Towed Hydrophone Array Research Articles

Automated tracking of multiple acoustic sources with towed hydrophone arrays

Line transect surveys often incorporate a towed hydrophone array to detect and localize marine mammals. The animals are typically tracked based on the estimated time difference of arrivals (TDOAs) of their calls between pairs of hydrophones. The estimated TDOAs or bearings are then tracked through time to obtain animal or group positions, a process often performed manually. This process can be especially challenging in the presence of multiple animal groups that are vocalizing simultaneously, but at the same time do not emit signals consistently through time. In addition, the process is hindered by missed detections and false alarms (false TDOAs). Here, an automated approach to TDOA tracking is outlined, based on a multi-target Bayesian framework, that incorporates target appearance, disappearance, missed detections and false alarms. The method is demonstrated on examples of line transect surveys from Western Canada [Norris et al., J. Acoust. Soc. Am.146, 2805 (2019)] and from Hawaii, USA. [In memory of Thomas F. Norris.]

The development and use of towed hydrophone arrays to inform Southern Resident killer whale Critical Habitat in outer coastal waters

Southern Resident killer whales (SRKW) were listed as endangered in 2005, requiring designation of Critical Habitat. At that time little was known about SRKW distribution in winter/spring along the outer coast. From 2004 to 2016, we conducted nine winter/spring surveys aboard NOAA ocean-class vessels to locate and follow SRKW pods using visual sightings, satellite telemetry, and passive acoustic monitoring using towed hydrophone arrays. We present information on the evolution of the towed hydrophone array system, and insights gained on the movements and acoustic behavior of this population. Beginning with a linear two-element hydrophone array system, operations evolved to include a secondary, five-element array towed in parallel, to resolve left-right bearing ambiguity to vocalizing animals. Acoustic localization allowed tracking of vocalizing whales when visual surveys were not possible (e.g., nighttime/inclement weather). When visual and/or satellite tag position data indicated whales were within acoustic range, vocalizations were detected only 45% of the time, a result that has implications for other PAM efforts. These surveys helped fill important gaps in our understanding of SRKW occurrence to inform Critical Habitat in the outer coastal portion of their range. Despite limited monitoring, we established the importance of this key portion of their range in winter/spring.

Whistle Classification of Sympatric False Killer Whale Populations in Hawaiian Waters Yields Low Accuracy Rates

Cetaceans are ecologically important marine predators, and designating individuals to distinct populations can be challenging. Passive acoustic monitoring provides an approach to classify cetaceans to populations using their vocalizations. In the Hawaiian Archipelago, three genetically distinct, sympatric false killer whale (Pseudorca crassidens) populations coexist: a broadly distributed pelagic population and two island-associated populations, an endangered main Hawaiian Islands (MHI) population and a Northwestern Hawaiian Islands (NWHI) population. The mechanisms that sustain the genetic separation between these overlapping populations are unknown but previous studies suggest that the acoustic diversity between populations may correspond to genetic differences. Here, we investigated whether false killer whale whistles could be correctly classified to population based on their characteristics to serve as a method of identifying populations when genetic or photographic-identification data are unavailable. Acoustic data were collected during line-transect surveys using towed hydrophone arrays. We measured 50 time and frequency parameters from whistles in 16 false killer whale encounters identified to population and used those measures to train and test random forest classification models. Random Forest models that included three populations correctly classified 42% of individual whistles overall and resulted in a low kappa coefficient, κ = 0.15, indicating low agreement between models and the true population. Whistles from the MHI population showed the highest correct classification rate (52%) compared to pelagic and NWHI whistles (42% and 36%, respectively). Pairwise random forest models classifying pelagic and MHI whistles proved slightly more accurate (62% accuracy, κ = 0.24), though a similar pelagic-NWHI model did not (56% accuracy, κ = 0.12). Results suggest that the time-frequency whistle characteristics are not suitable to confidently classify encounters to a specific false killer whale population, although certain features of whistles produced by the endangered MHI population allow for overall higher classification accuracy. Inclusion of other vocalization types, such as echolocation clicks, and alternative whistle variables may improve correct classification success for these sympatric populations.

Real-time passive acoustic monitoring results of the 2018 Pacific Region International Survey of Marine Megafauna off Western Canada

Real-time passive acoustic monitoring (PAM) was conducted as part of the first large-scale systematic survey of marine mammal distribution and abundance in territorial outer-coastal waters off western Canada. The survey was conducted for approximately five weeks, from early July to mid-August, 2018 using a 69-m Canadian Coast Guard and Fisheries Vessel. A four-element, towed hydrophone array and sonobuoys (type AN/SSQ-53F and AN/SSQ-53G) were deployed and monitored in real-time 24/7 by experienced bio-acousticians. The towed array was deployed for approximately 640 h (78% of the time underway). Approximately 145 independent detections of marine mammals were made. Sperm whales (n = 53) and porpoises (n= 49) represented just over two-thirds of all detections. Ten near real-time localizations of sperm whales and 31 localization of porpoises were made, representing just over 60% of all (n = 67) all high-quality localizations. Sonobuoy effort consisted of semi-systematic deployments day and night and when sightings of large baleen whales occurred. A total of 112 sonobuoys were deployed successfully and monitored in directional (DIFAR) mode. Marine mammal calls were recorded on 32% (36) of sonobuoys. Sonobuoy detections included killer whales calls and clicks, sperm whale clicks, dolphin clicks, fin whale calls, and blue whale calls (both B and D type calls).

Automated classification of dolphin echolocation click types from the Gulf of Mexico.

Delphinids produce large numbers of short duration, broadband echolocation clicks which may be useful for species classification in passive acoustic monitoring efforts. A challenge in echolocation click classification is to overcome the many sources of variability to recognize underlying patterns across many detections. An automated unsupervised network-based classification method was developed to simulate the approach a human analyst uses when categorizing click types: Clusters of similar clicks were identified by incorporating multiple click characteristics (spectral shape and inter-click interval distributions) to distinguish within-type from between-type variation, and identify distinct, persistent click types. Once click types were established, an algorithm for classifying novel detections using existing clusters was tested. The automated classification method was applied to a dataset of 52 million clicks detected across five monitoring sites over two years in the Gulf of Mexico (GOM). Seven distinct click types were identified, one of which is known to be associated with an acoustically identifiable delphinid (Risso’s dolphin) and six of which are not yet identified. All types occurred at multiple monitoring locations, but the relative occurrence of types varied, particularly between continental shelf and slope locations. Automatically-identified click types from autonomous seafloor recorders without verifiable species identification were compared with clicks detected on sea-surface towed hydrophone arrays in the presence of visually identified delphinid species. These comparisons suggest potential species identities for the animals producing some echolocation click types. The network-based classification method presented here is effective for rapid, unsupervised delphinid click classification across large datasets in which the click types may not be known a priori.

Influences of spatial variability in pelagic scattering layers on sperm whales behavior

The central slope of the Gulf of Mexico (GOM) is home to over 20 species of marine mammals. Prior to the Deep Horizon Oil Spill in 2010, sperm whales were the predominately sighted large whale in the area. This study investigated the distribution and utilization of the north-central GOM by sperm whales two years after the spill by comparing sperm whale presence to micronekton distribution. Micronekton, organisms 2–20 cm, might be a key component in understanding sperm whale behavior as they are potentially an essential link in the food web between primary producers and higher trophic levels, including cephalopods—primary prey items of sperm whales. A Simrad EK60 echosounder operating at 38 kHz recorded the backscatter of micronekton throughout the northern GOM. The spatial distribution of micronekton backscatter was highly variable and may be driven by proximity to the Mississippi Delta. Sperm whales were sighted by trained visual observers and foraging was detected using a towed hydrophone array. The sperm whales were sighted most often in shallow slope waters less than 900 meters in areas with higher micronekton backscatter, suggesting that the waters above sloping bottoms with dense patches of mesopelagic micronekton support the prey of sperm whales.

Using multipath reflections to obtain dive depths of beaked whales from a towed hydrophone array.

Beaked whales are deep divers, emitting echolocation clicks while at depth. Little is known about the dive behavior of most species; however, passive acoustic data collected with towed hydrophone arrays can provide depth information using multipath reflections of clicks coupled with a two-dimensional localization of the individual. Data were collected during a shipboard survey in the western North Atlantic Ocean using a towed linear hydrophone array. Beaked whale tracks were classified as either Cuvier's (Ziphius cavirostris) or Gervais'/True's (Mesoplodon europaeus/Mesoplodon mirus). Weighted species average depths and weighted species standard deviations were 1158 m ± 287 m for Cuvier's (n = 24), and 870 m ± 151 m for Gervais'/True's (n = 15). Depth uncertainties ranged from 3% to 142% of the average depth. Slant ranges were corrected for depth to provide average horizontal perpendicular distance estimates. The average horizontal perpendicular distance distribution exhibited fewer detections in the first bin than the second. This is the first report of dive depths for Gervais'/True's beaked whales and use of this method to obtain depths for beaked whales using a towed linear array.

High-Frequency Modulated Signals Recorded Off the Antarctic Peninsula Area: Are Killer Whales Emitting Them?

High-frequency modulated signals with a stereotyped down-swept contour were recorded in the northwestern Antarctic Peninsula using an autonomous recorder and a towed hydrophone array. Signals have a mean start frequency at 21.6 kHz, end frequency at 15.7 kHz, −10 dB bandwidth of 5.9 kHz, and duration of 65.2 ms. Bouts of signals were generally recorded with a median inter-signal interval of 2.1 s. HFM signals partially modulated in the non-ultrasonic range similar to the ones described in this paper have already been reported for killer whales in the North Pacific, Western South Atlantic and Western Australian coast. The HFM signals were recorded in the presence of other odontocete sounds such as whistles, echolocation clicks and burst-pulsed sounds. The similarities of these sounds with vocalizations described for killer whales in the Western Australian coast lead us to strongly believe that the described HFM signals were produced by Antarctic killer whales. This paper described for the first time HFM signals in Antarctica and discussed evidence suggesting that Antarctic type A killer whales are the most probable candidates to produce such signals. However, a visual confirmation is still needed and the function of the HFM signals remains unknown.

German Navy sonar development during the two world wars and interwar years

In the German Navy, as in the UK and America, the development of underwater acoustical detection was shaped by institutional, political, and technical constraints and in response to tactical events. German hydrophone development in WWI was less advanced than that of the Allies, who focussed on combating an all-out U-boat war. The German Navy did not develop the variety of hydrophones developed by the Allies in particular the towed hydrophone array of the American Navy, which nevertheless inspired their passive sonar arrays known as “Gruppenhorchgerät” (GHG), German for “group listening device” in the interwar years, developed for the long-range protection of their capital ships, until 1935 with the signing of the Anglo German Naval Agreement when they commenced their submarine building program. For strategic reasons, Germany continued to concentrate on pro-submarine research so that at the start of WWII the German Navy had developed sophisticated GHG systems available which they improved further into the “Balkon” system of the later high-speed U-boats, Type XXI and XXVI. Their operational echo-ranging Asdic equipment was tactically inferior to that of the Royal Navy, and some interesting prototypes in development did not reach active service.

Estimation of minke whale abundance from an acoustic line transect survey of the Mariana Islands

AbstractThe minke whale is one of the most abundant species of baleen whales worldwide, yet is rarely sighted in subtropical waters. In the North Pacific, they produce a distinctive sound known as the “boing,” which can be used to acoustically localize individuals. A vessel‐based survey using both visual and passive acoustic monitoring was conducted during the spring of 2007 in a large (616,000 km2) study area encompassing the Mariana Islands. We applied line transect methods to data collected from a towed hydrophone array to estimate the abundance of calling minke whales in our study area. No minke whales were sighted, but there were hundreds of acoustic detections of boings. Computer algorithms were developed to localize calling minke whales from acoustic recordings, resulting in over 30 independent localizations, a six‐fold increase over those estimated during the survey. The two best estimates of abundance of calling minke whales were determined to be 80 and 91 animals (0.13 and 0.15 animals per 1,000 km2, respectively; CV = 34%). These are the first density and abundance estimates for calling minke whales using towed hydrophone array surveys, and the first estimates for this species in the Mariana Islands region. These are considered minimum estimates of the true number of minke whales in the study area.

Estimating the location of baleen whale calls using dual streamers to support mitigation procedures in seismic reflection surveys.

In order to mitigate against possible impacts of seismic surveys on baleen whales it is important to know as much as possible about the presence of whales within the vicinity of seismic operations. This study expands on previous work that analyzes single seismic streamer data to locate nearby calling baleen whales with a grid search method that utilizes the propagation angles and relative arrival times of received signals along the streamer. Three dimensional seismic reflection surveys use multiple towed hydrophone arrays for imaging the structure beneath the seafloor, providing an opportunity to significantly improve the uncertainty associated with streamer-generated call locations. All seismic surveys utilizing airguns conduct visual marine mammal monitoring surveys concurrent with the experiment, with powering-down of seismic source if a marine mammal is observed within the exposure zone. This study utilizes data from power-down periods of a seismic experiment conducted with two 8-km long seismic hydrophone arrays by the R/V Marcus G. Langseth near Alaska in summer 2011. Simulated and experiment data demonstrate that a single streamer can be utilized to resolve left-right ambiguity because the streamer is rarely perfectly straight in a field setting, but dual streamers provides significantly improved locations. Both methods represent a dramatic improvement over the existing Passive Acoustic Monitoring (PAM) system for detecting low frequency baleen whale calls, with ~60 calls detected utilizing the seismic streamers, zero of which were detected using the current R/V Langseth PAM system. Furthermore, this method has the potential to be utilized not only for improving mitigation processes, but also for studying baleen whale behavior within the vicinity of seismic operations.

Evaluation of UAVs as an underwater acoustics sensor deployment platform

ABSTRACTMarine surveys carried out by Passive Acoustic Monitors conventionally use towed hydrophone arrays, which requires dedicated surface observation boats. This is a costly and slow process, which could be made cheaper and quicker by using Unmanned Aerial Vehicles (UAVs). Presented in this paper are the initial findings from using UAVs to capture underwater acoustic signals from an acoustic test tank.

Studying deep diving odontocetes potential prey fields' densities and size structure in Hawaii using a DIDSON imaging sonar

Deep diving odontocetesodontocetes feed in the deep ocean on mesopelagic squid and fish. The distribution of these prey are poorly known but does affect the distribution of deep diving odontocete predators. Hence, it is important to collect data on the density and composition of potential prey to identify deep sea habitats that attract deep diving odontocetes. We used a DIDSON imaging sonar to investigate the density and size structure of mesopelagic animals along the Kona coast of Hawaii, where beaked, sperm and pilot whales are regularly seen. Data were collected at multiple stations during three research cruises by lowering the sonar in the water column to sample at different depths and monitoring the mesopelagic biomass with an EK60 38 kHz echosounder. Foraging activity of whales was monitored at few stations using passive acoustic recorders and a towed array of hydrophones. We were able to count and size 7068 pelagic animals. Density and size varied in space, with higher densities and bigger animals at the offshore stations. The DIDSON was effective at detecting targets bigger than 10 cm, a size range that includes potential prey of deep diving odontocetes. Preliminary data suggest differences may exists in habitat selection by each deep diving odontocetes.

Environmentally adaptive autonomy for acoustic communication and navigation in the new Arctic

The particular sensitivity of the Arctic to climate change is well established. and the significance to undersea operations can be dramatic. As part of the recent ICEX16 US Navy Exercise in the Beaufort Sea, MIT deployed an autonomous underwater vehicle with a towed hydrophone array below the ice cover for assessing the climate induced changes to the undersea ambient noise environment. The safe underwater operation depended on navigation updates from the submarine tracking range being communicated to the vehicle for fusion with the onboard inertial navigation. However, the changes in the environment severely deteriorated the tracking performance compared to previous deployments. The reason was clearly associated with a previously observed neutrally buoyant layer of warm Pacific water persistently spreading throughout the Beaufort Sea, which severely alters the acoustic environment with dramatic effects for both long and short range acoustic sensing, communication and navigation. This paper describes the effects observed and discusses how robust acoustic connectivity in this environment makes it paramount that the manned or unmanned undersea platforms are capable of adapting to the environment for sensing, communication, and navigation. [Work supported by the Office of Naval Research.]

Prototype of a low-cost towed array with data-logging and short-range telemetry

Passive Acoustic Monitoring studies have been benefited from open source software initiatives such as PAMGuard and ISHMAEL, but their widespread use is often limited by availability of low cost hardware. This work presents the design of a towed hydrophone array that can be built with off the shelf components. The system comprises a Raspberry Pi computer to log GPS, three-axis accelerometer, magnetometer, gyroscope, and audio from the hydrophone array (20 Hz-44 kHz). Data can be analyzed in PAMGuard, either in real time, through a short range (10 m) RF link, or retrieved later from the data logger. The array has been tested in La Paz bay Mexico, where semi-resident populations of long-beaked common dolphin (Delphinus capensis) and bottlenose dolphin (Tursiops truncatus) have been reported. Results and performance of the array in four modalities are presented; as a drift array, as a secondary array deployed from a small vessel, as an array towed by a Kayak, and as an array towed by an Autonomous Surface Veh...

Monitoring the acoustic effects of pile driving for the first offshore wind farm in the United States

The Block Island Wind Farm, the first offshore wind farm in the United States, consists of five turbines in water depths of approximately 30 m. The turbines have a jacket type substructure with piles driven to the bottom to pin the structure to the seabed. A number of acoustic sensors were deployed to monitor the acoustic properties of the pile driving activity. The acoustic sensor systems consisted of an eight element towed hydrophone array, two fixed moorings with four hydrophones each, and a fixed sensor package for measuring particle velocity. The towed array was towed from 1 to 8 km from the pile driving location. The fixed moorings were deployed at 7.5 and 15 km from the pile location. The particle velocity sensor package was deployed at 500 m from the pile driving location. This sensor package consisted of a three-axis geophone on the seabed and a tetrahedral array of four low sensitivity hydrophones at 1 m from the bottom. Data collected on these sensor systems will be presented. Acoustic levels and particle velocity observations will be compared and their implications in the context of effects on marine life will be discussed. [Work supported by Bureau of Ocean Energy Management (BOEM).]

Passive and active, predator and prey: using acoustics to study interactions between cetaceans and forage fish

AbstractFisheries acoustics surveys provide platforms for deploying passive acoustic equipment to detect cetacean vocalizations. Passive acoustic methods are developing as viable alternatives to visual surveys, particularly for small, inconspicuous species such as the harbour porpoise (Phocoena phocoena). Passive acoustic monitoring using a towed hydrophone array was carried out during an acoustic survey of clupeids in the Clyde Sea and surrounding sea lochs to identify spatial relationships between porpoises and their prey. Methods were developed to process passive acoustic data, successfully identifying porpoise echolocation clicks while discriminating them from the transmitted 120-kHz echosounder pulse and its reflections. To date, this has been a confounding factor which has made these survey techniques potentially incompatible. The highest biomass of pelagic fish was detected in the northernmost parts of the survey region, as were the largest number of porpoises. A moving average was used to examine the scale of the relationships identified, and it was found that while porpoises show no significant preferences for pelagic prey numbers at the smallest scales, they do show significant avoidance of larger areas (5+ km) with very low pelagic fish biomass. This study demonstrates that high-frequency passive acoustic monitoring can be used effectively alongside multifrequency fisheries echosounder surveys to provide novel insights into the trophic interactions between these species, and that further work will hopefully prove useful in improving the efficacy of management strategies for harbour porpoises.

Baleen whale localization using a dual-line towed hydrophone array during seismic reflection surveys

Three dimensional seismic reflection surveys use multiple towed hydrophone arrays for imaging the structure beneath the seafloor. Since most of the energy from seismic reflection surveys is low frequency, their impact on Baleen whales may be particularly significant. To better mitigate against this potential impact, safety radii are established based on the criteria defined by the National Marine Fisheries Service. Marine mammal observers use visual and acoustic techniques to monitor safety radii during each experiment. However, additional acoustic monitoring, in particular locating marine mammals, could demonstrate the effectiveness of the observations, and improve knowledge of animal responses to seismic experiments. In a previous study (Abadi et al., 2014), data from a single towed seismic array was used to locate Baleen whales during a seismic survey. Here, this method is expanded to a pair of towed arrays and the locations are compared with an alternative method. The experimental data utilized in this presentation are from the seismic experiment conducted by the R/V Marcus G. Langseth near Alaska in summer 2011. Results from both the simulation and experiment are shown and data from the marine mammal observers conducted simultaneously with the experiment are used to verify the analysis. [Sponsored by NSF.]

Towing the line: Line-transect based density estimation of whales using towed hydrophone arrays

Towed hydrophone arrays have been used to monitor marine mammals from research vessels since the 1980s. Although towed hydrophone arrays have now become a standard part of line-transect surveys of cetaceans, density estimation exclusively using passive acoustic has only been attempted for a few species. We use examples from four acoustic line-transect surveys that we conducted in the North Pacific Ocean to illustrate the steps involved, and issues inherent, in using data from towed hydrophone arrays to estimate densities of cetaceans. We will focus on two species of cetaceans, sperm whales and minke whales, with examples of beaked whales and other species as needed. Issues related to survey design, data-collection, and data analysis and interpretation will be discussed using examples from these studies. We provide recommendations to improve the survey design, data-collection methods, and analyses. We also suggest areas where additional research and methodological development are required in order to produce robust density estimates from acoustic based data.

Detection, bearing estimation, and telemetry of North Atlantic right whale vocalizations using a wave glider autonomous vehicle

Assessing and mitigating the effects of anthropogenic noise on marine mammals is limited by the typically employed technologies of archival underwater acoustic recorders and towed hydrophone arrays. Data from archival recorders are analyzed months after the activity of interest, so assessment occurs long after the events and mitigation of those activities is impossible. Towed hydrophone arrays suffer from nearby ship and seismic air gun noise, and they require substantial on-board human and computing resources. This work has developed an acoustic data acquisition, processing, and transmission system for use on a Wave Glider, to overcome these limitations by providing near real-time marine mammal acoustic data from a portable and persistent autonomous platform. Sea tests have demonstrated the proof-of-concept with the system recording four channels of acoustic data and transmitting portions of those data via satellite. The system integrates a detection-classification algorithm on-board, and a beam-forming algorithm in the shore-side user interface, to provide a user with aural and visual review tools for the detected sounds. Results from a two-week deployment in Cape Cod Bay will be presented and future development directions will be discussed.