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A model-based analysis of sound transmission in a deep ice-covered Arctic ocean recorded during the Ice Experiment 2014 is presented. A source of opportunity transmitted mid-frequency (3500 Hz) 5 s duration continuous wave pulses. The source and receiver were omnidirectional, located under ice at a ∼30 m depth at a ∼719 m distance from each other. Recorded acoustic intensity time series showed a clear direct blast signal followed by an about 30 s duration reverberation coda. The model considers several types of arrivals contributing to the received signal at different time intervals. The direct signal, corresponding to a short-range nearly horizontal propagation, is strongly affected by the presence of a weak near-surface (within 50 m depth) acoustic channel. Reverberation coda that follows the direct signal corresponds to medium-range bottom- and ice-bounced arrivals from steep angles which are controlled by reflectivity and scattering strengths of ice and bottom, their physical properties, and acoustical parameters.

Two experiments on midfrequency acoustic transmission under ice in the Beaufort Sea are discussed. The APL-ICEX14 measurements were made using 3500 Hz 5s-long CW pulses for fixed geometry of source and receiver at ∼30 m depths and ∼720 m range. The NPS-ICEX16 experiment was performed using CW pulses and LFM sweeps from two mobile 950–3000 Hz transmitters and five spatially separated hydrophones at different combinations of source-receiver ranges (0.3–10km) and depths (45–183m). For analysis of recorded timeseries, a modeling approach is suggested that considers several types of arrivals contributing to the received signal at different time intervals. The direct arrivals corresponding to nearly horizontal propagation are described using a hybrid PE-&ray-based approach, which employs a fast PE code for propagation in a stratified ocean with flat ice-free surface to account for effects of structure and dynamics of the arctic surface duct and then adds a field of an image source whose strength is defined by a complex reflection loss factor of the ice cover at small grazing angles. The direct signal is followed by reverberation coda that is modeled by bottom- and ice-bounced arrivals with corresponding reflectivity and scattering strengths at steep angles. Potential applications to remote sensing in ice-covered environments are discussed. [Work supported by ONR.]

This research utilizes linear and nonlinear ultrasonic techniques to establish a linkage between microstructure and macroscale mechanical properties of additively manufactured (AM) stainless steel 316L samples. The specimens are manufactured using two methods: laser-powder bed fusion and traditional wrought manufacturing. Using the nonlinear ultrasonic method of second harmonic generation, the acoustic nonlinearity parameter is estimated in samples with different heat treatment levels intended to alter microstructural and mechanical properties. Linear ultrasonic parameters including wave speed and resonant frequency are additionally measured. Mechanical properties are obtained through tensile testing of coupons corresponding to the test samples. Microstructural information for the samples is obtained using electron backscatter diffraction to help elucidate the relationships between microstructure, mechanical properties, and ultrasonic response. Results indicate correlations between the nonlinearity parameter and both ultimate tensile strength and yield strength, where nonlinearity generally decreases as sample strength increases, particularly in the AM samples. We hypothesize that microstructural evolution of grain characteristics across different heat treatments influences trends in measured nonlinearity, as well as substructures at smaller scales such as dislocations. These results show promising evidence for the feasibility of AM parts qualification using nondestructive nonlinear ultrasonic testing.

Mid-frequency (3.5 kHz) pulses (5 pings, 5 s-long, one per minute) were radiated by an omnidirectional source of opportunity in a deep (3 km) Arctic ocean at 30m depth under ice during the ICEX14 experiment, ALAS component [Williams et al., IEEE-JOE 43, 145–159 (2018)]. Data analysis and modeling results are presented for normalized (re source level) acoustic intensityreceived by an omnidirectional hydrophone located at a 719 m distance from and the same depth as the source. Received time series showed a clear direct blast signal followed by an about 30 s-long reverberation coda. Modeling of the under-ice propagation shows that magnitude of the direct signal is strongly affected by presence of a weak near-surface (within 50 m depths) acoustic channel and by reflectivity of the ice, controlled by its physical properties and acoustical parameters, particularly the ice layer thickness. Analysis of the reverberation coda shows effects of reflections and scattering from rough and/or heterogeneous bottom, as well as reflectivity of the ice layer. Some other effects that may be caused by variability of the sound speed profile and absorption in water are considered as well. Possibilities of inversions for water column, bottom and ice parameters based on this and similar experiments are discussed. [Work supported by ONR.]

The backscattering spectrum versus azimuthal angle, also called the "acoustic color" or "acoustic template," of solid cylinders located in the free water column have been previously studied. For cylinders lying proud on horizontal sand sediment, there has been progress in understanding the backscattering spectrum as a function of grazing angle and the viewing angle relative to the cylinder's axis. Significant changes in the proud backscattering spectrum versus the freefield case are associated with the interference of several multipaths involving the target and the surface. If the cylinder's axis has a vertical tilt such that one end is partially buried in the sand, the multipath structure is changed, thus modifying the resulting spectrum. Some of the changes in the template can be approximately modeled using a combination of geometrical and physical acoustics. The resulting analysis gives a simple approximation relating certain changes in the template with the vertical tilt of the cylinder. This includes a splitting in the azimuthal angle at which broadside multipath features are observed. A similar approximation also applies to a metallic cylinder adjacent to a flat free surface and was confirmed in tank experiments.

Previously, a combined finite element/physical acoustics model for proud targets [K. L. Williams, S. G. Kargl, E. I. Thorsos, D. S. Burnett, J. L. Lopes, M. Zampolli, and P. L. Marston, J. Acoust. Soc. Am. 127, 3356-3371 (2010)] was compared to both higher fidelity finite element models and to experimental data for a proud 2:1 aluminum cylinder. Here that expression is generalized to address the case of a target buried in a layered media. The result is compared to data acquired for the same 2:1 cylinder but half buried in a mud layer that covers the sand sediment (considered here as infinite in extent below the mud layer). The generalized expression reduces to both the previous proud result and to the result for a target buried in an infinite medium under the appropriate limiting conditions. The model/data comparisons shown include both the previous proud model and data results along with the ones for the half buried cylinder. The comparison quantifies the reduction in target strength as a function of frequency in the half buried case relative to the proud case.

Understanding the physics governing the interaction of sound with targets in an underwater environment is essential to improving existing target detection and classification algorithms. To illustrate techniques for identifying the key physics, an examination is made of the acoustic scattering from a water-filled cylindrical shell. Experiments were conducted that measured the acoustic scattering from a water-filled cylindrical shell in the free field, as well as proud on a sand-water interface. Two modeling techniques are employed to examine these acoustic scattering measurements. The first 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 far field 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. 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. Examining these model results on a mode-by-mode basis offers easy visualization of the mode dynamics and helps distinguish the different physics driving the target response.

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.

It has been shown that when studying the acoustic scattering from a cylinder near a flat interface, there exists multiple paths by which sound can travel to, and subsequently scatter from, the cylinder. Specifically, sound can couple into a number of surface elastic waves (SEW), some of which give rise to an observed enhancement to the backscattering. Previously, the coupling conditions for these mechanisms have been derived, which are dependent on the relative angles between the target, the source/receiver and the interface [Plotnick, et al., JASA 140(3), 2016]. Here, a non-singular Helmholtz Kirchhoff Integral method has been implemented in a hybrid finite element/propagation model in order to predict the first and second order scattering mechanisms that exist when the cylinder is near an air-water interface, with its axis oblique with respect to the interface. This model, combined with the coupling conditions for SEWs, is used to dissect tank measurements that previously were only partially understood. Various multiple scattering mechanisms are identified and discussed. [Work supported by the Office of Naval Research.]It has been shown that when studying the acoustic scattering from a cylinder near a flat interface, there exists multiple paths by which sound can travel to, and subsequently scatter from, the cylinder. Specifically, sound can couple into a number of surface elastic waves (SEW), some of which give rise to an observed enhancement to the backscattering. Previously, the coupling conditions for these mechanisms have been derived, which are dependent on the relative angles between the target, the source/receiver and the interface [Plotnick, et al., JASA 140(3), 2016]. Here, a non-singular Helmholtz Kirchhoff Integral method has been implemented in a hybrid finite element/propagation model in order to predict the first and second order scattering mechanisms that exist when the cylinder is near an air-water interface, with its axis oblique with respect to the interface. This model, combined with the coupling conditions for SEWs, is used to dissect tank measurements that previously were only partially understood....

Under the “all models are wrong but some are useful” philosophy, one may at times allow some abuse of the basic physics if the predictions are accurate. However, few would start with a flawed model when they need not. We start with a thought experiment that indicates a fundamental requirement for capturing the correct dynamics in modeling porous granular media and show that this physics is captured in Biot and Pierce/Carey models as well as models that use them as a starting point. Ocean and laboratory propagation experiments that test the predictions of those models are then described. Since other modeling choices can, given free parameters, predict both granular sound speed and attenuation, we continue by looking at both back and forward scattering results. These results further mandate the need for a model that correctly treats the sediment dynamics. Finally, we present possible additional laboratory experiments that could alter some of the most important parameters inherent in Biot-like models and thu...

Broadband, multi-aspect time domain data of scattering from targets placed on sediment interfaces contains a wealth of information about the target and the modifications to the target scattering due to its location in the ocean environment. Different data products derived from the data can be used together to gain a better insight into the overall physical processes at play. We use a 2:1 aluminum cylinder and a 2:1 aluminum pipe to demonstrate the utility of different types of data products. Domains examined include imaging in the horizontal plane, frequency response as a function of look angle, and time domain response as a function of look angle. We then examine these same domains using relatively narrow windows of time with varying start times. This analysis allows visualization of the evolution of the physical mechanisms involved. [Sponsored by the Office of Naval Research and the Strategic Environmental Research and Development Program.]Broadband, multi-aspect time domain data of scattering from targets placed on sediment interfaces contains a wealth of information about the target and the modifications to the target scattering due to its location in the ocean environment. Different data products derived from the data can be used together to gain a better insight into the overall physical processes at play. We use a 2:1 aluminum cylinder and a 2:1 aluminum pipe to demonstrate the utility of different types of data products. Domains examined include imaging in the horizontal plane, frequency response as a function of look angle, and time domain response as a function of look angle. We then examine these same domains using relatively narrow windows of time with varying start times. This analysis allows visualization of the evolution of the physical mechanisms involved. [Sponsored by the Office of Naval Research and the Strategic Environmental Research and Development Program.]

The scintillation index and the intensity cumulative distribution function of mid-frequency (2-10 kHz) sound propagation are presented at ranges of 1-9 km in a shallow water channel. The fluctuations are due to water column sound speed variability. It is found that intensity is only correlated over a narrow frequency band (50-200 Hz) and the bandwidth is independent of center frequency and range. Furthermore, the intensity probability distribution peaks at zero for all frequencies, and follows an exponential distribution at small values.

Weekly sound surveys (n = 63) were collected, using 5 s sampling intervals, for two modern neonatal intensive care units (NICUs). Median weekly equivalent sound pressure levels (LEQ) for NICU A ranged from 61 to 63 dB (A weighted), depending on the level of care. NICU B L(EQ) measurements ranged from 55 to 60 dB (A weighted). NICU B was recently built with a focus on sound abatement, explaining much of the difference between the two NICUs. Sound levels exceeded 45 dB (A weighted), recommended by the American Academy of Pediatrics, more than 70% of the time for all levels of care. Hourly L(EQ)s below 50 dB (A weighted) and hourly L10s below 55 dB (A weighted), recommended by the Sound Study Group (SSG) of the National Resource Center, were also exceeded in more than 70% of recorded samples. A third SSG recommendation, that the 1 s L(MAX), should not exceed 70 dB (A weighted), was exceeded relatively infrequently (11% of the time). Peak impulse measurements exceeded 90 dB for 6.3% of 5 s samples recorded from NICU A and 2.8% of NICU B samples. Twenty-four h periodicities in sound levels as a function of regular staff activities were apparent, but short-term variability was considerable.

As part of a recent ocean sediment acoustics experiment, a number of independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range and were used to test both Biot-Stoll theory and Buckingham’s more recent grain-to-grain shearing model. While Biot theory was able to model the sound speed well, it was unable to predict the attenuation measured above 50kHz. This paper presents a series of measurements made in the laboratory on a simple glass-bead sediment. One goal of these measurements was to test the hypothesis that the attenuation measured at-sea was a result of scattering from shells within the sediment. The laboratory sediments used were saturated with fluids with different viscosities in order (assuming that Biot-Stoll theory is correct) to shift the dispersion into the frequency range of the measurement system. The measured attenuation in the glass-bead sediments exhibited the same frequency dependence as obser...

The parabolic wave equation (PE) code of Rosenberg [J. Acoust. Soc. Am. 105, 144–153 (1999)] is used as a benchmark to study acoustic propagation in an ocean waveguide with a rough air/water interface. The PE results allow a close examination of the ability of a ray code [i.e., Gaussian RAy Bundle (GRAB)] to accurately estimate coherent field propagation using a coherent reflection coefficient derived from scattering theory. Comparison with PE implies that the Beckmann–Spizzichino model, as given within the GRAB software package, does not give accurate predictions of the coherent field at long ranges. Three other coherent reflection coefficient approximations are tested: the perturbation, the small slope, and the Kirchhoff approximations. The small slope approximation is the most accurate of the models tested. However, the Kirchhoff approximation is perhaps accurate enough for some purposes and would be simpler to implement as a module within GRAB.

Seafloor roughness in shallow waters can be both spatially and temporally dynamic. While wind-driven waves and bottom currents can generate bedforms such as ripples on sand sediments, bottom-feeding fish tend to limit the lifetime of these anisotropic features. This greatly reduces the time during which high frequency sonars can exploit the ripples to coherently penetrate the sediment at subcritical angles. Sonar performance prediction in these environments therefore relies heavily on sediment transport models to generate roughness predictions that accurately reflect changes on these time scales. The competition between the hydrodynamics and biology of the environment can be further complicated by the natural introduction of materials which can locally suppress the bioturbation. Two examples are discussed here: The first comes from the Sediment Acoustics Experiment in 2004 where a storm deposited a layer of mud over some areas of seafloor protecting the bedforms from fish. The second is from the recent Ta...

A perturbation model is developed for sound scattering by a poroelastic seafloor having roughness small compared to the acoustic wavelength. The sediment is assumed to be homogeneous and isotropic with wave propagation described by Biot’s equations. When applied to sandy sediments, the model predicts backscattering levels that are substantially lower than those of a fluid model having the same roughness, density, sound speed, and attenuation.

For decades, broadband sonar has been a key tool in the underwater detection and classification of man-made targets. The DoD’s recent focus to clean up discarded WWII-era munitions has pushed a flurry of research and advancements in the areas of target scattering, environmental noise modeling, signal processing, and automatic target recognition (ATR). It is the interdisciplinary nature of this problem that makes it incredibly challenging, namely, the fact that the ability to classify an object is heavily influenced by the operational choices (grazing angle and vehicle altitude), surrounding environmental factors (target type, burial state, sea state, and bottom topography), and the processing tools (data products, feature-set, and ATR training/testing). This talk will outline the key steps underlying the process of trying to detect and classify underwater unexploded ordnance (UXO). It begins by pushing raw time domain data, which can be generated either experimentally or via models, through a set of signal processing tools to create data products (acoustic color and SAS images). These data products are fed through feature extraction processes, followed by ATR algorithms, in order to arrive at the final classification of an object. Examples to be presented include objects of interest to the remediation of underwater UXO. [Research supported by SERDP and ONR.].

A model is presented for bistatic scattering from ocean sediments. It treats scattering due to both roughness of the seabed and volume inhomogeneities within the sediment. Accordingly, the scattered intensity is assumed to be a sum of two terms, one proportional to the roughness-scattering cross section and the other proportional to the volume-scattering cross section. The model is tested against data acquired as part of the Coastal Benthic Boundary Layer (CBBL) research program. As part of that program, an autonomous, circularly scanning sonar system was deployed in well-characterized regions. This sonar operated at 40 kHz, had a 5° horizontal beam, and acquired backscattering data over a 50-m radius. During part of the deployment, it operated in conjunction with a mobile receiving array so as to acquire bistatic data. The experimental apparatus and procedures are presented, and results are compared with model predictions.

The monopole response of a gas-filled, spherical cavity in a sediment is investigated. The sediment is either a fluid, elastic solid, or saturated poroelastic medium. The present method entails the scattering of an incident displacement field that preferentially excites the monopole resonance of the cavity. The main result demonstrates that a gas-filled, spherical cavity in a saturated poroelastic medium can exhibit two distinct monopole resonances. These resonances arise from the two distinct longitudinal modes of propagation in saturated poroelastic medium as described by Biot’s theory.

A previous study of high-frequency acoustic backscattering data collected at Eckernfoerde Bay, Germany revealed that scattering is mainly due to methane gas bubbles buried about a meter beneath the seafloor [Tang et al., J. Acoust. Soc. Am. 96, 2930–2936 (1994)]. A backscattering model was developed [Tang, Geo-Marine Lett. 16, 161–169 (1996)] where the gas bubbles were approximated by oblate spheroids. In this paper, a bistatic scattering model is proposed as an extension of the previously developed backscattering model. In this model, gas bubbles are again assumed to be oblate spheroids with varying aspect ratios and a single-scattering approximation is used. The model is compared to bistatic data acquired in Eckernfoerde Bay, Germany. In particular, the azimuthal dependence of the bistatic scattering strength predicted by the model is tested against experimental data and it is found that both the model and the bistatic scattering strength data exhibit a mild azimuthal dependence. Best agreement between model and data requires a 35% reduction in areal bubble density relative to that used in the backscattering model/data comparison. Possible reasons for this are discussed including multiple scattering effects.

Previously, an effective density fluid model (EDFM) was developed by the author [J. Acoust. Soc. Am. 110, 2276–2281 (2001)] for unconsolidated granular sediments and applied to sand. The model is a simplification of the full Biot porous media model. Here two additional effects are added to the EDFM model: heat transfer between the liquid and solid at low frequencies and the granularity of the medium at high frequencies. The frequency range studied is 100 Hz–1 MHz. The analytical sound speed and attenuation expressions obtained have no free parameters. The resulting model is compared to ocean data.

Seafloor roughness can cause acoustic energy to propagate into sediments for ‘‘subcritical’’ incident grazing angles, that is, grazing angles smaller than the critical angle determined by the compressional wave speed. Such effects must be considered in interpreting experimental data on sound penetration into sediments. The sediment is modeled as a fluid supporting only compressional waves and a twofold theoretical approach is used. First, an exact integral equation is solved numerically to obtain the penetrating field in two dimensions. Physical insights are abstracted from these results which are also used to show that perturbation theory is valid for problems of interest. Using perturbation theory, a numerically tractable three‐dimensional model is compared to some of the experimental data of Chotiros and colleagues [N. P. Chotiros, J. Acoust. Soc. Am. 97, 199–214 (1995)]. The model and data match well subject to plausible assumptions about roughness statistics. [Work supported by ONR.]

In March 2014, an Arctic Line Arrays System (ALAS) was deployed as part of an experiment in the Beaufort Sea (approximate location 72.323 N, 146.490 W). The water depth was greater than 3500 m. The background noise levels in the frequency range from 1 Hz to 25 kHz were measured. The goal was to have a three-dimensional sparse array that would allow determination of the direction of sound sources out to hundreds of kilometers and both direction and range of sound sources out to 1–2 km from the center of the array. ALAS started recording data at 02:12 on March 10, 2014 (UTC). It recorded data nearly continuously at a sample rate of 50 kHz until 11:04 on March 24, 2014. Background noise spectral levels are presented for low and high floe-drift conditions. Tracking/characterization results for ice-cracking events (with signatures typically in the 10–2000-Hz band), including the initiation of an open lead within about 400 m of the array, and one seismic event (with a signature in the 1–40-Hz band) are presented. Results from simple modeling indicate that the signature of a lead formation may be a combination of both previously hypothesized physics and enhanced emissions near the ice plate critical frequency (where the flexural wave speed equals that of the water sound speed). For the seismic event, the T-wave arrival time results indicate that a significant amount of energy coupled to T-wave energy somewhere along the path between the earthquake and ALAS.

The fluid-structure interaction technique provides a paradigm for solving scattering from elastic targets embedded in a fluid by a combination of finite and boundary element methods. In this technique, the finite element method is used to compute the target’s elastic response and the boundary element method with the appropriate Green’s function is used to compute the field in the exterior medium. The two methods are coupled at the surface of the target by imposing the continuity of pressure and normal displacement. This results in a boundary element equation that can be used to compute the scattered field anywhere in the surrounding environment. This method reduces a finite element problem to a boundary element one with drastic reduction in the number of unknowns, which translates to a significant reduction in numerical cost. In this talk, the derivation of the technique will be outlined; the method will be applied to compute scattering from various targets, including unexploded ordnance (UXO) in complex ...

Synthetic aperture sonar (SAS) data were collected throughout May of 2014 during an experiment in St. Andrew's Bay, Panama City, FL. A brief overview is given of the SAS rail system and procedure used during the “BAYEX14” experiment. BAYEX14 was carried out in a shallow water, layered, environment, where water depth was approximately 8 m with a thin layer of mud (i.e., 0.15-0.30 m thickness) covering a sandy bottom. All targets settled into the mud and came to rest on the mud-sand interface. Scattering data from an aluminum finite cylinder, an aluminum pipe, and solid replicas of an 100-mm unexploded ordnance will be presented and compared to data taken previously in a sand-only environment and to model results. To model the scattered pressure, the ray paths are assumed to experience negligible refraction in passing through the mud layer to the mud-sand interface. However, the much higher attenuation in mud relative to water must be considered along the portion of the ray path in mud. For targets at long ...

Previously, a hybrid, axisymmetric, finite element (FE)—propagation model has shown utility in simulating the response of objects in contact with a flat, undisturbed sand/water interface [Espana et al., J. Acoust. Soc. Am. 136, 190-124, (2014)]. Here, we offer a three-way comparison between data, the hybrid FE/propagation model and a coupled finite element/boundary element model (Coupled FEBE) for the scattering from a replica of a 100-mm unexploded ordnance (UXO) proud, partially buried and fully buried in a sand sediment for geometries where reflection from and refraction into the sand dominate. This comparison offers a more in-depth analysis of the implications of various approximations of the hybrid model, such as the effect of improper fluid loading on the target surface in the partially buried situation. These results will be contrasted to the more complicated mud-over-sand layered environment of the BAYEX14 experiment. Preliminary model results will be compared to data, giving further insight into the dominant physical mechanisms affecting the UXO’s acoustic response in the more complicated layered environment. [Research sponsored by ONR and SERDP.]

High‐frequency (40‐kHz) bottom backscattering data obtained by the Benthic Acoustic Measurement System from two different shallow water locations were analyzed. One of the sites has a hard sandy bottom, and the other a soft silty bottom. Since the data were recorded on two slightly separated arrays, the differential phase technique was used to locate the scatterers. It is found that scattering at the sandy site originated from the seafloor interface, while the scattering at the silty bottom was caused by a scattering layer about 1 m beneath the seafloor (believed to be methane gas voids). The backscattering strengths at the silty bottom were about 3 dB higher than those at the sandy bottom. A simple sub‐bottom scattering model for the silty bottom is assumed and compared with the data. When the sound refraction and attenuation are taken into account, the model‐data fit yields reasonable results.

A high‐frequency forward scattering experiment carried out under multiyear ice is outlined. The experiment employed an omnidirectional transmitter emitting short tone bursts centered at either 25 or 45 kHz. The receiver was a four quadrant array, and the quadrants were recorded separately. Each quadrant had a broadbeam pattern, greater than 40 (70) deg at the 3‐dB down points in the horizontal (vertical) direction at 45 kHz. A large number of transmitter/receiver geometries were obtained by varying both the transmitter depth and horizontal range. Horizontal transmitter/receiver separations ranged from 400 to 2800 m. Model/experiment comparisons are shown for horizontal ranges less than 500 m, since these ranges can be most readily compared with our high‐frequency scattering models developed to date, which ignore refraction in the water column. Both individual realizations and ensemble experimental results are shown and compared with models. Specular point theory [D. E. Funk and K. L. Williams, J. Acoust. ...

The Kirchhoff theory is applied for the target strength of a rough, circular surface whose roughness is characterized by a two‐dimensional, isotropic power‐law wave number spectrum, W2(κ) = η2κ−p2. The reflection coefficient for ice and three nondimensional parameters are found to govern the target strength. These parameters are ζ=κ0a, η≡η2ap2−4, and p1=p2−1, where κ0 is the acoustic wave number, a is the radius of the surface, and p1 is the spectral exponent of the one‐dimensional power‐law wave‐number spectrum from which W2(κ) is derived. The general influence of ζ, p1, and η on the target strength is discussed. Calculations of average target strength of the ice/water interface of a submerged cylindrical block of ice are shown, which are then compared with individual realizations of measured target strengths of ice blocks for ζ between 25 and 100, corresponding to frequencies between 20 and 80 kHz for a=0.29 m. Data and theory show that the (smooth surface) form function for a finite surface does not de...

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° to 90° 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. © 2010 Acoustical Society of America.

Results of an in‐situ experiment to examine acoustic parameters and scattering properties of growing and newly submerged arctic sea ice are presented. The primary emphasis is on the acoustic properties of highly porous sea ice. High resolution (to about 1 cm) vertical profiles of the longitudinal wave speed in growing and submerged sea ice are derived from the experimental data. The results indicate a skeletal layer about 3 cm thick at the bottom of the growing ice. The time dependence of the vertical sound‐speed profile in a submerged block reveals a long time scale process of up to 80 h and a short process of a few hours. The former process is block warming, the latter is thought to be due to the disturbance of the hydrostatic equilibrium as the ice is submerged. Concurrently with the wave speed measurements, scattering from the underside of the ice was measured at several frequencies. A comparison with predictions based on the sound‐speed data demonstrate the ability to predict normal incidence reflect...

Elastic moduli of sea ice have been determined from high‐frequency velocity measurements on small samples extracted from the arctic pack ice. The experimental apparatus allowed quick in‐field observations of the velocities. The experimental procedure included measurement of the ice samples’ salinity and temperature; the relations derived here can be used to predict elastic moduli from knowledge of these more easily obtainable physical parameters. The elastic moduli determined are those appropriate for a specialized Biot model in which the ice is viewed as an isotropic material with closed pores. For this model, seven parameters are needed to predict acoustic velocities, all of which are easily interpreted physically and are given here in terms of their salinity and temperature dependencies.

A simulation technique using specular point theory (SPT) is developed to examine high‐frequency signals forward scattered from a rough surface. SPT results in treatment of the forward scattered signal as a summation of point sources, a result convenient for many simulators. This method is used to derive the spatial coherence for an infinite ensemble of signals forward scattered from random surfaces with Gaussian wave number spectra. The results are compared with those of previously published methods. An SPT single realization procedure is formulated and implemented for frequencies of 30 and 300 kHz using a Gaussian spectrum approximation to the random rough surface. Both the spatial coherence and the temporal structure of finite ensembles of forward scattered signals are compared for the Gaussian case. The relation between the results for infinite and finite ensembles is then examined, and the practical implications and limitations of SPT for simulating forward scattering are discussed. [Work supported by NAVSEA with technical management by NOSC and ONT with technical management by NCSC.]

An experimental/theoretical study of the scattering from large aspect ratio, finite elastic cylinders is presented. The study focuses on the excitation mechanisms of the axisymmetric resonances. It is found that significant coupling of the incident field to these resonances occurs in the region of geometric shadow at low kD/2 (D is the cylinder diameter). The implications of this coupling for the relative excitation strengths of the resonances and for the associated bistatic beam pattern are examined. It is argued on physical grounds that the results must apply to all large aspect ratio targets (both solids and shells).

Pulse transmission ultrasound was used to determine the longitudinal wave speed along the direction of trabecular alignment in 32 water-saturated anisotropic tibial bovine cancellous bone samples and in one cortical bone sample also from the bovine tibia. These results are compared to published ultrasound wave-speed data obtained from bovine femoral specimens. Nonlinear regression was used to fit Biot's theory to the data. The correlation coefficient for regression analysis between the experimental ultrasound velocities and the velocities predicted by Biot's theory was r = 0.78.

Recently, Pierce and Carey [JASA‐EL 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 (i.e., the effective density fluid model (EDFM)). The resulting expression for attenuation allows identfication of the nondimensional parameter in the Pierce/Carey result in terms of physical parameters. One of the important aspects of the Pierce/Carey derivation is that they show how to account for the inertial effects due to the relative grain/fluid density. The agreement between the Pierce/Carrey and EDFM low‐frequency attenuation expressions motivates further frequency and relative grain/fluid density analyses of attenuation and sound speed. The general inference of these analyses is that any model that ignores inertial effects due to differences in grain and fluid density is missing a key piece of physics that makes it incomplete. Wood’s equation is one such model and since the Buckingham model uses Wood’s equation, it is another example. The point being that the use of the mass density to handle inertia in a porous medium is a low‐frequency approximation. [Work supported by the Office of Naval Research.]

Preliminary results are presented from an analysis of mid- frequency acoustic transmission data collected at range 550 m during the Shallow Water 2006 Experiment. The acoustic data were collected on a ver- tical array immediately before, during, and after the passage of a nonlinear internal wave on 18 August, 2006. Using oceanographic data collected at a nearby location, a plane-wave model for the nonlinear internal wave's posi- tion as a function of time is developed. Experimental results show a new acoustic path is generated as the internal wave passes above the acoustic source.

Mid-frequency (1-10 kHz) sound propagation was measured at ranges 1-9 km in shallow water in order to investigate intensity statistics. Warm water near the bottom results in a sound speed minimum. Environmental measurements include sediment sound speed and water sound speed and density from a towed conductivity-temperature-depth chain. Ambient internal waves contribute to acoustic fluctuations. A simple model involving modes with random phases predicts the mean transmission loss to within a few dB. Quantitative ray theory fails due to near axial focusing. Fluctuations of the intensity field are dominated by water column variability.

An acoustic ray model, recently presented by Kargl et al. (IEEE J. Ocean. Eng., DOI: 10.1109/JOE.2014.2356934), describes the scattering of sound from a target within a homogeneous waveguide. For a proud target, four ray paths account for the interaction of incident sound with the target and its local environment. In marine environments, a target at the water-sediment boundary can be partially or completely buried. Ray paths that may reach a target then depend on the burial state. Modifications to the ray model are presented. The simplest approach introduces a Heaviside step function into the model, which abruptly turns a ray contribution off as a ray's point of impact on a target transitions from above the boundary to below. In geometric acoustics, the Heaviside step function is a zeroth order approximation to the Fresnel integral for the diffraction of sound at a shadow boundary into a shadow region. The Fresnel integral provides a smooth transition in the intensity across the boundary. Similarly, a smooth transition should occur within a modified ray model for partially buried targets. The modified ray model will be compared to finite-element model results for a solid aluminum cylinder and an aluminum replica of a underwater unexploded ordnance. [Research supported by SERDP and ONR.]

Professor Joe Henderson of the University of Washington physics department formed the Applied Physics Laboratory during WWII. The lab’s initial efforts were to redesign the magnetic influence exploders used in US torpedoes. One of the lab’s first Underwater Acoustics (UA) successes was development of transducers used in the Bikini Atoll Able test (1946). Those transducers, used to trigger other instrumentation, proved critical. Combining UA and torpedo expertise brought APL-UW to the forefront of tracking range design, construction and deployment in Dabob Bay, Nanoose, and St. Croix in the 1950s and 1960. Understanding the torpedo behavior seen in tracking ranges required measuring both the ocean environment and the acoustics within that environment. Making those measurements, as well as development and testing of models based on those measurements, also became standard operating procedure at APL, led in the 50’s by Murphy and Potter. This blueprint of applied research motivating basic research, and the pursuit of basic research via ocean experiments and high fidelity modeling, continues to this day. The presentation will follow this evolution. APL-UW ocean experiments carried out during that time, as well as notable APL-UW research papers, technical reports, computer codes and textbooks, will be used as guideposts.

In this paper we present an acoustic propagation model that approximates a porous medium as a fluid with a bulk modulus and effective density derived from Biot theory. Within the framework of Biot theory it is assumed here that the porous medium has low values of frame bulk and shear moduli relative to the other moduli of the medium and these low values are approximated as zero. This leads to an effective density fluid model. It is shown that, for saturated sand sediments, the dispersion, transmission, reflection, and in-water backscattering predicted with this effective density fluid model are in close agreement with the predictions of Biot theory. In this agreement we demonstrate that the frame bulk and shear moduli play only a minor role in determining several aspects of sand acoustics. Thus, for many applications the effective density fluid model is an accurate alternative to full Biot theory and is much simpler to implement.

The Buried Object Scanning Sonar (BOSS) integrated into a Bluefin12 autonomous underwater vehicle was used to collect seafloor sub-bottom data along the TREX reverberation track. BOSS is a downward looking sonar and employs an omni-directional source to transmit a 3 to 20 kHz linear frequency modulated (LFM) pulse. Backscattered signals are received by two 20-channel linear hydrophone arrays. The BOSS survey was carried out to support long-range reverberation measurements at 3 kHz. The data were beamformed in three dimensions and processed into 10cm x 10cm x 10cm voxel maps of backscattering to a depth of 1 m. Comparison of the BOSS imagery with 400 kHz multibeam sonar imagery of the seafloor allows muddy regions to be identified and shows differences rationalized by the differences in sediment penetration of the two frequency ranges utilized. Processed BOSS data are consistent with observations from diver cores and the reverberation data collected by stationary arrays deployed on the seafloor. Specifically, stronger and deeper backscattering from muddy regions is observed (relative to near-by sandy regions). This correlates well with the large amounts of detritus (e.g., shell fragments) and complicated vertical layering within cores, and the enhanced reverberation, from those regions. [Work supported by ONR.]

The Target and REverberation EXperiment 2013 (TREX13) was carried out off the coast of Panama City, Florida, from 22 April to 16 May, 2013. Two fixed-source/fixed-receiver acoustic systems were used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Beamformed RL data are categorized to facilitate studies emphasizing (1) bottom reverberation; (2) sea surface impact; (3) biological impact; and (4) target echo. This presentation is an overview of RL over the entire experiment, summarizing major observations and providing a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis will be placed on the dependence of RL on local geoacoustic properties and sea surface conditions. [Work supported by ONR.]

The apparatus and experimental procedure used during the target portion of TREX13 are described. A primary goal of the TREX13 target experiments was to test the high speed modeling methods developed and previously tested as part of efforts in more controlled environments where the sediment/water interface was flat. At issue is to what extent the simplified physics used in our models can predict the changes seen in acoustic templates (target strength versus angle and frequency) as a function of grazing angle, i.e., the Target-In-the-Environment-Response (TIER), for a target proud on an unprepared “natural” sand sediment interface. Data/model comparisons for a 3 ft. long, 1 ft. diameter cylinder are used as a case study. These comparisons indicate that much of the general TIER dependence is indeed captured and allows one to understand/predict geometries where the broadest band of TIER information can be obtained. This case study indicates the predictive utility of dissecting the target physics at the expens...

Target scattering within a waveguide is recast into a ray model, where time-of-flight wave packets are tracked. The waveguide is replaced by an equivalent set of image sources and receivers, where rays are associated with these images and interactions with the waveguide's boundaries are taken into account. By transforming wave packets into the frequency domain, scattering becomes a multiplication of a wave packet's spectrum at the target location and the target's free-field scattering amplitude. Data- and model-model comparisons for an aluminum replica of a 100-mm unexploded ordnance will be discussed. For the data-model comparisons, synthetic aperture sonar (SAS) data were collect during Pond Experiment 2010 from this replica, where it was placed on a water-sand sediment boundary. The model-model comparisons use the results from a hybrid 2-D/3-D model. The hybrid model combines a 2D finite-element model to predict the scattered pressure and its derivatives in the near-field of the target, and then a 3D H...

During the 2014 St. Andrews Bay experiments some canonical metallic targets (a hollow sphere and some circular cylinders) were viewed with a synthetic aperture sonar (SAS) capable of acquiring data using a 110–190 kHz chirped source. The targets rested on mud-covered sand and were typically at a range of 20 m. Fast reversible SAS processing using an extension of line-scan quasi-holography [K. Baik, C. Dudley, and P. L. Marston, J. Acoust. Soc. Am. 130, 3838–3851 (2011)] was used to extract relevant signal content from images. The significance of target elastic responses in extracted signals was evident from the frequency response and/or the time-domain response. For example, the negative group velocity guided wave enhancement of the backscattering by the sphere was clearly visible near 180 kHz. [For a ray model of this type of enhancement see: G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994).] In another example, the timing of a sequence of near broadside echoes from ...

Previous work has shown that hybrid finite element (FE)/propagation models are a viable tool for estimating the Target-In-The-Environment-Response, or TIER, for simple shapes such as cylinders and pipes on a flat, undisturbed sand/water interface [K. L. Williams et al., J. Acoust. Soc. Am 127, 3356–3371 (2010)]. Here we examine their use for more complicated targets located in complicated ocean environments. The targets examined include various munitions and ordnance-like targets, with intricate internal structure and filled with either air or water. A hybrid FE/propagation model is used to predict their TIER on flat, undisturbed sand. Data acquired during the target portion of TREX13 is used to validate the model results. Next, the target response is investigated in a more complicated environment, being partially buried with their axis tilted w.r.t. the flat sand interface. Again model results are validated using TREX13 data, as well as data acquired in a controlled tank experiment. These comparisons highlight the feasibility of using hybrid models for complex target/environment configurations, as well possible limitations due to the effects of multiple scattering.

The ensemble-averaged field scattered by a smooth, bounded, elastic object near a penetrable surface with small-scale random roughness is formulated. The formulation consists of combining a perturbative solution for modeling propagation through the rough surface with a transition (T-) matrix solution for scattering by the object near a planar surface. All media bounding the rough surface are assumed to be fluids. By applying the results to a spherical steel shell buried within a rough sediment bottom, it is demonstrated that the ensemble-averaged "incoherent" intensity backscattered by buried objects illuminated with shallow-grazing-angle acoustic sources can be well enhanced at high frequencies over field predictions based on scattering models where all environmental surfaces are planar. However, this intensity must compete with the incoherent intensity scattered back from the interface itself, which can defeat detection attempts. The averaged "coherent" component of the field maintains the strong evanescent spectral decay exhibited by flat interface predictions of shallow-angle measurements but with small deviations. Nevertheless, bistatic calculations of the coherent field suggest useful strategies for improving long-range detection and identification of buried objects.