fluid pressure in a series of azimuthal Fourier modes. To facilitate elastic mode identification, the calculations for the scattered pressure from a nearly rigid pipe of identical size are subtracted coherently from the elastic model results. Plotting these "subtracted" pressure amplitudes and particle displacements on the wet surface of the pipe allows for visualization of the elastic modes. Subsequently, the model is extended for the pipe in contact with sand, via a superposition of freefield results along with the water/sand reflection coefficient. The method is used to study the modal scattering in the presence of the interface. Where applicable, finite element results are validated by predictions from ray theory and experimental results. [Work supported by ONR.].
The Journal of the Acoustical Society of America 10/2011; 130(4):2332. · 1.55 Impact Factor
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ABSTRACT: Previous work has illustrated the potential benefit of using low frequency sound as a means for detecting and classifying objects in contact with a sand sediment. In these situations, the wavelength of sound is on the order of the object dimensions, thus coupling to the objects resonant modes. This leads to an acoustic signature rich in physical phenomena unique to the object shape and elastic properties. Hybrid 2-D/3-D finite element models have been developed for unexploded ordnance in contact with a sand sediment. Previous work has demonstrated these models are in good agreement with data collected during experiments conducted in a test pond in 2010 [A. L. España et al., J. Acoust. Soc. Am. 129, 2685 (2011)]. In this paper, the finite element models are used as a means for mode identification and physical interpretation. These modes are visualized through plots of the pressure amplitudes and displacements along the UXO exterior and are explained using insights derived from physical acoustics. Finally, a full 3-D finite element model was developed to investigate the changes to the acoustic response in situations where the symmetry of the problem is broken and the hybrid 2-D/3-D method is no longer viable. [Research supported by ONR and SERDP.].
The Journal of the Acoustical Society of America 10/2011; 130(4):2330. · 1.55 Impact Factor
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ABSTRACT: The 3-D scattering from axisymmetric objects illuminated by nonaxisymmetric incident acoustic fields can be computed efficiently using finite element models, in which the field variables are decomposed via azimuthal Fourier series expansions. The number of azimuthal modes needed is determined by the convergence of the decomposition of the incident field. For simple cases, the field decomposition can be described analytically. For more complex incident fields, however, a closed form azimuthal Fourier series representation of the incident field is often not possible. A pre-processing step is presented, in which the incident field is decomposed numerically at the Gauss points on the wet surface of the target. This approach makes it possible to treat general cases, such as scattering from objects included inside heterogeneous media, at shallow grazing angle, when the symmetry axis of the object is not perpendicular to the interface between the two media. Other applications include decomposing the boundary scattered field re-incident on an object. The model is applied to scattering from a 2:1 aluminum cylinder, included in a fast fluid medium above a slow fluid medium, with the source in the slow medium, and the results are compared to ultrasonic tank measurements. [Work supported by ONR/ONRG.].
The Journal of the Acoustical Society of America 10/2011; 130(4):2330. · 1.55 Impact Factor
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ABSTRACT: Backscattering of sound by a solid aluminum cylinder was measured in the free field and with the cylinder near a flat surface. The target was suspended just below the surface of a water tank to simulate some aspects of backscattering when resting on the seabed. Measurements were compared with predictions made by an approximate hybrid approach based on multiple two-dimensional finite element calculations and the use of images. Many of the spectral features present in the tank data were present in the model. Comparing numerical model predictions with experimental data serves to build credibility for the modeling approach and can assist in developing insight into the underlying physical processes.
The Journal of the Acoustical Society of America 08/2011; 130(2):669-72. · 1.55 Impact Factor
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ABSTRACT: Sediment acoustics experiments in 1999 (SAX99) and 2004 (SAX04) were conducted in shallow water off the Florida Panhandle near Fort Walton Beach, Florida. Measurements were made of the frequency dependence of sound speed and attenuation in sandy sediments, supplemented by detailed environmental characterization. Following SAX99 laboratory measurements were conducted to further investigate the physical mechanisms contributing to the frequency dependences observed. An overview will be given of these sound speed and attenuation results, and the implications for sediment acoustics modeling will be discussed. [Work supported by ONR.].
The Journal of the Acoustical Society of America 10/2010; 128(4):2294. · 1.55 Impact Factor
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ABSTRACT: Detection and classification of objects on or imbedded in the ocean sediment are difficult problems due to the complexity of the sediment. The measured scattering will include interactions of the incident sound with the sediment interface as well as the possibility for multiple interactions between the target and sediment. Recent experiments conducted in the test pond at the Naval Surface Warfare Center, Panama City Division, investigated the impact the environment has on the measured acoustic response from various targets. A particular subset of these experiments is presented, focusing on the acoustic scattering from a water-filled aluminum pipe having length-to-diameter ratio equal to 2. Initial measurements examined the scattering from the pipe in a proud configuration on the flattened sand sediment. Subsequent measurements placed the pipe in contact with interfaces with known reflection coefficients, including suspending the pipe just below the air∕water interface and placing it in a proud configuration on a flat acrylic panel on the sand sediment. Acoustic templates depicting the absolute target strength as a function of azimuthal angle over the frequency range 1-30 kHz are compared for the three different configurations. [Work supported by the Office of Naval Research.].
The Journal of the Acoustical Society of America 10/2010; 128(4):2461. · 1.55 Impact Factor
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ABSTRACT: The aspect dependence of the spectral response of simple objects is sometimes examined in scattering research. The use of reversible imaging algorithms for the approximate de-coupling of the impulse response and associated spectral-responses of two adjacently located targets with overlapping signals was previously demonstrated [T. M. Marston, P. L. Marston, and K. L. Williams, J. Acoust. Soc. Am. 127, 1749 (2010)]. The current research concerns how resonant features of targets can be enhanced by using related imaging and filtering algorithms. When resonant targets are imaged using synthetic aperture techniques, the resonant portion is often distinct enough from specular or edge-diffraction portions so that it is possible to design spatial filters that isolate the resonant from the non-resonant portions of the signal in the image domain. The filtered image is then returned to the original domain, and frequency response is plotted with the non-resonant portions of the data filtered out. The resulting plots show enhanced resonant features. Demonstrations using line-scan SAS and circular SAS data will be shown. [Work supported by ONR.].
The Journal of the Acoustical Society of America 10/2010; 128(4):2461. · 1.55 Impact Factor
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ABSTRACT: Reverberation in shallow waters can be generally considered as a combination of two-way forward scatter and a single backscatter. With increasing range, reverberation consists of backscattered signals at ever smaller grazing angles. First-order perturbation theory is a suitable choice to model boundary roughness scatter if the rms roughness height is small compared to the acoustic wavelength. With decreasing grazing angle, the first-order theory works even better. To apply the first-order approximation, a zeroth-order problem is assumed known, which is often conveniently chosen to be the case where the boundary is flat. However, if the boundary has a low-wavenumber component, the flat boundary assumption for the zeroth-order field can result in modeling error. To quantitatively assess such error, perturbation results are compared to exact numerical solutions using COMSOL. It is found that such errors are found when grazing angles are on the order of a few degrees, because at such small grazing angles the true local grazing angle is sensitive to variations of the local boundary slopes.
The Journal of the Acoustical Society of America 10/2010; 128(4):2326. · 1.55 Impact Factor
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ABSTRACT: Bubbles rising from the seabed evolve as they rise through the water column due to gas transfer and changes in hydrostatic pressure. Consequently, the bubble's acoustic response can change quite dramatically as it rises. For depth integrated measurements of target strength from these bubble clouds, depth dependent changes can confound acoustic inversions for bubble size distribution, void fraction, and the amount of bubbles (and hence gas) reaching the ocean surface. One approach to solving this problem is to implement a model for the evolution of the bubble as it rises through the surface and to then compute an effective, depth integrated scattering response for the bubble plume as a function of bubble size at the source. This effective scattering strength can then be used for inversions of frequency dependent, depth integrated target strength measurements to find the source distribution of bubbles. Such an inversion procedure has been implemented using an incoherent forward model, and tested for a steady stream of bubbles pumped through an orifice at a depth of 11 m. Broadband (1-30 kHz) target strength estimates were made at a distance of approximately 10 m. Inversions for the source distributions of bubbles are compared with estimates made directly with underwater video. Results of this acoustic inversion, and possible coherent bubble cloud effects, will be discussed.
The Journal of the Acoustical Society of America 10/2010; 128(4):2279. · 1.55 Impact Factor
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ABSTRACT: Understanding acoustic scattering from objects placed on the interface between two media requires incorporation of scattering off the interface. Here, this class of problems is studied in the particular context of a 61 cm long, 30.5 cm diameter solid aluminum cylinder placed on a flattened sand interface. Experimental results are presented for the monostatic scattering from this cylinder for azimuthal scattering angles from 0 degrees to 90 degrees and frequencies from 1 to 30 kHz. In addition, synthetic aperture sonar (SAS) processing is carried out. Next, details seen within these experimental results are explained using insight derived from physical acoustics. Subsequently, target strength results are compared to finite-element (FE) calculations. The simplest calculation assumes that the source and receiver are at infinity and uses the FE result for the cylinder in free space along with image cylinders for approximating the target/interface interaction. Then the effect of finite geometries and inclusion of a more complete Green's function for the target/interface interaction is examined. These first two calculations use the axial symmetry of the cylinder in carrying out the analysis. Finally, the results from a three dimensional FE analysis are presented and compared to both the experiment and the axially symmetric calculations.
The Journal of the Acoustical Society of America 06/2010; 127(6):3356-71. · 1.55 Impact Factor
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ABSTRACT: Pressure signals scattered from a target and received by a line array can be buried in noise having similar frequency content, such as reflections from nearby interfaces or objects adjacent to the target. In such circumstances, it may be difficult to isolate the desired signal through the use of standard linear time-invariant filtering techniques, because the frequency content of the signal and noise are identical. Supersonic line-scan holography, however, can be used in many such circumstances to achieve signal isolation by backpropagating the acoustic signal to a focal region, spatially and temporally filtering the signal around the focal region, and forward-propagating the isolated signal to the original array location. This technique was used to measure the bistatic angular scattering pattern of a small target driven at resonance. This was done in a water tank using a hydrophone scanned along a line. A demonstration of how this method was used to isolate the signal from larger, overlapping reflections will be given. The method was also applied to backscattering measured with a scanned transducer and fixed proud cylinder resting on sediment. [Work supported by ONR.].
The Journal of the Acoustical Society of America 03/2010; 127(3):1749. · 1.55 Impact Factor
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ABSTRACT: During the Shallow Water 2006 experiment, simultaneous measurements were made of the sound speed structure associated with nonlinear internal waves and acoustic propagation at frequencies of 2-10 kHz over a 1 km path. The internal waves were measured by a towed CTD chain in order to get high resolution. These measurements were coordinated so that the nonlinear waves can be interpolated onto the acoustic path, allowing predictions of their effects on the acoustics. An internal wave train was measured that passed the acoustic path on August 13. When the wave train was in between the sound source and receiver, distinctive arrival time oscillations on three acoustic paths were measured, which are all rays having an upper turning point. Using the CTD chain data, a deterministic explanation is given to the arrival time oscillations.
The Journal of the Acoustical Society of America 05/2009; 125(4):2512. · 1.55 Impact Factor
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ABSTRACT: A 2:1 aspect ratio solid aluminum cylinder is placed on the planar interface between two fluids (watersand and waterair), with a point source radiating at frequencies for which the acoustic wavelengths range from 0.5 to 12 cylinder lengths. The distance between the source and the target is approximately 100 wavelengths, relative to the center frequency, and a vertical receive array is placed near the source. The problem is studied using a finite-element target scattering model, which is capable of treating axially symmetric objects via the decomposition of the unknowns into a Fourier basis around the axis of symmetry. Since the axis of the cylinder is parallel to the planar interface, the overall problem is not axially symmetric. Nevertheless, an approximate solution is obtained, which takes into account the interface reflection of the incident field, as well as the first bounce of the scattered field (via the Helmholtz-Kirchhoff integral with layered medium Greens functions). Small variations in the source and receiver positions cause large changes in the received signal, which can be explained by a Lloyd mirror-like interference resulting from the coherent addition of the point-sources and image point-sources with which the incident field and the scattered field can be described.
The Journal of the Acoustical Society of America 05/2009; 125(4):2701. · 1.55 Impact Factor
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ABSTRACT: Cylindrical targets of finite length can be used as reference targets not only for calibrating an existing SAS system, but more importantly, for testing new classification and identification algorithms. With only a few well-characterized measurements available for proud and buried cylindrical targets, numerical simulations of the acoustic response of these targets offer the potential to realize an unlimited set of target orientations with respect to the source and receiver locations. This paper discusses recent progress with our acoustic scattering models and the generation of a set of pings suitable for SAS processing. SAS images generated from numerical simulations are compared to SAS images generated from data collected during the recent pond experiment 2009 (Pondex09) at NSWC-PCD's facility 383. The target is a solid aluminum cylinder with a 0.3 m diam and length of 0.61 m. [Work sponsored by ONR and SERDP.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2734. · 1.55 Impact Factor
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ABSTRACT: Recently, Pierce and Carey [J. Acoust. Soc. Am. 124, EL308-EL312 (2008)] presented a low frequency analysis of sound propagation in sand/silty sediments. Here, equivalent expressions are presented using a low frequency expansion of an unconsolidated version of Biot porous medium theory. The resulting expression for attenuation allows identification of the non-dimensional parameter beta in the Pierce/Carey result in terms of physical parameters. The agreement of these two derivations motivates further analyses. The results imply that porous media propagation models that treat the medium's inertia via a single component approximation disregard a fundamental physical effect resulting from the relative inertia of the grains and fluid and are thus incomplete.
The Journal of the Acoustical Society of America 05/2009; 125(4):EL164-70. · 1.55 Impact Factor
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ABSTRACT: When using sound to detect objects buried beneath the seafloor, situations arise in which the incident acoustic wave has a significant evanescent component. This situation has been simulated in tank experiments [Osterhoudt et al., IEEE J. Ocean. Eng. (in press)] and the simulation was used to investigate scattering mechanisms. In prior work, the backscattering of evanescent and ordinary-propagating waves from small solid aluminum cylinders was studied [Espana and Marston, J. Acoust. Soc. Am. 124, 2584 (2008)]. It was determined that a strong low frequency mode could be excited when the cylinder was highly tilted in a horizontal plane. With increasing simulated burial depth, the spatial decay rate of the backscattering was enhanced compared to the spatial decay rate of the evanescent soundfield. This resonance has since been driven by evanescent waves when the cylinder is highly tilted in a vertical plane. This alternate method of excitation also showed an enhanced spatial decay rate with increasing simulated burial depth. FEM simulation of the free-field response of the cylinder reveals that this mode causes a rocking motion of each end of the cylinder and it is plausible that evanescent waves could also excite this type of response. [Work supported by ONR.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2733. · 1.55 Impact Factor
