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.

A full-field perturbation approach is modified for an ice-covered ocean and applied to estimating narrowband long-range reverberation caused by roughness of the ice–water interface. First-order approximation of the approach is used which requires the roughness amplitudes be small compared to the acoustic wavelength. To obtain the zeroth-order Green's function and transmission loss field used in the reverberation model, elastic parabolic equation solutions are generated in range-independent environments. Ice is represented by an isospeed layer on top of a linear transition layer. Effects of ice properties are discussed and demonstrated by comparing reverberation calculated for different ice layer thicknesses and wave speeds for typical ice values.

A perturbation approach to roughness scattering and reverberation in range-dependent environments is developed treating each interface as a superposition of a smooth reference interface, which may include large-scale deterministic features (such as bathymetry changes), and small compared to the acoustic wavelength vertical deviations from this interface that are considered as random roughness perturbations. The reference interface is assumed to be smooth enough to allow analytic or numerical solution for the field in the vicinity of this interface that can then be used in perturbation theory. Expressions for both the reverberation field and average reverberation intensity in a general case of an arbitrary number of rough interfaces are obtained in a form convenient for numerical simulations. In the case of long-range ocean reverberation, several approximations for these expressions are developed, relevant to various environmental scenarios and different types of interfaces: sea-surface, water-sediment interface, buried sediment interfaces, and bottom basement. The results are presented in a simple form and provide a direct relationship of the reverberation intensity with three critical characteristics defined at each interface: (1) local spectrum of roughness, (2) local contrast of acoustic parameters, and (3) two-way full-field transmission intensity calculated taking into account only large-scale changes of the environment.

The long-term goals of this research are to better understand and accurately model low- to mid-frequency reverberation in shallow water environments. Specific goals are to develop a model of reverberation for conditions (1–10 kHz, ~ 20 m water depth, ~ 10 km range) corresponding to the ONR Target and Reverberation Experiment performed in spring 2013 (TREX2013), develop a code and conduct computer simulations with environmental inputs typical for the chosen location, and apply this model to analysis of available TREX2013 data. This report presents a Green's function modeling approach that allows fast estimations of volume reverberation in complex shallow water environments. A simplified first-order version of the approach is considered to show how far-field scattering solutions obtained for free space can be incorporated into reverberation in complicated bounded, range-dependent, and stratified environments. A higher-order modification of this approach is considered as well, using a MFSB (Multiple Forward Single Backscatter) approximation. Application to TREX2013 reverberation data and tentative model–data comparisons are presented.

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.

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).

Geological underwater processes are affected by physical properties of sediment particles, their size, shape, and spatial variability. In geological modeling, these parameters are normally provided by analysis of sediment cores and documented in certain geologically relevant terms. What is the set of parameters most relevant from acoustics standpoint, or in "acoustically relevant" terms, is an open question. It is addressed using an acoustic scattering model directly based on sediment particle analysis, geo-acoustic model of bottom interaction taking into account the sediment discrete heterogeneity. The particle size distribution in this model is comprised of central and coarse parts. The central part describes the sediment matrix and its large scale variability, or continuous heterogeneity, critical for modeling of acoustic propagation in the sediment. To account for discrete component of heterogeneity, the coarse part of size/shape distributions is attributed to 'inclusions' in the sediment matrix and provides direct inputs for evaluation of volume scattering in the sediment. An example of such analysis and modeling based on the SAX04 geo-acoustic data set is presented and used for calculating the frequency-angular dependences of bottom backscattering strength. Comparisons with acoustic backscatter data measured at this site and possibilities for geoacoustic inversions are discussed.

Hydrates have a strong effect on elastic properties of the seafloor and their spatial (vertical and lateral) variability. The contrast in velocity created by the hydrate-cemented zone produces a strong seismic and acoustic reflection, the "bottom simulating reflection," which is commonly used to locate gas hydrate deposits. Gas-hydrate-cemented strata also act as seals for trapped free gas. Hydrates may form complicated three-dimensional (3-D) structures and inclusions, such as nodes, veins, and flakes, resulting in significant volume heterogeneity of the sediment. This may cause a strong impact on the seabed scattering properties. Currently existing models of scattering from heterogeneous sea beds can be used for predicting this impact, and also serve as a base for development of new algorithms for remote acoustic characterization of gas hydrates and monitoring their stability. This possibility is discussed, and examples of such modeling are presented.

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.

Sound scattering from a target situated near a water-sediment interface was studied in laboratory conditions in order to control separately all the parameters involved in the scattering process. Targets of different sizes were ensonified with wide band transducers covering the frequency range 200 kHz to 1 MHz. First, the target scattering strength was measured in the free space conditions, and the scattering strength of the water–sediment interface was measured at oblique incidence. These characteristics were used to provide a rough estimate for the signal-to-noise ratio for the second set of experiments where the target was situated near the interface to study effects of target–boundary interactions. The intensity of the total scattered field was measured as a function of the beamwidth, transducer/object and object/interface distances, frequency, grazing angle, target size and the interface roughness parameters. The interface considered here is a flattened sand surface which was studied earlier [Ivakin and Sessarego, High frequency scattering from flattened sand sediments: effects of granular structure, J. Acoust. Soc. Am., 122, (5) 2007]. The targets were spherical glass beads of different size. Side scan sonar images are presented and possibilities of their qualitative interpretation are discussed.

Acoustic properties of water-saturated granular sediments at frequencies from 150 kHz to 8 MHz were studied in controlled laboratory conditions using broadband transducers. Two samples of medium sand sediments, with the same mean grain size, but with different width of the size distribution, were taken for the study, degassed, and their surface was flattened. Another sample of sediments was composed of glass beads of the same grain size. The main difference of glass beads from sand grains was their shape. Backscattering strength at normal and oblique incidence and reflection coefficient at normal incidence were measured for the three samples. The reflection experiments were made for different thicknesses of the samples, so that reflections from both first and second interfaces of the sediment layer were measured. This allowed also estimating sound speed and attenuation in the sediments. The results obtained for the three chosen types of the sediments were compared to demonstrate effects of the grain size distribution width and the grain shape on acoustic properties of the sediment.

A model of high frequency scattering from sediments with discrete inclusions (such as shells and shell fragments) having an arbitrary size–depth distribution is developed. The model assumes also knowledge of a depth-dependent individual scattering function of inclusions. It is more general than in previous models and includes discrete scatterers located both below and on the water–sediment interface (partially buried). Some simple results are obtained for the case of high enough frequencies using approaches of geometry acoustics. Frequency–angular dependencies of the bottom backscattering strength are calculated for sediments with different size–depth distributions of inclusions. Inputs for the size distribution of inclusions (shell fragments) are obtained from granulometric analysis of coarse fractions of the sediment samples taken at the SAX99/SAX04 site (near Ft. Walton Beach, Florida). It is shown that taking into account partially buried shells is important and can significantly enhance estimates of the bottom scattering strength especially at grazing angles below critical (about 30 degrees for sand).

A unified model of reverberation in a shallow-water waveguide caused by the volume heterogeneity and rough interfaces is proposed. Normal modes are used to describe propagation of a narrow-band signal from a point source to the scattering volume (a vertical column of the waveguide including both water and sediment) and from the scattering volume to the receiver. A local scattering matrix describes a process of reradiation from one normal mode to another within the scattering volume. The case of statistically axial symmetry, where the source and receiver are separated only in the vertical direction and the medium is statistically homogeneous and isotropic in the horizontal plane, is considered in more detail. A simple relationship of the temporal dependence of the reverberation intensity with the scattering matrix, attenuation, and group velocities of the normal modes is obtained. Contributions of different components of reverberation due to rough air–water and water–sediment interfaces, volume heterogeneity of the water column, and the sediment are discussed.

Sound backscattering from water-saturated granular sediments at frequencies from 150 kHz to 8 MHz at normal and oblique incidence was studied in controlled laboratory conditions. Two kinds of sediments, moderately sorted medium and coarse sands, were chosen for the study, degassed, and their surface was flattened. At oblique incidence, intrinsic scattering due to the sediment granular structure can be considered as a dominating mechanism of backscatter. Comparison of frequency dependencies of the backscattering strength for the two sediments with different mean grain size shows the existence of a persistent scaling effect that allows a description of the backscattering strength as a unique scaling function of one parameter, the mean grain size/wavelength ratio. For a wide range of this ratio, 0.15 to 1.3, the scaling function has been measured and presented, which is the major result of this work. Possible applications to remote sensing of marine sediments, and particularly determining the sediment mean grain size, are discussed.

Sound backscattering from water-saturated granular sediments at frequencies from 150 kHz to 8 MHz at oblique incidence was studied in controlled laboratory conditions. Two kinds of sediments, medium and coarse sands, were degassed, and their surface was flattened. In these conditions, the sediment granular structure can be considered as a controlling mechanism of backscattering. Comparison of frequency dependencies of backscatter for the two sediments with different mean grain size shows the existence of a persistent scaling effect that allows description of the backscattering strength as a function of one parameter, the mean grain size/wavelength ratio.