The twilight feeding migration of fish around a shallow water artificial reef (a shipwreck) was observed by a horizontal-looking, mid-frequency sonar. The sonar operated at frequencies between 1.8 and 3.6 kHz and consisted of a co-located source and horizontal line array deployed at 4 km from the reef. The experiment was conducted in a well-mixed shallow water waveguide which is conducive to characterizing fish aggregations at these distances. Large aggregations of fish were repeatedly seen to emerge rapidly from the shipwreck at dusk, disperse into the surrounding area during the night, and quickly converge back to the shipwreck at dawn. This is a rare, macroscopic observation of an ecologically-important reef fish behavior, delivered at the level of aggregations, instead of individual fish tracks that have been documented previously. The significance of this observation on sonar performance associated with target detection in the presence of fish clutter is discussed based on analyses of echo intensity and statistics. Building on previous studies of long-range fish echoes, this study further substantiates the unique utility of such sonar systems as an ecosystem monitoring tool, and illustrates the importance of considering the impact of the presence of fish on sonar applications.

Previous laboratory results have suggested that unheated TiO₂ nanowire arrays provide a means to detect trace level concentrations of nitro-group containing compounds in the gas phase with fast response and high chemical selectivity. The detection method is based on the chemiresistive effect, where an electron-deficient compound adsorbed to the surface of an n-type semiconductor causes a surface charge deficit on the semiconductor leading to an increase in its resistance. However, a major issue with this method is that chemiresistive sensors based on TiO₂ are also sensitive to the presence of water vapor, and this cross-sensitivity could lead to artifacts for sensors used under environmental conditions. Results are presented here, where thin planar arrays of TiO₂ nanowires were tested to determine the sensitivity towards water vapor, along with the detection limits and response times towards several nitrocontaining organic molecules as a function of gas-phase water vapor concentration. When water vapor concentrations were carefully controlled to ensure they remain constant throughout the testing cycle, it was found that TiO₂ nanowire devices could detect 2,4,6-trinitrotoluene at a concentration as low as a few tens of part per trillion by volume with a response time in the range of 10² – 10³ s, depending on experimental conditions.

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.

Transport theory has been developed for modeling shallow water propagation and reverberation at mid frequencies (1-10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Reverberation measurements made during ASIAEX in 2001 provide a useful test of transport theory predictions. Modeling indicates that the measured reverberation was dominated by bottom reverberation, and the reverberation level at 1 and 2 kHz was observed to decrease as the sea surface conditions increased from a low sea state to a higher sea state. This suggests that surface forward scattering was responsible for the change in reverberation level. By modeling the difference in reverberation as the sea state changes, the sensitivity to environmental conditions other than the sea surface roughness is much reduced. Transport theory predictions for the reverberation difference are found to be in good agreement with measurements.

During the Sediment Acoustics Experiment 2004 (SAX04), a synthetic aperture sonar (SAS) was used to detect simple targets that were either proud or buried below a water-sediment interface, where the nominal grazing angle of incidence from the SAS to the point above a buried target was well below the critical grazing angle. SAS images from other measurements below the critical angle have also produced target detections of buried spheres and finite cylinders. Models and numerical simulations are developed to investigate these proud and buried target detections. For buried targets, the simulations include estimates of reverberation from the rough seafloor, the subcritical penetration through the seafloor, scattering from a target, and propagation back to the SAS. For proud targets, the simulations include the scattering from the target where interaction with the seafloor is included through simple ray models. The simulations used environmental and material parameters measured during SAX04. The environmental measurements include profiles of small-scale surface roughness and superimposed ripple structure. The SAS simulations and model/measurement comparisons over a frequency range of 10-50 kHz further support scattering from sediment ripple structure as the dominant mechanism for subcritical penetration in this range.

A vast literature already existed on sea surface and seabed scattering as of 1982, and over the intervening thirty years that literature has steadily increased in size. We focus here on just four developments that we believe have been of particular importance over the past thirty years. First, numerical methods have been developed for obtaining rigorous solutions to certain simplified scattering problems, and this has led to a better understanding of the accuracy of rough surface scattering theory, and in particular of the two classical approximations: the Kirchhoff approximation and small-height perturbation theory. Second, a more general scattering theory approximation, the small-slope approximation, has been obtained which reduces smoothly to the two classical approximations in the appropriate limits, a long soughtafter goal. Third, modeling seabed scattering has been significantly improved by taking into account a more realistic description of the seabed, including, for example, volume heterogeneity, layering, and shear. Finally, methods are presently being developed for modeling reverberation that account for sea surface forward scattering, which can have important effects on the reverberation level.

n the time period from 1981 to 2011 understanding of the acoustic behavior of sand matured via the combination of experiments, modeling and data/model comparisons. At the core of the issues addressed is the question of whether the sand is best described as a viscoelastic solid or a porous medium. Progress in answering this question has involved examining transmissioninto/ propagation-within/scattering-from sand. A perspective is presented that is based on the premise that results of experiments examining transmission/propagation/scattering must be explained in terms of one unified physical model of sand. The 30 year time span will be divided into three periods: 1981-1997, 1997-2004, and 2004-2011. Experiments, modeling and data/model comparisons from each of these periods will be used to arrive at a perspective on the acoustic behavior of sand.

Transport theory has been developed for modeling shallow water propagation at mid frequencies (1-10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Transport theory has recently been extended to model shallow water reverberation, including the effect of forward scattering from the sea surface. Transport theory results will be compared with other solutions for reverberation examples taken from ONR Reverberation Modeling Workshop problems. These comparisons show the importance of properly accounting for multiple forward scattering in shallow water reverberation modeling.

To evaluate progress made in basic and applied underwater acoustic reverberation modeling and to make recommendations for transitions to operational systems, a series of two reverberation modeling workshops (RMWs) was held (the last in May 2008). A basic goal of the RMWs was to provide well-defined problems and consensus solutions to support verification and validation for new models, upgrades to Navy Standard models, and geoacoustic inversion techniques based on reverberation data. The basic problem in designing the workshop was that even the simplest reverberation problems of interest to the Navy do not have closed form solutions and are still (essentially) beyond our computational capabilities to solve using standard "exact" numerical techniques. All current, practical underwater reverberation models replace the physical problem by employing scattering and loss functions or tables. We discuss the development of a sequence of well-defined problems (physics-based), with the equivalent loss/scattering input, which increases in complexity. We also discuss the lessons learned in this process and point out some of the unexpected results from the workshops, and make recommendations for future benchmarking workshops.

Scattering of terahertz waves by surface roughness can obscure spectral signatures of chemicals at these frequencies. We demonstrate this effect using controlled levels of surface scattering on alpha-lactose monohydrate pellets. Furthermore, we show an implementation of wavelet methods that can retrieve terahertz spectral information from rough surface targets. We use a multiresolution analysis of the rough-surface-scattered signal utilizing the maximal overlap discrete wavelet transform (MODWT) to extract the resonant signature of lactose. We present a periodic extension technique to circumvent the circular boundary conditions of MODWT, which can be robustly used in an automated terahertz stand-off detection device.

At frequencies of about 1 kHz and higher, forward scattering from a rough sea surface (and/or a rough bottom) can strongly affect shallow water propagation and reverberation. The need exists for a fast, yet accurate method for modeling such propagation where multiple forward scattering occurs. A transport theory method based on mode coupling is described that yields the first and second moments of the field. This approach shows promise for accurately treating multiple forward scattering in one-way propagation. The method is presently formulated in two space dimensions, and Monte–Carlo rough surface PE simulations are used for assessing the accuracy of transport theory results.

The six papers in this third of a three-part special issue are devoted to sediment acoustic processes.

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 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.

Previously, it has been shown that scattering of terahertz waves by surface roughness of a target can alter the terahertz absorption spectrum and thus obscure the detection of some chemicals in both transmission and reflection geometries. In this paper it is demonstrated that by employing Maximal Overlap Discrete Wavelet Transform (MODWT) coefficients, wavelet-based methods can be used to retrieve spectroscopic information from a broadband terahertz signal reflected from a rough surface target. It is concluded that while the commonly used direct frequency domain deconvolution method fails to accurately characterize and detect the resonance in the dielectric constant of rough surface lactose pellets, wavelet techniques were able to successfully identify such features.

During the recent 2004 sediment acoustics experiment (SAX04), a buried hydrophone array was deployed in a sandy sediment near Fort Walton Beach, FL. The array was used to measure both the acoustic penetration into the sediment and sound speed and attenuation within the sediment while a smaller, diver-deployed array was also used to measure sound speed and attenuation. Both of these systems had been deployed previously during the 1999 Sediment Acoustics Experiment (SAX99). In that experiment, the buried array was used to make measurements in the 11-50-kHz range while the diver-deployed array made measurements in the 80-260-kHz range. For the SAX04 deployment, the frequency range for the measurements using the buried array was lowered to 2 kHz. The diver-deployed array was also modified to cover the 40-260-kHz range.

Unlike the SAX99 deployment, there were no obvious sand ripples at the SAX04 buried array site at the time of the measurements. To examine the role of sand ripples in acoustic penetration over this new frequency range, artificial ripple fields were created. For the high frequencies, the penetration was consistent with the model predictions using small-roughness perturbation theory as in SAX99. As the frequency of the incident acoustic field decreased, the evanescent field became the dominant penetration mechanism. The sound speed measured using the buried array exhibits dispersion consistent with the Biot theory while the measured attenuation exceeds the theory predictions at frequencies above 200 kHz.

This paper presents observations of a buried sphere detected with a low-frequency (5-35-kHz) synthetic aperture sonar (SAS). These detections were made with good signal-to-noise ratios (SNRs) at both above and below the critical grazing angle. The raw data for the below-critical-grazing angle detection shows that the acoustic penetration is skewed by the 29deg offset of the ripple field relative to the sonar path. This observed skew is in agreement with T-matrix calculations carried out to model penetration into the bottom via ripple diffraction. Additionally, measured SNRs over different frequency bands are compared to predictions made using both first- and second-order perturbation theory for ripple diffraction. Both the data and the models indicate a peak detection region around 25 kHz for the environmental conditions present during the test. These results confirm that ripple diffraction can play a critical role in long range (subcritical angle) buried target detection.

Several of the problems for the first Reverberation Modeling Workshop yielded interesting differences between solutions obtained with ray and normal mode methods. These particular problems were defined with high boundary scattering loss. A bottom reverberation case at 3.5 kHz with a down-refracting sound speed profile (Problem VI) will be considered as a case in point. The ray solutions show a "direct path" contribution unaffected by the bottom scattering loss as long as a direct path can reach the bottom, while the mode solutions obtained to date show a lower reverberation level during this period due to modal attenuation. These differences occur in both incoherent and coherent reverberation solutions for both rays and modes. Arguments will be presented that indicate the correctness of the ray solutions for this case. Suggestions will also be made on how the mode approach can be used to obtain solutions in agreement with the ray method.

The results from two bottom backscattering experiments are described in this paper. These experiments occurred within about 1 km of each other but were separated by approximately five years (1999 and 2004). The experimental methods used in the second experiment were changed based on lessons learned in the first experiment. These changes and the motivation for them are discussed. The sediment at each experiment site would generally be classified as the same (as a well-sorted medium sand sediment) before the weather events (Hurricane Ivan and Tropical Storm Matthew) that occurred in late September and early October 2004. As a result of these weather events, the sediment present during the October 18, 2004 experiments was much more complicated than that in 1999 and in many places had a mud/sand surface layer.

The environmental measurements in both experiments were sufficient to separate physical mechanisms responsible for scattering. For shallow grazing angles (less than 45deg), backscattering at frequencies between 20 and 150 kHz was attributable to sediment interface roughness in 1999, whereas volume scattering dominated in 2004. Furthermore, in 2004, volume heterogeneity within the mud/sand surface layer is a probable mechanism for the scattering feature seen in the data in the 20deg-30deg region. Above 200 kHz, the frequency dependence of both the 1999 data and the 2004 data indicates that a new scattering mechanism is coming into play. Other results within this issue [Ivakin, IEEE J. Ocean. Eng., vol. 34, no. 4, Oct. 2009] indicate that scattering from shells is a viable candidate for explaining the data above 200 kHz.

In some applications of underwater acoustics, it is important to know the ripple structure on shallow-water sediments. For example, the prediction of buried target detection via sound scattering by ripples depends critically on the ripple height and spatial wavelength. Another example is the study of sediment transport, where knowing the ripple structure and its evolution over time helps to understand the forcing on the bottom and the response of sediments.

Here, backscatter data from a 300-kHz system are used to show that ripple wavelength and height can be estimated from backscatter images via a simple inversion formula. The inversion results are consistent with in situ measurements of the ripple field using an independent measurement system. Motivated by the backscatter data, we have developed a time-domain numerical model to simulate scattering of high-frequency sound by a ripple field. This model treats small-scale scatterers as Lambertian scatterers distributed randomly on the large-scale ripple field. Numerical simulations are conducted to investigate the conditions under which remote sensing of bottom ripple heights, wavelength, and its power spectrum is possible.

Acoustic scattering from solid cylinders located near interfaces include effects due to energy interacting with those interfaces. Therefore, modeling cylinder response also requires models of scattering from and penetration across those interfaces. The simplest modeling can be carried out using a plane wave approximation. Using this approximation finite element results for a solid cylinder in the freefield are used to calculate the acoustic scattering of the same cylinder located near an interface. These calculations are compared to experimental data for cylinder target strength and possible reasons for differences seen are discussed. The physical mechanisms responsible for the cylinder's response are examined and cylinder surface displacements are shown.

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.

Measurement of incoherent rough surface scattering powers in terahertz frequency regime by means of time-domain spectroscopy has been for the first time demonstrated. Furthermore, applications of such incoherent measurements in spectroscopy and detection of chemicals are presented.

Acoustic signatures of elastic targets located near sediment interfaces include effects due to energy interacting with the sediment. Therefore, modeling target response also requires models of scattering from, penetration into and propagation within ocean sediments. We first describe at-sea and test pond measurements carried out on "proud" (target resting on the sediment) and buried targets at frequencies in the range of 2 to 50 kHz. The results from some of these measurements are then compared to models incorporating various levels of sophistication relative to both the target and the sediment physics.

The modeling hierarchy includes the following: (1) simple sonar equation estimates that treat the target physics via a frequency dependent target strength and use formally averaged results for sediment scattering, (2) realization level modeling that allows calculation of sediment and target scattering for individual pings with sufficient fidelity to carry out synthetic aperture processing (for a proud target only its geometrical scattering is considered while the elastic response can be included for a buried target), (3) T-matrix and finite element modeling in which the target elastic response is included but sediment scattering is treated using formal averages and/or flat surface approximations.

This paper summarizes results from modeling and measurement efforts investigating shallow grazing angle reverberation levels from a rippled bottom and subcritical detection of targets buried under such interfaces. The focus of this work is associated with frequencies less than 10 kHz where evanescent transmission is important. Measurements were performed in a 13.7-m deep, 110-m long, 80-m wide test-pool with a 1.5-m layer of sand on the bottom. Rippled contours were artificially formed with the aid of a sand scraper. A parametric sonar that generated difference frequency signals in the 1 to 20 kHz frequency range was placed onto a rail system permitting acquired data to be processed and displayed similar to that of a side scan sonar. The buried target was a solid aluminum cylinder. The seabed roughness was measured to assess ripple fidelity and to estimate the small-scale roughness spectrum which was used in scattering models to calculate the backscattered signal levels from the target and bottom. Acoustic backscatter data obtained for various ripples parameters (wavelengths, heights, orientation, etc.) were compared to model predictions based on perturbation theory.

Synthetic aperture sonar (SAS) is used often to detect targets that are either proud or buried below a sandy sediment interface where the nominal grazing angle of incidence from the SAS to the point above a buried target is below the critical grazing angle. A numerical model for scattering from simple targets in a shallow water environment will be described, and can be used to generate pings suitable for SAS processing. For buried targets, the model includes reverberation from the rough seafloor, penetration through the interface, target scattering, and propagation back to the SAS. The reverberation and penetration components are derived from first order perturbation theory where small-scale roughness and superimposed ripple can be accommodated. For proud targets, the simulations include the scattering from the target where interaction with the seafloor is included through simple acoustic ray models. The interaction of the target with an incident field is based on a free field scattering model. Simulations will be compared to both benchmark problems and measurements over a frequency range of 10–30 kHz. These comparisons further support sediment ripple structure as the dominant mechanism for subcritical penetration in this frequency range.

A technique is presented that uses an expansion in unperturbed modes to calculate acoustic scattering from ocean surface waves in a shallow water waveguide. The basic formalism as well as a useful extension to account for the difference between the water column and the domain in which the modes are calculated. The coupling between the modes due to the waves is local at the ocean surface, unlike the coupling of local modes. Numerical examples of the calculation are given for both a sinusoid surface wave and a random surface wave with a typical wind driven spectrum.

Recent advances in ultrafast optical laser technology have improved generation and detection of energy within the terahertz (THz) portion of the electromagnetic (EM) spectrum. One promising application of THz spectroscopy is the detection of explosive materials and chemical or biological agents. This application has been motivated by initial measurements that indicate that explosives may have unique spectral characteristics in the THz region thus providing a discernible fingerprint. However, since THz wavelengths are 10's to 100's of microns in scale, rough interfaces between materials as well as the granular nature of explosives can cause frequency-dependent scattering that has the potential to alter or obscure these signatures. For reflection spectroscopy in particular the measured response may be dominated by rough surface scattering, which is in turn influenced by a number of factors including the dielectric contrast, the angle of incidence and scattering, and the operating frequency. In this paper, we present measurements of THz scattering from rough surfaces and compare these measurements with analytical and numerical scattering models. These models are then used to predict the distortion of explosive signatures due to rough surface interfaces with varying surface height deviations and correlation lengths. Implications of scattering effects on the performance of THz sensing of explosive materials are presented and discussed.

The existence of unique absorption spectrum patterns for many chemicals at terahertz frequencies has opened an exciting avenue in non-contact safe detection of such materials by terahertz spectroscopy. However, scattering of THz waves, which have wavelengths on the order of material grain sizes, by surface roughness challenges the sensitivity of this detection scheme in practice. In this work, we present terahertz time domain spectroscopy results for materials with rough surfaces. Both reflection from and transmission through lactose, which has sharp absorption peaks in the terahertz regime, are studied and the effect of increasing scattering through controlled surface roughness is investigated. Such electromagnetic scattering can alter the terahertz absorption spectrum and thus obscure the detection of chemicals. Furthermore we examine electro-optic detection of terahertz signals reflected from randomly rough targets with a theoretical electromagnetic system perspective and provide a method to retrieve coherent reflection responses from rough surface targets.

Two workshops on reverberation modeling are being conducted under joint sponsorship from the Program Executive Office C4I, PMW 180 (as funded by the Office of the Oceanographer of the Navy) and the Office of Naval Research. The overall goal of these workshops is to evaluate recent progress made in reverberation-related research efforts and to make recommendations for further development. The first workshop was held in November 2006 and the second is scheduled for March 2008. The focus of the first workshop was on reverberation from the environment, while the second workshop will emphasize system characteristics. At the first workshop, fifteen different reverberation models were represented and extensive comparisons were carried out before, during, and after the workshop. We present the approach used to conduct the first workshop, discuss issues that have been identified, and outline tentative plans for the second workshop.

Participants at Reverberation Modeling Workshop I presented solutions to posed reverberation problems using a wide variety of methods. The reverberation problems were posed in both two- and three-space dimensions, and included cases with different levels of boundary roughness, different sound speed profiles, and in some cases, range dependence. A number of important reverberation modeling issues became evident when the results of this workshop were considered in detail following the workshop. These include the important role of coherent effects in determining reverberation structure at short ranges, and the important role of boundary reflection loss in affecting the reverberation level at long ranges. A summary of modeling issues highlighted by this workshop will be presented.

For many of the problems posed for Reverberation Modeling Workshop I, the boundary roughness was presented in terms of roughness spectra. Models and/or tables were also supplied for the bistatic scattering cross section and for the coherent reflection loss. Integral equation simulations were used to verify the accuracy of the bistatic scattering cross sections, and rough boundary PE simulations were used to verify the accuracy of the coherent reflection losses. The results of this work will be briefly summarized. The issue also arises for reverberation simulations whether the coherent reflection loss is appropriate for the boundary loss, or whether some other loss model is more appropriate, or whether the loss due to roughness should be simply ignored. This issue will be discussed in light of rough boundary PE simulations.

The effects of surface scattering on terahertz reflection spectrum for explosive detection are studied by measuring terahertz reflection pulses from sandpapers with different roughness coated with gold. The experimental results show that the amplitude decrease and pulse broadening of the detected signal caused by the surface scattering result in the width reduction of Gaussian distribution of the specular scattering coefficient spectrum. A simple analytical model is applied to the analysis of experimental results and good agreements are obtained.

Terahertz (THz) imaging is emerging as a potentially powerful method of detecting explosive devices, even in the presence of occluding materials. However, the characteristic spectral signatures of pure explosive materials may be altered or obscured by electromagnetic scattering caused by their granular nature. This paper presents THz transmission measurements of granular systems representative of explosives and presents results from dense media theory that accurately explain the observed scattering response.

This paper presents predictions of classical electromagnetic scattering from granular material illuminated by a terahertz (THz) source. Random media models are created to represent the explosive grains, air voids and filler material commonly found in explosive devices. These constituents can cause significant volume scattering that may alter or obscure the chemical response of the explosive, thus impacting THz detection of explosives. Furthermore, the air-explosive interface may have significant roughness, and scattering from this interface may be a dominant factor - particularly in reflection spectroscopy. The volume scattering is calculated using the Quasi-Crystalline Approximation (QCA) and a Finite Difference Time Domain (FDTD) calculation; the FDTD method is also used to estimate the rough surface scattering. Results from these calculations are provided for mixtures that are representative of common classes of explosives.

During SAX99 (for sediment acoustics experiment &$151; 1999) the sediment sound speed (125 Hz to 400 kHz) and attenuation (2.5 to 400 kHz) in sandy sediments were measured by a variety of techniques. The SAX99 site was 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18–19 m depth. SAX04 was held in the fall of 2004 at a site close to the SAX99 site, about 1 km from shore in water of 17 m depth. The sediment sound speed and attenuation were again measured over a broad frequency range by multiple techniques, with even more attention paid to the low frequency band from 1–10 kHz. The results and corresponding uncertainties from SAX99 will be reviewed, and the consistency with Biot model predictions and alternative models (e.g., Buckingham's model) will be discussed. An overview will then be presented of the recently completed SAX04 measurement program on sediment sound speed and attenuation.

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.

This paper describes recent results from an ongoing modeling and measurement effort investigating shallow grazing angle acoustic detection of targets buried in sand. The measurements were performed in a 13.7-m deep, 110-m long, 80-m wide test-pool with a 1.5-m layer of sand on the bottom. A silicone-oil-filled target sphere was buried under a rippled surface with contours formed by scraping the sand with a machined rake. Broad band (10 to 50 kHz) transducers were placed onto the shaft of a tilting motor, which in turn was attached to an elevated rail that enabled this assembly to be translated horizontally, permitting acquired data to be processed using synthetic aperture sonar (SAS) techniques. Acoustic backscatter data were acquired at subcritical grazing angles for various ripple wavelengths and heights. In addition, the backscattered signals from a calibrated free-field sphere and the transmitted signals received with a free-field hydrophone were recorded. For each bottom configuration, the seabed roughness over the buried target was measured to determine the ripple parameters and to estimate the small-scale roughness spectrum. This roughness information is used in scattering models to calculate the backscattered signal levels from the target and bottom. In previous work, measured signal-to-reverberation ratios were found to compare well with model predictions, demonstrating the accuracy of first-order perturbation theory (for the ripple heights used in those experiments) for frequencies up to 30 kHz. By taking advantage of the backscattered data collected using the free-field sphere and of the acquired transmitted data, more stringent comparisons of predicted buried target backscatter levels to measured levels are made here. Results of a second series of measurements using larger ripple heights to investigate the impact of higher-order scattering effects on buried target detection are presented.

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10° to 40°. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50–150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30°) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.

During the sediment acoustics experiment in 1999 (SAX99), several researchers measured sound speed and attenuation. Together, the measurements span the frequency range of about 125 Hz-400 kHz. The data are unique both for the frequency range spanned at a common location, and for the extensive environmental characterization that was carried out as part of SAX99. Environmental measurements were sufficient to determine or bound the values of almost all the sediment and pore water physical property input parameters of the Biot poroelastic model for sediment. However, the measurement uncertainties for some of the parameters result in significant uncertainties for Biot-model predictions. Here, measured sound-speed and attenuation results are compared to the frequency dependence predicted by Biot theory and a simpler "effective density" fluid model derived from Biot theory. Model/data comparisons are shown where the uncertainty in Biot predictions due to the measurement uncertainties for values of each input parameter are quantified. A final set of parameter values, for use in other modeling applications e.g., in modeling backscattering (Williams et al., 2002) are given, that optimize the fit of the Biot and effective density fluid models to the sound-speed dispersion and attenuation measured during SAX99. The results indicate that the variation of sound speed with frequency is fairly well modeled by Biot theory but the variation of attenuation with frequency deviates from Biot theory predictions for homogeneous sediment as frequency increases. This deviation may be due to scattering from volume heterogeneity. Another possibility for this deviation is shearing at grain contacts hypothesized by Buckingham; comparisons are also made with this model.

This paper presents observations of buried target detections made using a 20-kHz synthetic aperture sonar. At grazing angles below the critical angle, surprisingly high signal-to-noise detections were made of cylindrical targets buried at depths of 15 and 50 cm. During a separate set of measurements, buried spheres were clearly seen at steep grazing angles, but were generally not seen below the critical angle. Since scattering from wave-generated sand ripples may contribute to detections at grazing angles below critical angle, the information available on the ripple fields is discussed and used in acoustic backscatter simulations for the buried spheres. Lack of information on the ripple height precludes a definitive explanation for the absence of buried sphere detections at subcritical grazing angles.

Under certain conditions, Wood's equation can be used to predict sound speed in fluid/solid-grain suspensions if the bulk moduli and densities of the grains and fluid are known. In this paper, that relationship is used to estimate grain-bulk moduli in suspensions where sound speed, fluid density, fluid bulk modulus, grain density, and particle concentrations are known or accurately measured. Measured values of grain-bulk moduli for polystyrene beads suspended in water (mean = 4.15 x 10⁹ Pa) and soda-lime glass beads suspended in a "heavy liquid" (mean = 3.8 x 10¹⁰ Pa) are consistent with the values of bulk moduli for polystyrene beads and soda-lime glass beads found in the literature (3.6 to 4.2 x 10⁹ Pa and 3.4 to 4.0 x 10¹⁰ Pa, respectively). These measurements thus provide controls, which demonstrate the validity of the suspension technique to estimate values of particle bulk modulus. The values of bulk modulus, measured using the same suspension techniques, for Ottawa sand and quartz sand grains collected from the coastal sediments of the northeast Gulf of Mexico ranged between 3.8 and 4.7 x 10¹⁰ Pa, with 95% confidence limits between 3.0–5.7 x 10¹⁰ Pa. These measured values of bulk modulus are consistent with the range of handbook values for polycrystalline quartz (3.6 to 4.0 x 10¹⁰ Pa). The use of the lower bulk modulus (i.e., 7.0 x 10⁹ Pa) recently suggested by Chotiros is therefore inappropriate and traditional handbook values of sediment grain-bulk moduli should be used as inputs for sediment acoustic modeling.

As part of the effort to characterize the acoustic environment during the high frequency sediment acoustics experiment (SAX99), fine-scale variability of sediment density was measured by an in situ technique and by core analysis. The in situ measurement was accomplished by a newly developed instrument that measures sediment conductivity. The conductivity measurements were conducted on a three-dimensional (3-D) grid, hence providing a set of data suited for assessing sediment spatial variability. A 3-D sediment porosity matrix is obtained from the conductivity data through an empirical relationship (Archie's Law). From the porosity matrix, sediment bulk density is estimated from known average grain density. A number of cores were taken at the SAX99 site, and density variations were measured using laboratory techniques. The power spectra were estimated from both techniques and were found to be appropriately fit by a power-law. The exponents of the horizontal one-dimensional (1-D) power-law spectra have a depth-dependence and range from 1.72 to 2.41. The vertical 1-D spectra have the same form, but with an exponent of 2.2. It was found that most of the density variability is within the top 5 mm of the sediment, which suggests that sediment volume variability will not have major impact on acoustic scattering when the sound frequency is below 100 kHz. At higher frequencies, however, sediment volume variability is likely to play an important role in sound scattering.

During the sediment acoustics experiment, SAX99, a hydrophone array was deployed in sandy sediment near Fort Walton Beach, Florida, in a water depth of 18 m. Acoustic methods were used to determine array element positions with an accuracy of about 0.5 cm, permitting coherent beamforming at frequencies in the range 11–50 kHz. Comparing data and simulations, it has been concluded that the primary cause of subcritical acoustic penetration was diffraction by sand ripples that were dominant at this site. These ripples had a wavelength of approximately 50 cm and RMS relief of about 1 cm. The level and angular dependence of the sound field in the sediment agree within experimental uncertainties with predictions made using small-roughness perturbation theory.

A finite-difference time-domain (FDTD) method for scattering by one-dimensional, rough fluid–fluid interfaces is presented, modifications to the traditional FDTD algorithm are implemented which yield greater accuracy at lower computational cost. These modifications include use of a conformal technique, in which the grid conforms locally to the interface, and a correction for the numerical dispersion inherent to the FDTD algorithm, Numerical results are presented for fluid-fluid cases modeling water-sediment interfaces. Two different roughness spectra, the single-scale Gaussian roughness spectrum and a multiscale modified power-law spectrum, are used. The Gaussian results are calculated as a function of the dimensionless parameters kh and kl, where k is the wavenumber in water, h is the rms surface height, and l is the surface correlation length. For the modified power-law spectrum, statistical parameters consistent with an insonification frequency of 7.5 kHz are used. Results are compared with those obtained using an integral equation technique both for scattering from single-surface realizations and for Monte Carlo averages of scattering from an ensemble of surface realizations. Scattering strengths are calculated as a function of scattering angle for an incident angle of 70° (20° grazing). The results agree well over all scattering angles for the cases examined.

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18–19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10–300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30°. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results.

A 1-km² area located 2 km off the Florida Panhandle (30°22.6'N; 86°38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18–19 m of water, is characterized by 1–2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics.

As part of the SAX99 experiment, a buried hydrophone array was deployed together with a movable tower with attached sources covering the frequency range 11–50 kHz. This system was used to examine subcritical penetration into the sediment. For incident grazing angles below the critical angle, scattering dominates the penetrating field. Comparisons with simulations based on perturbation theory show that the penetration is predominately the result of diffraction by the low-amplitude ripple field prevalent at the SAX99 site. Simulations predict a cutoff effect as a function of frequency and grazing angle that is found in the data, and predict changes in penetration as a function of ripple field amplitude that are consistent with those observed.

Two new instruments were developed and deployed during SAX99 to measure surficial sediment variability at centimeter scales. Such data serve as input to acoustic models predicting sound scattering in the frequency range of 10–50 kHz. One instrument, IMP (In situ Measurement of Porosity) measures sediment conductivity at 1-cm resolution in the horizontal dimensions and at 3-mm resolution in the depth dimension. From this instrument the following information is derived: (1) 3-D porosity or density variation in the top 12 cm of sediments, and (2) 2-D bottom roughness and associated spectra. The second instrument, the Acoustic Imager (AI), is a 3-D sediment tomographic tool with 1-cm resolution operating at 170 kHz. Information derived from the AI includes (1) 3-D sediment sound speed variability, (2) 3-D variability of sediment attenuation coefficients, (3) the presence and distribution of discrete scatterers such as shell pieces, and (4) the temporal variability of the above parameters over 3 days. These results and their implications to the acoustic measurements taken during the SAX99 experiment will be discussed.

The main goals of the APL program in SAX99 were to measure and improve our ability to model acoustic propagation within, high-frequency backscattering from, and penetration into sand sediments. To prepare for these measurements, new equipment and experimental procedures were developed. For the penetration studies, simulations were used extensively to guide the experiment design in order to ensure that the measurements would be useful for addressing our goals. Illustrations will be given of how simulations were used to support the experiment design. The APL experimental equipment used in SAX99 will be described, and the experimental procedures will be presented. Finally, the resulting data set will be summarized.

As part of the SAX99 experiment, a buried hydrophone array was deployed together with a movable tower with attached sources covering the frequency range 11–50 kHz. With the tower placed to provide incident grazing angles well above the critical angle, this system was used to obtain data from which sediment sound speed and absorption were determined. The sound-speed data exhibit significant dispersion, while the absorption data show an approximate linear frequency dependence. When these data are combined with data at other frequencies from the same site, the dispersion and absorption are found to be consistent with causality and with the Biot model.

In the fall of 1999, "SAX99" (for sediment acoustics experiment—1999) was performed at a site 2 km from the shore on the Florida Panhandle in water 18–19 m in depth. The seafloor sediment was medium sand, and acoustic frequencies were mainly in the 10–300-kHz range. A wave-induced ripple field was present at the site. The main goals in SAX99 were to quantify acoustic backscattering from the sediment, acoustic penetration into the sediment (above and below the critical angle of 30 deg), and acoustic propagation within the sediment. Extensive environmental characterizations were made at the experiment site. Quantities that enter into a Biot model description of the sediment were measured. Sediment variability was measured to centimeter scales, giving the most complete coverage for the frequency range of 10–50 kHz. An overview of the entire SAX99 measurement program will be given. The range of acoustic measurements will be briefly described, as well as the scope of measurements of the seafloor physical properties and of biological processes within the sediment and the water column. Initial results of this work will be described in separate papers.

During the SAX99 experiment, acoustic backscattering measurements were made at frequencies from 20 to 300 kHz as a function of grazing angle. The results from these acoustic measurements will be presented and compared with backscattering models that use the environmental measurements of other SAX99 researchers as input. In the 20–50-kHz range these comparisons indicate that surface roughness plays a dominant role in acoustic backscattering with a very distinctive reduction in backscattering at grazing angles above the critical angle of the sediment. Above 50 kHz this critical angle feature is less evident. Possible reasons for this change with frequency will be discussed. The backscattering models used here were originally developed for frequencies from 10 to 100 kHz. SAX99 data give some indication that further modeling is needed above 100 kHz.

Recent low-frequency (<1500 Hz) underwater acoustic measurements have demonstrated that scattering processes at the air–sea interface depend primarily on the wind speed and the surface wave spectrum. The scattering strength from the rough interface is proportional to the spectral density at the Bragg wavelength with modifications due to tilt and modulation by longer waves. These modifications are accounted for in the small slope approximation. When wave breaking becomes significant, rough interface scattering is augmented by bubble cloud scattering, which depends primarily on wind speed. In this regime, bubble cloud scattering dominates at low grazing angles, and rough interface scattering dominates at high grazing angles. A physics-based empirical model is used to describe bubble scattering. The mean-frequency-shift characteristics of acoustic signals scattered from both the moving sea surface and bubble clouds have been successfully modeled given the 2-D surface wave spectrum. These scattering-strength and frequency-shift models are used to explore the sensitivity of low-frequency scattering to environmental variables obtainable by remote sensing.

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.

Recent experimental results reveal acoustic penetration into sandy sediments at grazing angles below the critical angle. A mechanism for this subcritical penetration is described based on scattering at a rough water–sediment interface. Using perturbation theory, a numerically tractable three-dimensional model is used for simulating experiments. The rough interface scattering has been treated using formally averaged methods as well as with single rough surface realizations. Data-model comparisons show that scattering by interface roughness is a viable hypothesis for the observed subcritical penetration.