England Mud Patch Research Articles

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.

Direct measurements of acoustic and physical properties in mud and comparison to sediment acoustics models

Sediment acoustics models relate seabed physical properties to acoustic wave speed and attenuation, and direct measurements of sediment acoustic and physical properties provide a means to test such models. We examine direct measurements from two field experiment sites with muddy sediments that contain different ratios of clay, silt, sand, and organic components, comparing the acoustic data with sediment acoustic model predictions using parameters obtained from cores as model inputs. In the New England Mud Patch, sediment sound speed profiles up to several meters below the seabed surface were measured using in situ acoustic probes mounted to a gravity corer. At a second site near Mobile Bay, in situ probe measurements of sound speed, attenuation, and shear speed were obtained in the upper 20 cm of sediment, and diver cores were collected and scanned with an acoustic core logger. Sediments from both sites were analyzed for porosity and grain size distribution. The applicability of various models to the mud found at each site will be discussed, as well as effects of infauna and organic matter, which were also measured, but are not explicitly included in the models. [Work sponsored by ONR.]

Impacts of infauna on sediment heterogeneity and acoustic propagation

The sea floor is inhabited by diverse communities of animals that mix sediments through bioturbation and modify the physical properties of sediments. Infaunal activities, such as burrowing, construction of burrows and tubes, and ingestion and compaction of sediments into fecal pellets, can compact and dilate sediments and change grain size distribution on small scales. These impacts increase heterogeneity of sediment structure, potentially leading to increased acoustic attenuation, especially at higher frequencies, and increased variability in sound speed. We examined the impacts of infaunal communities from Mobile Bay, AL, and the New England Mud Patch on acoustic properties of sediments through measurements on sediment cores collected at each site. We measured sound speed and attenuation at six different orientations within a core and characterized the infaunal community within the core to determine how infauna affect variability in acoustic propagation. We hypothesize that infauna increase variability both on very small scales, among orientations within a core, and on local scales, among cores collected from the same site. These data are compared with acoustic measurements over a broader frequency range to determine how infauna affect variability in acoustic propagation both over different spatial scales and across a range of frequencies.

Potential and kinetic energy of underwater noise measured below a passing ship and response to sub-bottom layering

For underwater sound energy emitted by a passing vessel and received at a relatively steep angles, such as vessel directly overhead, there can be a significant excess in potential acoustic energy relative to kinetic energy with the opposite also occurring. Observations when expressed as a ratio of kinetic to potential energy in decibels are interpretable upon inspection, yielding an estimate of the sensor height above the seabed, and information on properties of the seabed. The effect was studied as part of the recently completed experiment on the New England Mud Patch. The R/V Neil Armstrong underwent controlled transits directly overhead a vector sensor, positioned 1.45 m above the seabed. A model for kinetic and potential energy developed from method of images combined with a layered seabed, is used to invert the data. The low-speed mud-layer is identified along with higher-speed transition layer separating the mud substrate from a sediment basement, with parameters consistent with other recent studies. Similar observations made in Puget Sound are briefly discussed with the higher-impedance seabed identified being consistent with thin Holocene sediments characteristic of region. Both examples illustrate the standing wave features of such data.

Quantifying data information content to resolve seabed structure in geoacoustic inversion

This paper considers the importance of quantitative model selection and general parameterizations in estimating and interpreting seabed profiles in geoacoustic inversion, with application to data collected on the New England Mud Patch. In particular, the seabed structure that can be resolved depends on the information content of the acoustic data set under consideration, which varies with a number of factors, including the physics of the seabed acoustic interaction, frequency content of the data, and measurement and theory errors. Quantitative model selection applied to general parameterizations ensures the inclusion of seabed structure that is reliably sensed by the data while avoiding spurious structure. Data sets considered here include ship noise, modal dispersion, and wide-angle reflection coefficients. In each case, seabed models consistent with the data information content are estimated as part of the inversion using trans-dimensional and/or Bernstein-polynomial parameterizations. Results for all data sets indicate a low sound-speed mud layer over higher-speed sand; however, the ability to resolve structure within the mud layer, such as a transition to higher speeds near the mud base and possibly a weak positive gradient in the upper mud, depends on the information content of the various data sets. [Supported by the Office of Naval Research.]

Overview of the seabed characterization experiment

The seabed characterization experiment (SBCEX) is a multi-disciplinary, multi-institutional international research project devoted to the following scientific goals: (1) to understand the physical mechanisms that control acoustic propagation in fine-grained sediments, (2) to quantify uncertainties in the estimation of seabed parameters, (3) to correlate the observed horizontal variations in the acoustical field with the measured horizontal variability of the seabed, and (4) to assess the performance of the resulting geoacoustic models, inversions, and statistical inference methods. To facilitate these scientific goals, research cruises were undertaken in 2015, 2016, 2017, 2021, and 2022 primarily at a site known as the New England Mud Patch. The work of the project through 2019 was documented in a special issue of the IEEE Journal of Oceanic Engineering, Vol. 45, No. 1 that appeared in January 2020, as well as in peer-reviewed papers in other journals. Here a brief overview of the results through the 2020 special issue will be presented, as well as an overview of the new field work competed in 2021 and 2022. [Work supported by ONR.]

Sound speed profile estimation in a thick mud layer from travel time data

The sound speed profile in a surficial mud layer is of importance because it controls what is acoustically illuminated within and below the mud layer. A variety of estimates have been made at the New England Mud Patch (NEMP) with some near-surface sound speed gradients as large as 10 s−1 and larger. Here, single bounce seabed reflection travel time data are used to infer what mud sound speed gradients are and are not plausible near the central NEMP region where the mud thickness is ∼10 m. Time series data were collected on a moored hydrophone from a broadband source with a 1 second repetition rate as the tow ship transited on a radial from the hydrophone to a distance of ∼1 km at a speed of 2 m/s. The water column was essentially isovelocity and grazing angles ranged from 4.5° to 84°. It is known that at the base of the mud, there is a mud-sand transition layer (of order 1 m thick) where sound speed gradients can be large. However, above this layer, the travel time data clearly indicate that a mud 10 s−1 sound speed gradient is too large. [Research supported by ONR Ocean Acoustics Program.]

Bayesian geoacoustic inversion of 1-2 kHz seabed reflection data for layered muddy sediments

This paper presents trans-dimensional Bayesian geoacoustic inversion of seabed reflection data for sub-bottom geoacoustic profiles and associated uncertainty estimation at the New England Mud Patch. The data considered here are wide-angle seabed reflection coefficients as a function of grazing angle and frequency measured during the 2017 Seabed Characterization Experiment. Chirp pulses over a frequency band of 1–6 kHz were transmitted by an omnidirectional acoustic source towed by a research vessel at a speed of about 4 knots and recorded at a bottom-moored hydrophone. High signal-to-noise-ratio reflection coefficients from 1–2 kHz and angular coverage of ∼15–25° are considered here for geoacoustic inversion. This frequency range is higher than for previous reflection-coefficient data sets on the Mud Patch. The angular range, although relatively narrow, includes strong Bragg resonances which provide information on the sediment layering properties. The inversion applies the viscous grain-shearing sediment acoustics model, which provides dispersive (frequency-dependent) results for sound speeds and attenuations. The seabed structure estimated here is compared to previous inversion results and to core measurements in the vicinity. [Work supported by the Office of Naval Research.]

An end-to-end deep learning approach for joint detection, source localization and environmental characterization using a single hydrophone in shallow water

Environmental characterization of underwater environments is crucial for modeling acoustic propagation in the ocean, which in turn enables underwater applications such as performing environmental impact studies on the effect of ocean noise on marine life, evaluating sonar performance and more. In this work, the aim is to automate and improve run-time performance of environmental characterization, compared to classical methods which employ high-cost computational schemes and sometimes require manual inputs from a skilled operator. To do so, we use 1D Convolutional Neural Networks (1D-CNN), a type of Deep Learning (DL) model, to acoustically characterize the underwater environment using unprocessed pressure time-series recorded on a single hydrophone. We notably propose an end-to-end approach to detect, classify and localize acoustic sources along with estimating environmental parameters using a single DL model. The experimental data used for testing the resulting 1D-CNN models are signals that were generated by navy explosives (SUS charges) deployed during the Seabed Characterization Experiment performed in the New England Mud-patch off the coast of Massachusetts.

Measurements of Geologic Characteristics and Geophysical Properties of Sediments From the New England Mud Patch

The characterization of physical, geological, and geophysical properties of sediments within the New England Mud Patch (NEMP) was undertaken to provide a physical basis for acoustic inversions associated with the SeaBed Characterization EXperiment 2017 (SBCEX17). Using a suite of 89 sediment cores (piston/trigger, gravity [acoustic], and vibracore), a comprehensive database of laboratory-based sediment analyses, geophysical core logs, and the results of seismic reflection profiling, we formulate a three-layer lithostratigraphic model of the area within and immediately adjacent to the SBCEX17 focus area, referred to as the seabed experiment area (SEA). The uppermost lithostratigraphic unit, Unit 1, is relatively homogenous clayey- to sandy silt, with consistent downcore textural, mineralogical, and physical property attributes. Unit 2 is a variable-thickness transitional layer between Unit 1 and Unit 3, whose properties reflect a decrease in proximal erosion and transition to a lower energy depositional environment. Unit 3 is clean quartz sand containing abundant shells and shell fragments that was regionally deposited during Holocene sea-level rise. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">210</sup> Pb and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> C radiocarbon geochronologies spanning the past 13 000 years are used to facilitate intercore comparison across the SEA. Analytical results and laboratory methods used in the derivation of those results are described in detail, serving as a reference for ongoing and future investigation of the SEA and entire NEMP. Although the derived lithostratigraphic model of the SEA is in good agreement with past evaluations of the regional sedimentology, comparisons of the lithostratigraphic and seismostratigraphic models highlight several significant incompatibilities that remain to be satisfactorily explained.

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.

Seabed Observations at the New England Mud Patch: Reflection and Scattering Measurements and Direct Geoacoustic Information

Seabed reflection and scattering measurements were conducted at the New England Mud Patch to better understand the acoustic properties of fine-grained (muddy) sediments. The measurement philosophy and the measurements themselves are summarized. In addition, geoacoustic information accessed directly from the data in the time and frequency domains is presented. The main result is the existence of an angle of intromission. This observation proves that the mud sound speed is less than that of the water and yields a sediment sound speed ratio 0.9865 with outer bounds {0.985 0.989}. Another result is the observation of strong scattered arrivals from within the mud volume at/near normal incidence but not at low grazing angles. These are likely due to anisotropic sediment heterogeneities with a large horizontal to vertical scale. Evidence is also presented for a highly heterogeneous mud–sand horizon with lateral variability down to scales of order meters. Finally, the reflection measurements successfully capture Bragg interference patterns. Their importance is their substantial geoacoustic information content, which can be accessed by several inversion methods.

Hybrid Seabed Parameterization to Investigate Geoacoustic Gradients at the New England Mud Patch

This article applies Bayesian geoacoustic inversion with a hybrid seabed-model parameterization to modal-dispersion data from the New England Mud Patch to estimate gradient structure in the upper mud layer. The hybrid parameterization comprises an upper layer with general smooth (continuous) gradients represented by Bernstein polynomials (BPs) for the mud, above an unknown number of discrete (uniform) layers. The Bayesian information criterion is applied to estimate BP orders for sound-speed and density profiles in the mud, and trans-dimensional (trans-D) inversion is applied for the underlying layered structure. The data, collected during the 2017 Seabed Characterization Experiment, include high-order modes (up to mode 21) extracted via warping time-frequency analysis from recordings of a combustive sound source at a vertical hydrophone array. Inversion results for the hybrid parameterization are compared to those from trans-D inversion with no gradient layer. Hybrid-inversion results indicate a nearly iso-speed mud layer with a rapidly increasing gradient near its base, consistent with increasing sand content in the mud above a sand interface, as indicated by cores. The sound-speed ratio of surficial sediments to bottom seawater is found to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt;$</tex-math></inline-formula> 1 with high probability, which differs from trans-D inversion results, indicating the significance of the choice of parameterization in interpreting structure.

Transdimensional Inversion on the New England Mud Patch Using High-Order Modes

This article presents geoacoustic inversion results for modal-dispersion data collected during the 2017 Seabed Characterization Experiment on the New England Mud Patch, an area where the seabed is characterized by an upper layer of mud. The experiment utilized a combustive sound source and a vertical line array of receivers at 5.4-km range. Using a careful combination of source deconvolution and warping time–frequency analysis, modal dispersion data (arrival time as a function of frequency) are estimated for 18 modes between modes 1 and 21. The modal dispersion data are then used to estimate seabed geoacoustic profiles and uncertainties via transdimensional Bayesian inversion. This article demonstrates the capacity to estimate high-order modes using warping. Comparing inversion results obtained with subsets of (lower order) modes to those obtained with the full set of available modes highlights the rich data information content carried by high-order modes. The results suggest a small sound-speed increase over the first 8 m of the seabed, the upper portion of the mud layer, which some earlier studies found to be isospeed. Overall, the inversion results are consistent with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> measurements, as well as with previous geoacoustic inversion results.

Statistical Inference of Sound Speed and Attenuation Dispersion of a Fine-Grained Marine Sediment

Acoustic recordings of signals in the 1.5–4.0-kHz band were analyzed for information about the sound speed and attenuation frequency dispersion of a fine-grained sediment found in the New England Mudpatch. Analysis of piston cores established prior bounds for a geophysical parameterization of a seabed model that predicts Kramers–Kronig dispersion relations. Sediment layers are described by the Buckingham viscous grain shearing (VGS) model that accounts for the effects of overburden pressure of compressional and shear speeds and attenuations. A statistical inverse problem was solved by using multiple samples of received levels recorded on two vertical line arrays as a function time and hydrophone depth for six frequencies in the 1.5–4.0-kHz band. A statistical inference model that assumed both model parameters and data samples are random variables quantified information content from marginalization of a conditional posterior probability distribution for the geophysical parameters that characterize the mud layer. From the inferred geophysical parameter point estimates the sediment sound speed and attenuation frequency dispersion are predicted and compared to previously reported direct measurements. Also, the predicted sound-speed gradient in the mud sediment from the VGS model is compared to a previous inference that utilized explosive sources.

Seabed Characterization Experiment: Analysis of Broadband Data

Analysis of data acquired during the seabed characterization experiment conducted in the New England mud patch area in March 2017 is presented in this article. A particular feature of these data is the presence of the Airy phase in some of the modes. The mode dispersion data along with the Airy phase information are extracted using the time warping method. The mode dispersion data obtained from the analysis of the acoustic data are then used to estimate the compressional wave speed and density profiles of the sediment layers using linear inversion methods. The validity of these results is investigated, and their validity is demonstrated. The results are consistent with the results obtained by the analysis of similar data in which a nonlinear inversion method was used to estimate the sediment properties. These results further show that meaningful results can be obtained using the linearized inversion procedure.

Acoustic resonances within the surficial layer of a muddy seabed.

This is an investigation of sound propagation over a muddy seabed at low grazing angles. Data were collected during the 2017 Seabed and Bottom Characterization Experiment, conducted on the New England Mud Patch, a 500 km2 area of the U.S. Eastern Continental Shelf characterized by a thick layer of muddy sediments. Sound Underwater Signals (SUS), model Mk64, were deployed at ranges of 1-15 km from a hydrophone positioned 1 m above the seafloor. SUS at the closest ranges provide measurements of the bottom reflection at low grazing angles (<3 deg). Broadband analysis from 10 Hz to 10 kHz reveals resonances in the bottom reflected signals. Comparison of the measurements to simulated signals suggest a surficial layer of mud with a sound speed lower than the underlying mud and overlying water. The low sound speed property at the water-mud interface, which persists for less than 1 m, establishes a sound duct that impacts mid-frequency sound propagation at low grazing angles. The presence of a low-speed surficial layer of mud could be universal to muddy seabeds and, hence, has strong implications for mid-frequency sound propagation wherever mud is present.

Maximum entropy inference of seabed properties using waveguide invariant features from surface ships.

Acoustic data were recorded on two vertical line arrays (VLAs) deployed in the New England Mud Patch during the Seabed Characterization Experiment 2017 in about 75 m of water. The sound recorded during the passage of merchant ships permits identification of singular points for the waveguide invariant β for mode pairs (1,n):β1,n,for n=2,3,4,5, in the 15-80 Hz band. Using prior geophysical information and an acoustic data sample from the merchant ship KALAMATA, a geoacoustic model M of the seabed was developed. Then, using data samples from other merchant ships, a feature-ensemble maximum entropy method is employed to infer the statistical properties of geoacoustic parameter values for the sound speeds in a surface mud layer and a deep sand layer. Technical challenges include a sparsity of observed singular points, the unique identification of mode pairs for an observed singular point, and the deviation of the waveguide from horizontal stratification. A geoacoustic model M is developed that reproduced the observed β≈-1 for f < 20 Hz and mode cutoff features at about 15 Hz. The statistical low-frequency inference of the singular point structure from multiple ships provides evidence of an angle of intromission at the water sediment interface with an average sound speed ratio of about 0.986 and an average sound speed for the deeper sand layer of about 1775 m/s.

Layered and gradient model parameterizations in geoacoustic inversion

This paper considers the importance of model parameterization in geoacoustic inversion and uncertainty estimation, including quantitative approaches to model selection as well as potential limitations of the information content of acoustic data to determine the form of geoacoustic profiles, e.g., differentiating between layered and gradient structures. In particular, general parameterizations are considered based on trans-dimensional (trans-D) inversion, which represents profiles as an unknown number of uniform layers, and Bernstein polynomial (BP) inversion, which represents smooth gradients as polynomials of unknown order. These approaches are illustrated and compared for the inversion of high-order modal-dispersion data collected at the New England Mud Patch. It is shown that while the data constrain the sound-speed profile in the mud layer to reasonably high precision, the data cannot differentiate between trans-D layered or BP gradient representations. However, simpler (fixed) parameterizations, such as a homogeneous layer or linear gradient, can be ruled out based on the Bayesian information criterion. Furthermore, the prior choice of parameterization (layers or gradient) has implications on whether the sound-speed ratio at the water–sediment interface is estimated to be less or greater than one with high probability (an issue other acoustic datasets may share). [Work supported by the Office of Naval Research]

Quantifying and transferring environmental uncertainties in underwater acoustic modeling

Uncertainties in environmental inputs represent a major source of uncertainty in underwater acoustic model outputs and applications thereof (e.g., transmission-loss estimation, source localization). Seabed geoacoustic parameters are often estimated by the inversion of ocean acoustic data. Hence, rigorous quantification of geoacoustic inversion uncertainties and the transfer of these uncertainties to modeling applications are of key importance. Uncertainty estimation in geoacoustic inversion is naturally accommodated in a Bayesian formulation, which combines data and prior information to form the posterior probability density (PPD) of seabed parameters. Important components of this approach include quantitative model selection for seabed parameterizations consistent with the information content of the data; an appropriate model for residual data errors that specifies the likelihood function; and nonlinear estimation of the PPD, which is normally carried out using Markov-chain Monte Carlo (MCMC) methods. MCMC characterizes the PPD using a large ensemble of dependent random samples of the geoacoustic parameters, which can be computationally demanding. However, these uncertainties can be transferred efficiently to subsequent propagation-modeling applications using a much-smaller, randomly chosen (independent) subset from the ensemble. The approach is illustrated using simulations and inversion of ship noise recorded on a horizontal array of hydrophones at the New England Mud Patch.