Industrial development in Cook Inlet, Alaska, has raised concerns about the decline of the resident beluga whale ( Delphinapterus leucas) population over the past decades. Anthropogenic noise sources, such as commercial vessels, can cause auditory masking of cetacean vocalizations used in communication, navigation, and foraging. This study includes a source-level spectrum of a containership, positioned in Cook Inlet within the primary shipping channel, and audiograms obtained from a live, stranded beluga calf in Cook Inlet, in sound transmission loss modeling. The model uses a hybrid parabolic equation/raytracing solution to determine underwater sound pressure levels. Whereas the communication band of belugas can be fully masked by ambient noise and underwater radiated noise from a containership positioned at 5,000 m, the echolocation band (for navigation) is only partially masked at shorter ranges (≤2,500 m). Increased multipath reflections from the propagation of underwater noise in the upslope direction contributes significantly to the attenuation of high frequencies and minimizes masking in the echolocation band of the beluga whale. To assess the complex task of auditory masking, critical ratios were used to determine the difference between the energy of a pure tone and the noise in the masking band. A successful strategy to reduce underwater radiated noise must consider the interactions and contributions of mitigation measures that have already been shown to be effective. The technical and economic profitability of these measures will largely depend on the design, operating parameters, and mandatory requirements for a particular vessel.

The habitat of the endangered southern resident killer whale (SRKW) overlaps major international shipping lanes near the Port of Vancouver, British Columbia. Shipping is a dominant source of underwater noise, which can hinder SRKW key life functions. To reduce environmental pressure on the SRKWs, Vancouver Fraser Port Authority offers incentives for quieter ships. However, the absence of a widely accepted underwater radiated noise (URN) measurement procedure hinders the determination of relative quietness. We review URN measurement procedures, summarizing results to date from two Canadian-led projects aimed at improving harmonization of shallow-water URN measurement procedures: One supports the International Organization for Standardization (ISO) in the development of a URN measurement standard; the other supports the alignment of URN measurement procedures developed by ship classification societies. Weaknesses in conventional shallow-water URN metrics are identified, and two alternative metrics proposed. Optimal shallow-water measurement geometry is identified.

The shallow water Canada Basin Acoustic Propagation Experiment (SW CANAPE) was conducted to study the effects of oceanographic variability on broadband acoustic fields in the Arctic. The physics of the acoustic waveguide on the northeastern edge of the Chukchi Shelf are influenced by dynamic boundary conditions and spatio-temporal fluctuations in the water column temperature and salinity profiles. Several oceanographic and acoustic receiving arrays were deployed across the Chukchi Shelf out to the shelf break region. Linear frequency modulated (LFM) signals were transmitted by two sources on the shelf for a long period of time. The influence of small scale, short-term water column variability, and dynamic upper boundary conditions including open water, marginal, and solid ice zones on shallow water propagation is shown for a 10 km source-receiver separation with well-defined water column properties measured at the source, receiver, and a mid-point along the cross-shelf acoustic path. [Work supported by ONR 321OA.]

The environment in a recent experiment exhibited short time scale isotherm depressions and elevations in the temperature profile of the water column. This dynamic behavior is significantly pronounced over a 2 hour period (between 70–90 m depth) during a 24 hour deployment. High frequency broadband transmissions (22–28 kHz) were sent between a stationary source (5 m above the seafloor) and an 8-element vertical hydrophone array (4.5 m above the seafloor) in an approximate depth of 100 m with 1 km separation. Vertical beamforming of measured impulse response across all array elements and application of Gaussian steering revealed strong correlation between vertical temperature profiles and angular spread of the direct path receptions. Inherently a 3D problem, we consider a 2D approach to show beam fluctuations as a function of the environment. 2D PE modeling is driven by measured sound speed profiles to calculate the acoustic field between source and receiver and to beamform across an ideal vertical array for data/model comparison. Over time, fluctuations in the intensity of the acoustic beam, spatial path and angular spread of the direct path signal can be attributed to the vertical oscillations of isotherms in the water column. [Work supported by ONR321.]

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.]

A multi-institutional, acoustical oceanography experiment was conducted from October 2016 through November 2017 on the Chukchi continental shelf covering 100–700 m isobaths. Parallel to a deep-water experiment conducted during the same period, the Shallow Water Canada Basin Acoustic Propagation Experiment (SW CANAPE) was designed to assess basin scale acoustic signals on the shelf region while detailed oceanographic dynamic of the shelf break region, particularly the upwelling and other dynamic of the upper 500 m water column, was measured simultaneously. Multiple arrays of oceanographic sensors including upward looking ice profiler, current profiler, temperature, conductivity, and pressure profiles measured temporal and spatial dynamics of 500 m upper ocean in connection with acoustic measurements. Distributed in a 30 km2 area north of Barrow Alaska, vertical line arrays including an L-shaped array, covered upper 200 m of the water column. Two acoustic sources placed at 148 m and 193 m depths on the shelf emitted broadband acoustic signals in frequency bands (700–1100 Hz, and 1400–4000 Hz) along and across the shelf while the sound speed and current profile and surface ice were being measured continuously. Deep water low frequency signals were also recorded. This talk provides an overview of the SW CANAPE experiment and highlights some of the detailed measurements. [Work supported by ONR.]

During the Canada Basin Acoustic Propagation Experiment (CANAPE) in 2016, two extended shipboard oceanographic measurements were conducted simultaneously with the acoustic propagation from deep water to the Chukchi shelf-break region. These shipboard measurements were aimed at understanding the oceanographic variability including along the shelf eddy formation and upwelling around the shelf break region. We utilize the measured oceanographic data to construct the environmental input for acoustic models. While there was no ice formation or coverage during the observation period, the effects of upwelling and the eddy formation on acoustic propagation were present. This paper demonstrates the results measured in the experiment in the context of temporal and spatial variability of the water column in the Chukchi shelf region. [Work supported by ONR Ocean Acoustics].

In shallow water regions, the environment has complicated temporal and spatial variability including changes of bathymetry, sediment layer structure, bottom property, and physical oceanographic spatial and temporal changes due to processes like internal waves. All these effects can influence sound propagation in the waveguide. The azimuth angle dependence of sound propagation has been studied using the broadband acoustic signals measured at the Atlantic Generation Station site on the New Jersey continental Shelf, where two distinctive geologic/geoacoustic regions exist [J. Acoust. Soc. Am. 96(6), 1994]. The current paper revisits this idea by analyzing the modal dispersion of broadband acoustic signals deployed along circular tracks at the site of the Seabed Characterization Experiment 2017, where the seabed shows strong azimuthal dependent sub-mud layer ridges overlaid by a relatively uniform mud layer with variable thickness along different directions. The results of this paper can be utilized to assess the azimuthal dependence of the sound propagation in the mud patch region. [Work supported by ONR Ocean Acoustics.]

While 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 from acoustic measurements. The ONR-sponsored Seabed Characterization Experiment (SBCE), conducted in a mud patch on the New England continental shelf in the spring of 2017, provides an opportunity to make substantial improvements in understanding the physical mechanisms controlling sound propagation in muddy sediments. Acoustic signals (e.g., 31g explosive and combustive source signals) detonated at various ranges, depths and azimuths were measured in SBCE. This paper utilizes these measured signals to extract the acoustic normal mode characteristics including modal dispersive curve with Airy phase structure, modal amplitude, modal attenuation coefficient, and mode depth function. These normal mode characteristics are used in geo-acoustic inversion algorithms to estimate low-frequency sound speed and attenuation in muddy sediments as a function of frequency. The performance of different inversion methods using different normal mode characteristics is discussed. [Work supported by ONR Ocean Acoustics.]

During the 2015 Canada Basin Acoustic Propagation Experiment (CANAPE) expedition on the southern edge of Canada basin (northern Chukchi Sea shelfbreak), shipboard suspended underwater sound sources were deployed to transmit acoustic signals to hydrophone arrays moored on the deep basin as well as the shallow shelf. In this paper, numerical simulations utilizing the Parabolic Equation method are conducted to provide physical insights into the variability of signals propagating over the shelfbreak and slope and recorded on a vertical array on the Chukchi Sea shelf. The numerical models simulate sound propagating over the slope via the Pacific Halocline duct, which is a water-borne vertical sound duct formed between the layers of Pacific summer water and Atlantic water. The source to receiver distance is about 130 km, and realistic variability is introduced in the numerical models. The previous studies reported in the literature have concluded that the shelfbreak circulation, specifically upwelling, and the sub-mesoscale eddies spun off the shelfbreak jet are the two major causes of the temporal and spatial variability of the Pacific Summer Water Layer over the slope in the region. The numerical simulation study also emphasize on the scattering and attenuation effects caused by ice cover and roughness. A preliminary data-model comparison is made and discussed in this paper. [Work supported by the Office of Naval Research.]

During the summer of 2015 a pilot experiment for the Canadian Basin Acoustic Propagation Experiment (CANAPE) was conducted between deep Arctic basin and shallow Chukchi shelf. A vertical line array (VLA) was deployed on the shelf (72.336 N, 157.449 W) at a depth of 161 m from July 26 to August 13, 2015. Sound sources were deployed from R/V Sikuliaq at locations ranging from 131 to 375 km from the VLA. M-Sequences centered at 250 Hz (bandwidth of 62.5 Hz) were transmitted from each location. Discrete shipboard CTD data were conducted at each transmission location and continuous CTD data were recorded along the VLA. Water column data suggests a sound channel located vertically between Pacific and Atlantic waters near the depth of Arctic halocline layer. They also show upward shoaling of lower halocline waters onto the shelf. This upwelling over the continental slope moves the sound channel from offshore to onshore near the bottom. The axis of this sound duct is 100 m below the surface and is about 100 m thick. Received acoustic signals on the shelf increase rapidly in intensity at time scales that range from minutes to days. In this paper we show the acoustic variations are due to the variable sound speed profile. Signal intensity, noise, and temporal variability for geotime scale of minutes to hours are reported. Numerical simulations are conducted to investigate the associated acoustic effects in data. [Work supported by ONR 322OA.]

Arrival time fluctuations of coherent reflections from surface gravity waves are examined. A two-dimensional ray model with an evolving rough sea surface is used to explain the mechanism and formation of the deterministic striation patterns due to the surface reflection. Arrival time predictions from the ray model match qualitatively well with the measurements from bidirectional acoustic transmissions in a water depth of 100 m.