Sound is usually used for sensing in underwater environments, because it is absorbed less by seawater than electromagnetic waves and thus travels longer distances underwater. In shallow ocean regions, one key factor that influences the propagation of sound waves underwater is the seabed. In such regions, the characteristics of the sound heard at any location and how it depends on the sound transmitted at other locations, is shaped by the numerous interactions the sound waves undergo with the seabed and sea surface in terms of reflection, refraction, absorption and scattering.

The type and properties of the seabed such as the density of its sediment, its soundspeed, and its absorption of sound, can affect how far sound waves travel in the ocean and how they are scattered or received at other points. Consequently, it helps us understand how underwater acoustic equipment such as sonars or communication modems may work, and also helps us strategize how to operate our underwater vehicles and plan their navigation. All these can have serious implications for underwater sensing and exploration. Thus, characterizing the seabed is an important step towards understanding the properties of the sound field and how this impacts our acoustic sensing, communication and navigation capabilities underwater. Information on the type and properties of the seabed could also be used for studies in varied fields such as marine biology, geology and chemistry.

In January 2020, IEEE Journal of Oceanic Engineering has published a special issue on seabed characterization. The special issue features a guest editorial and 15 articles on the Seabed Characterization Experiment - an extensive scientific exercise conducted in 2015 and 2016 to understand the seabed better. The special issue also has other technical articles related to the topic of seabed characterization.

Here, we list out the articles published under this special issue and their abstracts.

The seabed impacts sound propagation via mechanisms such as reflection, refraction and scattering and absorption. ¹

Guest Editorial: An Overview of the Seabed Characterization Experiment - P. S. Wilson, D. P. Knobles, and T. B. Neilsen

In littoral ocean environments, knowledge of the acoustic properties of the seabed is generally required to predict the acoustic field. An ability to make accurate predictions of the acoustic field is important in the areas of sonar, acoustic communication, and navigation. Much effort has been expended over the past half-century to understand and model the acoustic behavior of sandy sediments, composed primarily of medium or coarse grains, over the entire frequency range of interest to underwater acoustics, O(10) Hz to O(1) MHz. Extensive measurements of sound speed and attenuation in sandy sediments have been collected over large portions of the world and compared to models [see, for example, item 1) of theAppendix], but such is not the case for fine-grained sediments. The work described in this special issue seeks to address this deficiency.

Short-Range Signatures of Explosive Sounds in Shallow Water Used for Seabed Characterization - P. S.Wilson, D. P. Knobles, P. H. Dahl, A. R. McNeese, and M. C. Zeh

Small explosions were used as sound sources in the Seabed Characterization Experiment conducted in spring 2017 in the New England Mud Patch, a shallow-water region with a depth of approximately 75 m. The sources were U.S. Navy Signal Underwater Sound (SUS) Mk 64 charges, which contained 31.18 g of the explosive 2,4,6-Trinitrophenylmethylnitramine, commonly referred to as Tetryl. Source recordings were obtained by two hydrophones deployed from the same ship that deployed the SUS. The recordings were analyzed for bubble period, energy spectral density, and the variability of these parameters, and compared to previous results from the literature, including the prediction of a historic spectral model, and a new semiempirical time-domain model assembled using measured data from the literature. The new model describes the source level measurements in the 25–275-Hz band and in the 400-Hz octave band to within 0.5 dB, and agrees with similar measurements from the literature to within 0.6 dB. The standard deviation of the band-limited source levels was found to be about 1 dB, some of which is ascribed to uncertainty and variation in the source-to-receiver distance. The observed source level variation is similar to previously reported values.

In Situ Measurements of Compressional Wave Speed During Gravity Coring Operations in the New England Mud Patch - M. S. Ballard, K.M. Lee, A. R.McNeese, P. S.Wilson, J. D. Chaytor, J. A. Goff, and A. H. Reed

This paper presents measurements of sediment sound-speed profiles measured in situ using the acoustic coring system (ACS). The reported measurements were obtained from seven gravity cores collected in the New England Mud Patch. The ACS uses two sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. The in situ sound-speed profiles are interpreted in the context of stratigraphic layering measured by a seismic survey, and the measured sound-speed profiles are consistent with the geophysical description. The in situ sound-speed profiles measured by the ACS were compared to conventional measurements of sound speed on the recovered cores using the multisensor core logger (MSCL). The MSCL data displayed both random and systematic errors that were attributed to disturbance from the coring process and/or the handling of the sediments after collection. Finally, using porosity and grain size distributions measured from discrete samples of the cored sediments, the in situ sound-speed measurements were compared to empirical regressions based on independent data sets.

Measurement of Sound Speed in Fine-Grained Sediments During the Seabed Characterization Experiment - J. Yang and D. R. Jackson

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 × 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.

Estimation of the Geoacoustic Properties of the New England Mud Patch From the Vertical Coherence of the Ambient Noise in theWater Column - D. R. Barclay, D. A. Bevans, and M. J. Buckingham

The autonomous passive-acoustic lander Deep Sound was deployed at five locations during the Office of Naval Research (ONR)-supported Seabed Characterization Experiment, a multi-institutional field effort held at the New England Mud Patch, where the seabed is known to consist of a thick layer of silt and clay overlying a medium and coarse sand. The five deployments of Deep Sound were up to 9 h long, during which time ambient noise data, taken over an acoustic bandwidth of 5 Hz–30 kHz, were collected on four hydrophones arranged in an inverted “T” shape. Local temperature and conductivity were also recorded continuously at each location. A wave number integral model of wind-driven noise in a fluid waveguide over a two-layered elastic seabed was used to calculate the dependence of the vertical noise coherence on the geoacoustic properties in the overlying silt and clay layer, as well as the subbottom sand half-space. The modeled noise coherence was fitted to the data over the band 100 Hz–12 kHz, returning the compressional- and shear-wave speeds and densities of both layers and the thickness of the top layer.

Linearized Bayesian Inversion for Experiment Geometry at the New England Mud Patch  - J. Belcourt, S. E. Dosso, C. W. Holland, and J. Dettmer

This paper presents a linearized Bayesian approach to invert acoustic arrival-time data for high-precision estimation of experiment geometry and uncertainties for geoacoustic inversion applications. The data considered here were collected as part of the 2017 Seabed Characterization Experiment at the New England Mud Patch for the purpose of carrying out broadband reflection-coefficient inversion. The calculation of reflection coefficients requires accurate knowledge of the survey geometry. To provide this, a Bayesian ray-based inversion is developed here that estimates source–receiver ranges, source depths, receiver depths, and water depths at reflection points along the track to much higher precision than prior information based on GPS and bathymetry measurements. Near the closest point of approach, where rays are near vertical, data information is low and inaccurate range estimates are improved using priors from analytic predictions based on nearby sections of the track. Uncertainties are obtained using analytic linearized estimates, and verified with nonlinear analysis. The high-precision experiment geometry is subsequently used to calculate grazing angles, with angle uncertainties computed using Monte Carlo methods.

Depth-Dependent Geoacoustic Inferences With Dispersion at the New England Mud Patch via Reflection Coefficient Inversion - J. Belcourt, C. W. Holland, S. E. Dosso, J. Dettmer, and J. A. Goff

Depth-dependent geoacoustic properties are inferred from wide-angle frequency-domain reflection-coefficient data at two sites with different mud-layer thicknesses on the New England Mud Patch. A trans-dimensional Bayesian inversion is employed to estimate geoacoustic properties and uncertainties from these data using the viscous grain shearing sediment model and spherical-wave reflection-coefficient predictions. Results near the thick-mud (SWAMI) site show a nearly uniform sound velocity over the upper approximately 9.2 m, followed by a transition layer with velocity increasing nonlinearly by ∼ 280 m/s over 1.8 m. At the thin-sediment (VC31-2) site, the velocity profile exhibits a similar transition layer. Estimates of intrinsic velocity and attenuation dispersion are also obtained. Over the measurement band of about 400–1300 Hz, the velocity in the fine-grained sediments (mud) at both sites varies by only a few meters per second, i.e., velocity is nearly independent of frequency. The attenuation of the fine-grained sediments at both sites follows a nearly linear frequency dependence. The geoacoustic inferences compare reasonably closely with independent measurements including core measurements, chirp-reflection data, and angle of intromission data.

Multipath Broadband Localization, Bathymetry, and Sediment Inversion - Z.-H. Michalopoulou and P. Gerstoft

Transmission of linearly frequency modulated pulses generates receptions at a vertical line array that can be cross correlated with the source signal to provide estimates of the oceanic waveguide impulse response. For short ranges, distinct path arrivals can be identified including the direct, surface reflection, bottom reflection, and sediment reflection. Accurate estimation of arrival times of such paths is tightly related to successful inversion for source location and water column depth and sound speed and, subsequently, estimation of sediment sound speed and thickness. To achieve accurate estimation, particle filtering is applied to the received time series at 16 phones combined with a simple cross-correlation method. Using linearization, arrival time probability density functions are connected to the geometry and water column sound-speed parameters, providing point estimates as well as probability densities. These are then employed in sediment sound speed and thickness estimation. The results, obtained from the application of the method to data collected during the Seabed Characterization Experiment, are consistent with prior information on the site.

Effect of Shear on Modal Arrival Times - G. R. Potty and J. H. Mille

The sea bottom is generally modeled as a fluid for many of the shallow water acoustic propagation modeling applications. The inherent assumption is that the bottom does not support any shear wave propagation. This study explores the impact of this assumption on the dispersion behavior of acoustic normal modes. A sensitivity study is performed to investigate the effect of density, shear, and compressional wave speeds on modal dispersion. This study focuses on low-frequency (less than 100 Hz) and lower order modes (mode four and lower) in shallow water (70–90-m water depth). These modes and frequency bands are characterized by deeper penetration and larger depth averaging scales. This enabled the use of a simple half-space elastic model of the sea bottom for most of the analyses in this study. A depth-dependent bottom model was used to investigate the effect of a mud layer on modal dispersion. Sediment shear wave speeds were also estimated using broadband data from two shallow water experiments: Shelf Break Primer (1996) and Seabed Characterization Experiment (2017). The inversion results from these two experiments are compared and they are also compared with deep core data and prior inversions.

Trans-Dimensional Inversion of Modal Dispersion Data on the New England Mud Patch  - J. Bonnel, S. E. Dosso, D. Eleftherakis, and N. R. Chapman

This paper presents single receiver geoacoustic inversion of two independent data sets recorded during the 2017 seabed characterization experiment on the New England Mud Patch. In the experimental area, the water depth is around 70 m, and the seabed is characterized by an upper layer of fine grained sediments with clay (i.e., mud). The first data set considered in this paper is a combustive sound source signal, and the second is a chirp emitted by a J15 source. These two data sets provide differing information on the geoacoustic properties of the seabed, as a result of their differing frequency content, and the dispersion properties of the environment. For both data sets, source/receiver range is about 7 km, and modal time-frequency dispersion curves are estimated using warping. Estimated dispersion curves are then used as input data for a Bayesian trans-dimensional inversion algorithm. Subbottom layering and geoacoustic parameters (sound speed and density) are thus inferred from the data. This paper highlights important properties of the mud, consistent with independent in situ measurements. It also demonstrates how information content differs for two data sets collected on reciprocal tracks, but with different acoustic sources and modal content.

Vector Acoustic Analysis of Time-Separated Modal Arrivals From Explosive Sound Sources During the 2017 Seabed Characterization Experiment  - P. H. Dahl and D. R. Dall’Osto

The Intensity Vector Autonomous Recorder (IVAR) is a system that records four coherent channels of acoustic data continuously: one channel for acoustic pressure and three channels associated with a triaxial accelerometer from which acoustic particle velocity is obtained. IVAR recorded the vector acoustic field in broadband signals originating from Signal, Underwater Sound (SUS) (Mk-64) charges deployed at 5–13-km range from the fixed IVAR site (mean depth 74.4 m) as part of the 2017 Seabed Characterization Experiment (SBCEX) designed to study the acoustics of fine-grained muddy sediments. Sufficient geometric dispersion at these ranges permitted unambiguous identification of up to four modes as a function of frequency for frequencies less than 80 Hz. From time–frequency analysis of the dispersed arrivals, a single mode ( $n$ ) and single-frequency ( $f_{i})$ properties are identified at peaks in the narrowband scalar field, with time dependence corresponding to mode group speed. At these time–frequency addresses, four quantities derived from the vector acoustic measurements are formed by coherent combination of pressure and velocity channels: first, modal phase speed; second, circularity, a measure of the normalized curl of active intensity; third, depth-dependent mode speed of energy; and fourth, vertical component of reactive intensity normalized by scalar intensity. A means to compute these quantities theoretically is provided, and a comparison of model results based on a notional geoacoustic representation for the SBCEX experimental area consisting of a single low-speed mud layer over a half-space area versus a Pekeris representation based on the same half-space shows a striking difference, with the field observations also clearly at variance with the Pekeris representation. A fundamental property of mode 2, observed at the IVAR location, is a change in sign for circularity and vertical reactive intensity near 37 Hz that is posited as a constraint observation for mode 2 that must be exhibited by any geoacoustic model that includes a low-speed mudlike layer applied to this location.

Geoacoustic Inversion for a New England Mud Patch Sediment Using the Silt-Suspension Theory of Marine Mud - E.M. Brown, Y.-T. Lin, J. D. Chaytor, and W. L. Siegmann

This article provides an application of the silt-suspension theory to a Bayesian-inference inversion for the geo-acoustic parameters in marine mud. The theory, with consequences that have been developed recently, postulates a suspension of water and clay mineral card-houses that supports moderately dilute concentrations of silt particles. The approach is an example of a physically based model inversion, in which parameters representing physical mud-layer properties are obtained by inversion and used to produce estimates of geoacoustic properties, including their frequency dependence. The acoustic data are from a combustive source signal propagated along a track, located over several meters of fine-grained mud in the New England Mud Patch, to a single hydrophone on a receiver array during the 2017 Seabed Characterization Experiment. Data extracted from a nearby piston core inform the physical modeling, with selections of inversion parameters guided by both sensitivity analyses and bounds from archival and core measurements. Results show the feasibility of this inversion approach. The estimates of mud density and sound speed are close to values obtained independently. The frequency dependence of attenuation is estimated over the full low-frequency source band and has an approximate power exponent of 1.72.

Maximum Entropy Derived Statistics of Sound-Speed Structure in a Fine-Grained Sediment Inferred From Sparse Broadband Acoustic Measurements on the New England Continental Shelf - D. P. Knobles, P. S. Wilson, J. A. Goff, L. Wan, M. J. Buckingham, J. D. Chaytor, and M. Badiey

Marginal probability distributions for parameters representing an effective sound-speed structure of a fine-grained sediment are inferred from a data ensemble maximum entropy method that utilizes a sparse spatially distributed set of received pressure time series resulting from multiple explosive sources in a shallow-water ocean environment possessing significant spatial variability of the seabed. A remote sensing seabed acoustics experiment undertaken in March 2017 off the New England Shelf was designed so that multiple independent analyses could infer the statistical properties of the seabed. The current analysis incorporates the measured horizontal variability from interpretations of a subbottom profiling survey of the experimental area. An idealized range- and azimuth-dependent parameterization of the seabed is derived from identification of horizons within the seabed that define multiple sediment layers. A sparse set of explosive charges were deployed on circular tracks with radii of about 2, 4, and 6.5 km with an acoustic array at the center to correlate a set of random measurements to physical acoustic processes that characterize the seabed. The mean values of a surface sound speed ratio and a linear sound speed gradient for the fine-grained sediment layer derived from 12 data samples processed in the 25–275-Hz band provide an estimate of the effective sound-speed structure in a 130-km 2 area. The inferred sediment sound speed values are evaluated by predicting measured time series data not used in the statistical inference, and are also compared to historical measurements. Finally, the low-frequency maximum entropy estimate of the sediment sound speed along with physical measurements derived from piston core measurements are utilized to estimate the sediment grain bulk modulus.

Broadband Waveform Geoacoustic Inversions With Absolute Travel Time -  Y.-T. Lin, J. Bonnel, D. P. Knobles, and P. S. Wilson

Numerical and experimental studies were conducted to investigate the advantage of utilizing absolute travel time information in Bayesian geoacoustic inversions of broadband acoustic data. It is shown that inversions using absolute travel time can yield smaller uncertainties compared to inversions using relative arrival time and maximum-likelihood estimates for clock time synchronization. Experimental data collected in the Seabed Characterization Experiment on the New England Mud Patch in the Middle Atlantic Bight were used for real data demonstration, and it is shown that inversions using relative arrival times have greater uncertainty in estimating source distance, which consequently affects the overall posterior probability distribution of inverted parameters. Numerical study enables investigation of performance dependence on the signal-to-noise ratio, and it is found that absolute travel time information may have more profound advantages when the signal-to-noise ratio is low.

Ship-of-Opportunity Noise Inversions for Geoacoustic Profiles of a Layered Mud-Sand Seabed - D. Tollefsen, S. E. Dosso, and D. P. Knobles

This paper considers the use of broadband noise from a ship-of-opportunity in statistical inference for estimating geoacoustic parameters of a layered mud-sand seabed model via trans-dimensional (trans-D) Bayesian matched-field inversion, with applications to data collected with a bottom-moored horizontal array in the 2017 Seabed Characterization Experiment conducted on the New England Shelf. The trans-D approach applied here samples probabilistically over possible model parameterizations (different numbers of seabed layer interfaces), and provides quantitative uncertainty estimates of seabed geoacoustic profiles. Inversions are carried out for acoustic data sets collected both when the ship-of-opportunity (a container ship) was oriented with its bow and with its stern towards the array. A third inversion involved combining data from a series of segments along the ship track. Inversion results image an upper sediment layer 3–7 m thick with low-sound speed (close to the water sound speed) over higher speed sediment, with indication of a transition layer above the interface. Sediment parameter estimates from the inversions are in good agreement with direct measurements from sediment cores and other geophysical data collected in the experiment area.

Estimates of Low-Frequency Sound Speed and Attenuation in a Surface Mud Layer Using Low-Order Modes  - L. Wan, M. Badiey, D. P. Knobles, P. S. Wilson, and J. A. Goff

Whereas there have been numerous theoretical and experimental studies on the properties of marine granular sands, there are significantly fewer studies on sediments classified as muds. The validity of geoacoustic models for muddy sediments has not been successfully tested due to the lack of inverted low-frequency sound speed and attenuation values. The geoacoustic properties of a surface fine-grained mud layer, overlaying three sand transition layers, and a half-space basement within the New England Mud Patch, were studied using explosive signals from long-range along-shelf sound propagation tracks. The sound-speed profile of the mud layer in the low-frequency band (100–500 Hz) was estimated using acoustic normal mode characteristics including the mode shapes and the modal dispersion curves of low-order modes, which mainly propagated in the water column and the surface mud layer. The ambiguity of sound speed at the top of the mud layer and sound-speed gradient was approximately removed. It was found that the resultant sound-speed ratio at the water-sediment interface was close to unity and the sound-speed gradient was 1.8 1/s with a standard deviation of 1.0 1/s. The attenuation in the mud layer was inverted using the attenuation coefficient of the first mode extracted from explosive signals at three source locations. The estimated attenuation at 150 Hz had a mean of 0.006 dB/m and a standard deviation of 0.003 dB/m.

¹"Seabed" by IsabelleAcatauassu is licensed under CC BY 2.0