Three-dimensional localization of sperm whales using a single hydrophone

Exploiting environmental asymmetry for vessel localization from the vertical coherence of radiated noise

A method to determine the range and bearing of a moving broadband acoustic source, such as a surface vessel, using the coherence measured on two omni-directional, vertically separated hydrophones is demonstrated using acoustic data recorded near Alvin Canyon on the New England shelf break. To estimate the vessel's range, two theoretical approaches, a half-space model and a Pekeris waveguide model based on normal modes, establish simple relationships between the broadband signal coherence and frequency, source range, and the vertical separation of the receiver hydrophones. A brute force inversion produces a passive acoustic estimate of vessel range. Rapidly changing bathymetry with large features, such as that near Alvin Canyon, produces azimuthal asymmetry in the plan-view coherence pattern about the receivers due to horizontal refraction, focussing, and the up- (down-) slope compression (extension) of modal interference patterns. For vessels with a constant speed and heading, this generates an asymmetry in the received power and vertical coherence fringing pattern. This effect is first demonstrated using reciprocal three-dimensional parabolic equation and raytracing models in an idealized Gaussian canyon, then observed in Alvin Canyon measurements. By comparing the experimental observations to the modeled coherence, the vessel's bearing and range relative to the receivers are obtained.

Where's Whaledo: A software toolkit for array localization of animal vocalizations

Where's Whaledo is a software toolkit that uses a combination of automated processes and user interfaces to greatly accelerate the process of reconstructing animal tracks from arrays of passive acoustic recording devices. Passive acoustic localization is a non-invasive yet powerful way to contribute to species conservation. By tracking animals through their acoustic signals, important information on diving patterns, movement behavior, habitat use, and feeding dynamics can be obtained. This method is useful for helping to understand habitat use, observe behavioral responses to noise, and develop potential mitigation strategies. Animal tracking using passive acoustic localization requires an acoustic array to detect signals of interest, associate detections on various receivers, and estimate the most likely source location by using the time difference of arrival (TDOA) of sounds on multiple receivers. Where's Whaledo combines data from two small-aperture volumetric arrays and a variable number of individual receivers. In a case study conducted in the Tanner Basin off Southern California, we demonstrate the effectiveness of Where's Whaledo in localizing groups of Ziphius cavirostris. We reconstruct the tracks of six individual animals vocalizing concurrently and identify Ziphius cavirostris tracks despite being obscured by a large pod of vocalizing dolphins.

Range estimation of a moving source using interference patterns in deep water

The frequency-range interference patterns of the acoustic field in the shadow zone of deep water correlate with the source location. However, extraction of such interference structure requires a broadband source and fails for narrowband signals. In this work, the narrowband depth-time interference patterns of the acoustic field from a moving source are investigated. Two types of time intervals in the observed patterns are derived based on the ray theory. These time intervals are correlated with the multipath arrival angles, which in turn imply the source range. The simulations and the experimental results demonstrate stable range estimations using the interference patterns.

Source level and vocalizing depth estimation of two blue whale subspecies in the western Indian Ocean from single sensor observations

The source level (SL) and vocalizing source depth (SD) of individuals from two blue whale (BW) subspecies, an Antarctic blue whale (Balaenoptera musculus intermedia; ABW) and a Madagascar pygmy blue whale (Balaenoptera musculus brevicauda; MPBW) are estimated from a single bottom-mounted hydrophone in the western Indian Ocean. Stereotyped units (male) are automatically detected and the range is estimated from the time delay between the direct and lowest-order multiply-reflected acoustic paths (multipath-ranging). Allowing for geometric spreading and the Lloyd's mirror effect (range-, depth-, and frequency-dependent) SL and SD are estimated by minimizing the SL variance over a series of units from the same individual over time (and hence also range). The average estimated SL of 188.5 ± 2.1 dB re 1μPa measured between [25-30] Hz for the ABW and 176.8 ± 1.8 dB re. 1μPa measured between [22-27] Hz for the MPBW agree with values published for other geographical areas. Units were vocalized at estimated depths of 25.0 ± 3.7 and 32.7 ± 5.7 m for the ABW Unit A and C and, ≃20 m for the MPBW. The measurements show that these BW calls series are stereotyped in frequency, amplitude, and depth.

Acoustic localization of terrestrial wildlife: Current practices and future opportunities

Autonomous acoustic recorders are an increasingly popular method for low-disturbance, large-scale monitoring of sound-producing animals, such as birds, anurans, bats, and other mammals. A specialized use of autonomous recording units (ARUs) is acoustic localization, in which a vocalizing animal is located spatially, usually by quantifying the time delay of arrival of its sound at an array of time-synchronized microphones. To describe trends in the literature, identify considerations for field biologists who wish to use these systems, and suggest advancements that will improve the field of acoustic localization, we comprehensively review published applications of wildlife localization in terrestrial environments. We describe the wide variety of methods used to complete the five steps of acoustic localization: (1) define the research question, (2) obtain or build a time-synchronizing microphone array, (3) deploy the array to record sounds in the field, (4) process recordings captured in the field, and (5) determine animal location using position estimation algorithms. We find eight general purposes in ecology and animal behavior for localization systems: assessing individual animals' positions or movements, localizing multiple individuals simultaneously to study their interactions, determining animals' individual identities, quantifying sound amplitude or directionality, selecting subsets of sounds for further acoustic analysis, calculating species abundance, inferring territory boundaries or habitat use, and separating animal sounds from background noise to improve species classification. We find that the labor-intensive steps of processing recordings and estimating animal positions have not yet been automated. In the near future, we expect that increased availability of recording hardware, development of automated and open-source localization software, and improvement of automated sound classification algorithms will broaden the use of acoustic localization. With these three advances, ecologists will be better able to embrace acoustic localization, enabling low-disturbance, large-scale collection of animal position data.

Fin whale density and distribution estimation using acoustic bearings derived from sparse arrays

Passive acoustic monitoring of marine mammals is common, and it is now possible to estimate absolute animal density from acoustic recordings. The most appropriate density estimation method depends on how much detail about animals' locations can be derived from the recordings. Here, a method for estimating cetacean density using acoustic data is presented, where only horizontal bearings to calling animals are estimable. This method also requires knowledge of call signal-to-noise ratios, as well as auxiliary information about call source levels, sound propagation, and call production rates. Results are presented from simulations, and from a pilot study using recordings of fin whale (Balaenoptera physalus) calls from Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) hydrophones at Wake Island in the Pacific Ocean. Simulations replicating different animal distributions showed median biases in estimated call density of less than 2%. The estimated average call density during the pilot study period (December 2007-February 2008) was 0.02 calls hr^-1 km² (coefficient of variation, CV: 15%). Using a tentative call production rate, estimated average animal density was 0.54 animals/1000 km² (CV: 52%). Calling animals showed a varied spatial distribution around the northern hydrophone array, with most detections occurring at bearings between 90 and 180 degrees.

The seasonal occupancy and diel behaviour of Antarctic sperm whales revealed by acoustic monitoring

The seasonal occupancy and diel behaviour of sperm whales (Physeter macrocephalus) was investigated using data from long-term acoustic recorders deployed off east Antarctica. An automated method for investigating acoustic presence of sperm whales was developed, characterised, and applied to multi-year acoustic datasets at three locations. Instead of focusing on the acoustic properties of detected clicks, the method relied solely on the inter-click-interval (ICI) for determining presence within an hour-long recording. Parameters for our classifier were informed by knowledge of typical vocal behaviour of sperm whales. Sperm whales were detected predominantly from Dec-Feb, occasionally in Nov, Mar, Apr, and May, but never in the Austral winter or early spring months. Ice cover was found to have a statistically significant negative effect on sperm whale presence. In ice-free months sperm whales were detected more often during daylight hours and were seldom detected at night, and this effect was also statistically significant. Seasonal presence at the three east Antarctic recording sites were in accord with what has been inferred from 20th century whale catches off western Antarctica and from stomach contents of whales caught off South Africa.

A double-difference method for high-resolution acoustic tracking using a deep-water vertical array

Ray-tracing is typically used to estimate the depth and range of an acoustic source in refractive deep-water environments by exploiting multipath information on a vertical array. However, mismatched array inclination and uncertain environmental features can produce imprecise trajectories when ray-tracing sequences of individual acoustic events. "Double-difference" methods have previously been developed to determine fine-scale relative locations of earthquakes along a fault [Waldhauser and Ellsworth (2000). Bull. Seismolog. Soc. Am. 90, 1353-1368]. This technique translates differences in travel times between nearby seismic events, recorded at multiple widely separated stations, into precise relative displacements. Here, this method for acoustic multipath measurements on a single vertical array of hydrophones is reformulated. Changes over time in both the elevation angles and the relative arrival times of the multipath are converted into relative changes in source position. This approach is tested on data recorded on a 128-element vertical array deployed in 4 km deep water. The trajectory of a controlled towed acoustic source was accurately reproduced to within a few meters at nearly 50 km range. The positional errors of the double-difference approach for both the towed source and an opportunistically detected sperm whale are an order of magnitude lower than those produced from ray-tracing individual events.

Estimating range to a vocalizing fin whale using the timing and amplitude of multipath arrivals

A semi-automated method is described to range to vocalizing fin whales using the timing and amplitude of multipath arrivals measured on seafloor receivers. Calls are detected and multipath arrivals identified with a matched filter. Multipath times and relative amplitudes are predicted as a function of range by ray tracing. Because the direct and first water-column multiple arrivals are not always observed, different hypotheses for the observed arrival paths must be considered. For two arrivals, an amplitude threshold is used to determine if the first arrival is the direct path and if not, the call is disregarded as distant. When three or more arrivals are detected, three hypotheses for the paths of arrivals are considered; the solution is the hypothesis and range that minimizes the timing and optionally, amplitude ratio or absolute amplitude residual. The method is tested with data from two ocean bottom seismometers, one on the Juan de Fuca Ridge and the other in the Cascadia Basin. Solutions obtained by minimizing a combined residual from timing and an empirical absolute amplitude model extracted from the data yield reliable ranges up to 5 km at both sites, and are sufficient to estimate call density using point transect distance sampling.

Vast assembly of vocal marine mammals from diverse species on fish spawning ground

Observing marine mammal (MM) populations continuously in time and space over the immense ocean areas they inhabit is challenging but essential for gathering an unambiguous record of their distribution, as well as understanding their behaviour and interaction with prey species. Here we use passive ocean acoustic waveguide remote sensing (POAWRS) in an important North Atlantic feeding ground to instantaneously detect, localize and classify MM vocalizations from diverse species over an approximately 100,000 km(2) region. More than eight species of vocal MMs are found to spatially converge on fish spawning areas containing massive densely populated herring shoals at night-time and diffuse herring distributions during daytime. We find the vocal MMs divide the enormous fish prey field into species-specific foraging areas with varying degrees of spatial overlap, maintained for at least two weeks of the herring spawning period. The recorded vocalization rates are diel (24 h)-dependent for all MM species, with some significantly more vocal at night and others more vocal during the day. The four key baleen whale species of the region: fin, humpback, blue and minke have vocalization rate trends that are highly correlated to trends in fish shoaling density and to each other over the diel cycle. These results reveal the temporospatial dynamics of combined multi-species MM foraging activities in the vicinity of an extensive fish prey field that forms a massive ecological hotspot, and would be unattainable with conventional methodologies. Understanding MM behaviour and distributions is essential for management of marine ecosystems and for accessing anthropogenic impacts on these protected marine species.

Sound source localization technique using a seismic streamer and its extension for whale localization during seismic surveys

Marine seismic surveys are under increasing scrutiny because of concern that they may disturb or otherwise harm marine mammals and impede their communications. Most of the energy from seismic surveys is low frequency, so concerns are particularly focused on baleen whales. Extensive mitigation efforts accompany seismic surveys, including visual and acoustic monitoring, but the possibility remains that not all animals in an area can be observed and located. One potential way to improve mitigation efforts is to utilize the seismic hydrophone streamer to detect and locate calling baleen whales. This study describes a method to localize low frequency sound sources with data recoded by a streamer. Beamforming is used to estimate the angle of arriving energy relative to sub-arrays of the streamer which constrains the horizontal propagation velocity to each sub-array for a given trial location. A grid search method is then used to minimize the time residual for relative arrival times along the streamer estimated by cross correlation. Results from both simulation and experiment are shown and data from the marine mammal observers and the passive acoustic monitoring conducted simultaneously with the seismic survey are used to verify the analysis.

Three-dimensional tracking of Cuvier's beaked whales' echolocation sounds using nested hydrophone arrays

Cuvier's beaked whales (Ziphius cavirostris) were tracked using two volumetric small-aperture (∼1 m element spacing) hydrophone arrays, embedded into a large-aperture (∼1 km element spacing) seafloor hydrophone array of five nodes. This array design can reduce the minimum number of nodes that are needed to record the arrival of a strongly directional echolocation sound from 5 to 2, while providing enough time-differences of arrivals for a three-dimensional localization without depending on any additional information such as multipath arrivals. To illustrate the capabilities of this technique, six encounters of up to three Cuvier's beaked whales were tracked over a two-month recording period within an area of 20 km(2) in the Southern California Bight. Encounter periods ranged from 11 min to 33 min. Cuvier's beaked whales were found to reduce the time interval between echolocation clicks while alternating between two inter-click-interval regimes during their descent towards the seafloor. Maximum peak-to-peak source levels of 179 and 224 dB re 1 μPa @ 1 m were estimated for buzz sounds and on-axis echolocation clicks (directivity index = 30 dB), respectively. Source energy spectra of the on-axis clicks show significant frequency components between 70 and 90 kHz, in addition to their typically noted FM upsweep at 40-60 kHz.

A two-hydrophone range and bearing localization algorithm with performance analysis

An automated, passive algorithm for detecting and localizing small boats using two hydrophones mounted on the seabed is outlined. This extends previous work by Gebbie et al. [(2013). J. Acoust. Soc. Am. 134, EL77 - EL83] in which a similar two-hydrophone approach is used to produce an ambiguity surface of likely target locations leveraging multipath analysis and knowledge of the local bathymetry. The work presented here improves upon the prior approach using particle filtering to automate detection and localization processing. A detailed analysis has also been conducted to determine the conditions and limits under which the improved approach can be expected to yield accurate range and unambiguous bearing information. Experimental results in 12 m of water allow for a comparison of different separation distances between hydrophones, and the Bayesian Cramér-Rao lower bound is used to extrapolate the performance expected in 120 m water. This work demonstrates the conditions under which a low cost, passive, sparse array of hydrophones can provide a meaningful small boat detection and localization capability.

Moving source localization with a single hydrophone using multipath time delays in the deep ocean

Localizing a source of radial movement at moderate range using a single hydrophone can be achieved in the reliable acoustic path by tracking the time delays between the direct and surface-reflected arrivals (D-SR time delays). The problem is defined as a joint estimation of the depth, initial range, and speed of the source, which are the state parameters for the extended Kalman filter (EKF). The D-SR time delays extracted from the autocorrelation functions are the measurements for the EKF. Experimental results using pseudorandom signals show that accurate localization results are achieved by offline iteration of the EKF.

Range estimation of bowhead whale (Balaena mysticetus) calls in the Arctic using a single hydrophone

Bowhead whales generate low-frequency calls in shallow-water Arctic environments, whose dispersive propagation characteristics are well modeled by normal mode theory. As each mode propagates with a different group speed, a call's range can be inferred by the relative time-frequency dispersion of the modal arrivals. Traditionally, at close ranges modal arrivals are separated using synchronized hydrophone arrays. Here a nonlinear signal processing method called "warping" is used to filter the modes on just a single hydrophone. The filtering works even at relatively short source ranges, where distinct modal arrivals are not separable in a conventional spectrogram. However, this warping technique is limited to signals with monotonically increasing or decreasing frequency modulations, a relatively common situation for bowhead calls. Once modal arrivals have been separated, the source range can be estimated using conventional modal dispersion techniques, with the original source signal structure being recovered as a by-product. Twelve bowhead whale vocalizations recorded near Kaktovik (Alaska) in 2010, with signal-to-noise ratios between 6 and 23 dB, are analyzed, and the resulting single-receiver range estimates are consistent with those obtained independently via triangulation from widely-distributed vector sensor arrays. Geoacoustic inversions for each call are necessary in order to obtain the correct ranges.

Ranging bowhead whale calls in a shallow-water dispersive waveguide

This paper presents the performance of three methods for estimating the range of broadband (50-500 Hz) bowhead whale calls in a nominally 55-m-deep waveguide: Conventional mode filtering (CMF), synthetic time reversal (STR), and triangulation. The first two methods use a linear vertical array to exploit dispersive propagation effects in the underwater sound channel. The triangulation technique used here, while requiring no knowledge about the propagation environment, relies on a distributed array of directional autonomous seafloor acoustics recorders (DASARs) arranged in triangular grid with 7 km spacing. This study uses simulations and acoustic data collected in 2010 from coastal waters near Kaktovik, Alaska. At that time, a 12-element vertical array, spanning the bottom 63% of the water column, was deployed alongside a distributed array of seven DASARs. The estimated call location-to-array ranges determined from CMF and STR are compared with DASAR triangulation results for 19 whale calls. The vertical-array ranging results are generally within ±10% of the DASAR results with the STR results providing slightly better agreement. The results also indicate that the vertical array can range calls over larger ranges and with greater precision than the particular distributed array discussed here, whenever the call locations are beyond the distributed array boundaries.

Passive localization of noise-producing targets using a compact volumetric array

A technique is presented for passively localizing multiple noise-producing targets by cross-correlating the elevation beams of a compact volumetric array on separate bearings. A target's multipath structure inherently contains information about its range; however, unknown, random noise waveforms make time separation of individual arrivals difficult. Ocean ambient noise has previously been used to measure multipath delays to the seabed by cross-correlating the beams of a vertical line array [Siderius, Song, Gerstoft, Hodgkiss, Hursky, and Harrison, J. Acoust. Soc. Am. 127, 2193-2200 (2010)], but this methodology has not been applied to distant noise sources having non-vertical arrivals. The technique presented in this paper uses a compact volumetric array mounted to an autonomous underwater vehicle to measure the three-dimensional directionality and time delays of multipath arrivals, while adaptively rejecting clutter and multi-target interference. This is validated with experimental results in a shallow ocean environment in which a small workboat maneuvered in the vicinity. Short ranges could be estimated reliably using straight ray paths, but longer ranges required accounting for ray refraction.

Using a coherent hydrophone array for observing sperm whale range, classification, and shallow-water dive profiles

Sperm whales in the New England continental shelf and slope were passively localized, in both range and bearing, and classified using a single low-frequency (<2500 Hz), densely sampled, towed horizontal coherent hydrophone array system. Whale bearings were estimated using time-domain beamforming that provided high coherent array gain in sperm whale click signal-to-noise ratio. Whale ranges from the receiver array center were estimated using the moving array triangulation technique from a sequence of whale bearing measurements. Multiple concurrently vocalizing sperm whales, in the far-field of the horizontal receiver array, were distinguished and classified based on their horizontal spatial locations and the inter-pulse intervals of their vocalized click signals. The dive profile was estimated for a sperm whale in the shallow waters of the Gulf of Maine with 160 m water-column depth located close to the array's near-field where depth estimation was feasible by employing time difference of arrival of the direct and multiply reflected click signals received on the horizontal array. By accounting for transmission loss modeled using an ocean waveguide-acoustic propagation model, the sperm whale detection range was found to exceed 60 km in low to moderate sea state conditions after coherent array processing.

Offshore killer whale tracking using multiple hydrophone arrays

To study delphinid near surface movements and behavior, two L-shaped hydrophone arrays and one vertical hydrophone line array were deployed at shallow depths (<125 m) from the floating instrument platform R/P FLIP, moored northwest of San Clemente Island in the Southern California Bight. A three-dimensional propagation-model based passive acoustic tracking method was developed and used to track a group of five offshore killer whales (Orcinus orca) using their emitted clicks. In addition, killer whale pulsed calls and high-frequency modulated (HFM) signals were localized using other standard techniques. Based on these tracks sound source levels for the killer whales were estimated. The peak to peak source levels for echolocation clicks vary between 170-205 dB re 1 μPa @ 1 m, for HFM calls between 185-193 dB re 1 μPa @ 1 m, and for pulsed calls between 146-158 dB re 1 μPa @ 1 m.

Underwater behavior of sperm whales off Kaikoura, New Zealand, as revealed by a three-dimensional hydrophone array

Observations are presented of the vocal behavior and three dimensional (3D) underwater movements of sperm whales measured with a passive acoustic array off the coast of Kaikoura, New Zealand. Visual observations and vocal behaviors of whales were used to divide dive tracks into different phases, and depths and movements of whales are reported for each of these phases. Diving depths and movement information from 75 3D tracks of whales in Kaikoura are compared to one and two dimensional tracks of whales studied in other oceans. While diving, whales in Kaikoura had a mean swimming speed of 1.57 m/s, and, on average, dived to a depth of 427 m (SD = 117 m), spending most of their time at depths between 300 and 600 m. Creak vocalizations, assumed to be the prey capture phase of echolocation, occurred throughout the water column from sea surface to sea floor, but most occurred at depths of 400-550 m. Three dimensional measurement of tracking revealed several different "foraging" strategies, including active chasing of prey, lining up slow-moving or unsuspecting prey, and foraging on demersal or benthic prey. These movements provide the first 3D descriptions underwater behavior of whales at Kaikoura.

Range estimation of cetaceans with compact volumetric arrays

Passive acoustic monitoring is the method of choice to detect whales and dolphins that are acoustically active and to monitor their underwater behavior. The NATO Science and Technology Organization Centre for Maritime Research and Experimentation has recently implemented a compact passive acoustic monitor (CPAM), consisting of three arrays of two hydrophones each that are combined in a fixed three-dimensional arrangement and that may be towed at depths of more than 100 m. With its volumetric configuration, the CPAM is capable of estimating the three-dimensional direction vector of arriving sounds and under certain conditions on relative geometry between the whale and hydrophone array, the CPAM may also estimate the range to echolocating animals. Basic ranging methods assume constant sound speed and apply straightforward geometry to obtain depth and distance to the sound source. Alternatively, ray-tracing based methods may be employed to integrate the information provided by real sound speed profiles. Both ranging methods combine measurements of sound arrival angles and surface reflection delays and are easily implemented in real-time applications, whereby one could promote the ray-tracing approach as the preferred method because it may integrate real sound speed profiles.

Acoustic tracking of sperm whales in the Gulf of Alaska using a two-element vertical array and tags

Between 15 and 17 August 2010, a simple two-element vertical array was deployed off the continental slope of Southeast Alaska in 1200 m water depth. The array was attached to a vertical buoy line used to mark each end of a longline fishing set, at 300 m depth, close to the sound-speed minimum of the deep-water profile. The buoy line also served as a depredation decoy, attracting seven sperm whales to the area. One animal was tagged with both a LIMPET dive depth-transmitting satellite and bioacoustic "B-probe" tag. Both tag datasets were used as an independent check of various passive acoustic schemes for tracking the whale in depth and range, which exploited the elevation angles and relative arrival times of multiple ray paths recorded on the array. Analytical tracking formulas were viable up to 2 km range, but only numerical propagation models yielded accurate locations up to at least 35 km range at Beaufort sea state 3. Neither localization approach required knowledge of the local bottom bathymetry. The tracking system was successfully used to estimate the source level of an individual sperm whale's "clicks" and "creaks" and predict the maximum detection range of the signals as a function of sea state.

Underwater passive acoustic localization of Pacific walruses in the northeastern Chukchi Sea

This paper develops and applies a linearized Bayesian localization algorithm based on acoustic arrival times of marine mammal vocalizations at spatially-separated receivers which provides three-dimensional (3D) location estimates with rigorous uncertainty analysis. To properly account for uncertainty in receiver parameters (3D hydrophone locations and synchronization times) and environmental parameters (water depth and sound-speed correction), these quantities are treated as unknowns constrained by prior estimates and prior uncertainties. Unknown scaling factors on both the prior and arrival-time uncertainties are estimated by minimizing Akaike's Bayesian information criterion (a maximum entropy condition). Maximum a posteriori estimates for sound source locations and times, receiver parameters, and environmental parameters are calculated simultaneously using measurements of arrival times for direct and interface-reflected acoustic paths. Posterior uncertainties for all unknowns incorporate both arrival time and prior uncertainties. Monte Carlo simulation results demonstrate that, for the cases considered here, linearization errors are small and the lack of an accurate sound-speed profile does not cause significant biases in the estimated locations. A sequence of Pacific walrus vocalizations, recorded in the Chukchi Sea northwest of Alaska, is localized using this technique, yielding a track estimate and uncertainties with an estimated speed comparable to normal walrus swim speeds.

Effect of towed array stability on instantaneous localization of marine mammals

Reliable localization of marine mammals using towed arrays is often required for mitigation, population density estimates, and bioacoustics research. The accuracy of the range estimates using towed arrays is often not well quantified. Triangulation methods using multiple hydrophones allow for fast range estimates but are sensitive to the species type, location of the animal with respect to the array, sound propagation conditions, and array stability. A simple model is presented that is used to estimate the range accuracy of towed arrays for different vocalizations and is compared to measured range accuracies of sperm whale clicks recorded with a 15 m baseline towed array. The ranging performance is particularly sensitive to hydrophone position errors which are found to dominate. Hydrophone position errors could be estimated using heading sensors placed in the array and are taken into account in the model. A good agreement is found between the empirical range errors and theoretically predicted ones. Extrapolation of the model to other species suggests that species emitting high frequency clicks and calls can be localized from distances out to a few kilometers with a baseline of 15 m, but baleen whales transmitting low frequency calls require longer baselines to obtain range estimates.

Calling depths of baleen whales from single sensor data: development of an autocorrelation method using multipath localization

Multipath localization techniques have not previously been applied to baleen whale vocalizations due to difficulties in application to tonal vocalizations. Here it is shown that an autocorrelation method coupled with the direct reflected time difference of arrival localization technique can successfully resolve location information. A derivation was made to model the autocorrelation of a direct signal and its overlapping reflections to illustrate that an autocorrelation may be used to extract reflection information from longer duration signals containing a frequency sweep, such as some calls produced by baleen whales. An analysis was performed to characterize the difference in behavior of the autocorrelation when applied to call types with varying parameters (sweep rate, call duration). The method's feasibility was tested using data from playback transmissions to localize an acoustic transducer at a known depth and location. The method was then used to estimate the depth and range of a single North Atlantic right whale (Eubalaena glacialis) and humpback whale (Megaptera novaeangliae) from two separate experiments.

Methods for tracking multiple marine mammals with wide-baseline passive acoustic arrays

Most methods used to track marine mammals with passive acoustics require that time differences of arrivals (TDOAs) are established and are associated between hydrophone pairs. Consequently, multiple animal trackers commonly apply single-animal TDOA localization methods after performing a call separation and/or TDOA association step. When a wide-baseline array is used with multiple animals that make similar calls with short inter-call-intervals, the separation/association step can be challenging and potentially rejects valid TDOAs. This paper extends a model-based TDOA method to deal with multiple-animal datasets in a way that does not require a TDOA association step; animals are separated based on position. Advantageously, false TDOAs (e.g., a direct path associated with a multipath arrival) do not need to be removed. An analogous development is also presented for a model-based time of arrival tracking method. Results from simulations and application to a multiple sperm whale dataset are used to illustrate the multiple-animal methods. Although computationally more demanding than most track-after-association methods because separation is performed in a higher-dimensional space, the methods are computationally tractable and represent a useful new tool in the suite of options available for tracking multiple animals with passive acoustics.

Localization of a noisy broadband surface target using time differences of multipath arrivals

Previous studies [Tiemann et al., J. Acoust. Soc. Am. 120, 2355-2365 (2006)] have reported the localization of marine mammals in 3-D from their clicks using multipath arrivals. Bathymetric variations were advantageously used to predict multipath arrival times with a raytracer. These arrivals are directly discernible from the time series for impulsive sources, such as whale clicks, but extension of the method to continuous broadband sources presents additional complications. By pulse compressing noise emitted from a small boat using two hydrophones, the hyperbolic direct-arrival ambiguity can be refined in both range and bearing. Acoustic-derived results are validated with target GPS measurements.

Bayesian three-dimensional reconstruction of toothed whale trajectories: passive acoustics assisted with visual and tagging measurements

The author describes and evaluates a Bayesian method to reconstruct three-dimensional toothed whale trajectories from a series of echolocation signals. Localization by using passive acoustic data (time of arrival of source signals at receptors) is assisted by using visual data (coordinates of the whale when diving and resurfacing) and tag information (movement statistics). The efficiency of the Bayesian method is compared to the standard minimum mean squared error statistical approach by comparing the reconstruction results of 48 simulated sperm whale (Physeter macrocephalus) trajectories. The use of the advanced Bayesian method reduces bias (standard deviation) with respect to the standard method up to a factor of 8.9 (13.6). The author provides open-source software which is functional with acoustic data which would be collected in the field from any three-dimensional receptor array design. This approach renews passive acoustics as a valuable tool to study the underwater behavior of toothed whales.

Assessing the Deepwater Horizon oil spill impact on marine mammal population through acoustics: endangered sperm whales

Long-term monitoring of endangered species abundance based on acoustic recordings has not yet been pursued. This paper reports the first attempt to use multi-year passive acoustic data to study the impact of the Deepwater Horizon oil spill on the population of endangered sperm whales. Prior to the spill the Littoral Acoustic Demonstration Center (LADC) collected acoustic recordings near the spill site in 2007. These baseline data now provide a unique opportunity to better understand how the oil spill affected marine mammals in the Gulf of Mexico. In September 2010, LADC redeployed recording buoys at previously used locations 9, 25, and 50 miles away from the incident site. A statistical methodology that provides point and interval estimates of the abundance of the sperm whale population at the two nearest sites is presented. A comparison of the 2007 and the 2010 recordings shows a decrease in acoustic activity and abundance of sperm whales at the 9-mile site by a factor of 2, whereas acoustic activity and abundance at the 25-mile site has clearly increased. This indicates that some sperm whales may have relocated farther away from the spill. Follow-up experiments will be important for understanding long-term impact.

Tracking of Pacific walruses in the Chukchi Sea using a single hydrophone

The vocal repertoire of Pacific walruses includes underwater sound pulses referred to as knocks and bell-like calls. An extended acoustic monitoring program was performed in summer 2007 over a large region of the eastern Chukchi Sea using autonomous seabed-mounted acoustic recorders. Walrus knocks were identified in many of the recordings and most of these sounds included multiple bottom and surface reflected signals. This paper investigates the use of a localization technique based on relative multipath arrival times (RMATs) for potential behavior studies. First, knocks are detected using a semi-automated kurtosis-based algorithm. Then RMATs are matched to values predicted by a ray-tracing model. Walrus tracks with vertical and horizontal movements were obtained. The tracks included repeated dives between 4.0 m and 15.5 m depth and a deep dive to the sea bottom (53 m). Depths at which bell-like sounds are produced, average knock production rate and source levels estimates of the knocks were determined. Bell sounds were produced at all depths throughout the dives. Average knock production rates varied from 59 to 75 knocks/min. Average source level of the knocks was estimated to 177.6 ± 7.5 dB re 1 μPa peak @ 1 m.

Localization of sperm whales in a group using clicks received at two separated short baseline arrays

In this paper, a sperm whale click analysis scheme is proposed in order to calculate the position of individual sperm whales in a group using data received at two arrays deployed near the surface. The proposed method mainly consists of two parts: short baseline (SBL) with classification and long baseline (LBL) with class matching. In SBL with classification, a click is automatically detected, and its direction of arrival is calculated. The clicks are then classified based on their direction vectors. The class data are then sent together with direction data and matched to the other array's class data. LBL with class matching is used for localization. The classification algorithm can be used to estimate the number of whales clicking and to list potential candidates for LBL matching. As a result, the proposed method is able to localize the positions of the whales in a group. The performance of the proposed method is evaluated using data recorded off Ogasawara islands with two arrays near the surface. The three-dimensional underwater trajectories of six sperm whales are extracted to demonstrate the capability of the proposed method.

A large-aperture low-cost hydrophone array for tracking whales from small boats

A passive sonar array designed for tracking diving sperm whales in three dimensions from a single small vessel is presented, and the advantages and limitations of operating this array from a 6 m boat are described. The system consists of four free floating buoys, each with a hydrophone, built-in recorder, and global positioning system receiver (GPS), and one vertical stereo hydrophone array deployed from the boat. Array recordings are post-processed onshore to obtain diving profiles of vocalizing sperm whales. Recordings are synchronized using a GPS timing pulse recorded onto each track. Sensitivity analysis based on hyperbolic localization methods is used to obtain probability distributions for the whale's three-dimensional location for vocalizations received by at least four hydrophones. These localizations are compared to those obtained via isodiachronic sequential bound estimation. Results from deployment of the system around a sperm whale in the Kaikoura Canyon in New Zealand are shown.

Observations of potential acoustic cues that attract sperm whales to longline fishing in the Gulf of Alaska

Sperm whales (Physeter macrocephalus) have learned to remove fish from demersal longline gear deployments off the eastern Gulf of Alaska, and are often observed to arrive at a site after a haul begins, suggesting a response to potential acoustic cues like fishing-gear strum, hydraulic winch tones, and propeller cavitation. Passive acoustic recorders attached to anchorlines have permitted continuous monitoring of the ambient noise environment before and during fishing hauls. Timing and tracking analyses of sperm whale acoustic activity during three encounters indicate that cavitation arising from changes in ship propeller speeds is associated with interruptions in nearby sperm whale dive cycles and changes in acoustically derived positions. This conclusion has been tested by cycling a vessel engine and noting the arrival of whales by the vessel, even when the vessel is not next to fishing gear. No evidence of response from activation of ship hydraulics or fishing gear strum has been found to date.

A Bayesian method to estimate the depth and the range of phonating sperm whales using a single hydrophone

Some bioacousticians have used a single hydrophone to calculate the depth/range of phonating diving animals. The standard one-hydrophone localization method uses multipath transmissions (direct path, sea surface, and seafloor reflections) of the animal phonations as a substitute for a vertical hydrophone array. The standard method requires three multipath transmissions per phonation. Bioacousticians who study foraging sperm whales usually do not have the required amount of multipath transmissions. However, they usually detect accurately (using shallow hydrophones towed by research vessels) direct path transmissions and sea surface reflections of sperm whale phonations (clicks). Sperm whales emit a few thousand clicks per foraging dive, therefore researchers have this number of direct path transmissions and this number of sea surface reflections per dive. The author describes a Bayesian method to combine the information contained in those acoustic data plus visual observations. The author's tests using synthetic data show that the accurate estimation of the depth/range of sperm whales is possible using a single hydrophone and without using any seafloor reflections. This method could be used to study the behavior of sperm whales using a single hydrophone in any location no matter what the depth, the relief, or the constitution of the seafloor might be.