The Cabled Observatory Vent Imaging Sonar (COVIS) was installed on the Ocean Observatories Initiative's Regional Cabled Array observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. The acoustic backscatter data recorded by COVIS in August–September 2018, in conjunction with in situ temperature measurements, are used to showcase and verify the use of COVIS for long‐term, quantitative monitoring of hydrothermal discharge. Specifically, sonar data processing generates three‐dimensional backscatter images of the buoyant plumes above major sulfide structures and two‐dimensional maps of diffuse flows within COVIS's field‐of‐view. The backscatter images show substantial changes of plume appearance and orientation that mostly reflect plume bending in the presence of ambient currents and potentially the variations of outflow fluxes. The intensity of acoustic backscatter decreases significantly for highly bent plumes as compared to nearly vertical plumes, reflecting enhanced mixing of plume fluids with seawater driven by ambient currents. A forward model of acoustic backscatter from a buoyancy‐driven plume developed in this study yields a reasonable match with the observation, which paves the way for inversely estimating the source heat flux of a hydrothermal plume from acoustic backscatter measurements. The acoustic observations of diffuse flows show large temporal variations on time scales of hours to days, especially at tidal frequencies, but no apparent long‐term trend. These findings demonstrate COVIS's ability to quantitatively monitor hydrothermal discharge from both focused and diffuse sources to provide the research community with key observational data for studying the linkage of hydrothermal activity with oceanic and geological processes.

Acoustic scattering from layered seafloors exhibits dependence on both the mean geoacoustic layering, as well as the roughness properties of each layer. Several theoretical treatments of this environment exist, including the small roughness perturbation approximation, the Kirchhoff approximation, and three different versions of the small slope approximation. All of these models give different results for the scattering cross section and coherent reflection coefficient, and there is currently no way to distinguish which model is the most correct. In this work, an integral equation for scattering from a layered seafloor with rough interfaces is presented, and compared with small roughness perturbation method, and two of the small slope approximations. It is found that the most recent small slope approximation by Jackson and Olson [J. Acoust. Soc. Am. 147(1), 56–73 (2020)] is the most accurate when the root-mean-square (rms) roughness is large, and some models are in close agreement with each other when the rms roughness is small.

The small-slope approximation (SSA) for rough-interface scattering is most commonly applied to the upper boundary of either impenetrable media or uniform half-space media, but has been recently developed for layered media in the acoustic and electromagnetic cases. The present work gives an overview of three forms of the SSA for layered media. The first has been previously presented in the acoustics literature. The second is from the electromagnetics literature and in the present work is converted to the fluid-sediment problem. A missing proof is supplied of a key consistency condition demanded of the small-slope ansatz. As is usual, these small-slope results are expressed in k-space. A third SSA for layered seafloors follows from conversion of the usual half-space formulation from k-space to coordinate space. This form turns out to be useful for reverberation simulations. The three different approaches are compared with respect to scattering strength and the coherent reflection coefficient, but an assessment of their relative merits will require comparison with exact calculations.

The Seabed Characterization Experiment was carried out from March 5 to April 10, 2017 (SBCEX17) on the New England Mud Patch, approximately 90 km south of Martha's Vineyard. The SBCEX17 experimental site covers an area of 11 km x 30 km with water depth in the range of 75–80 m. The Sediment Acoustic-speed Measurement System (SAMS) is designed to measure sediment sound speed and attenuation simultaneously over the surficial 3 m of sediments. During SBCEX17, SAMS was successfully deployed at 18 sites, which were chosen to coincide with coring locations, with the goal of developing a geoacoustic model for the study area. In this article, a summary of SAMS operation during SBCEX17 is presented, as well as preliminary results for sediment sound speed and its spatial variation in the frequency band of 2–10 kHz. It is found that in mud, the sound-speed ratio is in the range of 0.98–1. Little dispersion was observed in this frequency band. Using the preliminary SAMS sound-speed results measured at different depths, the sound-speed gradient in mud within the surficial 3 m favors an exponential rather than a linear dependence at SBCEX17 site. Large gradients are observed for depth shallower than 1.5 m. For the sandy basement beneath the mud layer, the sound-speed ratio is as high as 1.105.

A generalization of the conventional interface scattering cross section is introduced. This new object will be called the mutual scattering cross section, and, like the conventional cross section, can be used in narrow-band sonar applications. It can treat both sea-surface and seafloor scattering and is useful in cases where large arrays are employed as well as in multipath environments. The application to large arrays with uniform half-space water column and seafloor is examined briefly, but the bulk of this article is devoted to multipathing in the ocean waveguide. Comparisons with more accurate, but more numerically intensive, approaches in range-independent environments show that the mutual cross section can provide an efficient solution for the reverberation intensity time series. The mutual cross section incorporates interference effects causing oscillations in the reverberation time series. Such oscillations have been reported in the literature, but previous modeling efforts have been ad hoc, not based on scattering physics. The mutual cross section is shown to model backscattering enhancement due to multipathing, another phenomenon not seen in simpler models. Expressions for the mutual cross section are derived for seafloor roughness scattering and sediment volume scattering.

The primary goal of the Target and Reverberation Experiment in spring 2013 (TREX13) was to identify the major physical mechanisms responsible for midfrequency reverberation. While both the sea surface and seafloor can contribute to reverberation, the seafloor is typically dominant in shallow water environments. To determine the level of this contribution at the TREX13 site, the bottom backscatter sonar (BBS) was deployed from a dive boat at multiple locations around the site. The BBS consists of a source and a receiver mounted on a short bracket that is suspended above the seafloor to measure direct-path bottom backscatter at 3 kHz. Data near normal incidence were interpreted as bottom reflectivity, which was used to quantitatively explain the range-dependence of the sediment composition at the experiment site. Two factors restricted the estimates of the bottom backscatter strength to the minimum grazing angle of 21°: the currents at the experiment site made it difficult to position the system close to the seafloor, and the shallow water depth resulted in sea surface scatter contaminating small angle bottom backscatter. From the measured backscatter strength and by utilizing available environmental data, initial models of scattering strength indicate that at the shallow grazing angles of importance to reverberation, the scattering on the sand ridges is dominated by roughness scattering while in the muddy areas of the ridge swales, volume scattering dominates. The volume scattering from these mud areas is significantly stronger than the roughness scattering on the ridges by as much as 10 dB and may explain the substantial fluctuations observed in the reverberation as a function of range.

Bottom scattering is important for a number of underwater applications: it is a source of noise in target detection and a source of information for sediment classification and geoacoustic inversion. While current models can predict the effective interface scattering strength for layered sediments, these models cannot directly compute the ensemble averaged mean-square pressure. A model for bottom scattering due to a point source is introduced which provides a full-wave solution for mean-square scattered pressure as a function of time under first-order perturbation theory. Examples of backscatter time series from various types of seafloors will be shown, and the advantages and limitations of this model will be discussed.

Acoustic imaging of hydrothermal flow regimes started with the incidental recognition of a plume on a routine sonar scan for obstacles in the path of the human-occupied submersible ALVIN. Developments in sonar engineering, acoustic data processing and scientific visualization have been combined to develop technology which can effectively capture the behavior of focused and diffuse hydrothermal discharge. This paper traces the development of these acoustic imaging techniques for hydrothermal flow regimes from their conception through to the development of the Cabled Observatory Vent Imaging Sonar (COVIS). COVIS has monitored such flow eight times a day for several years. Successful acoustic techniques for estimating plume entrainment, bending, vertical rise, volume flux, and heat flux are presented as is the state-of-the-art in diffuse flow detection.

Continuous time-series observations are key to understanding the temporal evolution of a seafloor hydrothermal system and its interplay with thermal and chemical processes in the ocean and Earth interior. In this paper, we present a 26-month time series of the heat flux driving a hydrothermal plume on the Endeavour Segment of the Juan de Fuca Ridge obtained using the Cabled Observatory Vent Imaging Sonar (COVIS). Since 2010, COVIS has been connected to the North East Pacific Time-series Underwater Networked Experiment (NEPTUNE) observatory that provides power and real-time data transmission. The heat flux time series has a mean value of 18.10 MW and a standard deviation of 6.44 MW. The time series has no significant global trend, suggesting the hydrothermal heat source remained steady during the observation period. The steadiness of the hydrothermal heat source coincides with reduced seismic activity at Endeavour observed in the seismic data recorded by an ocean bottom seismometer from 2011 to 2013. Furthermore, first-order estimation of heat flux based on the temperature measurements made by the Benthic and Resistivity Sensors (BARS) at a neighboring vent also supports the steadiness of the hydrothermal heat source.

We present a 26 day time series (October 2010) of physical properties (volume flux, flow velocity, expansion rate) of a vigorous deep-sea hydrothermal plume measured using our Cabled Observatory Vent Imaging Sonar (COVIS), which is connected to the Northeast Pacific Time Series Underwater Experiment Canada Cabled Observatory at the Main Endeavour Field on the Juan de Fuca Ridge. COVIS quantitatively monitors the initial buoyant rise of the plume from inline image to inline image above the vents. The time series exhibits temporal variations of the plume vertical volume flux ( inline image), centerline vertical velocity component ( inline image) and expansion rate ( inline image); these variations have major spectral peaks at semidiurnal ( inline image cycle/day) and inertial oscillation ( inline image cycle/day) frequencies. The plume expansion rate (average inline image) is inversely proportional to the plume centerline vertical velocity component (coefficient of determination inline image). This inverse proportionality, as well as the semidiurnal frequency, indicates interaction between the plume and ambient ocean currents consistent with an entrainment of ambient seawater that increases with the magnitude of ambient currents. The inertial oscillations observed in the time series provide evidence for the influence of surface storms on the dynamics of hydrothermal plumes.

The small-slope approximation has found application to unlayered seabeds and is generally regarded as an improvement over methods that employ either small-roughness perturbation theory or the Kirchhoff approximation. Unfortunately, the usual small-slope ansatz fails when applied to layered seabeds, as it is inconsistent with perturbation theory. This ansatz is replaced by an alternative which is found to satisfy the criteria of reciprocity and consistency with the perturbation and Kirchhoff approximations. This approach will be illustrated by computation of the coherent reflection coefficient and scattering strength for a seabed consisting of a single rough fluid layer over a semi-infinite, elastic basement with flat upper boundary. Computation time is significantly longer than for the unlayered case, increasing as the desired accuracy increases. The results will be contrasted with those obtained using a variety of existing approximations.

In the Spring of 2012, high-frequency backscattering from a sandy sediment was measured in the Gulf of Mexico at the site of the upcoming, ONR-sponsored reverberation experiment. The measurements were made using an array of sources and receivers that collected data from 200 to 500 kHz and that could be rotated such that the incident grazing angles varied from 10 to 50 degrees. This array was used previously to measure scattering from a sand/mud sediment during the Sediment Acoustics Experiment 2004 (SAX04). To support data/model comparisons, the seabed roughness, sediment shell content, sediment sound speed, and sediment attenuation were also measured. For scattering below the critical grazing angle, sediment roughness is found to be the dominant scattering mechanism while above the critical angle, roughness scattering underpredicts the measured scattering strength. To understand the scattering strength at high grazing angles, scattering from shells and shell hash is considered. The measured scattering strengths and environmental properties at the experiment site are also compared to those made during SAX04.

A rough-interface reverberation model is developed for range-dependent environments. First-order perturbation theory is employed, and the unperturbed background medium can be layered and heterogeneous with arbitrary range dependence. To calculate the reverberation field, two-way forward scatter due to the slowly changing unperturbed environment is handled by fast numerical methods. Backscatter due to small roughness superimposed on any of the slowly varying interfaces is handled efficiently using a Monte Carlo approach. Numerical examples are presented to demonstrate the application of the model. The primary purpose of the model is to incorporate relevant physics while improving computational speed.

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.

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems many of which operate at mid-frequencies (1–10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data between 2 and 5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters density, compressional wave speed, and attenuation, was used for inversion by fitting the model to data in the least-square sense. Inverted sediment sound speed and attenuation were compared with direct measurements and with inversion results using different techniques carried out in SW06. Inverted results of the present work are consistent with other measurements, considering the known spatial variability in this area. The observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.

This communication corrects errors and supplies missing parameter values for a previous publication by the authors (ibid., vol. 35, no. 3, pp. 603-617, Jul. 2010) regarding the geoacoustic bottom interaction model (GABIM).

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems, many of which operate at mid-frequencies (1-10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data in the frequency band of 2-5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters, density, compressional wave speed, and attenuation, was used for geoacoustic inversion by fitting the model to data in the least-squares sense. Inverted sediment sound speed was compared with direct measurements and inversion results using different techniques in the same area. The comparison shows that bottom loss can be used to infer sediment geoacoustic parameters at mid-frequencies. In addition, observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.

The Cabled Observatory Vent Imaging Sonar has been deployed at the Main Endeavour Node of the Canadian Neptune cabled observatory and has acquired data on plume and diffuse hydrothermal flows. Based on the Reson 7125 multibeam sonar and operating at 200 and 400 kHz, two-dimensional and three-dimensional time series are produced using plume backscattering, Doppler shift, and acoustic scintillation. Hydrothermal plumes and diffuse flow are important as agents of transfer of heat, chemicals, and biological material from the mantle and crust into the ocean in quantitatively significant amounts. High-frequency sonar measurements offer the possibility of inversion to obtain fluxes of central importance in these processes. Long-term time series, obtainable in cabled systems, allow observations of hydrothermal response to tidal, tectonic, and volcanic forcing. Examples will be given of plume bending due to currents, determination of entrainment of ambient water, time variation of diffuse flows, and Doppler determination of volume flux.

Reflection off the rough sea surface typically introduces time spread in an acoustic signal. The details of channel response and hence the time spread will change as the sea surface evolves. Communications signals that are spread by the rough surface may still be recompressed and demodulated successfully by an equalizer providing that the channel response does not change too rapidly. To aid in designing an equalizer, it would be useful to know which acoustic paths should be treated as useful signal and which must be treated as noise because they change too rapidly. Viewed as a rough surface scattering problem, the classic Kirchhoff approximation should be appropriate for modeling reflected communications signals. Textbook descriptions of the Kirchhoff approximation are not promising, however, as they imply that the calculations depend on the details of the surface wave spectrum. In the present work, simplified expressions for the surface reflected communications signals are derived. The simplified results are tested in two ways: Predictions for the mutual coherence function are compared to numerically intense calculations, and predictions for communications performance are compared to experimental results.

A first-order perturbation model of scattering in an elastic medium is revisited and discussed. The material properties of the medium are defined by three spatially fluctuating variables, the density and two Lame parameters. The wave interaction process is described in terms of four mechanisms of scattering and energy conversion: two without change of the wave type, from compressional to compressional and from shear to shear, and two with the type conversion, from compressional to shear and vice versa. The model is applied to the case of acoustic scattering from and propagation in underwater sediments of different types, sand and rock. Wave interaction and attenuation due to various mechanisms of scattering in the sediment are considered. An improved method for calculation of the seabed scattering strength is proposed, which takes into account the so-called "windowing" effect. It allows more accurate accounting for the contribution of volume heterogeneities near the sediment surface and its comparison with the first-order roughness mechanism of scattering. The frequency-angular dependencies of the scattering strength for elastic sandy and rocky seafloors are calculated, and behaviors of the volume and roughness contributions are compared.

The geoacoustic bottom interaction model (GABIM) has been developed for application over the low-frequency and midfrequency range (100 Hz to 10 kHz). It yields values for bottom backscattering strength and bottom loss for stratified seafloors. The model input parameters are first defined, after which the zeroth-order, nonrandom problem is discussed. Standard codes are used to obtain bottom loss, uncorrected for scattering, and as the first step in computation of scattering. The kernel for interface scattering employs a combination of the Kirchhoff approximation, first-order perturbation theory, and an empirical expression for very rough seafloors. The kernel for sediment volume scattering can be chosen as empirical or physical, the latter based on first-order perturbation theory. Examples are provided to illustrate the various scattering kernels and to show the behavior predicted by the full model for layered seafloors. Suggestions are made for improvements and generalizations of the model.

As part of the effort to characterize the acoustic and physical properties of the seafloor during the high-frequency 2004 Sediment Acoustics Experiment (SAX04), fine-scale variability of sediment sound speed and density was measured in a medium quartz sand using diver cores and an in situ conductivity probe. This study has a goal of providing environmental input to high-frequency backscatter modeling efforts. Because the experiment was conducted immediately following exposure of the site to Hurricane Ivan, measurements revealed storm-generated sedimentary structure that included mud deposits and trapped sand pockets suspended in the mud.

Fluctuations of sediment sound speed and density were measured downcore at 1- and 2-cm increments, respectively, with standard laboratory techniques. Sediment density was also measured on a very fine scale with an in situ conductivity probe [in situ measurement of porosity (IMP2)] and by means of computed tomography (CT) imaging of a diver core. Overlap between the locations of the diver cores and the conductivity probe measurements allowed an examination of multiple scales of sediment heterogeneity and a comparison of techniques. Sediment heterogeneity was characterized using estimates of covariance corresponding to an algebraic form for the power spectrum of fluctuations obtained from core, conductivity, and CT measurements. Correcting for sampling brings the power spectra for density fluctuations determined from the various measurements into agreement, and supports description of heterogeneity at the site over a wide range of scales by a power spectrum having a simple algebraic form.

While Biot theory can successfully account for the dispersion observed in sand sediments, the attenuation at high frequencies has been observed to increase more rapidly than Biot theory would predict. In an effort to account for this additional loss, perturbation theory is applied to Biot's poroelastic equations to model the loss due to the scattering of energy from heterogeneities in the sediment. A general theory for propagation loss is developed and applied to a medium with a randomly varying frame bulk modulus. The theory predicts that these heterogeneities produce an overall softening of the medium as well as scattering of energy from the mean fast compressional wave into incoherent fast and slow compressional waves. This theory is applied to two poroelastic media: a weakly consolidated sand sediment and a consolidated sintered glass bead pack. The random variations in the frame modulus do not have significant effects on the propagation through the sand sediment but do play an important role in the propagation through the consolidated medium.

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.

The problem of sound scattering by surfaces having sinusoidal shape has been intensively studied, yet fundamental questions remain. The present work employs this problem as a testing ground for our understanding of small-roughness perturbation theory. Extending previous work by the author, the convergence of the perturbation series is studied using numerical calculations. As has been noted previously, the Rayleigh hypothesis does not set the bounds for convergence to the correct radiated field, but is relevant to determination of the surface field. In the present work, both topics are examined in some detail, and conformal mapping is used to support some of the conclusions.

Acoustic backscattering from a diver-smoothed sand sediment was measured at frequencies from 200 to 500 kHz as a function of grazing angle. The residual roughness of this smoothed surface was measured using a laser line scanning system capable of measuring sub-millimeter heights over a 4-meter track. Using the measured sediment roughness, perturbation theory underestimates the scattering strength at all frequencies for angles greater than the critical grazing angle. The absence of large, discrete scatterers in the sediment suggests that scattering from fluctuations in the sediment properties may be the dominant scattering mechanism for these angles. The sound speed and attenuation were also measured in this sediment and the attenuation was found to exhibit a linear frequency dependence similar to that observed for other sand sediments.

To account for this linear attenuation, a theory that incorporated scattering losses due to porosity fluctuations into the effective density fluid model has been developed. This theory suggests that the propagation losses may be connected by the same physical mechanism to the scattering of sound from the sediment. This connection is explored in the context of these scattering and propagation measurements.

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.

While the mechanism of Bragg scattering is well known, most experimental work has been concentrated in the area of narrow band sound sources and in the far-field. Motivated by underwater detection problems in the presence of sediment ripple fields, we report laboratory measurements of broadband sound scattering from a sinusoidal surface machined on a polyurethane board. The surface has a wavelength of 8 mm and peak-to-peak height of 2 mm. Coherently scattered sound data were taken in near-field geometries and in the frequency band of 150–400 kHz. The measurement geometry is such that a broad range of Bragg angles corresponding to the frequency band are covered. We observe that the scattered sound demonstrates a down chirp time dependence when the incident sound is a short pulse. Models based on first order perturbation theory were developed which explain the observed scattered sound in both magnitude and phase. In addition, we also measured second order Bragg scattering. This motivates modeling efforts on higher order Bragg scatter.

In this experimental effort, field data and modeling are used to investigate backscatter imaging of schools of pelagic fish in a shallow water oceanic waveguide. Two-dimensional (2-D) circular images of an oceanic waveguide are created using a monostatic circular receiving array and vertical line array source (12 kHz) deployed from a stationary ship. Horizontal multibeam images of a radial (depth integrated) cross section of the ocean are formed with targets and reverberation structure observed at ranges up to several km. Examples of waveguide images are presented for data taken in the Puget Sound of Washington State. In some instances, the backscatter from aggregations of fish was observed to be higher than the reverberation; in other cases, fish schools could not be resolved. Fish school locations were independently observed by vertical echo sounding. Preliminary experimental results are shown to illustrate the potential of resolving the 2-D horizontal structure of aggregations of fish using such methods. Modeling is used to interpret the effects of waveguide propagation on imaging. Operational issues related to using waveguide imaging for fisheries research and signal processing issues related to waveguide imaging will be discussed.

Sand sediments are inherently heterogeneous due to the random packing of the grains. For sound propagation through fluid-saturated sediments, these heterogeneities may lead to scattering from the coherent fast compressional wave into incoherent slow compressional waves or shear waves. This loss of energy from the fast compressional wave may account for the increase in high-frequency attenuation above that predicted by Biot theory.

In a previous talk, we presented preliminary results of applying perturbation theory to Biot theory in order to model scattering from heterogeneities in the porosity [Hefner et al., J. Acoust. Soc. Am. 120, 3098 (2006)]. This theory has since been refined to properly account for scattering into both the slow compressional wave and the shear wave. In order to apply this theory to a given sand sediment, knowledge of the covariance function for the spatial variations in the porosity is required. Results of the theory will be presented for several different analytic covariance functions, as well as for covariance functions measured in simulated and real unconsolidated granular materials.

Previously, we presented the results of applying perturbation theory to the problem of fast compressional wave propagation through a Biot medium with heterogeneities in the frame bulk modulus [B. T. Hefner et al., J. Acoust. Soc. Am. 119, 3447 (2006)]. It was found that the heterogeneities scattered energy into both the slow and fast compressional waves, thus increasing the attenuation of the fast compressional wave. This theory has since been generalized to account for heterogeneities in both the frame bulk and shear moduli. For the fast compressional wave, energy is now scattered into the shear wave as well as the fast and slow compressional waves, further increasing the attenuation of the coherent field. While shear wave propagation is unaffected by variations in the frame bulk modulus, scattering does occur when there are heterogeneities in the shear modulus. Energy in the shear wave is scattered into both shear and compressional waves as well. The generalized theory depends on the autocorrelation functions of both the shear and bulk moduli variations as well as the cross-correlation function between the moduli. Efforts are underway to estimate these statistics in simple random packings of spherical grains using discrete-element modeling.

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.

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.

A method is developed for using multibeam sonar to map the flow velocity field of black smoker plumes. The method is used to obtain two-dimensional cross-sectional maps of vertical velocity, but is capable of mapping velocity in three dimensions. This is in contrast to conventional current meters, which measure only at several points and acoustic Doppler current profilers, whose diverging beams cannot readily map the interior of a plume. Geometric corrections are used to estimate the vertical component of velocity, compensating for ambient current. The method is demonstrated using data from the main plume at the Grotto vent complex in the Main Endeavour Field, Juan de Fuca Ridge, and the errors due to noise, signal fluctuations, and fluctuations in plume structure are estimated.

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.

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 decision-directed extension to passive phase conjugation array demodulation is described. Initialisation by a training preamble is followed by block least-squares channel estimation via conjugate gradient and memoryless decisions, which are used by estimation in the next block. Excellent shallow-water performance is shown at ranges up to 46 km under windy conditions and shipping noise. The algorithm demonstrates low error rate and robust channel tracking.

A new method for underwater acoustic communication called passive phase conjugation is evaluated. The method begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. The parallel processing makes the method computationally efficient and allows near real-time communication.

We describe a novel decision-directed structure that extends the passive phase conjugation (PPC) array demodulation method of Rouseff et al.. The PPC transmission scheme is a single source and multiple receive sensors. Our algorithm uses an initial training period with known data, followed by block-update LS channel estimation via LSQR (Paige and Saunders, 1982) iterations, and subsequent memoryless decisions. Decisions are circulated back into the LS model for the next estimation block. Performance on a shallow-water acoustic channel is shown at ranges up to 4.6 km under windy surface conditions and shipping noise. The algorithm demonstrates good bit error rates, and robustly tracks time-varying channels. Its unquantized output can be further processed by conventional equalizers.

Active phase conjugation requires an array capable of both transmitting and receiving. A field incident on the array can be refocused both in space and time at the location of the original source. To do acoustic communication, an additional step in the processing is introduced; prior to backpropagation, the measured probe field is first convolved with a data stream. The direction of communication is from the active array to the location of the original point source. By contrast, in passive phase conjugation [D. R. Jackson et al., J. Acoust. Soc. Am. 108, 2607 (2000)] the direction of communication is from the point source to the passive array. Further results from an experiment conducted in Puget Sound are presented. The effects of array curvature and truncation are discussed.

A new method for coherent underwater acoustic communication called passive phase conjugation is evaluated. The method is so named because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean. In contrast to active techniques, however, the array in passive phase conjugation needs only receive. The procedure begins with a source transmitting a single probe pulse. After waiting for the multipathed arrivals to clear, the source then transmits the data stream. At each element in the distant receiving array, the received probe is cross-correlated with the received data stream. This cross-correlation is done in parallel at each array element and the results are summed across the array to achieve the final communication signal suitable for demodulation. As the ocean changes, it becomes necessary to break up the data stream and insert new probe pulses. Results from an experiment conducted in Puget Sound near Seattle are reported. Measurements were made at multiple ranges and water depths in range-dependent environments.

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.

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.

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.

A new method for coherent underwater communication called passive phase conjugation is evaluated. The technique takes its name because of conceptual similarities to active phase conjugation methods that have been demonstrated in the ocean [Kuperman et al., J. Acoust. Soc. Am. 103, 25-40 (1998)]. In contrast to active techniques, however, the array in passive phase conjugation need only receive. This makes the method plausible for scenarios where spatially compact sources might be communicating to a distant receive-only array. Compared to other approaches for coherent communication, the computational burden is low allowing the method to be evaluated in the field in nearly real-time. Results from an experiment conducted in Puget Sound near Seattle in May 2000 are reported. Various modulation schemes and array geometries were employed. Measurements were made at several ranges and water depths in a range-dependent environment.

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.