To track highly directional echolocation clicks from odontocetes, passive hydrophone arrays with small apertures can be used to receive the same high frequency click on each sensor. A four-hydrophone small-aperture array was coupled to an autonomous acoustic recorder and used for long-term tracking of high-frequency odontocete sounds. The instrument was deployed in the spring of 2009 offshore of southern California in a known beaked whale and dolphin habitat at about 1000 m depth. The array was configured as a tetrahedron with approximately 0.5 m sensor spacing. Time difference of arrival measurements between the six sensor-pairs were used to estimate three-dimensional bearings to sources. Both near-seafloor beaked whales and near-sea surface dolphins were tracked. The tracks observed using this technique provide swimming and diving behavioral information for free-ranging animals using a single instrument. Furthermore, animal detection ranges were derived, allowing for estimation of detection probability functions.

Beaked whale and dolphin tracking using a multichannel autonomous acoustic recorder.

The study of marine soundscapes is a growing field of research. Recording hardware is becoming more accessible; there are a number of off-the-shelf autonomous recorders that can be deployed for months at a time; software analysis tools exist as shareware; raw or preprocessed recordings are freely and publicly available. However, what is missing are catalogues of commonly recorded sounds. Sounds related to geophysical events (e.g. earthquakes) and weather (e.g. wind and precipitation), to human activities (e.g. ships) and to marine animals (e.g. crustaceans, fish and marine mammals) commonly occur. Marine mammals are distributed throughout Australia’s oceans and significantly contribute to the underwater soundscape. However, due to a lack of concurrent visual and passive acoustic observations, it is often not known which species produces which sounds. To aid in the analysis of Australian and Antarctic marine soundscape recordings, a literature review of the sounds made by marine mammals was undertaken. Frequency, duration and source level measurements are summarised and tabulated. In addition to the literature review, new marine mammal data are presented and include recordings from Australia of Omura’s whales (Balaenoptera omurai), dwarf sperm whales (Kogia sima), common dolphins (Delphinus delphis), short-finned pilot whales (Globicephala macrorhynchus), long-finned pilot whales (G. melas), Fraser’s dolphins (Lagenodelphis hosei), false killer whales (Pseudorca crassidens), striped dolphins (Stenella coeruleoalba) and spinner dolphins (S. longirostris), as well as the whistles and burst-pulse sounds of Australian pygmy killer whales (Feresa attenuata). To date, this is the most comprehensive acoustic summary for marine mammal species in Australian waters.

Review of Underwater and In-Air Sounds Emitted by Australian and Antarctic Marine Mammals

Dolphins are known to produce nearly omnidirectional whistles that can propagate several kilometers, allowing these sounds to be localized and tracked using acoustic arrays. During the fall of 2007, a km-scale array of four autonomous acoustic recorders was deployed offshore of southern California in a known dolphin habitat at ∼800 m depth. Concurrently with the one-month recording, a fixed-point marine mammal visual survey was conducted from a moored research platform in the center of the array, providing daytime species and behavior visual confirmation. The recordings showed three main types of dolphin acoustic activity during distinct times: primarily whistling during daytime, whistling and clicking during early night, and primarily clicking during late night. Tracks from periods of daytime whistling typically were tightly grouped and traveled at a moderate rate. In one example with visual observations, traveling common dolphins (Delphinus sp.) were tracked for about 10 km with an average speed of ∼2.5 m s−1 (9 km h−1). Early night recordings had whistle localizations with wider spatial distribution and slower travel speed than daytime recordings, presumably associated with foraging behavior. Localization and tracking of dolphins over long periods has the potential to provide insight into their ecology, behavior, and potential response to stimuli.

Tracking dolphin whistles using an autonomous acoustic recorder array

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.

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

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

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

Sperm whales are known to dive to a depth of over two thousand meters. They emit loud impulsive broadband sounds called "clicks" during their diving. To investigate the sperm whale behavior, we attempt to apply the autonomous underwater vehicle (AUV) as an observation platform for sperm whales. AUV recognizes sperm whales individually and tracks them by listening to their clicks. As an initial stage for these objectives, we set up an experiment to test our observation system using two boats without an AUV. The observation system consists of two sets of 4-hydrophone arrays. Each hydrophone array works as a short baseline (SBL) system. The SBL system detects sperm whale clicks and calculates the orientation to sperm whales. Two relative directions from the two SBL arrays separated over a long base line (LBL) tell us the position of whale by triangulation. In the middle of August 2003, we carried out an experiment of the test system off Chichijima Island in Ogasawara. We deployed two boats each with a hydrophone array. An acoustic link between the SBL arrays was established to obtain the LBL length. During the experiments over five days we encountered at least fourteen sperm whales and succeeded in tracking some sperm whales by our observation system. Later in September 2004, we will perform another experiment using an AUV that works as one of the SBL systems. We expect this AUV-based observation system will give us a better understanding of sperm whale diving behavior and eco-system

Development of AUV-based system for acoustic tracking of diving sperm whales

Sperm whales are known to dive to a depth of over two thousand meters and it is also known that they emit loud impulsive broadband sounds called "clicks" during their diving. However, little is known about sperm whales' behavior because of a shortage of methods to observe them during their dives. Our attempt is to use the autonomous underwater vehicle (AUV) as an observation platform for sperm whales. The tasks of AUV are to recognize sperm whales individually and track them in real time by listening to sperm whale's clicks with 4-hydrophone array and detecting its direction. As the initial stage for these objectives, we set up an experiment for our observation system using a boat instead of AUV and a support boat. In the middle of August 2003, we carried out an experiment to test our observation system off Ogasawara Islands. Two boats, each with a hydrophone array were deployed mainly in the west or southeast of Chichijima Island in an ocean depth of about 1000 meters. During the experiments over five days we encountered at least fourteen sperm whales. The result of this experiment indicates that our observation system is suitable for tracking sperm whales in sub-real time. We also acquired sufficient sound data of sperm whale's vocalization for off-line verification. The analysis of the results is presently underway. This experiment can be considered as the preliminary step to apply this kind of technique to develop innovative observation for underwater ecosystem system using AUVs.

Acoustic tracking of sperm whales using two sets of hydrophone array

A simple and easily implemented observation method to observe sperm whales that swim in groups and repeatedly dive to depths of more than 1,000 m, is desirable in order to understand their underwater behavior. In this paper, we propose a passive acoustic method of classification and localization of individual whales, which does not disturb them, by analyzing click sound data captured by two hydrophones suspended from two boats. We classify the whales based upon the time delay between the direct clicks and their surface reflections on each hydrophone, and also the time delay between the direct clicks observed at the hydrophones. The former time delays identify two hyperboloids of potential whale locations, since the reflection on the sea surface is equivalent to the signal received at a virtual hydrophone. A third hyperboloid is similarly obtained from the latter time delay. The three hyperboloids intersect at two points when the depths of two hydrophones and the distance between the two boats, which correspond to baselines of hyperbolic positioning, are known. The whale depths and locations can be determined with symmetric ambiguity about the baseline between the boats. One of the advantages of this method is that the depth of the whales can be calculated independently from the horizontal distance between the two hydrophones. An analysis has been carried out using data collected off the Islands of Ogasawara in the summer of 2003. This method effectively distinguished six whales in conformity with visual investigation

Automatic classification of diving sperm whales by analysis of click time delay using two hydrophones

A simple method using only two hydrophones, each loosely deployed from separate mobile platforms, has been developed to simultaneously track several vocalizing sperm whales. The separation distance of several hundred meters between the hydrophones implies that a particular whale could present vastly different beam orientations towards them, thus precluding use of any specific relationship between the click signal levels at the two locations for the purpose of whale localization. This method utilizes time‐of‐arrival of the direct clicks and their surface reflections and matches them at the two hydrophone locations. Whales are segregated on the basis of the set of observed time delays. Click parameters such as interpulse interval and average click frequency are proposed to be used as secondary data for the purpose of track refinement. Depth profile of the whales is obtained independent of the separation distance between the two hydrophones. However, knowledge of the separation distance between the hydrophones provides the 3D coordinates of the whales within a left–right ambiguity. Whale tracks have been obtained using actual data collected off Ogasawara Islands in Japan. These results demonstrate the utility of this method for studying the bioacoustics and behavior of deep diving sperm whales.

M. Sakata

Papers

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

Development of AUV-based system for acoustic tracking of diving sperm whales

Acoustic tracking of sperm whales using two sets of hydrophone array

Automatic classification of diving sperm whales by analysis of click time delay using two hydrophones

Passive tracking of multiple diving sperm whales using single hydrophones at two mobile locations