Vertical Hydrophone Array Research Articles

Estimation of Water-Like Bottom Length along an Acoustic Track in Shallow Water

Analytically and within the framework of numerical simulations, remote sensing of the water-like bottom in shallow water based on low-frequency sound propagation loss is considered. Water-like bottom sediments are understood as sediments in which the sound speed is close to the sound speed in water, but having a significantly higher density. A model statistical analysis of depth-averaged transmission loss is carried out for acoustic tracks of a fixed range in one of the areas of the Kara Sea with a known structure of the upper layer of the bottom, which includes water-like areas. A good correlation between low-frequency transmission loss and the length of water-like bottom is demonstrated. Based on this result, a method for remote integral estimation of the water-like bottom size at an acoustic track between a single source and a vertical array of hydrophones is proposed.

The active space of sperm whale codas: inter-click information for intra-unit communication.

Sperm whales (Physeter macrocephalus) are social mega-predators who form stable matrilineal units that often associate within a larger vocal clan. Clan membership is defined by sharing a repertoire of coda types consisting of specific temporal spacings of multi-pulsed clicks. It has been hypothesized that codas communicate membership across socially segregated sympatric clans, but others propose that codas are primarily used for behavioral coordination and social cohesion within a closely spaced social unit. Here, we test these hypotheses by combining measures of ambient noise levels and coda click source levels with models of sound propagation to estimate the active space of coda communication. Coda clicks were localized off the island of Dominica with a four- or five-element 80m vertical hydrophone array, allowing us to calculate the median RMS source levels of 1598 clicks from 444 codas to be 161 dB re. 1 μPa (IQR 153-167), placing codas among the most powerful communication sounds in toothed whales. However, together with measured ambient noise levels, these source levels lead to a median active space of coda communication of ∼4 km, reflecting the maximum footprint of a single foraging sperm whale unit. We conclude that while sperm whale codas may contain information about clan affiliation, their moderate active space shows that codas are not used for long range acoustic communication between units and clans, but likely serve to mediate social cohesion and behavioral transitions in intra-unit communication.

Hector's dolphins (Cephalorhynchus hectori) produce both narrowband high-frequency and broadband acoustic signals.

Odontocetes produce clicks for echolocation and communication. Most odontocetes are thought to produce either broadband (BB) or narrowband high-frequency (NBHF) clicks. Here, we show that the click repertoire of Hector's dolphin (Cephalorhynchus hectori) comprises highly stereotypical NBHF clicks and far more variable broadband clicks, with some that are intermediate between these two categories. Both NBHF and broadband clicks were made in trains, buzzes, and burst-pulses. Most clicks within click trains were typical NBHF clicks, which had a median centroid frequency of 130.3 kHz (median -10 dB bandwidth = 29.8 kHz). Some, however, while having only marginally lower centroid frequency (median = 123.8 kHz), had significant energy below 100 kHz and approximately double the bandwidth (median -10 dB bandwidth = 69.8 kHz); we refer to these as broadband. Broadband clicks in buzzes and burst-pulses had lower median centroid frequencies (120.7 and 121.8 kHz, respectively) compared to NBHF buzzes and burst-pulses (129.5 and 130.3 kHz, respectively). Source levels of NBHF clicks, estimated by using a drone to measure ranges from a single hydrophone and by computing time-of-arrival differences at a vertical hydrophone array, ranged from 116 to 171 dB re 1 μPa at 1 m, whereas source levels of broadband clicks, obtained from array data only, ranged from 138 to 184 dB re 1 μPa at 1 m. Our findings challenge the grouping of toothed whales as either NBHF or broadband species.

Evaluating machine learning architectures for sound event detection for signals with variable signal-to-noise-ratios in the Beaufort Sea.

This paper explores the challenging polyphonic sound event detection problem using machine learning architectures applied to data recorded in the Beaufort Sea during the Canada Basin Acoustic Propagation Experiment. Four candidate architectures were investigated and evaluated on nine classes of signals broadcast from moored sources that were recorded on a vertical line array of hydrophones over the course of the yearlong experiment. These signals represent a high degree of variability with respect to time-frequency characteristics, changes in signal-to-noise ratio (SNR) associated with varying signal levels as well as fluctuating ambient sound levels, and variable distributions, which resulted in class imbalances. Within this context, binary relevance, which decomposes the multi-label learning task into a number of independent binary learning tasks, was examined as an alternative to the conventional multi-label classification (MLC) approach. Binary relevance has several advantages, including flexible, lightweight model configurations that support faster model inference. In the experiments presented, binary relevance outperformed conventional MLC approach on classes with the most imbalance and lowest SNR. A deeper investigation of model performance as a function of SNR showed that binary relevance significantly improved recall within the low SNR range for all classes studied.

Azimuthal, range, and depth tracking of marine mammal sounds using a drifting acoustic vector sensor and hydrophone array platform

Deep-water acoustic tracking of marine mammals typically requires correlating hydrophone signals across both short and large-aperture hydrophone arrays. Here, we demonstrate how a single drifting, depth-controlled platform can obtain two and three-dimensional positional fixes of marine mammal sounds using an acoustic vector sensor and short-aperture arrays, both mounted on an autonomous drifting platform. In February and June 2023 two autonomous opto-acoustic drifters were deployed 50km off San Diego, CA at 250 m depth in 1030 m deep water. Both drifters were equipped with a CTD and acoustic recording system comprised of a 1.75 m aperture vertical hydrophone array, a tetrahedral hydrophone array, and either a 2-D Geospectrum M-35 or 3-D Wilcoxon VS-209 acoustic vector sensor. Throughout the two to three-day deployments, all acoustic sensors detected numerous marine mammal calls, including humpback whales and a pod of common dolphins. Estimates of azimuth and elevation were obtained from the simultaneously sampled acoustic vector sensor and hydrophone arrays, while multi-path and cross-platform processing were employed to extract range information. The results suggest that sparse deployments of drifting vector sensor platforms may be able to map biological distributions over spatial scales that would otherwise require larger numbers of hydrophone-only platforms. [Work supported by ONR TFO.]

Geographic distribution of the Cross Seamount beaked whale based on acoustic detections

AbstractBeaked whales produce frequency‐modulated echolocation pulses that appear to be species‐specific, allowing passive acoustic monitoring to play a role in understanding spatio‐temporal patterns. The Cross Seamount beaked whale is known only from its unique echolocation signal (BWC) with no confirmed species identification. This beaked whale spans the Pacific Ocean from the Mariana Archipelago to Baja California, Mexico, south to the equator, but only as far north as latitude 29°N. Within these warm waters, 92% of BWC detections occurred at night, 6% during crepuscular periods, and only 2% during daylight hours. Detections of BWC signals on drifting recorders with a vertical hydrophone array at 150 m depth demonstrated that foraging often occurred shallow in the water column (<150 m). No other species of beaked whale to date has been documented foraging in waters this shallow. Given their nocturnal, shallow foraging dives, this species appears to prefer prey that may be available in the water column only during those hours. The foraging behavior of Cross Seamount beaked whales appears to be unique among all beaked whales, and these findings contribute additional ecological and acoustic information which can help guide future efforts to identify this cryptic whale.

Application of deconvolution interferometry to receiver ghost reflection in vertical cable seismic survey

AbstractMarine vertical cable seismic is to probe targets near the submarine with use vertical hydrophone array suspending in deep sea water. Despite imaging of primary reflections from vertical cable seismic data can provide seismic profiles with high resolution, its submarine illumination is much narrower than that of conventional towed‐streamer seismic owing to the irregular geometry. In view of the fact that imaging of multiples can provides better subsurface illumination with fine resolution, we present a strategy for imaging of receiver ghost reflections from vertical cable seismic data, based on seismic interferometry by deconvolution. This method can convert first‐order receiver ghosts into virtual primaries from the towed‐streamer data without velocity model and source‐receiver position. We illustrate the deconvolution interferometry for the receiver ghost reflections with the real vertical cable seismic data in Shenhu area, South China Sea and further obtain virtual primary reflections. Finally, we use these virtual primaries to successfully construct a stacked image and obtain a post‐stack migration image with conventional seismic data process method. This stacked and migrated image significantly extends the submarine illumination with higher resolution and it is useful in characteristics recognition of hydrate‐bearing sediments and free gas.

Physics-based acoustic inversion of sound velocity and attenuation in low-velocity marine sediments

Using data from the Yellow Sea, arrival times of the direct wave and surface/bottom reflections from explosive sources to a vertical hydrophone array are used to precisely determine each explosive source’s location, the source energy spectral density (SESD), and the water depth. Long-range propagation waveforms reveal modal dispersion: the ground wave, water wave, Airy phase, etc. There are two high-frequency (HF) groups of water waves. One propagates with the sound speed in the water below the thermocline, the other with a speed close to the sound speed in the water above the thermocline. The HF group arrival times offer a time reference for dispersion analyses, including the ground wave speed at the cutoff frequency and the group velocity at Airy frequency. Associated with a top layer oflow-velocity sediments (LVS), seafloor reflections have two pulses: one from the water-sediment interface, one from the sediment-basement interface; Long-range transmission loss (TL) exhibits abnormal peaks at selected frequencies. Above-mentioned physics characteristics and the SESD-normalized TL(r) and TL(f) in 50–5000 Hz range up to 27.6 km are used for observationally driven inferences of seafloor geo-acoustic parameters, such as the sound velocity and attenuation in the LVS layer, its thickness, the sound velocity in the basement, etc.

Spatial and temporal variation of three-dimensional ship noise coherence in a submarine canyon.

Ship noise recorded by a vertical hydrophone array in the Mississippi Canyon region of the Gulf of Mexico is shown to contain the acoustic influence of bathymetric features, seabed properties, and water column sound speeds. Noise coherence is demonstrated to be an effective metric not just for identifying ship traffic in recorded data but also for "fingerprinting" the environment. A three-dimensional acoustics model adopting automatic identification system ship track information and realistic oceanographic conditions is used to compute noise coherence distributions across the canyon area and enables numerical study of the water column sound speed effects that can lead to temporal changes in noise coherence. The study shows the importance of including in situ sound speed measurements or constraints in passive ship noise localization from coherence measurements. Seasonal variability is also examined with models suggesting a strong influence of seasonal changes.

Angular Spectrum of Acoustic Pulses at Long Ranges

Long-range propagation of sound pulses in the deep ocean is considered. A new method for the estimation of the pulse angular spectrum is presented. The method is based on the Husimi transform of a wave field and can be realized with a short vertical array of nondirectional hydrophones. As a result, one obtains a diagram of the arrival pattern in the time–angle plane. The method is applied to a model of the underwater sound channel in the Sea of Japan. Special attention is paid to sound scattering on a cold synoptic eddy along the waveguide. It is shown that the synoptic eddy leads to a splitting of the individual ray’s arrivals into clusters with close angles and times. The random sound-speed perturbation induced by internal waves blurs these clusters into a fuzzy background and simultaneously broaden the angular spectrum of pulses. Nevertheless, it is found that the latter effect is relatively weak for short vertical arrays. In particular, it is shown that increasing the array length from 10 to 30 m results in the separation of the arrivals with opposite angles.

Mapping of surface-generated noise coherence

The performance of a hydrophone array can be evaluated by its coherent gain, which depends on the spatial correlation of both the signal of interest and the background noise between different array elements, where one hopes to maximize the former while minimizing the latter with array signal processing. In this paper, a computational vertical noise coherence map of the first zero-crossing is generated near Alvin Canyon, south of Martha’s Vineyard, Massachusetts, to study its dependence on the spatial variation in bathymetry, water column sound speed and sediment type. A two and three-dimensional Parabolic Equation propagation model based on reciprocity theory were used for the simulation. The results showed that the seabed parameters have the greatest impact on vertical noise coherence at the array location in the Alvin Canyon area, when compared to 3-D bathymetric and water column sound speed profile variability, especially in the shallower water. The analysis reveals the ideal spacing for a vertical hydrophone array for better signal detection in acoustic experiments. In the continental shelf and slope regions, the ideal spacing lies between 3λ⁄8 in deep water and λ⁄2 in shallow water, and for areas with strong bathymetric variations the ideal spacing can be determined by comprehensive numerical models.

Determining the speed dependent source level of a snowmobile traveling on sea-ice

As part of the Sustainable Nunatsiavut Futures project, a field experiment to determine the acoustic properties and underwater radiated sound level of a snowmobile was designed and executed. The fieldwork consists of lowering acoustic recorders under the sea ice and driving a snowmobile with a known position and velocity to evaluate its speed-dependent source level. This experiment is the first step toward collaborative Dalhousie and community research on underwater sound as it relates to the marine habitat, human use of the ocean, and sea-ice in Nunatsiavut. Due to COVID-19, the planning stages were coordinated virtually, and the fieldwork in Nunatsiavut was conducted by local Inuit Research Coordinators (IRCs), while a twin Dalhousie-led experiment was conducted in Caraquet, New Brunswick. A single hydrophone sensor was used in Nunatsiavut, and a vertical array of hydrophones was used in Caraquet to obtain underwater sound data from a moving snowmobile. Skidoo specifications for each site were recorded as well as sea-ice thickness, temperature, salinity, and sound-speed data were collected. Spectrograms of skidoos traveling at different speeds were computed. Comparisons between received levels at different velocities, sites, and ranges are shown, and the impact of sea ice and snowmobile specifications on received levels are discussed.

The coordinated arctic acoustic thermometry experiment—CAATEX

The coordinated arctic acoustic thermometry experiment (CAATEX) was a joint U.S.-Norwegian trans-Arctic acoustic propagation experiment with a design comparable to the 1994 TransArctic Propagation (TAP) experiment. The goal was to measure the changes in low-frequency sound propagation due to changes in ocean heat content and salinity, and ice conditions. Two 35 Hz acoustic transceiver moorings, one in the Nansen Basin and one in the Beaufort Sea, were deployed along with four other receiving moorings. All six moorings were equipped with vertical hydrophone arrays, recording transmissions every 36 h from fall 2019 to fall 2020. Each mooring also recorded temperature and salinity time series along the vertical extent of the hydrophone arrays, ice thickness using an upward-looking sonar, and ocean bottom pressure. The acoustic travel-times allow comparison of present-day heat content to the 1994 measurement, but there are several other points of comparison that are sensitive to environmental changes. These include transmission loss, acoustic scattering, and the acoustic arrival structure. These measurements of ocean and ice processes, and the acoustic propagation and ambient sound will improve our ability to monitor, communicate, and navigate in the Arctic Ocean.

Quantifying information content of modal dispersion data in geoacoustic inversion

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

New England shelf break acoustic (NESBA) experiment: Seabed analysis

The NESBA signals and noise experiment was conducted in April–May, 2021. The goal of the experiment was to assess the potential for improving sonar prediction through enhanced environmental awareness. In this talk, an overview of the experiment will be presented along with a description of the environmental conditions. Extensive measurements and modeling were used to help characterize the complex oceanography and seabed. The study region had water depths that varied from 70 m to 2000 m and included a diversity of seabed types and sub-bottom layering. Along the shelf break, there are regions where the seabed sub-bottom layering is particularly intricate. This talk will focus on the NESBA sub-goal of determining the properties of the seabed using passive sensing from two drifting, vertical hydrophone arrays. One array had 16 hydrophones spaced at 1 m and the other had 32 hydrophones with spacing of 0.1875 m. The seabed characterization is based on array processing techniques using the wind generated surface noise. Results will be presented for seabed characterization from several sites in the NESBA region with discussion on the potential impact on acoustic propagation. [Work supported by the Office of Naval Research.]

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.

Probabilistic Estimation of Merchant Ship Source Levels in an Uncertain Shallow-Water Environment

The estimation of ship source levels (SSLs) in shallow-water environments can be complicated by sound interaction with the seabed. Uncertainty in seabed properties influences SSL estimates, and it is of interest to mitigate and quantify such effects. This article proposes a probabilistic approach to ship radiated noise recorded on a vertical line array (VLA) of hydrophones to infer SSL and properties of a mud-sand shallow water seabed on the New England Shelf. The approach, trans-dimensional Bayesian marginalization, samples probabilistically over complex spectral source strengths, source depths/ranges, and number of seabed layers and geoacoustic parameters of each layer. The Bayesian information criterion is applied to determine the appropriate number of (point) sources used to describe a ship. Radiated noise due to two merchant ships passing the VLA at beam aspect at 3.2−3.4-km range is considered. The SSL estimates agree well with reference spectra from shallow-water studies on large ensembles of merchant ships. The average SSL uncertainty (in terms of one-half the interquartile range interval) is 3.2 dB/Hz for low-frequency narrowband (20−120 Hz) and 1.8 dB/Hz for broadband noise (190−590 Hz). Seabed layering and geoacoustic parameter estimates agree reasonably well with mud-over-sand seabed models from other inversions in the area.

UNDERWATER SOURCE LOCALIZATION USING CYLINDRICAL HYDROPHONE ARRAY AND RIEMANNIAN MATCHED FIELD PROCESSING

The Matched Field Processing (MFP) is for defining the range and the depth of an underwater source effectively. However the algorithm is seldom used for finding the azimuthal direction of the source and is still not considered for multiple field replicas due to the variation of sound velocity. If using cylindrical hydrophone array (CHA) with 32x10 elements (ten ring with each ring of 32 elements) the rough azimuthal angle resolution, , is obtained. Then using TDOA (Time difference of arrival) of the two selected consecutive beams of CHA the system could provide the bearing accuracy up to . Then, the range and the depth of the source are the solution of the proposed Riemannian MFP with the assumption of cylindrical spreading. The proposed RMFP is on the basis of Riemannian distance which is calculated directly by solving the geodesic equation on cylinder surface. The simulations show that only with the data from selected vertical hydrophone array of CHA (there are 32 vertical arrays on cylindrical array) is enough to locate the range and the depth of underwater source precisely.

Spatial variation in acoustic field due to submarine melting in glacial bays

Climate-change induced melting is leading to accelerating ice loss at tidewater glaciers worldwide. A significant component of the freshwater flux from these glaciers arises from submarine melting at the glacier-ocean interface. This melting causes a distinct acoustic signature due to the release of pressurized bubbles underwater, opening up the possibility of monitoring this phenomenon on a large scale using passive acoustic systems. To evaluate the use of sound in monitoring submarine glacier melting, we made acoustic measurements using vertical hydrophone arrays in four glacial bays in Svalbard in 2019. As the recording array was moved away from the glacier, the variation in the recorded acoustic level due to melting ice did not follow a uniformly decreasing trend as one might expect. Moreover, the acoustic intensities at different glaciers were clustered at different levels. These observations indicate that the geometry of the glacier-ocean interface, thermohaline structure of the underwater channel and presence of floating ice in the bay played a role in determining the acoustic field. Most of this variation can be explained through propagation modeling. Moving forward, this model-based interpretation of the field will play an integral part in inverting the sound to estimate the submarine melt rate.

Effects of nonlinear internal gravity waves on normal-incident reflection measurements of marine sediments

Sonar data acquired by sub-bottom profilers and multi-beam echosounder systems are widely used to estimate geoacoustic properties of marine sediments. However, the uncertainty of the seabed property estimates caused by water column variability may limit the application. In this study, we focus on the acoustic focusing and defocusing effects of nonlinear internal waves on normal-incident reflection measurements in the South China Sea. The field experiment was conducted using a transceiver mooring consisted of a sound source and a four channel vertical hydrophone array. The variation of acoustic signal reflections from sub-bottom layers during three days at the same site is observed. The effects of the nonlinear internal waves passing through the transceiver mooring are identified. The impact on the sub-bottom property estimates is investigated. [Work supported by the Office of Naval Research.]