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Recent changes in ice cover and ocean stratification have been so large that acoustic measurements made during the Cold War no longer reflect current conditions.

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.

This work examines three-dimensional (3D) propagation caused by interaction with the sea ice canopy using an approximate normal mode/parabolic equation hybrid model. The effects of horizontal refraction are assessed through comparison of two-dimensional (2D) and 3D solutions for the modal amplitudes and depth-averaged transmission loss. The following 3D effects are described: diffraction of sound into shadow zones behind ice keels, horizontal defocusing of sound behind ice keels, and horizontal focusing of sound that has propagated between ice keels. A statistical analysis shows that the 3D solution is characterized by 20% greater variance in the depth-averaged transmission loss than the 2D solution.

Two scale-model acoustic propagation experiments were conducted in a laboratory tank to investigate three-dimensional (3D) propagation effects induced by range-dependent bathymetry. The model bathymetry, patterned after measured bathymetric data, represents a portion of the Hudson Canyon at 1:7500 scale. The bottom condition in the scale model is nearly pressure release, and as a result, the bottom reflection and backscattering are stronger than that of the real ocean environment. Measurements are presented for propagation paths oriented along and across the axis of the canyon. The measured data are interpreted using both 3D adiabatic-mode and 3D ray models. For propagation along the canyon axis, horizontal focusing is observed, and the out-of-plane arrivals are identified using the vertical mode/horizontal ray analogy to determine which wall or walls of the canyon refracted the sound. For the across-canyon propagation, out-of-plane arrivals are observed for both forward scattered and backward scattered sound. Using the 3D ray model, an investigation of the horizontal and vertical launch angles is used to identify features on the canyon walls responsible for the measured out-of-plane propagation. For both the along- and across-canyon experiments, the 3D ray model produced a solution that was more accurate and less computationally intensive.

Seabed variability introduced by physical or biogenic processes contributes to multi-scale horizontal and vertical variability in seabed acoustic and geotechnical properties. An experiment was conducted near the mouth of Mobile Bay to investigate such spatial variability over a mud-sand gradient. A normal-incidence bottom loss survey was conducted on a 750 × 1500 m2 grid with 50-m track-line spacing, in which a broadband acoustic source transmitted frequency-modulated chirps (1–100 kHz) every second, resulting in dense sampling of the seabed reflection coefficient over a wide frequency band. Diver cores were collected in the survey area for direct analysis of near-surface seabed properties. Cores were acoustically logged to obtain vertical profiles of sound speed and attenuation (10–1000 kHz) and then analyzed for physical properties (porosity, grain size, carbon content), infauna community composition, and sediment strength properties. Additionally, in situ portable free fall penetrometer measurements were conducted within the survey area to characterize geotechnical properties within the upper 50 cm of the sediment. Spatial distributions of properties estimated from bottom loss measurements (e.g. porosity and sound speed) will be compared with vertical profiles from the cores and from the penetrometer to examine potential connections between seabed acoustic, geotechnical, and biological parameters. [Sponsored by ONR.]

The article of record may be found at https://doi.org/10.1121/10.0001970 The Pacific Arctic Region has experienced decadal changes in atmospheric conditions, seasonal sea-ice coverage, and thermohaline structure that have consequences for underwater sound propagation. To better understand Arctic acoustics, a set of experiments known as the deep-water Canada Basin acoustic propagation experiment and the shallow-water Canada Basin acoustic propagation experiment was conducted in the Canada Basin and on the Chukchi Shelf from summer 2016 to summer 2017. During the experiments, low-frequency signals from five tomo- graphic sources located in the deep basin were recorded by an array of hydrophones located on the shelf. Over the course of the yearlong experiment, the surface conditions transitioned from completely open water to fully ice- covered. The propagation conditions in the deep basin were dominated by a subsurface duct; however, over the slope and shelf, the duct was seen to significantly weaken during the winter and spring. The combination of these surface and subsurface conditions led to changes in the received level of the sources that exceeded 60 dB and showed a dis- tinct spacio-temporal dependence, which was correlated with the locations of the sources in the basin. This paper seeks to quantify the observed variability in the received signals through propagation modeling using spatially sparse environmental measurements. This work was supported by the Office of Naval Research Ocean Acoustics Program (ONR OA322) under Grant Nos. N00014-15-1-2144, N00014-15-1-2119, N00014-15-1-2017, N00014-15-1-2068, N00014-15-1-2110, N00014-19-1-2721, N00014-15-1-2898, N00014-15-1-2806, and N00014-18-1- 2140. The basin moored environmental data were supported by the ONR Arctic and Global Prediction Program (ONR AG322) under Grant No. N00014-15-1-2782. Mooring and hydrographic data were collected and made available by the Beaufort Gyre Exploration Program based at the Woods Hole Oceanographic Institution (http://www.whoi.edu/beaufortgyre) in collaboration with researchers from Fisheries and Oceans Canada at the Institute of Ocean Sciences. This work was supported by the Office of Naval Research Ocean Acoustics Program (ONR OA322) under Grant Nos. N00014-15-1-2144, N00014-15-1-2119, N00014-15-1-2017, N00014-15-1-2068, N00014-15-1-2110, N00014-19-1-2721, N00014-15-1-2898, N00014-15-1-2806, and N00014-18-1- 2140. The basin moored environmental data were supported by the ONR Arctic and Global Prediction Program (ONR AG322) under Grant No. N00014-15-1-2782. Mooring and hydrographic data were collected and made available by the Beaufort Gyre Exploration Program based at the Woods Hole Oceanographic Institution (http://www.whoi.edu/beaufortgyre) in collaboration with researchers from Fisheries and Oceans Canada at the Institute of Ocean Sciences.

Previous investigations related to acoustic propagation through seagrass have shown that sound waves are sensitive to the gas contained within seagrass tissue as well as free bubbles produced by photosynthesis. The acoustical effects include reduced low-frequency sound speed and increased sound attenuation. The effects are more pronounced when the density of vegetation is higher as well as during daylight hours when sunlight-driven photosynthesis occurs. However, the application of mathematical models to describe these phenomena have been limited to effective medium models for water containing spherical gas bubbles. These approaches neglect both the effects of elastic properties of seagrass tissue as well as the shape of the aspherical gas bodies constrained within the plant. In this work, a finite element model of an acoustic resonator containing seawater and seagrass blades was developed to explain previously published measurements. The model utilizes independently measured values of the elastic properties of seagrass tissue and microscopic cross-section imagery of the gas volumes contained within the seagrass tissue. These results represent a step towards defining an appropriate effective medium model for acoustic propagation through seagrass tissue.

An acoustic propagation experiment was conducted in Currituck Sound to characterize low-frequency propagation in a very-shallow-water estuarine environment. The water column properties were homogeneous over the study area, and the emphasis of this work is on understanding the propagation effects induced by the estuarine bed. During the experiment, low-frequency sound propagation measurements of waterborne sound and interface waves were acquired, and direct measurements of the compressional and shear wave properties were obtained at high frequencies. The propagation data consist of signals from a Combustive Sound Source recorded on bottom mounted geophones and a vertical line array of hydrophones. A statistical inference method was applied to obtain an estimate of the sediment compressional and shear wave speed profiles as a function of depth within the estuarine bed. The direct measurements were obtained in situ by inserting probes 30 cm into the sediment. Sediment acoustics models were fit to the high-frequency in situ data to enable comparison with the inferred low-frequency wave speeds. Overall, good agreement was found between the directly measured and inferred wave speeds for both the compressional and shear wave data.

Nearshore and estuarine sediments experience varying sediment inputs that create sharp gradients in sediment properties over fairly small vertical and horizontal distances. In this study, we explored the sand-mud transition in sediment cores collected from Mobile Bay, AL, with acoustic and geotechnical approaches. A custom built instrument was used to measure depth profiles of tensile strength at cm-scale resolution. These measurements were compared to acoustic sound speed and attenuation across a range of frequencies. Cores collected exhibited a horizontal gradient from sand to mud across the study area with a sandy layer on top of muddier sediment in most cores. This study explicitly examines the relationship between acoustic sound speed and attenuation and strength-related properties of sediments such as cohesion that can be measured in situ more easily than textural properties (e.g., porosity and grain size). We compare our data to in situ normal incidence measurements and portable free fall penetrometer data as well as infaunal community composition to better relate acoustic, geotechnical, and biological properties across a gradient of sediments.

Two Autonomous Underwater Multi-Dimensional Acoustic Recorders (AUMDAR) were deployed in the southeastern part of Lake Travis during the summer of 2018. Each AUMDAR system possessed a three-axis acoustic vector sensor capable of estimating the azimuthal and vertical arrival angles from discrete sound sources. A unique and complicated propagation environment existed during the experiment due to the mooring locations and the range-dependent lake bathymetry. The AUMDAR systems were almost entirely shielded from sound emanating from surface vessels to the south and southeast of the deployment location, while a larger, yet limited, direct acoustical field of view was realized to the north and northeast. During the evening hours, the low-frequency received level increased without a corresponding increase in the number of detected discrete surface vessels. During the same time, the predominant direction of the received sound pointed toward the bend in the river channel. A three-dimensional ray model was employed to assess the various arrival angles from a grid of source positions located throughout the lake. The model results are consistent with the observations and suggest that the ambient noise field originated from vessels physically located to the northwest of the sensors, but arriving at angles consistent with out-of-plane sound propagation.

Transmission loss measurements were conducted in the meltwater-modified surface layer near Hansbreen Glacier in Hornsund Fjord in southwestern Svalbard in September 2017 [Deane and Glowacki, JASA 143, 1711 (2018)]. An ITC-1007 (International Transducer Corporation) source emitting an m-sequence signal (149 dB re 1 µPa, 11 kHz carrier frequency) was tethered at 6.7 m depth to a boat allowed to drift 20 m to 200 m from a pair of stationary receivers. These two receivers, HTI-96-Min hydrophones (High Tech, Inc.), were tethered at 8 and 17 m to a buoy anchored 500 m from the glacier terminus. Within this environment, and typical for a glacierized fjord, regular calving events contributed to a collection of icebergs, or brash ice, at the water surface obstructing the transmission path, particularly the surface reflected path. A ray-based approach using Bellhop, a code available in the Acoustics Toolbox, was used to simulate the environment as a means of comparison to the measured data. The variability of the brash ice surface was included in the model through incorporating a simulated surface at the ocean surface. Comparisons between outcomes from increasingly complex iterations of this model with the collected data are discussed.

Underwater acoustic recordings often have contributions from anthropogenic, geophonic, and biophonic sources and deconstructing the sound field into constituent parts can be useful when characterizing the soundscape. This work focuses on the analysis of a yearlong acoustic measurement made on the Chukchi Shelf as part of the Canada Basin Acoustic Propagation Experiment (CANAPE). The data were recorded on a shallow-water array with horizontal and vertical apertures and span time periods including the open water season, marginal ice zone, and complete ice cover. A supervised convolutional neural network is trained on labeled spectra and demonstrates the ability to separate anthropogenic and biologic sources. Details of the model, labeling process, initial results, and lessons learned will be discussed in this talk.

Studies of ambient sound in the Arctic, both under ice and in the marginal ice zone, have spanned more than 50 years, but rapidly changing conditions with regards to declining ice cover have reduced the relevance of measurements taken in previous decades. Changes in the environment have resulted in changes in the ambient sound field, affecting both the sound generating mechanisms and the sound propagation. From October 2016 to October 2017, the Shallow Water Canada Basin Acoustic Propagation Experiment (SW CANAPE) was conducted on the Chukchi Shelf. One goal of CANAPE was to observe the changing soundscape. This talk presents acoustic recordings collected on the 150-m isobath with the Persistent Acoustic Observation System (PECOS), which contained a horizontal line array of hydrophones along the seabed and a vertical line array spanning a portion of the water column. This study examines the ambient sound level and uses k-means clustering to quantify the occurrence of six unique spectral shapes associated with different seasons and various sound generation mechanisms. The spectral clusters are correlated with environmental observations including sea concentration and thickness, wind speed, and air temperature. [Work supported by ONR.]

The Applied Research Laboratories at the University of Texas at Austin (ARL:UT) deployed two passive acoustic recording systems along the 150-m isobath of the Chukchi Shelf during the 2016-2017 Canada Basin Acoustic Propagation Experiment (CANAPE). The first system was a single-hydrophone recorder located on the seafloor, while the Persistent Acoustic Observation System (PECOS) contained a horizontal line array of hydrophones along the seabed and a vertical line array spanning a portion of the water column. The systems were deployed and recovered during open-water conditions, but remained in place during the ice-formation, ice-covered, and ice-melt time periods. This work presents initial findings of the statisti-cal ambient noise levels during the yearlong experiment and presents two examples of the dynamic noise environment present during the recordings.

Seabed layering and sediment properties impact sound propagation in ocean waveguides, particularly in environments where sound propagation paths repeatedly interact with the seafloor. As part of the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), experiments were designed to investigate seabed layering and sediment properties on the Chukchi Shelf. First, the shallow water experimental region was surveyed with a subbottom profiler to provide information about the overall sediment layering. Second, ship-radiated noise from a research vessel sailing specifically designed tracks was received on the Persistent Acoustic Observation System (PECOS). These recordings provide an opportunity for short- to mid-range geoacoustic inversion for sediment properties. Third, in-situ acoustic sound speed measurements were made with the Acoustic Coring System (ACS) while two to five meter long core samples were simultaneously collected. This talk presents initial findings of the seabed layering and sediment properties from these three experiments.

This paper discusses the design and operation of a measurement apparatus used to conduct scale-model underwater acoustic propagation experiments, presents experimental results for an idealized waveguide, and compares the measured results to data generated by two-dimensional (2D) and three-dimensional (3D) numerical models. The main objective of this paper is to demonstrate the capability of the apparatus for a simple waveguide that primarily exhibits 2D acoustic propagation. The apparatus contains a computer-controlled positioning system that accurately moves a receiving transducer in the water layer above a scale-model bathymetry while a stationary source transducer emits broadband pulsed waveforms. Experimental results are shown for a 2.133 m × 1.219 m bathymetric part possessing a flat-bottom bathymetry with a translationally invariant wedge of 10° slope along one edge. Beamformed results from a synthetic horizontal line array indicate the presence of strong in-plane arrivals along with weaker diffracted and horizontally refracted arrivals. A simulated annealing inversion method is applied to infer values for five waveguide parameters with the largest measurement uncertainty. The inferred values are then used in a 2D method of images model and a 3D adiabatic normal-mode model to simulate the measured acoustic data.

To investigate the effects of infauna on seabed geoacoustic properties, a field experiment was conducted during May 2017 in Petit Bois Pass, near the mouth of Mobile Bay, Alabama. In situ measurements of seabed geoacoustic properties were collected using hydrophones to generate and receive compressional waves from 5 kHz to 100 kHz and bimorph transducers to measure shear waves from 300 Hz to 1.5 kHz. The measurement system was deployed multiple times at three distinct sites within the pass, and acoustic measurements were conducted at depths ranging from 5 cm to 20 cm into the sediment. For each deployment of the acoustic system, diver cores were collected. A subset of the cores were sieved on site to collect infauna, and the remaining cores were taken back to the laboratory where they were sectioned and analyzed for porosity and grain size distribution. Comparison between compressional and shear wave speed and attenuation, the sediment geotechnical properties, and the distribution and abundance of infauna...

Benthic infauna perform important ecological functions such as water filtration, the recycling of organic matter, and forage for larger predators. However, how infauna affect bulk acoustical properties of the sediment is less studied and warrants further investigation. During a field experiment conducted in Petit Bois Pass, AL, multiple cores of fine-grained sediments containing infauna were collected by divers and analyzed using a core and resonance logger (CARL). CARL is a core-logger capable of performing both conventional pitch-catch measurements radially along the core as a function of depth and lower frequency resonance measurements. Resonance measurements were performed by exciting acoustic modes within the core and sensing them externally along the core's length. By utilizing both pitch-catch transducers as receivers, symmetric and asymmetric modes can be identified, and sound speed can be inferred as a function of frequency. The frequency ranges for the pitch-catch and resonance measurements were 100 kHz to 300 kHz and 8 kHz to 23 kHz, respectfully. Subsequently, the cores were sectioned and were either sieved for infauna or analyzed for porosity and grain size distribution. The measured sound speeds will be compared with biological and geological properties of each individual core.Benthic infauna perform important ecological functions such as water filtration, the recycling of organic matter, and forage for larger predators. However, how infauna affect bulk acoustical properties of the sediment is less studied and warrants further investigation. During a field experiment conducted in Petit Bois Pass, AL, multiple cores of fine-grained sediments containing infauna were collected by divers and analyzed using a core and resonance logger (CARL). CARL is a core-logger capable of performing both conventional pitch-catch measurements radially along the core as a function of depth and lower frequency resonance measurements. Resonance measurements were performed by exciting acoustic modes within the core and sensing them externally along the core's length. By utilizing both pitch-catch transducers as receivers, symmetric and asymmetric modes can be identified, and sound speed can be inferred as a function of frequency. The frequency ranges for the pitch-catch and resonance measurements were...

This paper presents observations of two classes of acoustic arrivals recorded on a sparsely populated vertical line array (VLA) moored in the center of the Catoche Tongue, a major reentrant in the Campeche Bank in the southeastern Gulf of Mexico. The acoustic signals were generated by signals underwater sound (SUS) located 50-80 km from the VLA. The first class of arrivals was identified as resulting from a direct (non-horizontally refracted) path. Then following a quiescent period, a second, more diffuse class of arrivals is observed and is believed to be the result of horizontal refraction from the margin of the Tongue. A spectral analysis of the measured data revealed that both classes of arrivals were characterized by the source spectrum associated with SUS. Additionally, the difference in time between the onset of the first and second class of arrivals observed as a function of range from the VLA is consistent with the relative difference in the length of the direct and refracted paths. The observations are further supported by a three-dimensional (3D) acoustic propagation computation that reproduces many of the features of the measured data and provides additional insight into the details of the 3D propagation.

Recent incidents at the nation's only operating deep geologic nuclear waste repository, the Waste Isolation Pilot Plant (WIPP), resulted in the release of americium and plutonium from one or more defense-related transuranic (TRU) waste containers into the environment. WIPP is a U.S. Department of Energy mined geologic repository that has been in operation since March, 1999. Over 85,000 m3 of waste in various vented payload containers have been emplaced in the repository. The primary radionuclides within the disposed waste are 239+240Pu and 241Am, which account for more than 99% of the total TRU radioactivity disposed and scheduled for disposal in the repository. For the first time in its 15 years of operation, there was an airborne radiation release from the WIPP at approximately 11:30 PM Mountain Standard Time (MST) on Friday, February 14, 2014. The radiation release was likely caused by a chemical reaction inside a TRU waste drum that contained nitrate salts and organic sorbent materials. In a recent news release, DOE announced that photos taken of the waste underground showed evidence of heat and gas pressure resulting in a deformed lid, in material expelled through that deformation, and in melted plastic and rubber and polyethylene in the vicinity of that drum. Recent entries into underground Panel 7 have confirmed that at least one waste drum containing a nitrate salt bearing waste stream from Los Alamos National Laboratory was breached underground and was the most likely source of the release. Further investigation is underway to determine if other containers contributed to the release. Air monitoring across the WIPP site intensified following the first reports of radiation detection underground to ascertain whether or not there were releases to the ground surface. Independent analytical results of air filters from sampling stations on and near the WIPP facility have been released by us at the Carlsbad Environmental Monitoring & Research Center and confirmed trace amounts of 241Am and 239+240Pu, at ratios reflecting the suspect waste stream. The highest activity detected offsite was 115.2 μBq/m3 for 241Am and 10.2 μBq/m3 for 239+240 Pu. These concentrations in air were very small, localized, and below any level of public health or environmental concern.

The overall goal of this work is to quantify the effects of environmental variability and spatial sampling on the accuracy and uncertainty of estimates of the three-dimensional ocean sound-speed field. In this work, ocean sound speed estimates are obtained with acoustic data measured by a sparse autonomous observing system using a perturbative inversion scheme [Rajan, Lynch, and Frisk, J. Acoust. Soc. Am. 82, 998-1017 (1987)]. The vertical and horizontal resolution of the solution depends on the bandwidth of acoustic data and on the quantity of sources and receivers, respectively. Thus, for a simple, range-independent ocean sound speed profile, a single source-receiver pair is sufficient to estimate the water-column sound-speed field. On the other hand, an environment with significant variability may not be fully characterized by a large number of sources and receivers, resulting in uncertainty in the solution. This work explores the interrelated effects of environmental variability and spatial sampling on the accuracy and uncertainty of the inversion solution though a set of case studies. Synthetic data representative of the ocean variability on the New Jersey shelf are used.

Infauna, animals living in marine sediments, modify sediment structure by burrowing, constructing burrows and tubes, and irrigating burrows. These activities can change the bulk porosity and density as well as create heterogeneity in sediment structure. We test the hypothesis that these activities alter sound speed and attenuation in sediments by manipulating homogenized sediments to mimic animal activities. Specifically, we examine the effects of burrow excavation, burrow wall compaction, burrow irrigation, and construction of tubes from shell hash on sound speed and attenuation at 100, 200, and 400 kHz. Wavelengths corresponding to these frequencies span the size of the burrows constructed, and measurements were conducted at several depths within the upper 10 cm of sediment in which infauna are commonly found. Each of these activities or functions is performed by multiple species of animals that comprise a functional group. Our results will help identify functional groups that have important impacts on sediment acoustics and will be used to interpret field data in which deviations from predicted sound speed and attenuation are correlated with different and diverse communities of infauna.Infauna, animals living in marine sediments, modify sediment structure by burrowing, constructing burrows and tubes, and irrigating burrows. These activities can change the bulk porosity and density as well as create heterogeneity in sediment structure. We test the hypothesis that these activities alter sound speed and attenuation in sediments by manipulating homogenized sediments to mimic animal activities. Specifically, we examine the effects of burrow excavation, burrow wall compaction, burrow irrigation, and construction of tubes from shell hash on sound speed and attenuation at 100, 200, and 400 kHz. Wavelengths corresponding to these frequencies span the size of the burrows constructed, and measurements were conducted at several depths within the upper 10 cm of sediment in which infauna are commonly found. Each of these activities or functions is performed by multiple species of animals that comprise a functional group. Our results will help identify functional groups that have important impacts on ...

During the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), acoustic transmissions were recorded from both deep- and shallow-water sources on a set of moored receivers, over the course of nearly one year, in ice-covered and open water conditions. One of the source-receiver geometries included LFM transmissions of 4000–1500 Hz and 1100–700 Hz recorded on an eight-element volumetric array at a distance of approximately 400 m. Surface, bottom, and sub-bottom reflections are observed in the data and are used to infer ice and sediment geo-acoustic properties. Some observations about bistatic scattering are also made.

Sediment cores provide valuable insight about the physical properties of the seabed, and laboratory measurements of sediment wave speed from cores are often considered “ground truth.” However, sound-speed estimates obtained from cores can be inaccurate due to changes in pressure, temperature, and mechanical properties of the sediment caused by the removal of the core from the seabed and its subsequent transport to the laboratory. To address these issues, the Acoustic Coring System (ACS) was developed. The system uses 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. Compressional wave measurements are obtained with rod-mounted piezoelectric cylinders, and shear wave measurements are obtained with bender elements mounted in flat blades. The ACS was deployed during the environmental survey for ONR Seabed Characterization Experiment (SBCEX) in 2016 and in support of the ONR Canada Basin Acoustic Propagation Experiment (CANAPE) in 2017. In both cases, the system provided measurements of the sediment sound-speed profile as a function of depth within the seabed. On-going work focused on the measurement of shear wave speed profiles and compressional and shear wave attenuation will also be presented. [Work supported by ONR.]Sediment cores provide valuable insight about the physical properties of the seabed, and laboratory measurements of sediment wave speed from cores are often considered “ground truth.” However, sound-speed estimates obtained from cores can be inaccurate due to changes in pressure, temperature, and mechanical properties of the sediment caused by the removal of the core from the seabed and its subsequent transport to the laboratory. To address these issues, the Acoustic Coring System (ACS) was developed. The system uses 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. Compressional wave measurements are obtained with rod-mounted piezoelectric cylinders, and shear wave measurements are obtained with bender elements mounted in flat blades. The ACS was deployed during the environmental survey for ONR Seabed Characterization Experiment (SBCEX) in 2016 and in support of the ONR Canada Basin Acoustic...

Transmission loss measurements were conducted in the meltwater-modified surface layer near Hansbreen Glacier in Hornsund Fjord in southwestern Svalbard in September 2017 [Deane and Glowacki, JASA 143, 1711 (2018)]. An m-sequence source signal (149 dB re 1 μPa, 11 kHz carrier frequency) was tethered at 7 m depth to a boat drifting from 0 to 200 m. This signal was received by two Hitech HTI-96 hydrophones at 8 and 17 m depth deployed from a stationary boat anchored 500 m from the glacier. Within this environment, and typical for a glacierized fjord, regular calving events contributed to an ice melange top boundary layer with larger icebergs occasionally obstructing the signal transmission path. The propagation environment was upward refracting, causing propagation sound to repeatedly reflect from the surface layer. A ray-based approach was applied to model the measured data. The variability of the top boundary was included in the model by incorporating surface scattering and inserting icebergs. Comparisons between several increasingly complex iterations of this model with the collected data will be presented. [Work supported by the NDSEG Fellowship, ONR Grant Nos. N00014-17-1-2633 and N00014-14-1-0213, and the Polish National Science Centre Grant No. 2013/11/N/ST10/01729.]

Previous work examined seabed geoacoustical properties near the 150 m isobath on the Chukchi shelf as observed during the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE) [Sagers and Ballard, JASA 144(3), 1666]. That work reported a water/sediment compressional sound speed ratio near 0.98 and a compressional sound speed gradient in the upper sediment around 6 s-1. This work extends the prior analysis by investigating additional locations throughout the larger shallow-water experimental site. Data from subbottom profile surveys, measurements from an in situ acoustic coring system and inferences made using ship-radiated noise received on Autonomous Multichannel Acoustic Recorders (AMARs) deployed by the Defence Research and Development Canada (DRDC) are examined to understand seabed layering and sediment properties throughout the experimental region. Particular interest is given to understanding whether geoacoustical properties on the Chukchi shelf exhibit range-dependence. [Work sponsored by ONR.] Previous work examined seabed geoacoustical properties near the 150 m isobath on the Chukchi shelf as observed during the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE) [Sagers and Ballard, JASA 144(3), 1666]. That work reported a water/sediment compressional sound speed ratio near 0.98 and a compressional sound speed gradient in the upper sediment around 6 s-1. This work extends the prior analysis by investigating additional locations throughout the larger shallow-water experimental site. Data from subbottom profile surveys, measurements from an in situ acoustic coring system and inferences made using ship-radiated noise received on Autonomous Multichannel Acoustic Recorders (AMARs) deployed by the Defence Research and Development Canada (DRDC) are examined to understand seabed layering and sediment properties throughout the experimental region. Particular interest is given to understanding whether geoacoustical properties on the Chukchi shelf exhibit range-dependence. [Work sponsored by...

As in the mid-latitudes, the variability of the oceanography in the arctic ocean causes azimuthal dependent acoustic fields induced by the environmental parameters, such as the variable sound speed profile and bathymetry. Preliminary analysis of a year-long experimental data from “deep-to-shallow” and the “shallow-water” Canada Basin Acoustic Propagation Experiment (SW CANAPE) shows strong azimuthal variability of broadband signals recorded on 11 spatially distributed acoustic receiver arrays on the Chukchi shelf from September 2016 to October 2017. Particular attention is paid on geotimes pertaining to the variable sound speed profiles around the dynamic shelf-break region. Although acoustic modeling of the propagation requires a physics based ocean model for a complete modeling exercise, preliminary determination of the azimuthal behavior of broadband propagation is conducted using multiple two-dimensional Parabolic Equation (Nx2D) with the collected oceanographic data. Calculations for long-range, as well as short-range, source-receiver pairs in the SW CANAPE data show that azimuthal variabilities occur frequently and are related to the spatial and temporal characteristics of signal dispersion for different range and geotime scales. [Work supported by ONR.]

In situ measurements of compressional and shear wave speed and attenuation were collected 30 cm below the water-sediment interface in Currituck Sound, North Carolina at two field locations having distinctly different sediment types: medium-to-fine-grained sand and fine-grained sand with approximately 10% mud content. Shear wave measurements were performed with bimorph transducers to generate and receive horizontally polarized shear waves in the 300 Hz to 1 kHz band, and compressional wave measurements were performed using hydrophones operated in the 5 kHz to 100 kHz band. Sediment samples were collected at both measurement sites and later analyzed in the laboratory to characterize the sediment grain size distribution for each field location. Compressional and shear wave speed and attenuation were estimated from the acoustic measurements, and preliminary comparisons to the extended Biot model by Chotiros and Isakson [J. Acoust. Soc. 135, 3264-3279 (2014)] and the viscous grain-shearing theory by Buckingham [J. Acoust. Soc. 136, 2478-2488 (2014)] were performed.

Three-dimensional propagation effects of low frequency sound from 100 to 400 Hz caused by seafloor topography and range-dependent bottom structure over a 20 km range along the New Jersey shelf are investigated using a hybrid modeling approach. Normal modes are used in the vertical dimension, and a parabolic-equation approximate model is applied to solve the horizontal refraction equation. Examination of modal amplitudes demonstrates the effect of environmental range dependence on modes trapped in the water column, modes interacting with the bottom, and modes trapped in the bottom. Using normal mode ray tracing, topographic features responsible for three-dimensional effects of horizontal refraction and focusing are identified. These effects are observed in the measurements from the Shallow Water 2006 experiment. Specifically, signals from a pair of fixed sources recorded on a horizontal line array sitting on the seafloor show an intensification caused by horizontal focusing due to the seabed topography of 4 dB along the array.

An acoustic propagation model is applied to predict measurements of three-dimensional (3-D) effects recorded off the southeast coast of Florida. The measured signal is produced by a low frequency source that is towed north parallel to the shelf from a fixed receiving array. The acoustic data show the direct path arrival at the bearing of the tow ship and a second refracted path arrival as much as 30° inshore of the direct arrival. Notably, the refracted arrival has a received level more than 25 dB greater than that of the direct arrival. A geoacoustic model of the environment is created to explain the data. It is shown that the topography of the seafloor plays the largest role in controlling horizontal refraction effects, whereas the range-dependent sediment properties have the most influence on the received level. The modeling approach is based on a 3-D adiabatic mode technique in which the horizontal refraction equation is solved using a parabolic equation in Cartesian coordinates. A modal decomposition of the field provides insight into the variability in the arrival angle and received level of the measured signal.

The Pacific Arctic Region, has experienced decadal changes in atmospheric conditions, seasonal sea-ice coverage, and seawater temperature. From the October 2016 to September 2017, the Canada Basin Acoustic Propagation Experiment (CANAPE) was conducted to understand the changing soundscape and to explore the use of acoustic signals as a remote sensing tool in the modern Arctic. During the experiment, low-frequency signals from five tomographic sources located in the Canada Basin were recorded by an array of hydrophones with both horizontal and vertical apertures located on the Chukchi Shelf at the 150 m isobath. The propagation distances ranged from 240 km to 520 km, and the propagation conditions changed from persistently ducted in the basin to seasonally upward refracting on the continental shelf. The water column properties and ice draft were measured by oceanographic sensors on the basin tomography moorings and by an array of oceanographic moorings on the continental shelf and slope to characterize the temporal and spatial variability of the environment. This talk examines the range-dependent measurements and explains the observed variability in the received signals through propagation modeling using the oceanographic measurements. [Work sponsored by ONR.]

One of the main objectives of the Shallow Water (SW) CANAPE experiment was to gain a thorough understanding of a yearlong propagation of broadband signals from deep to shallow water with simultaneous oceanographic and acoustic measurements together along the and across the shelf break region. Using more than eleven acoustic arrays and seven oceanographic moorings in a 30 km2 region on the Chukchi shelf this task is being done by assessing both deep water sound signatures and shallow water source transmissions. In this paper with present analysis of acoustic signals from both shallow and deep water sources on the Chukchi continental shelf for a specific time period between June and August 2017 where a 20 dB intensity drop from along-the-shelf source (S2) at 150 m water depth was observed for more than few weeks. This intensity drop is strongly correlated with occurrence of a large oceanographic event spanning the top 150 m water column due to Pacific Water outflow from Bering sea and retreat of Marginal Ice Zone (MIZ). During the same period, cross-the-shelf source (S1) was not transmitting signal but the reception from the deep water acoustic transmitters also show variability that could be correlated with the basin scale water column variability and the ice coverage. [Work supported by ONR.]

The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.

The Applied Research Laboratories at the University of Texas at Austin (ARL:UT) deployed two passive acoustic recording systems along the 150 m isobath of the Chukchi Shelf during the 2016–2017 Canadian Basin Acoustic Propagation Experiment (CANAPE). The first system was a single-hydrophone recorder located on the seafloor, while the Persistent Acoustic Observation System (PECOS) contained a horizontal line array of hydrophones along the seabed and a vertical line array spanning a portion of the water column. The systems were deployed and recovered during open-water conditions, but remained in place during the ice-formation, ice-covered, and ice-melt time periods. This talk presents initial findings of the statistical ambient noise levels during the year-long experiment, presents beam-noise levels recorded by PECOS, and qualitatively discusses the natural, biologic, and anthropogenic sounds present in the acoustic recordings. [Work sponsored by ONR.]

This presentation describes the apparatus, procedure, and results from a field experiment conducted in July 2017 in the seagrass beds of the Lower Laguna Madre. The Laguna Madre is a shallow hypersaline lagoon on the western coast of the Gulf of Mexico. The relatively flat bathymetry is covered by a thick meadow of Thalassia testudinum. Acoustic piezoelectric transducer probes measured compressional wave speed and attenuation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the underlying sediment. Geoacoustic parameters were measured at depths between 5 cm and 20 cm in 5 cm increments. Compressional wave measurements were made over a frequency range of 20 kHz to 100 kHz, and shear wave measurements were made at 500 Hz and 1 kHz. The compressional wave data from this experiment indicates dispersion within the seagrass canopy and sediment. This dispersion is likely due to the high presence of gas bubbles in both the seagrass and supporting sed...

A 3-D model of sediment sound speed for a 90-km2 area on the New Jersey shelf was constructed by application of a geoacoustic inversion technique. This approach is based on a combination of seismic reflection measurements and a perturbative inversion scheme using horizontal wave number estimates. In a two-step process, seismic reflection measurements were used to identify depths at which discontinuities in the sound-speed profile (SSP) likely occur. Then, the perturbative inversion algorithm made use of this a priori information by employing qualitative regularization, an optimal method for addressing stability and uniqueness issues associated with solving the ill-posed inversion problem that provides for resolution of the layered seabed structure.

This paper presents results of a range-independent perturbative inverse approach applied to data from the Shallow Water Experiment 2006. The inversion technique is based on a linearized relationship between sound speed in the sediment and modal eigenvalues. Horizontal wave numbers were estimated from data collected from two distinct source/receiver tracks oriented along and across the shelf. The specific inversion algorithm used is based on qualitative regularization and uses known information about the environment to constrain the solution. Locations of the R reflector and other layering information are used as a priori information for the inversion to emphasize the layered structure of the sediment.

Analysis of modeled time series data is presented to provide insight into propagation physics of horizontally refracted sound in the Catoche Tongue region of the Gulf of Mexico. The analysis is motivated by the observation of out-of-plane arrivals in measured time series data. In particular, the extended duration of the refracted arrivals is shown to be caused by interaction with multiple locations along the steep sides of the Tongue. Comparison of the modeled time series is made to previous work by Sturm [J. Acoust. Soc. Am. 117(3), 1058–1079 (2005)], who examined the frequency dependence of out-of-plane modal arrivals for the wedge-shaped ocean.

This paper further demonstrates the ability of piezoceramic bimorph bender elements to preferentially generate and receive near-surface shear waves for in situ sediment characterization measurements, in terrestrial as well as marine clay soils. The bimorph elements are housed in probe transducers that can manually be inserted into the sediment and are based on the work of Shirley [J. Acoust. Soc. Am. 63 (5), 1643-1645 (1978)] and of Richardson et. al. [Geo. - Marine Ltrs., 196-203 (1997)]. The transducers can discretely generate and receive horizontally polarized shear waves, within their bimorph directivity patterns. The use of multiple probes allows one to measure the shear wave velocity and attenuation parameters in the sediment of interest. Measured shear wave data on a hard clay terrestrial soil, as well as on soft marine sediments, are presented. These parameters along with density and compressional wave velocity define the elastic moduli (Poisson's ratio, shear modulus, and bulk modulus) of the sediment, which are of interest in various areas of geophysics, underwater acoustics, and geotechnical engineering. Discussion will focus on use of the probes in both terrestrial and marine sediment environments. [Work supported by the US Army Engineer Research and Development Center, Vicksburg, and by ARL:UT Austin]

The Pacific Arctic Region, encompassing the Bering, Chukchi, Western Beaufort, and Eastern Siberian shelves and seas, has experienced decadal changes in atmospheric conditions, seasonal sea-ice coverage, and seawater temperature. In the summer of 2016, the Canada Basin Acoustic Propagation Experiment (CANAPE) was conducted to understand the changing soundscape and to explore the use of acoustic signals as a remote sensing tool in the modern Arctic. During the experiment, low-frequency signals from five tomographic sources located in the Canada Basin were recorded by a short vertical line array of hydrophones deployed from a research vessel. The recordings were made at seven stations located in the Canada Basin, on the continental rise, and on the Chukchi Shelf. The propagation distances ranged from 50 km to 500 km, and the propagation conditions changed from ducted by the Beaufort Lens in the basin to upward refracting on the continental shelf. Multiple measurements of the sound speed profile were acquire...

In the Canada Basin and Chukchi Sea regions, a vertical sound duct can be formed between the Pacific Summer Water Layer on the top and the Atlantic Water Layer on the bottom, providing an acoustic pathway connecting the deep basin and the shallow shelf over the Chukchi Sea continental slope. Previous studies have shown that the shelfbreak circulation (specifically upwelling), the sub-mesoscale eddies spun off the shelfbreak jet, and the ice coverage are the three major causes of the temporal and spatial variability of the Pacific Summer Water Layer in the region. In this paper, numerical simulations utilizing the Parabolic-Equation (PE) method are conducted to investigate the sound propagation variability over the Chukchi Sea shelfbreak and slope, along with an idealized ocean circulation model providing the fundamental basis of water-column fluctuations. The sound pressure sensitivity kernel derived from the acoustic propagator in the PE method is also used to provide physical insights into the sound pro...

Naturally occurring marine sediments can possess poorly sorted or multimodal particle size distributions. Furthermore, while well-sorted coarse grained sediments are typically characterized according to their grain size, for mixed or fine grained sediments the shape of the particles can have significant influence on the acoustical properties. In this work, particle size and morphology are investigated using scanning electron microscopy (SEM). Images of dry particles are analyzed to determine the sphericity and angularity of sand and silt grains and to estimate the shape (e.g. platelets or needles) and aspect ratio of clay particles. SEM images of wet samples are analyzed to understand the sediment microstructure, including the interaction between sand, silt, and clay particles. Additionally, the effect of the proportionally varying composition of sand, silt, and clay particles on the bulk properties of the sediment, including the porosity and compressional wave speed, will be examined using a multiphase effective medium model. These techniques are applied to sediments collected in the New England Mud Patch as part of the Seabed Characterization Experiment.

This talk presents preliminary results from an experiment conducted in the Lower Laguna Madre, Texas to characterize the physical and acoustical properties in a meadow of Thalassia testudinum. Concurrent measurements were collected using (1) acoustic probes, (2) side-scan and parametric sonar, (3) broadband propagation, and (4) sediment cores. The acoustic probes provided localized, direct measurements of sound propagation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the seagrass-bearing sediment. The side-scan and parametric sonars were used to survey for seagrass abundance and sub-bottom layering. Broadband signals produced by a combustive sound source were recorded at several ranges by hydrophones and geophones and were used to infer geoacoustic properties of the seagrass and underlying sediment for rapid environmental assessment. The sediment cores were analyzed in the laboratory using both low-frequency resonator measurements and hig...

Scale-model tank experiments are beneficial because they offer a controlled environment in which to make underwater acoustic propagation measurements that can provide high-quality data for comparison with numerical models. This talk presents results from a 1:7500 scale model experiment for a wedge with a 10° slope fabricated from closed-cell polyurethane foam to investigate three-dimensional (3D) propagation effects. A 333 μs second long pulse allows the acoustic field to obtain a steady-state, continuous-wave signal. A computer controlled positioning system accurately moves the receiving hydrophone in 3D space to create a dense field of vertical line arrays, which are used to mode filter the measured time series. The single-mode fields show the classical interference pattern resulting from rays launched up and along the slope. The measured data are compared to an exact, closed-form solution for a point source in wedge with impenetrable boundaries. The finite size of the source and the departure from the ...

Scale-model acoustic propagation experiments were conducted in a laboratory tank to investigate three-dimensional (3D) propagation effects induced by range-dependent bathymetry. The model bathymetry, patterned after measured bathymetric data, represents a portion of the Hudson Canyon at 1:7500 scale and was fabricated from closed-cell polyurethane foam using a computer-numerically controlled (CNC) milling machine. In the measurement apparatus, a computer-controlled positioning system precisely locates the receiving hydrophone which permits the creation of synthetic horizontal line arrays. Results are shown for propagation paths along and across the axis of the canyon. The measurements are explained using both a hybrid method known as vertical modes/horizontal parabolic equation and a 3D ray model known as Bellhop3D. For propagation along the canyon axis, horizontal focusing is observed and discussed in the context of normal modes. For the across canyon propagation, the reflective foam walls of the canyon ...