Passive Acoustic Array Research Articles

A decade of declines in toothed whale densities following the Deepwater Horizon oil spill

Shortly after the Deepwater Horizon oil spill began in April 2010, a widely spaced passive acoustic monitoring array was deployed in the northeastern Gulf of Mexico to document the impacts of this unprecedentedly large and deep offshore oil spill on oceanic marine mammals. The array was subsequently maintained for over a decade. Here we document decadal density declines for seven of eight monitored species groups, including sperm whales (up to 31%), beaked whales (up to 83%), and small delphinids (up to 43%). Declines were observed both within and outside of the surface oil footprint. Though not conclusively linked to the oil spill, the broad spatial and temporal scale of these declines observed for disparate marine mammal species is consistent with Deepwater Horizon impacts. These declines have exceeded and outlasted post-spill damage assessment predictions, suggesting that the offshore ecosystem impacts of Deepwater Horizon may have been larger than previously thought.

Click detection rate variability of central North Pacific sperm whales from passive acoustic towed arrays.

Passive acoustic monitoring (PAM) is an optimal method for detecting and monitoring cetaceans as they frequently produce sound while underwater. Cue counting, counting acoustic cues of deep-diving cetaceans instead of animals, is an alternative method for density estimation, but requires an average cue production rate to convert cue density to animal density. Limited information about click rates exists for sperm whales in the central North Pacific Ocean. In the absence of acoustic tag data, we used towed hydrophone array data to calculate the first sperm whale click rates from this region and examined their variability based on click type, location, distance of whales from the array, and group size estimated by visual observers. Our findings show click type to be the most important variable, with groups that include codas yielding the highest click rates. We also found a positive relationship between group size and click detection rates that may be useful for acoustic predictions of group size in future studies. Echolocation clicks detected using PAM methods are often the only indicator of deep-diving cetacean presence. Understanding the factors affecting their click rates provides important information for acoustic density estimation.

Vocal fingerprinting reveals a substantially smaller global population of the Critically Endangered cao vit gibbon (Nomascus nasutus) than previously thought

The cao vit gibbon (Nomascus nasutus) is one of the rarest primates on Earth and now only survives in a single forest patch of less than 5000 ha on the Vietnam–China border. Accurate monitoring of the last remaining population is critical to inform ongoing conservation interventions and track conservation success over time. However, traditional methods for monitoring gibbons, involving triangulation of groups from their songs, are inherently subjective and likely subject to considerable measurement errors. To overcome this, we aimed to use ‘vocal fingerprinting’ to distinguish the different singing males in the population. During the 2021 population survey, we complemented the traditional observations made by survey teams with a concurrent passive acoustic monitoring array. Counts of gibbon group sizes were also assisted with a UAV-mounted thermal camera. After identifying eight family groups in the acoustic data and incorporating long-term data, we estimate that the population was comprised of 74 individuals in 11 family groups, which is 38% smaller than previously thought. We have no evidence that the population has declined—indeed it appears to be growing, with new groups having formed in recent years—and the difference is instead due to double-counting of groups in previous surveys employing the triangulation method. Indeed, using spatially explicit capture-recapture modelling, we uncovered substantial measurement error in the bearings and distances from field teams. We also applied semi- and fully-automatic approaches to clustering the male calls into groups, finding no evidence that we had missed any males with the manual approach. Given the very small size of the population, conservation actions are now even more urgent, in particular habitat restoration to allow the population to expand. Our new population estimate now serves as a more robust basis for informing management actions and tracking conservation success over time.

Tracking pygmy blue whales in the Perth Canyon using passive acoustic observatories

Eastern Indian Ocean pygmy blue (EIOPB) whales were localised and tracked through the Perth Canyon area utilising a passive acoustic tracking array. Vocalising animals were tracked using the Type II unit of the EIOPB whale song across recordings for an average of 109.3 min (95% CI ± 27 min), suggesting that animals move slowly through the array area. Comparisons of the distribution of animals between sample years revealed non-uniform distribution of animals suggesting that environmental variables may drive pygmy blue whale spatial distribution within foraging areas. Analysis of the movement patterns of acoustically tracked animals found that animals tracked between January and early March generally exhibited directional movement while singing animals tracked between March and May exhibited directional and milling movement patterns, indicating that they may be foraging, and thus that animals may sing between feeding bouts. Comparisons of visual and acoustic localisations found animals appeared to utilise similar areas around the passive acoustic receiver array on days where survey effort overlapped, though acoustic survey methods returned consistently higher numbers of detections. Further, acoustic and visual detections of animals within the same day were not within the same location highlighting the need for holistic studies of species habitat utilisation. The inclusion of in-situ environmental data collection in the design of future acoustic tracking studies would be valuable to understand whether there is a link between environmental factors at the local scale and the localised movement of whales around the listening area.

Active acoustic field modulation of ultrasonic transducers with flexible composites

The simple acoustic field generated by conventional transducers limits the development of ultrasound applications. Current methods rely on passive acoustic lenses or active arrays to manipulate ultrasonic waves, but they face challenges such as low transmission efficiency with bulky morphology for lenses, and complex systems with high-cost for arrays. Here, we propose a method exploiting flexible piezoelectric ultrasonic transducers (FPUT) with 1-3 PZT8/PDMS composites to achieve a high-frequency and diversified ultrasonic field. The FPUT at a center frequency of 1.5 MHz exhibits a high electromechanical coupling coefficient (kt ~ 0.74), excellent transmission efficiency, and mechanical conformability. We showcase two dynamic functionalities of our setup, namely variable acoustic focus and multi-order vortex generated by circular and spiral shape transducers. Finally, we show that the FPUT achieves high-resolution underwater ultrasonic imaging at a wide spatial range (>12λ) via ultrasonic collimation, offering a viable technological alternative for active acoustic fields manipulation and ultrasonic applications.

Performance Assessment of the Innovative Autonomous Tool CETOSCOPE© Used in the Detection and Localization of Moving Underwater Sound Sources

The detection and localization of acoustic sources remain technological challenges in bioacoustics, in particular, the tracking of moving underwater sound sources with a portable waterproof tool. For instance, this type of tool is important to describe the behavior of cetaceans within social groups. To contribute to this issue, an original innovative autonomous device, called a CETOSCOPE©, was designed by ABYSS NGO, including a 360° video camera and a passive acoustic array with 4 synchronized hydrophones. Firstly, different 3D structures were built and tested to select the best architecture to minimize the errors of the localizations. Secondly, a specific software was developed to analyze the recorded data and to link them to the acoustic underwater sources. The 3D localization of the sound sources is based on time difference of arrival processing. Following successful simulations on a computer, this device was tested in a pool to assess its efficiency. The final objective is to use this device routinely in underwater visual and acoustic observations of cetaceans.

Tidal effects on periodical variations in the occurrence of singing humpback whales in coastal waters of Chichijima Island, Ogasawara, Japan

Marine organisms inhabiting coastal waters are known to be driven by periodic cycles such as diel, tidal, and seasonal changes. Humpback whales (Megaptera novaeangliae) breed in shallow and warm coastal waters, with males singing complex songs during the breeding season. To investigate periodic variations in humpback whale singing activities, we conducted fixed passive acoustic monitoring in the Ogasawara (Bonin) Islands, Japan, from winter to spring during 2016–2018. The singing activity and individual number of singers were observed throughout the day and night using a very long baseline passive acoustic array. The occurrence of singers peaked before sunrise and in the evening and was reduced during the daytime. The frequency of song reception depended on the tidal phase. A generalised additive model demonstrated that the occurrence of singers increased during the flood tide and decreased during the ebb tide in the waters west of Chichijima Island. These results suggest that the singing behaviour of humpback whales in breeding areas is affected by the diel and tidal cycles. Male humpback whales may change their behaviour or singing location depending on the strength and direction of the tidal current, considering that the selection of a stable location is beneficial for singing whales.

Thickness-designable acoustic metamaterial for passive phased arrays

Passive phase-controlled acoustic arrays based on metamaterial surfaces artificially manipulate acoustic waves, which can be used in biomedical imaging and nondestructive testing. One of the most representative applications is ultrasonic therapy using a passive phased array within a complete 2π phase. In contrast, the surface thickness is 1/10 the wavelength of the operating frequency. However, conventional design methods feature a fixed array thickness for a specific frequency. This becomes necessary for different application scenarios where different thicknesses are needed. This study proposed a labyrinth resonant cavity metascreen phased array based on Helmholtz resonators and convoluted labyrinth acoustic metamaterials. The thickness of this array can be optimized to accommodate a specific operating frequency to achieve full 2π phase control by adjusting only one parameter. Theoretical simulations and experimental results verified the acoustic wave manipulation effect. The proposed method may find utility in applications for noninvasive ultrasonic therapy, including nondestructive testing, acoustic communication, and biomedical imaging.

A continuous underwater bubble localization method using passive acoustic array beamforming

The bubble is an effective carrier of underwater gas leakage detection, and its rapid localization is of great significance. In this paper, a continuous underwater bubble localization method using passive acoustic array beamforming is proposed. A specific array of hydrophones was constructed to capture the bubble acoustic signal. The signal of finite length is obtained by introducing the time window. Then, the time delay compensation is carried out according to the positions of the hydrophones in the array to estimate the direction of arrival. The influence of shape, number, and aperture of the hydrophone array on the algorithm performance is studied by simulation. Moreover, the experiment was carried out in a cuboid transparent tempered glass water tank in the laboratory, and a field programmable gate array is used as the core control unit of the data processing system to complete the signal acquisition and the algorithm execution. The localization results show that the passive acoustic cross array beamforming algorithm can accurately and stably estimate the leakage direction at different positions.

Residual Learning for Marine Mammal Classification

The passive acoustic monitoring of marine mammals is an essential tool for researchers tracking the populations of individual species in threatened environments. Given the large quantity of audio data generated by passive acoustic arrays, it is desirable to automate the process of identifying marine mammals present in the recordings. Utilizing acoustic data from the William A. Watkins Marine Mammal Sounds Database, we present an approach using residual learning networks (ResNets) for classifying the marine mammal vocalizations of up to 32 species. We first determine the optimal methods for converting acoustic recordings into discrete spectrograms suitable for input into neural networks. A series of configurations for spectrographic window functions, preprocessing augmentations, and multi-channel spectrogram generation are examined. Each configuration’s spectrographic output is used to train a residual learning network. Its multi-class classification performance is ranked using the harmonic mean of precision and recall to calculate a weighted F1-score. Configurations specifying 512×256 spectrograms created with a Hann window of 1024 and utilizing horizontal roll demonstrate superior performance. We use the top-performing configurations to generate training data as input for a series of single and multi-channel residual neural networks. These networks are trained to high precision before evaluating their multi-class classification performance. A single-channel network performed the best, obtaining an F1-score of 0.867 with an AUC of 0.9281 on a 32-class classification task. Our multi-channel configuration obtained an F1-score of 0.846 with an AUC of 0.9169. While we demonstrate that networks may learn more information from multi-channel spectrographic inputs, we find that single-channel spectrograms offer superior classification performance overall.

Spawning Intervals, Timing, and Riverine Habitat Use of Adult Atlantic Sturgeon in the Hudson River

AbstractThe Hudson River currently supports the largest population of Atlantic Sturgeon Acipenser oxyrinchus in the United States despite currently supporting less than 1% of historic abundance. Federally listed as endangered throughout much of their range, Atlantic Sturgeon are still impacted by multiple threats, resulting in habitat destruction or direct mortality that has inhibited the rebuilding of populations despite more than 20 years without direct fishing pressure. To assist in management and reduction in anthropogenic impacts to adult Atlantic Sturgeon in the Hudson River, we have examined sex‐specific patterns related to spawning. Utilizing hundreds of adult Atlantic Sturgeon telemetered along the Atlantic coast and an extensive passive acoustic array from 2010 to 2016, we were able to estimate arrival, departure, and duration of occupancy for four segments of the Hudson River for male, female, and unknown‐sex individuals. Male Atlantic Sturgeon arrived above river kilometer (RKM; RKM 0 = Battery Park, New York) 100 at an average date of May 27, 12 d prior to females on June 8. While there were only a few significant differences in the timing of departure, the average date from RKM 100 was July 11 for males, June 29 for females, and July 23 for unknown‐sex individuals, as such occupancy was significantly longer for male Atlantic Sturgeon. This study shows that female Atlantic Sturgeon often spawned in consecutive years and had much shorter mean intervals between spawning appearances (females 1.66 years, males 1.28 years) than the historical literature suggests. While the timing of spawning is consistent with the historic record, we provide a timeline with finer resolution and much greater detail for adults in the Hudson River, as well as update the historic estimate of spawning return rates. Conservation measures to reduce direct impacts to spawning habitat and mature Atlantic Sturgeon would likely provide the most benefit if conducted between mid‐May through the end of July.

Tom and Thode’s excellent adventure: Practical guidance on surviving seedy bars, preparing for a DJ career, dismantling the U.S. Panama Canal zone, and other useful tips when working with towed acoustic arrays in the Eastern Tropical Pacific

Tom Norris used many acoustic tools in his research career, but it is safe to say that towed passive acoustic arrays were one of his specialties. In 1998 the National Marine Fisheries Service conducted the Stenella Population and Abundance Monitoring (SPAM) cruise. The NOAA ship Endeavor was to survey enormous dolphin populations in the Eastern Tropical Pacific (ETP), starting from Lima, Peru, and ending in Panama City, Panama. Jay Barlow, one of my PhD thesis committee members, permitted me to participate in the cruise as a member of his towed array passive acoustic monitoring team. The first time I ever met Tom, the acoustics team leader, was when he was carrying a surfboard into his cabin, and I wondered how in the world he had managed to ship a surfboard into Peru. It was the first of many times I have enjoyed both working with and wondering about Tom, and our first trip together taught me practical things they forgot to teach in graduate school. Here, I will review Tom’s tips on Mackie mixer boards, buying drinks in dangerous bars, tracking dolphin whistles, not antagonizing visual observers, and liberating furniture from the U.S. Panama canal zone.

Habitat selection, fine-scale spatial partitioning and sexual segregation in Rajidae, determined using passive acoustic telemetry

Habitat selection is the process by which an individual makes an active decision to make use of a particular habitat when others are available. The ability to infer habitat selection therefore requires observations of movement through space and time which can be particularly challenging for marine species that are cryptic and do not regularly visit the sea surface. Rajidae (skates) are benthic mesopredators that inhabit turbid coastal waters and exhibit site fidelity, making them an ideal group for studying habitat selection and resource partitioning using a fixed, passive acoustic receiver array in the western English Channel, UK. Using network analysis, significant differences were found in the way 4 species of Rajidae occupied different parts of the array; for example,Raja microocellataandR. clavataoccupied shallower habitat thanR. brachyuraandR. montagui. R. montaguiandR. brachyurawere further separated, with each species detected more frequently at different receivers. Males and females of all 4 species were also detected at different receivers and at different times. These results demonstrate habitat selection, resource partitioning among species and sexual segregation in these 4 species of Rajidae. Our findings are important evidence for management of fisheries, such as the designation of marine protected areas, and further highlight the potential of this method for tracking other mobile marine species in temperate, open coastal regions.

Determining effective acoustic array design for monitoring presence of white sharks Carcharodon carcharias in nearshore habitats

Inferences regarding animal presence from passive acoustic receiver arrays are driven by the spatial configuration of receivers. Large, dense arrays provide more information, but maintenance of multiple receivers is costly. Configuring acoustic receiver arrays to maximise coverage while minimising cost is therefore paramount. This study used data from a dense acoustic receiver array within a white shark Carcharodon carcharias nursery area on the east coast of Australia to assess how detection data of tagged white sharks in the area was affected by reducing the array size. Receivers were sub-sampled post hoc by simple random sampling, clustered random sampling, and sampling of the top performing receivers. Using the top performing receivers, array size could be reduced by 60% (10 out of 25 receivers) while still detecting a median of 100% of white sharks detected with the full array. With random and clustered sampling methods, a 40% reduction in array size (15 out of 25 receivers) detected a median of 100% of sharks. Reducing the array size by 60% using the top performing receivers resulted in a 35% decrease in the median number of detections per visit of the tagged sharks (67 out of 102.5 detections). In comparison, reducing the array by the same amount with random and clustered sampling methods resulted in a 57% decrease (44 out of 102.5 detections). The post hoc sampling methods used in this study are an empirical approach for optimising placement of limited receiver resources with broad application for establishing cost-effective monitoring.

An autonomous hydrophone array to study the acoustic ecology of deep-water toothed whales

For vocal animals with distinctive calls, passive acoustic monitoring can be used to infer presence, distribution, and abundance provided that the calls and calling behaviour are known. Key to enabling quantitative acoustic surveys are calibrated recordings of identified species from which the source parameters of the sounds can be estimated. Obtaining such information from free-ranging aquatic animals such as toothed whales requires multi-element hydrophone arrays, the use of which is often constrained by cost, the logistical challenge of long cables, and the necessity for attachment to a boat or mooring in order to digitise and store multiple channels of high-sample rate audio data. Such challenges are compounded when collecting recordings or tracking the diving behaviour of deep-diving animals for which the array must be deployed at depth. Here we report the development of an autonomous drifting deep-water vertical passive acoustic array that uses readily available off-the-shelf components. This lightweight portable array can be deployed quickly and repeatedly to depths of up to 1000 m from a small boat. The array comprises seven ST-300 HF SoundTrap autonomous recorders equally spaced on an 84 m electrical-mechanical cable. The single-channel digital sound recordings were configured to allow for synchronisation in post-processing using an RS-485 timing signal logged by all channels every second. We outline how to assemble the array, and provide software for time-synchronising the acoustic recorders. To demonstrate the utility of the array, we present an example of short-finned pilot whale clicks localised on the deep-water (700 m) array configuration. This array method has broad applicability for the cost-effective study of source parameters, acoustic ecology, and diving behaviour of deep diving toothed whales, which are valuable not only to understand the sensory ecology of deep-diving cetaceans, but also to improve passive acoustic monitoring for conservation and management.

Tracking of multiple surface vessels based on passive acoustic underwater arrays

This paper introduces an approach to localize and track an unknown number of non-cooperative surface vessels based on passive acoustic sensing of their noise radiated underwater. Time-Difference of Arrival (TDOA) measurements are extracted from pairs of hydrophones, and a recently introduced Bayesian framework for multi-object tracking is employed to detect and track vessels from TDOA measurements. Results based on data from a three-dimensional compact hydrophone array towed by an autonomous vehicle confirm that non-cooperative surface vessels can be detected and tracked.

Performance of a Passive Acoustic Linear Array in a Tidal Channel

Baseline ambient sound level assessment is important in quantifying anthropogenic noise contributions. Static acoustical sensing in high-flow conditions is complicated by pseudosound, or flow noise, caused by turbulent flow on the surface of a hydrophone. Signal processing methods are used to identify and suppress flow noise at low frequencies (<; 500 Hz) in data collected on a four-element horizontal hydrophone array in Minas Passage, a tidal channel in the Bay of Fundy, in October 2016. Spectral slope analysis is used to identify the frequencies at which the flow noise and ambient noise contributions to the recorded signal are equal, between 180 and 275 Hz for current speeds of 0.1 to 2.6 m/s, respectively. The maximum frequencies at which flow noise is observed, between 100 and 125 Hz over the same current speed range, are determined using analysis of the spatial coherence. The array's performance in the Minas Passage is quantified by an empirical relationship between flow speed and the spectral critical frequencies of the coherent output from the linear array. Coherent averaging (broadside beamforming) is demonstrated as a potential flow noise suppression technique, improving low-frequency passive acoustic monitoring in a high-energy tidal channel.

Characterization of sound waves propagating in a fluctuating ocean: experimental validation with ALMA

The authors present an acoustic system, designed by DGA Naval Systems, dedicated to the study of sound propagation in challenging environments. The system is called ALMA for Acoustic Laboratory for Marine Applications. Shallow and coastal waters, where oceanographic phenomena (such as linear internal waves, tides and 3-D effects) interact with acoustic propagation, represent the main area of deployments. Since 2014, 5 at-sea campaign have been successfully conducted in the Mediterranean Sea and in the Atlantic Ocean. They mainly consisted in propagating sound waves in the 1–15 kHz frequency band using fixed or towed sources towards a modular passive acoustic array. The latter is composed of 8 rigid arms carrying 16 hydrophones each. These arms can be, and actually were, arranged in various shapes, depending on the goal of the experiment. Environmental sensing using thermistor strings and CTD cast complete the experimental equipment. The analysis of the 2016 campaign demonstrates the ability of the system to gather data representative what was described as the “saturation” theory by S. Flatté in the 1980s. In fact, criteria based on the mutual coherence function and the normalized acoustic intensity probability density function allow to explain the observed variability of detection performance on a 4x32-hydrophones vertical comblike passive acoustic array. A global progress report on the use of data gathered by the ALMA system, as well as future plans—including deployment in high latitudes environments—will be discussed.

Signal and array processing of biological transients

Studies of biological transients in the time domain require removing the distorting effects of the data acquisition system (DAQ) system. Unfortunately, amplitude and phase calibration across frequency (equivalently, the DAQ impulse response) rarely is determined for bioacoustic systems. Calibration is not needed for certain types of processing, e.g., detection since signal and noise undergo the same distortion. Under certain assumptions (mainly pertaining to the properties of the noise), the optimal detector for transients is a member of the family of power-law detectors. Therefore, a challenge is determining the pre-processing approach that converts ocean acoustic noise into noise with the assumed properties. A direction-of-arrival estimation approach that is optimal under certain conditions is the minimum variance, distortionless response beamformer. However, transients typically are snapshot-deficient for estimating the data cross spectral matrix. Quantitative performance of sub-optimal but robust approaches with passive acoustic arrays deployed in a variety of ocean environments are investigated. A non-acoustic biological transient is the infrared radiation from the heat of a marine mammal blow. Results from an automated processor based on the power-law detector for infrared video are presented.[Support from the Office of Naval Research, the Joint Industry Programme, and the Walton Foundation].

Localization and tracking of fish sounds with a 4-element underwater passive acoustic array

Localization and tracking are parameter estimation problems. As such, estimates are evaluated by their statistical distribution, in particular, their bias (equal to the mean of estimated positions minus the true position) and variance (variability in the measured positions about their mean). A variety of methods are used to localize sounds recorded by acoustic arrays. Here, we quantitatively compare the performance of four algorithms: (1) plane-wave-front frequency-domain beamforming; (2) curved-wave-front frequency domain beamforming; (3) time-difference-of-arrivals (TDOAs) using waveform cross-correlation; and (4) TDOAs using spectrogram cross-correlation, for fish sounds generated with a controlled underwater source at various GPS-located positions around a 4-element array. The array consisted of a SoundTrap ST4300 (Ocean Instruments, Auckland, NZ) four-channel acoustic recorder, equipped with four HTI-96-MIN hydrophones (High Tech, Inc., Long Beach, MS). The hydrophones were arranged in a tetrahedral-shaped configuration with a 20-m inter-element spacing. The objective is to optimize the localization and tracking of individual soniferous fish to better understand their small-scale spawning movements and reproductive behavior. Research supported by the Office of Naval Research, the Scripps graduate department, and a Natural Sciences and Engineering Research Council of Canada (NSERC) Postgraduate Scholarship-Doctoral (PGS D-3).