The Target and Reverberation EXperiment 2013 (TREX13) included a comprehensive reverberation field project in the frequency band of 2–10 kHz, and was carried out off the coast of Panama City, FL, USA, from April 21 to May 17, 2013. A spatially fixed transmit and receive acoustic system was 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. Extensive in situ measurements, including a multibeam bathymetric survey, chirp sonar subbottom profiling, gravity/diver cores, sediment sound speed and attenuation, interface roughness, wind-generated sea surface waves, and water column properties, were made to support studies of environmental effects on RL. Beamformed RL data are categorized to facilitate studies emphasizing physical mechanisms of 1) bottom reverberation; 2) sea surface impact; and 3) biological impact. This paper is an overview of RL over the entire sea trial, intending to summarize major observations and provide both a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis is placed on the dependence of RL on local geoacoustic properties and sea surface 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.

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.

Detection of an object in shallow water has seen a resurgence in importance due to concerns for harbor security. When the horizontal range to an object is large compared to the nominal water depth, then the response of an object to active sonar must necessarily include possible interactions with the boundaries of the waveguide. As an initial step toward the development of detection algorithms, we consider an object in a homogeneous waveguide with planar boundaries. Reflection of the transmitter, receiver, and their images through boundaries allows the scattering problem to be recast into a superposition of many free field scattering problems. An overview of our model and its application to a cylindrical target in littoral waters are given.

In March 2010, a series of measurements were conducted to collect synthetic aperture sonar (SAS) data from objects placed on a water-sediment interface. The processed data were compared to two models that included the scattering of an acoustic field from an object on a water-sediment interface. In one model, finite-element (FE) methods were used to predict the scattered pressure near the outer surface of the target, and then this local target response was propagated via a Helmholtz integral to distant observation points. Due to the computational burden of the FE model and Helmholtz integral, a second model utilizing a fast ray model for propagation was developed to track time-of-flight wave packets, which propagate to and subsequently scatter from an object. Rays were associated with image sources and receivers, which account for interactions with the water-sediment interface. Within the ray model, target scattering is reduced to a convolution of a free-field scattering amplitude and an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicated target can be reduced to a (complex) scattering amplitude, and this amplitude then can be used within the ray model via interpolation. The ray model permits the rapid generation of realistic pings suitable for SAS processing and the analysis of acoustic color templates. Results from FE/Helmholtz calculations and FE/ray model calculations are compared to measurements, where the target is a solid aluminum replica of an inert 100-mm unexploded ordnance (UXO).

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.

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.

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.

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.

A fast ray model for propagation in a homogenous water column tracks time-of-flight wavepackets from sources to targets and then to receivers. The model uses image sources and receivers to account for interactions with the water column boundaries, where the layer of water lies between an upper semi-infinite halfspace of air and a lower semi-infinite halfspace of a homogenous sediment. The sediment can be either an attenuating fluid with a frequency-independent loss parameter or a fluid consistent with an effective density fluid model (i.e., a fluid limit to Biot's model for a fluid-saturated poroelastic medium). The target scattering process is computed via convolution of a free-field scattering form function with the spectrum of an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicate target can be reduced to a scattering form function, and this form function then can be used within model via interpolation. The fast ray-based model permits the generation of sets of realistic pings suitable for synthetic aperture sonar processing for proud and partially buried target. Results from simulations are compared to measurements where the targets are an inert unexploded ordnance and aluminum cylinder.

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.

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.

The scattering from elastic targets in the low- to mid-frequency regime is affected by the environment surrounding the target. For axisymmetric targets with the axis of symmetry parallel to the water-sediment boundary, previous work has dealt with the change in the target strength as a function of frequency and aspect angle in relation to the burial depth in the sediment. The present work deals with the extension of a finite element model, based on the decomposition of the acoustic and elastic fields into azimuthal Fourier modes, to the case of a target buried at a tilt angle. The interaction between the target and the sediment is represented by the model up to the first order of the scattering series, which means that the scattering of the incident field and of the target reflected field is taken into account, but the rescattering of the boundary reflected echo from the target is neglected. Model results up to 30 kHz are compared to experimental data for a 2 foot long aluminum cylinder of 1 foot diameter buried in sand at a tilt angle.

Low frequency sound is a viable means for the detection of elastic targets in contact with the ocean floor. The incoming sound, with wavelengths on the order of the target dimensions, can excite resonant modes of the target leading to enhancements in the scattered field. A hybrid model has been developed to predict the acoustic scattering from cylinders, pipes and unexploded ordnance (UXO) in proud or buried configurations in the ocean sediment. The model exploits the symmetry by decomposing the 3-D problem into a sum of 2-D independent Fourier modal sub-problems. This hybrid modeling technique has been shown to agree well with experimental measurements conducted in a pond [A.L. España et al., J. Acoust. Soc. Am. 130, 2330 (2011)]. Presently, these hybrid model results are used to examine the target response on a mode-by-mode basis. A modal map is generated by keeping track of the number of dominant modes contributing to the bright features observed in the acoustic template. For features that are predominantly due to one or two modes, simple analytical models can be used to predict their evolution as a function of target/sensor geometry within the ocean waveguide.

The environment and the location of the target within that environment affect the scattering from elastic targets in ocean waveguides. Computational power is now realizable to compute the target scattering, in-situ, via finite elements. However, these calculations still require high cost computer facilities and in the end do not offer physical insight into processes involved. Here we compare two models, with different levels of simplification, to data acquired from an Aluminum target machined to replicate an Unexploded Ordnance (UXO). The first model treats the scattering using two-fluid Green's function propagators in combination with finite element calculations of the target scattering as placed within the waveguide. The second model uses free field, plane wave incidence, finite element results for the target scattering in conjunction with simple ray based propagation to account for the waveguide environment. The data/model comparisons are discussed in light of the physical insight they can help provide, the speed of the calculation and the level of fidelity they achieve.

Details of the surrounding environment, for examples, sediment conditions and nearby clutter, influence the ability to successfully detect and classify proud or buried targets. These issues were investigated during experiments conducted in March 2010 in a fresh water pond, during which targets were placed at varying distances from each other, in proud and buried configurations within a sand sediment. This paper will focus on a subset of these experiments involving the acoustic scattering from unexploded ordnances (UXOs) in proud and fully buried configurations in which the incident grazing angle of the sonar onto the water-sediment interface is above the critical angle. Monostatic synthetic aperture sonar (SAS) data will be presented for the case of a single, isolated UXO, as well as the situation where multiple UXOs are in close proximity to each other, hence simulating a cluttered environment. To supplement the data, finite element models have been developed for the UXO with varying levels of complexity in both target specifications (shape and material composition) and general experimental setup. These simulations reveal the level of fidelity required to achieve good data-model agreement.

Monostatic and bistatic scattering measurements were conducted on a set of targets near a fresh water-sand sediment interface. The measurements were performed during March 2010 and are referred to as the Pond Experiment 2010 (PondEx10). Monostatic synthetic aperture sonar (SAS) data were collected on a rail system with a mobile tower, while a stationary sonar tower simultaneously collected bistatic SAS data. Each tower is instrumented with receivers while the sources are located only on the mobile tower. For PondEx10, 11 targets, including 6 underwater munitions, were deployed at 2 ranges from the mobile tower system. Initially, the data were processed using standard SAS techniques, and then, the data were further processed to generate acoustic templates for the target strength as a function of frequency and aspect angle. Results of the data processing from proud targets are presented. Finite element model (FEM) predictions of the scattering from an ordnance in the free field and proud on the interface are also discussed. A processing technique that separates an individual target's response from nearby targets is also briefly discussed.

A series of monostatic and bistatic acoustic scattering measurements were conducted to investigate discrimination and classification capabilities based on the acoustic response of targets for underwater unexploded ordnance (UXO) applications. The measurements were performed during March 2010 and are referred to as the Pond Experiment 2010 (PondEx10), where the fresh water pond contained a sand sediment. The measurements utilized a rail system with a mobile tower and a stationary sonar tower. Each tower is instrumented with receivers while the sources are located on the mobile tower. For PondEx10, eleven targets were deployed at two distinct ground ranges from the mobile tower system.

Acoustic data were initially processed using synthetic aperture sonar (SAS) techniques, and the data were further processed to generate acoustic templates for the target strength as a function of frequency and aspect angle. Preliminary results of the processing of data collected from proud targets are presented. Also presented are the results associated with a processing technique that permits isolation of the response of an individual target, which is in close proximity to other targets.

Recent experiments conducted in a fresh water pond investigated the monostatic scattering from an aluminum pipe (length-to-diameter ratio of 2) in the free field, as well as in a proud configuration on a flattened sand sediment. Synthetic aperture sonar (SAS) techniques are used to process the data. Absolute target strength is calculated over various spatial filter boundaries of the SAS images in order to isolate the specular and elastic responses of the pipe. A finite element (FE) model has been developed for the aluminum pipe in the free field, making use of the exact geometry associated with the pond experiment. The absolute target strength from these FE calculations is plotted in a similar manner to the experimental data, whereby the specular and elastic contributions are identified and compared to the data.

Features present in synthetic aperture sonar (SAS) images associated with elasticity and the structural resonances of viewed objects are sometimes neglected when using SAS data for classification purposes. Other ways of processing sonar data sometimes emphasize the frequency response of the viewed object. The research described here concerns a hybrid approach based on a reversible SAS algorithm in which the acoustic spectral content contributing to a specified region of the SAS image can be extracted. The SAS algorithm uses deconvolution.

An example from a small scale tank experiment using a transducer scanned along a line illustrates some applications of the method in which quantitative ray theory is useful for interpreting the image and spectral features. The method is also proved to be useful for investigating spectral responses of several objects placed proud on sand viewed simultaneously in SAS scans in a fresh water pond. The hybrid processing technique simplifies the acquisition of spectral data by reducing the spectral contamination from adjacent sufficiently separated objects.

During the 2006 Shallow Water (SW06) experiment, simultaneous measurements were made of the sound-speed field as a function of range and depth associated with nonlinear internal waves and acoustic propagation at frequencies of 2–10 kHz over a 1-km path. The internal waves were measured by a towed conductivity-temperature-depth (CTD) chain to get high resolution. These measurements were coordinated so that the nonlinear waves could be interpolated onto the acoustic path, allowing predictions of their effects on the acoustics. Using the measured sound-speed field, the acoustic arrivals under the influence of the internal waves are modeled and compared to data. The largest impact of measured moderate amplitude internal waves on acoustics is that they alter the arrival time of the rays which turn at the thermocline.

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.

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.

Connecting changes in acoustic scattering from the seafloor with changes in seafloor topography is essential for modeling the time dependence of the scattering and the development of acoustics as a tool for the remote sensing of benthic activity. An equation is derived that links the decorrelation of scattered acoustic power with the decorrelation of seafloor roughness spectral estimates. The result is assessed through a comparison of decorrelation values generated by processing topographical data recorded by a digital photogrammetry system and backscattering data acquired with a translating source/receiver assembly. Both data sets were collected off the western coast of Florida as part of the U.S. Office of Naval Research (ONR)-sponsored sediment acoustics experiment (SAX04), during which the primary mechanism of topographical change at the frequencies of interest appeared to be fish feeding. Although decorrelation curves proved to be both space and time dependent, and the collection of data sets was neither collocated nor synchronized, the agreement between averaged topographical and acoustic decorrelation values was reasonable. Both types exhibited a strong frequency dependence, which should prove beneficial in classifying and quantifying sources of seabed transformation if it is mechanism specific.

A statistical model for the time evolution of seafloor roughness due to biological activity is applied to photographic and acoustic data. In this model, the function describing small scale seafloor topography obeys a time-evolution equation with a random forcing term that creates roughness and a diffusion term that degrades roughness. When compared to acoustic data from the 1999 and 2004 Sediment Acoustics Experiments (SAX99 and SAX04), the model yields diffusivities in the range from 3.5 times 10^-11 to 2.5 times 10^-10 m² s^-1 (from 10 to 80 cm² yr^-1), with the larger values occurring at sites where bottom-feeding fish were active. While the experimental results lend support to the model, a more focused experimental and simulation effort is required to test several assumptions intrinsic to the model.

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.

During the Shallow Water 2006 experiment, simultaneous measurements were made of the sound speed structure associated with nonlinear internal waves and acoustic propagation at frequencies of 2–10 kHz over a 1 km path. The internal waves were measured by a towed CTD chain in order to get high resolution. These measurements were coordinated so that the nonlinear waves can be interpolated onto the acoustic path, allowing predictions of their effects on the acoustics. An internal wave train was measured that passed the acoustic path on August 13. When the wave train was in between the sound source and receiver, distinctive arrival time oscillations on three acoustic paths were measured, which are all rays having an upper turning point. Using the CTD chain data, a deterministic explanation is given to the arrival time oscillations.

Recently, Pierce and Carey [ J. Acoust. Soc. Am. 124, EL308–EL312 (2008) ] presented a low frequency analysis of sound propagation in sand/silty sediments. Here, equivalent expressions are presented using a low frequency expansion of an unconsolidated version of Biot porous medium theory. The resulting expression for attenuation allows identification of the non-dimensional parameter B in the Pierce/Carey result in terms of physical parameters. The agreement of these two derivations motivates further analyses. The results imply that porous media propagation models that treat the medium's inertia via a single component approximation disregard a fundamental physical effect resulting from the relative inertia of the grains and fluid and are thus incomplete.

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.

In this paper, modeling results are presented demonstrating that, using an ensemble of forward-scattering measurements from a rippled sand/water interface, it is possible to accurately estimate the plane wave, flat surface reflection coefficient. The modeling effort was carried out in preparation for a sediment acoustics experiment in 2004 (SAX04). Guided by the modeling results, forward-scattering measurements were made during SAX04. The measurement instrumentation and procedure are presented. The plane wave reflection coefficients derived from these measurements are given and compared to reflection coefficients calculated using a fluid model and an approximation to the Biot porous medium model for the sand known as the effective density fluid model (EDFM). The model reflection coefficients were calculated using acoustic parameters determined from environmental measurements carried out by other researchers involved in SAX04. The reflection coefficient data/model comparison indicates that the sand at the SAX04 site is most accurately viewed as a porous medium for acoustic modeling purposes.

Knowledge of sediment sound speed is crucial for predicting sound propagation. During the Shallow Water '06 experiment, in situ sediment sound speed was measured using the Sediment Acoustic-speed Measurement System (SAMS). SAMS consists of ten fixed sources and one receiver that can reach a maximal sediment depth of 3 m. Measurements were made in the frequency range 2&$150;35 kHz. Signal arrival times and propagation distances were recorded, from which sediment sound speed was determined. Preliminary results from three deployments show that SAMS was capable of determining sediment sound speed with uncertainties less than 1.6%. Little dispersion in sediment sound speed was observed.

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.

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.

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 vertical 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 position 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.

A unique Sediment Acoustic-speed Measurement System (SAMS) was developed to directly measure sediment sound speed. The system consists of ten fixed sources and one receiver. In a typical deployment, the SAMS is deployed from a ship that is dynamically positioned. The sources are arranged just above the sea bottom and the receiver is drilled into the sediment with controlled steps by a vibro-core. The maximal sediment penetration depth is 3 meters. At each receiver depth, the 10 sources transmit to the receiver at different angles in the frequency range of 2–35 kHz, providing 10 estimates of sound speed through time-of-flight measurements from the known source-to-receiver geometry. SAMS was deployed three times during the recent Shallow Water Experiment 2006 (SW06) on the New Jersey shelf at 80 m water depth. Preliminary results of sediment sound speed are 1618 ± 11, 1598 ± 10, and 1600 ± 20 m/s at three separate deployment locations. Little dispersion in sediment sound speed was observed.

The comparison of sediment sound speed and attenuation measurements to predictions has been the primary method used to test Biot theory and the grain-shearing model. Examples of data–model comparisons will be shown. Subsequent refinements made to these models result in similar predictions for sound speed and attenuation. However, the underlying physics is substantially different suggesting other, more indirect means for discriminating between sediment propagation theories. One technique that has received recent attention is the measurement of forward scattering from the sediment interface. Model predictions of these measurements depend not only the sound speed and attenuation, but also on the acoustic impedance of the medium. Examination of the physics incorporated into Biot Theory shows that the "effective density" seen by the acoustic wave is lower than the bulk density, thus lowering the acoustic impedance. This results in a difference in the predicted flat surface reflection coefficient for Bio-type models as compared to grain-shearing models. The flat surface reflection coefficients derived from experiment will be compared with predictions using the Biot model and the viscosity grain shearing (VGS) model for a sand sediment. The validity of obtaining reflection coefficients using forward scattering from rough surfaces will also be discussed.

As part of the ONR-sponsored SW06 experiment, mid-frequency sound propagation was measured at ranges 1–10 km in the frequency band of 2–10 kHz in August, 2006. The water depth is 80 m and the source depth is 30 m, close to the minimum of a duct with a thermocline above and a warm salty water below. The receivers are clustered into two groups, one at 25 m depth, the other at 50 m. The region has active internal wave activity during this time. Because the source is near the axis of the sound channel, it is observed that propagation is dominated by trapped modes and behaves similar to sound propagation in a deep water duct. Amplitude fluctuations and cross-frequency correlations are estimated. The scintillation index as a function of frequency and bandwidth is calculated.

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.

On two occasions during the SW06 experiment, towed CTD chain measurements were made close to an acoustic propagation path. The acoustic path was 1 km long, oriented roughly in the direction of propagation of large nonlinear internal waves. On the first occasion, large nonlinear internal waves were absent, and on the second occasion, they were present. The CTD chain was towed in loops around the acoustic path, roughly 200 m on either side of the path. On the first occasion, 17 loops were made in about 5.5 hr, and on the second occasion, 7 loops were made in about 2.5 hr. Throughout these time periods, acoustic transmissions between 2 kHz and 10 kHz were carried out. The acoustic environment on the path is estimated by space and time interpolation between the tows on the two sides of the path. The acoustic data is compared with propagation modeling in this environment.

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.

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 50 kHz. 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 observed in the ocean experiment even though no shells were present. The laboratory results motivated development of a sediment model which incorporates both fluid viscosity and grain-to-grain interactions as embodied in a simple frequency-dependent, imaginary frame modulus first suggested by Biot.

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.

Efforts to model sound speed and attenuation in sandy sediments have centered on the use of theories for which either the relative motion of the pore fluid is the dominant attenuation mechanism, such as Biot theory, or the dominant loss mechanism is grain-to-grain friction. A recent model which attempts to incorporate grain-to-grain loss mechanisms into a model of sandy sediments was proposed by Buckingham. This model can fit the frequency dependence of the attenuation measured in ocean sediments and laboratory glass bead sediments, but it does not capture the sound speed dispersion as effectively as Biot theory. The relative success of each model suggests that both attenuation mechanisms may play important roles in sediment acoustics. In order to explore this possibility, a hybrid model has been developed which incorporates Buckingham's grain-to-grain shearing mechanisms into the frame moduli used in Biot theory. In the hybrid model, the grain-to-grain losses dominate at high and very low frequencies while pore fluid attenuation dominates at mid-frequencies where the sound speed dispersion is the most pronounced. As a consequence, the hybrid model is able to describe both the measured sound speed and attenuation in ocean and laboratory sediments.

Three bottom mounted sonar systems will be described that were built over a span of fifteen years. The complexity of deployment and sophistication of the tasks performed increased with each system. The first system is an autonomous tower with rotating sonar designed to examine backscattering from an area within 50 m radius of the tower. The second is a ship-cabled system that includes a diver movable tower and separate buried array for examining both backscattering and acoustic penetration into sediments. The last is a ship-cabled rail/tower system designed to carry out forward scattering and synthetic aperture backscattering measurements. All three systems are designed to remain deployed for time periods up to a couple of months. After describing the systems, their deployment and some example results, recent efforts will be described that are aimed at transitioning these types of systems to cabled ocean observatories. The overall goal of the talk is to indicate both the level of complexity that can be envisioned for bottom mounted systems as well as the new issues that must be addressed in moving to cabled ocean observatories.

For sediment interfaces that are very rough on the scale of the acoustic wavelength (i.e., kh greater than 1 where k is 2*n/wavelength and h is the rms roughness of the water/sediment interface) it is possible to estimate what the reflection coefficient would be if the interface were flat. In order to do so, a large ensemble of forward scattering measurements are needed in order to reduce the statistical uncertainty of the estimated reflection coefficient. In addition to the statistical uncertainty there can be biases in the estimate (for some grazing angles) that must be taken into account. The above conclusions will be supported through the use of monte carlo simulations of scattering from a rippled interface. The simulations are carried out in the context of discriminating between alternative acoustic models of sand sediments.

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.

As part of the environmental characterization to model acoustic bottom scattering during the high-frequency sediment acoustics experiment (SAX99), fine-scale sediment roughness of a medium sand was successfully measured within a 600 x 600-m area by two methods: stereo photography and a technique using a conductivity system. Areal coverage of the two methods, representing approximately 0.16 m² of the sea floor, was comparable, resulting in the depiction and quantification of half-meter wavelength sand ripples. Photogrammetric results were restricted to profiles digitized at 1-mm intervals; sediment conductivity results generated gridded micro-bathymetric measurements with 1- to 2-cm node spacing. Roughness power spectra give similar results in the low-spatial-frequency domains where the spectra estimated from both approaches overlap. However, spectra derived from higher resolution photogrammetric results appear to exhibit a multiple-power-law fit. Roughness measurements also indicate that spectrum changes as a function of time. Application of statistical confidence bounds on the power spectra indicates that roughness measurements separated by only 1-2 m may be spatially nonstationary.

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 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.

An experiment was carried out over a nine day period from August 18 to 27, 1996 to examine acoustic wave propagation in random media at frequencies applicable to synthetic aperture sonar. The objective was to test experimentally the hypothesized imaging effects of variations in the sound speed along two different acoustic paths as put forth by F.S. Henyey et al. (1997). The focus of this paper is on describing the experiment and carrying out an initial analysis of the data in the context of the effect of ocean internal waves on imaging resolution. The oceanography is summarized to the extent needed to discuss important aspects relative to the acoustics experiment. In the acoustics experiment transmissions at 6, 20, 75, and 129 kHz between sources and receiver arrays were carried out. Source to receiver separation was about 815 m. All sources and receivers were mounted on bottom-deployed towers and were at least 9 m off the seafloor. The analysis concentrates on the 75-kHz data acquired during one day of the experiment. The time span examined Is sufficient to examine a diurnal tidal cycle of the oceanographic conditions. The results indicate the IW phase perturbations would have a significant effect on imaging for even the most benign conditions of the experiment if no autofocusing scheme is used. Also, though autofocusing should be useful in recovering the focus for these conditions, there are conditions (e.g., for the path that has a turning point at the thermocline and during times when solibores are present), where more sophisticated compensation schemes would be needed.

A field experiment was carried out to examine the time variation of scattering from man-made objects placed near the water-sediment interface and within the sediment. The objects (spheres) were monitored for a period of about two months using a sonar system capable of measuring scattering levels, bottom bathymetry, and correlation of scattering over time. In addition, divers performed focalized biological treatments that were also monitored over extended periods. The results of these monitoring activities are presented and related to previous studies that used the same data sets for other purposes. One notable result is that the buried sphere becomes undetectable (by scattering level alone) within two days of deployment. The rapid changes in the first few days after the buried sphere is introduced are quantified relative to the rate of changes for undisturbed regions of the sediment.

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.

Biological and hydrodynamic processes can both create and destroy seafloor microtopography. As part of the SAX99 experiments, natural and artificial temporal changes in seafloor roughness were monitored acoustically and quantified using bottom stereo photographs. Feeding activities of benthic megafauna and fish destroyed large-scale roughness features generated by ocean surface gravity waves within a period of weeks to months; whereas, fine-scale roughness created by raking the seafloor decayed to background levels within 24 h. The effects of fine-scale roughness increased acoustic scattering centered at one-half the acoustic wavelength (a Bragg wavelength of 2 cm) by 12–18 dB in artificial manipulations of the bottom. These changes were restricted to roughness that was oriented predominantly orthogonal to the incident acoustic waves. Alternatively, seafloor roughness generated by ocean surface gravity waves had wavelengths of 50–100 cm and wave heights of 10–15 cm. These predictable large-scale roughness features should, by analogy, dramatically increase scattering at lower acoustic frequencies (near 1–2 kHz) and decay within weeks to months after storm events.

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.

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.

Several mechanisms have been proposed by which an incident acoustic wave could be coupled into the ocean floor at angles below the compressional critical angle. Recent observations of acoustic transmissions from water into sediment with arrival times and amplitudes inconsistent with the refractive compressional path have been interpreted as the excitation of a Biot slow wave in the sediment. Another hypothesis attributes the observed signals to scattering at the rough water–sediment interface. A third entails scattering of evanescent waves by volume inhomogeneities in the sediment. The existence of multiple hypotheses, each of which could account for the received energy, invited further investigation. A series of well-controlled laboratory measurements were made and compared with the results of a first-order perturbation theory model in order to evaluate the accuracy of the rough interface hypothesis. The experimental measurements of transmission through adjacent flat and rough interfaces show good agreement with the results of the numerical model, giving clear evidence of subcritical penetration in an environment incompatible with the requirements of the Biot slow wave and volume scattering hypotheses.

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.