Geoacoustic Properties Research Articles

Efficient reversible-jump Markov-chain Monte Carlo sampling in trans-dimensional Bayesian geoacoustic inversion

This paper considers efficient computational approaches to estimate the posterior probability density (PPD) of seabed geoacoustic profiles in the Bayesian inversion of ocean acoustic data, a numerically intensive problem. Trans-dimensional (trans-D) inversion is applied, which samples probabilistically over an unknown number of seabed layers as well as the layer geoacoustic properties and parameters of the error model (variances and autoregressive coefficients). Sampling is based on the reversible-jump Markov-chain Monte Carlo algorithm, the efficiency of which depends strongly on the formulation of the proposal density by which new candidate models are generated for probabilistic acceptance/rejection. A highly efficient proposal density is presented which combines principal-component (PC) reparameterization with parallel tempering. PC reparameterization applies an adaptive linearized approximation to the PPD as the proposal density, which provides effective directions and length scales for model perturbations in high-dimensional parameter spaces. Parallel tempering considers a series of interacting Markov chains with successively relaxed likelihood functions, which greatly improves the sampling of multi-modal parameter spaces and trans-D transitions. These approaches are combined by computing different PC reparameterizations for each Markov chain in the parallel tempering formulation. Inversion results are presented as marginal probability profiles for geoacoustic properties, marginalized over the number of layers.

Geo-acoustic inversion using ground waves of explosive sounds

The ground waves of explosive sounds exhibit strong dispersion in shallow water waveguides. Their travel time and amplitude are related to the geo-acoustic properties in the seabed and could be utilized to construct cost functions for geo-acoustic inversion. Following the analysis of the Airy phase [Wan et al., JASA, 143, EL199-205, (2018)], this research focuses on the extraction and utilization of ground waves, which propagate at frequencies below the corresponding Airy frequency. First, the ground waves of low order modes are identified in the received signals from explosive SUS charges deployed at various depths/ranges and recorded by vertical line arrays during the series of Seabed Characterization Experiments (SBCEX) conducted in the New England continental shelf and slope region. Then, the extracted ground waves, showing strong dispersive characteristics in a bandwidth of a few Hertz (i.e., between cut-off frequency and Airy frequency), are fed to a proposed inversion algorithm to obtain the estimated geo-acoustic parameters. Finally, the advantage of using ground waves rather than water waves is discussed and a physical explanation is provided. [Work supported by ONR Ocean Acoustics.]

Inversion of ocean acoustic modal-dispersion and amplitude data for seabed geoacoustic models including attenuation

The long-range acoustic field due to an initial sound source in a shallow-water waveguide can be expressed as the sum of a number of dispersive, propagating modes. Modal-dispersion data (arrival time as a function of frequency), as extracted with warping time-frequency analysis, have been inverted to estimate seabed sediment sound-speed and density profiles. In this work, measurements of modal amplitudes as a function of frequency are combined with modal-dispersion data in a joint (simultaneous) inversion which also provides sensitivity to frequency-dependent sound attenuation coefficients in the sediment layers. Since the source signature (spectrum) is often unknown in practice, the relative amplitudes between modes can be considered as data, or the source spectrum can itself be included as additional unknowns in the inversion. Here, trans-dimensional Bayesian inversion is applied to modal-dispersion and amplitude data to estimate marginal probability profiles for sediment geoacoustic properties, including attenuation. The signal processing and inversion methods are demonstrated using modal-dispersion and amplitude data extracted from acoustic measurements made by a hydrophone-equipped underwater glider during the 2017 Seabed Characterization Experiment (Work supported by the Office of Naval Research).

Broadband measurements and modeling of low-grazing angle bottom reflections from muddy marine sediments

In 2017 and 2022, a series of experiments using explosive sound sources were conducted on the New England Mud Patch (NEMP), an area of the Eastern US Continental shelf defined by its thick muddy seafloor sediments. Broadband measurements from the near-bottom Intensity Vector Acoustic Recorders (IVARs) reveal resonant frequencies in the bottom reflected signals, which are excited at very low-grazing angles. A signal model based on a layered-bottom reflection coefficient is used to examine the geoacoustic properties of the marine mud, which leads to the conclusion that the sound-speed contrast across the water-mud interface (i.e. index of refraction, n) is less than one (n < 1), and also an increase in sound-speed occurs within the deeper mud-sediments (n > 1). Measurements in May 2022, replicating the same source and receiver locations from March 2017, also show resonances in the bottom-reflected signal. Differences between the two data sets (2017 and 2022) are primarily caused by the complex oceanography encountered during the spring-time measurements, absent during the isothermal winter conditions. Comparison of the two datasets examines the stability of the sound-speed contrast, including the effect of time-varying temperature at the water-mud interface.

Simultaneous detection and ranging of baleen whale impulsive vocalizations using a temporal convolutional network

In shallow-water environments, low-frequency acoustic signals exhibit dispersive propagation due to interactions with the sea surface and seabed. The received signal can then be modeled as a set of propagating modes. Single-hydrophone modal dispersion has been used to range baleen whale vocalizations and estimate shallow-water geoacoustic properties. However, these algorithms require preliminary signal detection and human labor to estimate the modal dispersion. Here, we apply a temporal convolutional network (TCN) to time-frequency representations of baleen whale gunshots (impulsive calls) to simultaneously detect and range them in large single-hydrophone passive acoustic monitoring datasets. The TCN jointly learns ranging and detection by training using both synthetic gunshots simulated across multiple environments/ranges and experimental noise. The synthetic data is informed by only the experimental dataset’s water column depth, sound speed, and density, while other waveguide parameters vary within empirically observed bounds. The method is applied to an experimental North Pacific right whale dataset collected in the Bering Sea using a single hydrophone. To evaluate model performance, 50 calls are manually ranged using a state-of-the-art physics-based inversion method. The TCN closely matches the physics-based range estimations and detects dispersive gunshots among noise-only examples with high precision and recall. [Work supported by the ONR].

Broadband core and resonance logger measurements of sound speed and attenuation in the New England mud patch

Extensive measurements of sound speed and attenuation have been performed on sandy sediments over the last few decades, which have motivated a variety of physical sediment acoustics models. Recently, the seabed characterization experiment (SBCEX) was performed in 2015–2017, in part, to quantify the geoacoustic properties of a clayey-silt accumulation, known as the New England Mud Patch (NEMP). Due to the variability and frequency range reported by the various direct measurements and geoacoustic inversions, the sound speed measured in the NEMP lacked any noticeable frequency dependence. In 2022, an additional coring cruise was completed in the NEMP area and included shipboard core and resonance logger (CARL) measurements, expanding upper limit of the band to 1 MHz. CARL measured sound speed and attenuation as a function of depth from 100 kHz to 1 MHz using a time-of-flight technique, and sound speed along the entire core length from 12 kHz to 16 kHz using a resonance technique. Positive dispersion and depth dependance was observed in the 200 kHz to 1 MHz band. The frequency and depth dependence of the CARL measurements will be presented and compared with previously published model fits to the 2015–2017 SBCEX dataset. [Work sponsored by ONR.]

Evidence of 3D effects for along shelf acoustic propagation during SBCEX21

The 2021 Seabed Characterization Experiment (SBCEX21) was performed on the New England Shelf Break (NESB) in May 2021 with the main objective of characterizing the NESB sediment geoacoustic properties. In this talk, we focus on low-frequency propagation (f<100 Hz) along the 200 m isobath line. Time-frequency dispersion of normal modes is estimated using a SUS charge signal recorded on a distant single hydrophone TOSSIT system. Geoacoustic inversion is performed, but two-dimensional (2D) acoustic propagation models fail to capture the observed behavior of higher order acoustic modes, and predict much shorter travel times than are seen in the data. By introducing a three-dimensional (3D) model environment, both parabolic equation and modal ray calculations confirm that the gradual shelf slope has a nontrivial influence on modal travel times. This comparison between 2D and 3D simulations of SBCEX21 acoustics and experimental data emphasizes the importance of 3D effects from sloped bathymetry on understanding propagation in a shelfbreak environment. These effects will need to be accounted for when doing geoacoustic inversion of SBCEX21 along-shelf low-frequency data.

High-resolution transdimensional geoacoustic inversion using autonomous underwater vehicle data

We invert reflection coefficient measurements of muddy sediment layers along a 12-km seabed transect on the Malta Plateau in the Mediterranean Sea using 1711 source transmissions recorded on a 32-element linear hydrophone array with both source and array towed by an autonomous underwater vehicle. Trans-dimensional Bayesian inference using reversible jump Markov chain Monte Carlo sampling is applied to obtain posterior probability densities of the number of homogeneous sediment layers, their depths, and their geoacoustic parameters. The forward sediment acoustics model is based on the grain-shearing model which obeys physical causality and provides correlation between important geoacoustic properties. Each dataset was treated as one-dimensional seabed structure inversion carried out on high performance clusters, and inversion results for multiple data sets were combined to yield a two-dimensional subsurface profile including full uncertainty analysis. Comparisons of inversion results to piston and gravity core estimates show agreement in both geoacoustic parameter values and depths of discontinuities. In the range-dependent model constructed from inverting the entire data set, dipping and terminating layers are observed along the track with high vertical resolution on the order of 10 cm.

Quantifying information content of modal dispersion data in geoacoustic inversion

This paper illustrates the information content of ocean acoustic modal-dispersion data to constrain parameters and uncertainties of seabed geoacoustic models. In particular, time-frequency warping analysis is applied to extract dispersion data consisting of arrival times as a function of frequency for 18 of the first 21 propagating modes from recordings of an impulsive sound source on a vertical hydrophone array. To quantify the information content of these dispersion data to resolve seabed structure, a Bayesian inversion formulation is applied that includes rigorous approaches to model selection and data error modeling. Model selection considers both layered and gradient representations of seabed profiles using trans-dimensional inversion and Bernstein-polynomial basis functions, respectively. In both cases, model parameterizations are determined probabilistically from the data as part of the inversion. The error model assumes a multi-variate Gaussian distribution with unknown variance and covariance for each mode; covariance estimation is formulated in terms of trans-dimensional sampling of zeroth- and first-order autoregressive processes. The applicability of these assumptions/approaches is validated with qualitative (graphical) and quantitative residual analyses. Results are considered as marginal probability profiles for geoacoustic properties, which quantify the resolution of seabed structure versus sub-bottom depth. [Work supported by the Office of Naval Research.]

Use of the rupture induced, underwater sound source in the New England mud patch

The rupture induced, underwater sound source (RIUSS) is a device utilized in underwater acoustics experiments and surveys to create high amplitude, broadband acoustic pulses. The device functions by placing a rupture disk over an evacuated chamber and mechanically breaking the disk (either by striking on demand or via hydrostatic pressure) at a specified depth to initiate a volume collapse that produces an impulsive acoustic waveform. A new configuration of the device has been developed such that multiple chambers can simultaneously be deployed along with acoustic and oceanographic recorders. This device was deployed in the New England Mud Patch in support of the 2022 seabed characterization experiment along with a variety of acoustical recorders to investigate the frequency dependent, geoacoustic properties of the muddy seabed. Discussion will focus on the newly developed source and the various methods for which the source was configured and deployed in support of the experimental requirements. Results from a number of measurements will be discussed to demonstrate the ability of the source to generate long-range propagation signals, create interface waves in the seabed, and facilitate novel methods of measuring the angle of intromission within the seabed. [Work Supported by ONR.]

Coupled effects of nonlinear internal gravity waves and seabed properties on underwater sound propagation

Underwater sound propagation can be affected by strong sound speed gradients induced by nonlinear internal waves in the ocean. Meanwhile, geoacoustic properties of the seabed control acoustic reflections. Experimental data collected at the shelfbreak on the northeast of the South China Sea were analyzed to investigate the joint effects. Both two-dimensional (2D) and three-dimensional (3D) numerical sound propagation models with realistic seafloor and oceanographic inputs have been established to study the nonlinear internal wave effects observed on hydrophone vertical line array moorings. Compared to the 3D model, the prediction errors and uncertainties of the 2D scheme neglecting the transversal/azimuth energy propagation are discussed. In addition, the impact of sediment properties on 3D propagation effects affecting underwater soundscapes in the area is investigated. [Work supported by the Office of Naval Research.]

Inference of both source and seabed characteristics from broadband signals on the New England continental slope

In this study the characteristics of the modal dispersion of acoustic waves in littoral ocean environments are utilized to infer both source properties (range, depth, speed, and source level) and geoacoustic properties of the seabed. An analysis is made of broadband acoustic data (10–4500 Hz) taken during 2021–2022 on the New England continental slope over water depths from 200 to 500 m. The acoustic data are generated by both moving broadband sources and explosive sources over range scales of 1–30 km. Such range scales can reveal the nature of the modal dispersion. Conductivity, Temperature, and Depth (CTD) and Expendable Bathythermographs (XBTs) measurements provide a sparse sampling of the temporal and spatial variability of the ocean during the acoustic measurements. Along with bathymetry measurements the sound speed profiles derived from the CTDs and XBTs provide prior information for an inverse problem from which insight can be obtained on the effects of the seabed on the dispersion of broadband propagation.

Modeling the effect of random roughness on synthetic aperture sonar image statistics.

A model has been developed to predict the effect of random seafloor roughness on synthetic aperture sonar (SAS) image statistics, based on the composite roughness approximation-a physical scattering model. The continuous variation in scattering strength produced by a random slope field is treated as an intensity scaling on the image speckle produced by the coherent SAS imaging process. Changes in image statistics caused by roughness are quantified in terms of the scintillation index (SI). Factors influencing the SI include the seafloor slope variance, geo-acoustic properties of the seafloor, the probability density function describing the speckle, and the signal-to-noise ratio. Example model-data comparisons are shown for SAS images taken at three different sites using three different high-frequency SAS systems. Agreement between the modeled and measured SI show that it is possible to link range-dependent image statistics to measurable geo-acoustic properties, providing the foundation necessary for solving problems related to the detection of targets using high-frequency imaging sonars, including performance prediction or adaptation of automated detection algorithms. Additionally, this work illustrates the possible use of SAS systems for remote sensing of roughness parameters such as root mean square slope or height.

High-resolution modal wavenumber estimation in range-dependent shallow water waveguides using vertical line arrays

Estimation of modal wavenumbers is important for inference of geoacoustic properties and matched field processing in shallow water waveguides. However, it is challenging in a range-dependent environment, because modal content varies locally in response to changes in the environment. Moreover, the scales of the spatial variations in the waveguide may be on the same order as the range aperture required for resolvability of the individual modes. To this end, high-resolution (HR) wavenumber estimation methods have been widely used. In this paper, the matrix pencil and MUSIC algorithms are generalized to geometry involving a synthetic horizontal aperture (SHA) formed by a towed acoustic source and a fixed full-spanning vertical line array (VLA). The performance of the proposed methods is evaluated by simulated data in a noisy shallow water environment. Numerical results show that, when compared with the previous methods, the proposed methods significantly outperform the previous methods in terms of aperture requirement.

Passive Acoustic Glider for Seabed Characterization at the New England Mud Patch

Acoustic payload-equipped underwater gliders are proving to have great potential for maritime intelligence, surveillance, and reconnaissance missions, as well as oceanic environment characterization. This article demonstrates their capabilities for seabed characterization using broadband signals received on a hydrophone-equipped Teledyne Webb Research Slocum glider during the 2017 Seabed Characterization Experiment (SBCEX) conducted on the New England Mud Patch. In the experiment, a source ship maintained a fixed position while combustive sound-source signals were emitted at about 2 min intervals. The glider was programmed to follow a sawtooth-like track through the water column approximately 8 km from the source in an area where the water was ∼72 m deep. Two transmissions were received by the glider at depths separated by about 15 m. Trans-dimensional Bayesian geoacoustic inversion was applied to modal-dispersion data extracted from the received signals via a time-warping technique to study the consistency of the inversion results for signals received at different depths, and the advantages of including signal receptions at different depths in simultaneous inversion. The inferred geoacoustic properties are in good agreement with independent core measurements collected during a geophysical survey, and with other inversion results using data collected by dedicated bottom-moored receivers in the vicinity.

Estimating geoacoustic properties of sediments with a wide grain-size distribution using sediment organic content and the viscous grain shearing model

Interstitial organic matter (OM) is a ubiquitous constituent of marine sediment, particularly prevalent in sediments with significant silt/clay fractions, and is often quantified across a wide range of disciplines. OM suspends silt and clay particles in the sediment matrix, adsorbs onto mineral surfaces, and resides primarily between mineral contacts, all of which are hypothesized to alter geoacoustic properties, yet OM has not been included in sediment acoustics models. Recent studies have shown that OM content (OC) correlates well with sediment geoacoustic properties, such as porosity and index of impedance, from sediments found in the Baltic Sea to seagrass-bearing sediments in Texas to mud banks in Brazil. In Mobile Bay, Alabama, OC correlated well not only with porosity and index of impedance, but also with grain-shearing coefficients described by the viscous grain shearing (VGS) model. Using empirical regressions derived from a subset of the Mobile Bay biogeoacoustic data and VGS, OC will be used to estimate geoacoustic properties of Mobile Bay sediments with a wide grain-size distribution. The efficacy and potential of using OC to predict sediment geoacoustic properties (and vice versa) in Mobile Bay and in other sediments with similar composition will be discussed. [Work supported by ONR.]

Feature selection for seabed characterization using non-linear regression and decision trees

Knowledge of the seabed properties in an oceanic waveguide facilitates the understanding of wave propagation in the medium and, eventually, the implementation of tasks such as target detection and localization. Seabed characterization is, thus, a topic of extensive interest in the ocean acoustics research community. In this work, two approaches are implemented towards the task of seabed identification after the application of a preprocessing step that extracts characteristic features from received time-series following the transmission of a broadband sound signal. First, non-linear regression is applied, employing the extracted features and exploiting the relationship between those and geoacoustic properties. This approach leads to estimates of parameters relating to the sediment. Second, a decision tree is implemented, along with Principal Component Analysis, for identifying sediment types using the same features (or subsets of those). This technique is a classification task which maps propagation environments to specific types. Both approaches, tested on synthetic data from five sediment classes, lead to the successful extraction of information in regards to sediment properties. [Work supported by NSF and ONR.]

Source Level of Antarctic Blue and Fin Whale Sounds Recorded on Sonobuoys Deployed in the Deep-Ocean Off Antarctica

The source levels, SL, of Antarctic blue and fin whale calls were estimated using acoustic recordings collected from directional sonobuoys deployed during an Antarctic voyage in 2019. Antarctic blue whale call types included stereotyped song and downswept frequency-modulated calls, often, respectively, referred to as Z-calls (comprising song units-A, B, and C) and D-calls. Fin whale calls included 20 Hz pulses and 40 Hz downswept calls. Source levels were obtained by measuring received levels (RL) and modelling transmission losses (TL) for each detection. Estimates of SL were sensitive to the parameters used in TL models, particularly the seafloor geoacoustic properties and depth of the calling whale. For our best estimate of TL and whale-depth, mean SL in dB re 1 μPa ± 1 standard deviation ranged between 188–191 ± 6–8 dB for blue whale call types and 189–192 ± 6 dB for fin whale call types. These estimates of SL are the first from the Southern Hemisphere for D-calls and 40 Hz downsweeps, and the largest sample size to-date for Antarctic blue whale song. Knowledge of source levels is essential for estimating the detection range and communication space of these calls and will enable more accurate comparisons of detections of these sounds from sonobuoy surveys and across international long-term monitoring networks.

Geoacoustic Estimation of the Seafloor Sound Speed Profile in Deep Passive Margin Setting Using Standard Multichannel Seismic Data

Seafloor geoacoustic properties are important in determining sound propagation in the marine environment, which broadly affects sub-sea activities. However, geoacoustic investigation of the deep seafloor, which is required by the recent expansion of deep-water operations, is challenging. This paper presents a methodology for estimating the seafloor sound speed, c0, and a sub-bottom velocity gradient, K, in a relatively deep-water-compacting (~1000 m) passive-margin setting, based on standard commercial 2D seismic data. Here we study the seafloor of the southeastern Mediterranean margin based on data from three commercial seismic profiles, which were acquired using a 7.2 km-long horizontal receiver array. The estimation applies a geoacoustic inversion of the wide-angle reflections and the travel times of the head waves of bending rays. Under the assumption of a constant positive K, the geoacoustic inversion converges to a unique set of parameters that best satisfy the data. The analysis of 24 measurement locations revealed an increase in the average estimates of c0 from 1537 ± 13 m s−1 to 1613 ± 12 m s−1 for seafloor depths between ~1150 m and ~1350 m. K ranged between 0.75 and 0.85 m s−1 with an average of 0.80 ± 0.035 s−1. The parameters were consistent across the different locations and seismic lines and they match the values that were obtained through depth-migration-velocity analysis and empiric relations, thereby validating our estimation methodology.

Biogeoacoustic sediment properties along a horizontal sand-mud gradient

Whether deposited from the water column or generated by benthic infauna, interstitial organic matter is a ubiquitous constituent of marine sediment and is particularly prevalent in sediments with significant silt/clay fractions. Interstitial organic matter suspends silt and clay particles in the sediment matrix, adsorbs onto mineral surfaces, and resides between mineral contacts, all of which are hypothesized to alter geoacoustic properties. However, the extent to which interstitial organics alter geoacoustic properties is understudied and warrants further investigation. To address this, diver cores were collected along a horizontal sand-mud gradient in Mobile Bay, Alabama. The sediment was subsequently processed in the laboratory, where compressional wave speed (10 kHz to 1 MHz), compressional wave attenuation (100 kHz to 1 MHz), density, porosity, grain-size distribution, and organic carbon and nitrogen content were measured. In addition, physics-based geoacoustic models, such as VGS(λ) and mCREB, were fit to the wide range of sediment types. Measured geoacoustic properties and fit parameters inherent to each sediment acoustics model are compared with organic sediment properties. The extent to which organics control sediment geoacoustic properties and considerations to include organics in future modeling efforts will be discussed. [Work sponsored by ONR.]