Martin Gassmann

High-frequency modulated signals of killer whales (Orcinus orca) in the North Pacific

Killer whales in the North Pacific, similar to Atlantic populations, produce high-frequency modulated signals, based on acoustic recordings from ship-based hydrophone arrays and autonomous recorders at multiple locations. The median peak frequency of these signals ranged from 19.6-36.1 kHz and median duration ranged from 50-163 ms. Source levels were 185-193 dB peak-to-peak re: 1 μPa at 1 m. These uniform, repetitive, down-swept signals are similar to bat echolocation signals and possibly could have echolocation functionality. A large geographic range of occurrence suggests that different killer whale ecotypes may utilize these signals.

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.

Delphinid behavioral responses to incidental mid-frequency active sonar

Opportunistic observations of behavioral responses by delphinids to incidental mid-frequency active (MFA) sonar were recorded in the Southern California Bight from 2004 through 2008 using visual focal follows, static hydrophones, and autonomous recorders. Sound pressure levels were calculated between 2 and 8 kHz. Surface behavioral responses were observed in 26 groups from at least three species of 46 groups out of five species encountered during MFA sonar incidents. Responses included changes in behavioral state or direction of travel, changes in vocalization rates and call intensity, or a lack of vocalizations while MFA sonar occurred. However, 46% of focal groups not exposed to sonar also changed their behavior, and 43% of focal groups exposed to sonar did not change their behavior. Mean peak sound pressure levels when a behavioral response occurred were around 122 dB re: 1 μPa. Acoustic localizations of dolphin groups exhibiting a response gave insight into nighttime movement patterns and provided evidence that impacts of sonar may be mediated by behavioral state. The lack of response in some cases may indicate a tolerance of or habituation to MFA sonar by local populations; however, the responses that occur at lower received levels may point to some sensitization as well.

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

Cuviers 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 Cuviers 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. Cuviers 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.

Tracking dolphins with hydrophone arrays deployed from the floating instrument platform R/P FLIP in the Southern California Bight.

Dolphin movements were studied with hydrophone arrays and visual observations using the R/P FLIP moored North of San Clemente Island in Fall 2008. A total of 14 hydrophones distributed as two L‐shaped arrays at 36 m depth and one vertical line array at 139 m depth were deployed from FLIP. The data were sampled at 192 kHz continuously for 4 weeks. Sound localizations were realized by estimating vertical and horizontal angles from automatically detected dolphin echolocation clicks and whistles to compute ranges, depths, and bearings. While angles to broadband clicks were estimated by cross‐correlation, angles to narrow‐band whistles were obtained by coherently frequency‐averaging conventional frequency domain beamformer outputs. Sound refraction errors were corrected using Snell’s law. The localization methods were groundtruthed by successfully tracking ships and by comparing acoustic and visual positions for dolphin groups. First results reveal continuous trajectories of common dolphin schools for as long ...

Direction-finding techniques for a small-aperture hydrophone array

A volumetric array of hydrophones was coupled to a long-term, autonomous acoustic recorder and deployed ~130 km offshore of Southern California to the seafloor (~1300 m depth) to track continuous-wave (CW) and transient underwater sound sources. The array was composed of four, wide-band, omnidirectional hydrophones closely-spaced ~1 m apart. Sampling was continuous at 100 KSamples/s for each hydrophone over a period of more than two months. To track CW sound sources, conventional and adaptive beamforming techniques were implemented. The array’s beam pattern characteristics as a function of frequency (10—1000 Hz) were investigated by simulating the arrival of plane waves from directions of interest. Beamforming techniques were opportunistically applied to nearby (<5 km) transiting commercial ships with automated identification system (AIS) transmitted locations. Discrepancies were within the physical dimension of the transiting ships. Source levels for each of the transiting ships were estimated at the shi...

Underwater sound directionality of commercial ships

The underwater sound radiated by commercial ships is an unintended by-product of their operation and one of the most significant contributors to man-made noise at low frequencies in the ocean. To estimate the directionality of underwater sound radiated by current commercial ships, a seafloor array of five high-frequency acoustic recording packages (HARPs) deployed to 1 km depth with a maximum horizontal aperture of 1 km was used. As a ship of opportunity passed over the HARP array, the directions from the ship to each HARP along with the corresponding source levels were estimated for each ship location. Ships were tracked via satellites (Automatic Identification System—AIS) and acoustically by a frequency domain beamformer that was implemented for one of the HARPs configured with a volumetric hydrophone array (2 m maximum aperture). The directionality estimates of contemporary commercial ships exhibit significant stern-bow asymmetries among other quantitative characteristics that will be discussed.

Tracking Cuvier’s beaked whales using small aperture arrays

Cuvier beaked whales are deep-diving animals that produce strongly directional sounds using high frequencies (>30 kHz) at which attenuation due to absorption and scattering is high (>8 dB/km). This makes it difficult to track beaked whales in three dimensions with standard large-aperture hydrophone arrays. By embedding two volumetric small-aperture (~1 m element spacing) arrays into a large-aperture (~1 km element spacing) array of five nodes, individuals and even groups of Cuvier beaked whales were tracked in three dimensions continuously up to one hour within an area of 10 km2 in the Southern California Bight. This passive acoustic tracking technique provides a tool to study the characteristics of beaked whale echolocation, and their behavior during deep-diving.

Tracking dolphins using long-term autonomous acoustic recorders

Tracking marine mammals over long periods can provide information on their movement patterns including base-line behavior and responses to natural and anthropogenic stimuli. Autonomous acoustic recorders provide a cost effective and portable means of tracking these sounds over long periods, but until recently these devices have been restricted to tracking low-frequency large whales because of limited recording capabilities. In this paper, we will present long-term, passive acoustic tracking of high-frequency dolphin whistles and clicks using autonomous hydrophone recording arrays with kilometer- and meter-scale apertures.

Passive acoustic tracking of marine mammals and anthropogenic sound sources with autonomous three-dimensional small-aperture arrays

Marine mammals produce a wide range of frequency-modulated sounds at low and high frequencies as well as directional broadband echolocation sounds in a refractive ocean environment. This creates several challenges for passive acoustic long-term tracking of the various marine mammal species. To overcome these, three-dimensional small-aperture hydrophone arrays coupled to seafloor multi-channel recording packages were deployed in a large aperture array in the Southern California Bight. Taking advantage of the experimental setup in the oceanic waveguide, time and frequency-domain tracking methods will be presented and tracks of marine mammals as well as anthropogenic sources will be shown. This provides a tool to study over long timescales behavioral responses of tracked marine mammals to tracked anthropogenic sources.