Observations and modeling of range-dependent propagation in low-frequency tones emitted by a container shipa).

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.

Observations of underwater noise emission from a merchant ship are made as a function range in waters of depth 77 m on the New England Mud Patch (NEMP), as part of the 2022 Seabed Characterization Experiment with primary objective being study of sound propagation involving seafloors consisting of fine-grained sediments. In this region the seafloor is characterized by a mud-like layer of order 10 m thickness, commencing at the water–sediment interface. The ship traversed the NEMP producing a 30 km observation transect for which the water and mud-layer depth slowly varied with range. Data are forward-modeled using adiabatic normal modes based on a range-varying geoacoustic model for the upper sediments that incorporates stratigraphic information from the experimental area. For the deep sediment structure, the compressional speed is a constant, sand-like layer of speed 1810 m/s for O(100) m, after which speed increases before ultimately terminating at a basalt basement of speed 5000 m/s. For simplicity, compressional attenuation is estimated upon assumption of realistic values for the Poisson ratio and shear speed quality factor Q. For frequencies near 10 Hz, estimated compressional wave attenuation is reduced by approximately 30% relative to corresponding estimates that exclude elastic effects.

A method for measuring in situ compressional wave attenuation exploiting the spectral decay of reflection coefficient Bragg resonances is applied to fine-grained sediments in the New England Mud Patch. Measurements of layer-averaged attenuation in a 10.3 m mud layer yield 0.04 {0.03, 0.055} dB/m/kHz (braces indicate outer bounds); the attenuation is twice as large at a site with 3.2 m mud thickness. It is shown that both results are heavily influenced by a ∼1 m sand-mud transition interval created by geological and biological processes that mix sand (at the base of the mud) into the mud. Informed by the observations, it appears that the spatial dependence of mud layer attenuation across the New England Mud Patch can be predicted by accounting for the transition interval via simple scaling. Further, the ubiquity of the processes that form the transition interval suggests that the scaling may be applied to any muddy continental shelf. In principle, attenuation predictions in littoral environments could be substantively improved with a modest amount of geologic and biologic information.

An analysis of plane wave reflection is developed for a two-layer sediment, the top layer consisting of a fine-grained material (mud) with an upward refracting linear sound speed profile. Beneath is a homogeneous basement, and above is homogeneous seawater. A rather curious, exact analytical expression for the reflection coefficient is derived, involving easy to evaluate integrals over finite limits, of the modified Bessel functions of low-integer order. The expression is generally valid for any linear profile with positive gradient in the surficial mud layer and for any sound speed in the basement, either greater than or less than that in the seawater. For "fast" basements, a critical angle always exists that is independent of the sound speed in the mud layer. With a "slow" basement, a quasi-angle of intromission may exist, which depends only weakly on both frequency and the gradient of the profile in the mud, a conclusion that may be relevant to the conditions of the Seabed Characterization Experiment (2017) performed over the New England Mud Patch. With both types of basement, fast and slow, the reflection coefficient, as a function of grazing angle, exhibits fluctuations that are strongly frequency dependent, associated with resonances and anti-resonances in the mud layer.

Measurements of acoustic pressure and particle velocity were made during the Seabed Characterization Experiment (SBCEX-2017) in the New England Mud Patch south of Cape Cod in about 70 m of water. This experimental location is characterized by a “soft” layer of surficial sediment consisting of mud. University of Rhode Island and Woods Hole Oceanographic Institution deployed the “geosled” with a four-element geophone array, a tetrahedral array of four hydrophones, and several hydrophone receive units (SHRUs) as data acquisition packages. In addition, a new low frequency source, interface Wave Sediment Profiler (iWaSP) was deployed to excite interface waves (Scholte waves). The geophone array was localized using the known locations of the acoustic sources and noise from the research vessel. Modal arrivals from broadband sources on geophones and hydrophones were used to invert for compressional and shear wave speeds and attenuation in the mud layer and the underlying sand layer. Effect of shear conversion effects on the modal attenuation estimates and it’s frequency dependence will be explored. The role of shear conversion along the mud-sand interface will be discussed. [Work supported by the Office of Naval research, code 322 OA.]

Biology is prevalent on and within many ocean bottom sediments. Organisms can include animals dwelling at or near the water-sediment interface or infauna living within surficial sediments. Bioturbation from burrowing, tube building, or other activities can have physical effects on the sediment acoustic properties. As part of the Sediment Characterization Experiment, a survey cruise was conducted in August 2015 in the New England Mud Patch, a region in the Atlantic continental shelf characterized by a layer of mud up to 12 m thick overlying a sandy subbottom. In addition to gravity coring operations to determine the properties of the mud layer, box cores and multicores were collected to examine the surficial sediment properties. Infauna were prevalent in the surficial sediment samples and were collected and characterized for body size, hardness, and potential for bioturbation or structuring. Shipboard measurements of shear and compressional waves were performed on the box core samples using time-of-flight measurements. Preliminary results from the infauna analysis and the shipboard acoustic measurements will be presented. [Work supported by ONR.]

Biology is prevalent on and within many ocean bottom sediments. Organisms can include animals dwelling at or near the water-sediment interface or infauna living within surficial sediments. Bioturbation from burrowing, tube building, or other activities can have physical effects on the sediment acoustic properties. As part of the Sediment Characterization Experiment, a survey cruise was conducted in August 2015 in the New England Mud Patch, a region in the Atlantic continental shelf characterized by a layer of mud up to 12 meters thick overlying a sandy subbottom. In addition to gravity coring operations to determine the properties of the mud layer, box cores and multicores were collected to examine the surficial sediment properties. Infauna were prevalent in the interfacial sediment samples and were collected and characterized for body size, hardness, and potential for bioturbation. Shipboard measurements of shear and compressional waves were performed on the box core samples using time-of-flight measurements. Preliminary results from the infauna analysis and the shipboard acoustic measurements are presented.

A new method for measuring in situ compressional wave attenuation exploiting the spectral decay of Bragg resonances is applied to sediments at the New England Mud Patch. Measurements of layer-averaged attenuation in a 10.3 m mud layer yield 0.04 {0.03 0.055} dB/mi kHz (braces indicate outer bounds); the attenuation is twice as large at a site with 3.2 m mud thickness. Both results are strongly influenced by a ∼1 m sand-mud transition interval, created by geological and biological processes which mix sand (at the base of the mud) into the mud. Above the transition interval, homogeneous mud exhibits an attenuation 0.01–0.02 dB/m kHz, lower than that in the sand-mud transition interval by a factor of 10. Informed by these and additional observations, mud attenuation in and above the transition interval appears to be roughly spatially invariant across the area, explaining the factor of two in attenuation between the two sites by simple depth scaling. Further, the ubiquity of the processes that form the transition interval suggests that the scaling may be broadly applied to other muddy continental shelves. In principle, attenuation predictions in shallow water could be substantively improved with a modest amount of geologic and biologic information. [Research supported by ONR Ocean Acoustics.]

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.]

This paper considers Bayesian geoacoustic inversion of broadband, wide-angle reflection-coefficient data including shear parameters in the seabed model to investigate the ability to estimate these parameters as well as effects on the estimation of other geoacoustic parameters, particularly compressional-wave attenuation. The seabed parameterization is based on the viscous grain-shearing (VGS) sediment-acoustic model, including the grain-to-grain shear modulus as an unknown parameter. VGS sediment parameters are transformed to density and frequency-dependent compressional- and shear-wave speeds and attenuations. Data prediction involves spherical-wave reflection-coefficient calculations. Trans-dimensional inversion, which samples probabilistically over the number of layers in the seabed model, is applied to combine quantitative model selection with parameter/uncertainty estimation. The inversion is applied to reflection-coefficient data sets collected on the New England Mud Patch, and inversion results are compared to those obtained under the common assumption of negligible shear effects (i.e., inverting for a fluid sediment model).

The study on the sound speed dispersion and the frequency dependence of sound attenuation in marine sediments is of great importance in understanding the physics of sound propagation in the sea bottom. Modal dispersion curves (MDC) have only been utilized to estimate the low-frequency sound speed. At higher frequencies (i.e., above a few hundred hertz), arrival time difference for different modes generally become smaller and MDC is difficult to extract because of the cross-mode interference. This paper applies time-frequency representations (TFR) with high time-frequency resolution (e.g., optimal-kernel and center-affine-filtered TFR) to broadband acoustic signals (e.g., 31g explosive and combustive source signals) detonated along the 15-km Northwest-Southeast tracks during the Seabed Characterization Experiment in the New England Mud Patch, which has a surface fine-grained sediment layer overlying a sandy bottom. The resulting high-resolution broadband (150–1500 Hz) MDC, in conjunction with TL data, are fed to a previously developed two-step dimension-reduced geoacoustic inversion algorithm including modal dispersion-based inversion for sound speed and energy-based inversion for attenuation [IEEE J. Ocean. Eng., 44, in print, (2019)]. The estimated sound speed and attenuation are compared with the theoretical results from seabed geoacoustic models for fine-grained sediments. [Work supported by ONR Ocean Acoustics (322OA).]The study on the sound speed dispersion and the frequency dependence of sound attenuation in marine sediments is of great importance in understanding the physics of sound propagation in the sea bottom. Modal dispersion curves (MDC) have only been utilized to estimate the low-frequency sound speed. At higher frequencies (i.e., above a few hundred hertz), arrival time difference for different modes generally become smaller and MDC is difficult to extract because of the cross-mode interference. This paper applies time-frequency representations (TFR) with high time-frequency resolution (e.g., optimal-kernel and center-affine-filtered TFR) to broadband acoustic signals (e.g., 31g explosive and combustive source signals) detonated along the 15-km Northwest-Southeast tracks during the Seabed Characterization Experiment in the New England Mud Patch, which has a surface fine-grained sediment layer overlying a sandy bottom. The resulting high-resolution broadband (150–1500 Hz) MDC, in conjunction with TL data, are ...

Muddy sediments cover significant portions of continental shelves, but their physical properties remain poorly understood compared to sandy sediments. This paper presents a generally applicable model for sediment-column structure and variability on the New England Mud Patch (NEMP), based on trans-dimensional Bayesian inversion of wide-angle, broadband reflection-coefficient data in this work and in two previously published reflection-coefficient inversions at different sites on the NEMP. The data considered here include higher frequencies and larger bandwidth and cover lower reflection grazing angles than the previous studies, hence, resulting in geoacoustic profiles with significantly better structural resolution and smaller uncertainties. The general sediment-column structure model includes an upper mud layer in which sediment properties change slightly with depth due to near-surface processes, an intermediate mud layer with nearly uniform properties, and a geoacoustic transition layer where properties change rapidly with depth (porosity decreases and sound speed, density, and attenuation increase) due to increasing sand content in the mud above a sand layer. Over the full frequency band considered in the new and two previous data sets (400-3125 Hz), there is no significant sound-speed dispersion in the mud, and attenuation follows an approximately linear frequency dependence.

Laboratory ultrasonic measurements of compressional wave velocity and attenuation were made as a function of effective pressure on samples of limestone, sandstone and siltstone taken from a shallow borehole test site. The results indicate that the sandstones are pervaded by grain contact microcracks which dramatically affect their compressional wave attenuations. Clean sandstone shows a compressional wave quality factor (Q{sub p}) of 24 {+-} 2 at 5 MPa effective pressure (close to the estimated in situ burial pressure) and a Q{sub p} of 83 {+-} 29 at 60 MPa. The Q{sub p} of limestones and siltstones at the site show negligible and small increases with pressure in the laboratory, respectively. The strong pressure dependence of Q{sub p} in clean sandstone was used to infer the presence of in situ microcracks. Sediment velocities measured in the laboratory at about 1 MHz were compared with those from the full waveform sonic log at about 10 kHz implies that they must also be highly attenuating over a significant part of the frequency range 10 kHz to 1 MHz, to account for the magnitude of the observed velocity dispersion. Assuming the laboratory Q{sub p} values measured at 5 MPa remain constant down to 10 kHzmore » predicts the observed dispersion quite well. Furthermore, the sonic log velocities of sandstones, limestones and siltstones (after normalizing each lithology for porosity and clay content) were found to reflect the same pressure (depth) trends observed in the laboratory. The results provide evidence for the existence of in situ microcracks in near-surface sediments.« less

The New England Mud Patch (NEMP) has been a region of interest in underwater acoustics for the past decade. Numerous geo-acoustic inversion methods have been used to estimate the compressional and shear wave speeds, and densities of four distinguishable sediment layers. While exact values vary, it is known that the upper-most layer is a relatively thin, fluid-like layer (layer 1) of mud. Below that lies more rigid mud (layer 2) that has varying physical properties with depth, followed by a sand layer (layer 3) of approximately 10 m. Below these sediment layers, an elastic half-space layer is assumed for modeling purposes. The Seabed Characterization Experiment (SBCEX22) utilized Ocean Bottom Recorders (OBX’s) to provide acoustic and seismic measurements taken at the water–sediment interface. The resulting acoustic pressure and particle velocity measurements indicate the presence of frequency dependent reverberation, likely from roughness at the mud-sand interface between layer 2 and layer 3. Analysis of the shear potential in the elastic layers of the seabed suggests that the frequency dependency seen in these data can be attributed to bottom loss from shear effects. Data from the NEMP and modeling results from a wavenumber integration model will be shown.

Experiments were conducted on the New England Mud Patch in 2017, 2021, and 2022. The 2017 Seabed Characterization Experiment (SBCEX17) utilized Signal Underwater Sound (SUS) charges, model Mk64, to produce an impulsive acoustic waveform. However, recent work in 2022 has additionally utilized the Rupture Induced Underwater Sound Source (RIUSS) to produce a high-amplitude, broadband waveform with minimal bubble oscillations. Results from these experiments suggested the presence of a surficial layer of mud with a sound speed lower than that of the underlying mud and overlying water. The SBCEX22 experiment included the deployment of nine RIUSS devices and the use of Ocean Bottom Recorders (OBX) to measure the acoustic pressure and three components of particle velocity at range of about 1 km. Conductivity, temperature, and depth measurements from CTD surveys were taken from several locations around the mud patch and used to generate sound speed profiles. These were input into the RAM parabolic equation model to analyze the effect of the sound speed in mud on propagation. Results from the RAM modeling indicates that at mid-frequencies (1-3 kHz) the lower sound speed at the top of the mud layer creates a duct where the transmission loss is reduced.