Passive Acoustic Detection Research Articles

Passive acoustic detection of closed-circuit underwater breathing apparatus in an operational port environment

Divers constitute a potential threat to waterside infrastructures. Active diver detection sonars are available commercially but present some shortcomings, particularly in highly reverberant environments. This has led to research on passive sonar for diver detection. Passive detection of open-circuit UBA (underwater breathing apparatus) has been demonstrated. This letter reports on the detection of a diver wearing closed-circuit UBA (rebreather) in an operational harbor. Beamforming is applied to a passive array of 10 hydrophones in a pseudo-random linear arrangement. Experimental results are presented demonstrating detection of the rebreather at ranges up to 120 m and are validated by GPS ground truth.

Characteristics of gunshot sound displays by North Atlantic right whales in the Bay of Fundy

North Atlantic right whales (Eubalaena glacialis) produce a loud, broadband signal referred to as the gunshot sound. These distinctive sounds may be suitable for passive acoustic monitoring and detection of right whales; however, little is known about the prevalence of these sounds in important right whale habitats, such as the Bay of Fundy. This study investigates the timing and distribution of gunshot sound production on the summer feeding grounds using an array of five marine acoustic recording units deployed in the Bay of Fundy, Canada in mid-summer 2004 and 2005. Gunshot sounds were common, detected on 37 of 38 recording days. Stereotyped gunshot bouts averaged 1.5 h, with some bouts exceeding 7 h in duration with up to seven individuals producing gunshots at any one time. Bouts were more commonly detected in the late afternoon and evening than during the morning hours. Locations of gunshots in bouts indicated that whales producing the sounds were either stationary or showed directional travel, suggesting gunshots have different communication functions depending on behavioral context. These results indicate that gunshots are a common right whale sound produced during the summer months and are an important component in the acoustic communication system of this endangered species.

Analysis of Directional Accuracy of Three Single Arrays

Acoustic sensoxr array technology is the one of the key technology of passive acoustic detection technology, whether the single array has a good performance on direction will directly affect the accuracy of the locating system. In order to choose the best single array on direction, in this paper, respectively selected representative plane array and spatial array from a number of typical single arrays to analyze, find out the advantages and disadvantages of two single arrays. On this basis, around the idea of integrating the advantage of symmetric spatial array and plane cross five-element array, according to characteristics of the structure of this two kinds of arrays to design a new locating model—spatial five-element array. And compared with two kinds of single array mentioned above, the results of comparison show that the locating accuracy of spatial five-elements array is superior to both kinds of mentioned, and have high engineering value.

Dual Mode Sensing with Low-Profile Piezoelectric Thin Wafer Sensors for Steel Bridge Crack Detection and Diagnosis

Monitoring of fatigue cracking in steel bridges is of high interest to many bridge owners and agencies. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In this paper, we presented a dual mode sensing methodology integrating acoustic emission and ultrasonic wave inspection based on the use of low-profile piezoelectric wafer active sensors (PWAS). After introducing the research background and piezoelectric sensing principles, PWAS crack detection in passive acoustic emission mode is first presented. Their acoustic emission detection capability has been validated through both static and compact tension fatigue tests. With the use of coaxial cable wiring, PWAS AE signal quality has been improved. The active ultrasonic inspection is conducted by the damage index and wave imaging approach. The results in the paper show that such an integration of passive acoustic emission detection with active ultrasonic sensing is a technological leap forward from the current practice of periodic and subjective visual inspection and bridge management based primarily on history of past performance.

Monitoring a small marine cetacean population using passive acoustic techniques: Vaquita, a critically endangered species

In 2008, vaquita population was estimated in only 245 individuals. Between 1997 and 2007, a passive acoustic semi-autonomous system was used for monitoring detection rate of this species. The analysis resulted in decline of approximately 58%. Sighting and acoustic data from a 2008 research cruise, compared to the 1997 estimate of abundance, resulted in a decline of approximately 57%. Hence, passive acoustic detection probed reliable to monitor the population. The Mexican Government implemented a recovery plan, which includes monitoring population trends. At current level, the population can grow at maximum rates lower than 4% annually. The severely reduced population level, and scarcity of acoustic detections, made unreliable to continue the use of the methods applied till 2007. An increase of sampling effort was identified as the key to implement a reliable system, only achievable using completely autonomous detectors. Field test of autonomous detectors identified C-POD as very reliable. Using data collected with this equipment it was determined that an annual effort of 5000 C-POD days is needed to detect population increase. A sampling grid of 62 stations was designed. Work to design and test reliable mooring devices was done. Currently, the first of five years of sampling has started.

Listening for a needle in a haystack: passive acoustic detection of dolphins at very low densities

Passive acoustic surveys have potential for detecting trends in abundance and habitat use by rare cetaceans. We deployed commercially available acoustic data loggers (T-PODs) in 4 har- bours on the west coast of New Zealand's North Island between 2005 and 2008 to investigate the dis- tribution of Maui's dolphin Cephalorhynchus hectori maui and assess whether current protection measures are sufficient. A set of decision rules was developed to minimise the potential for false pos- itive detections. Over 3211 'T-POD days' of acoustic monitoring, 39 click trains which satisfied all of our decision rules were detected, indicating the presence of Maui's dolphins in Manukau and Kaipara Harbours. Data from the site with the most detections were fitted to 3 models, showing that the number of detections varied temporally (p < 0.001). The models were also used to show to what degree dolphins could have been present at monitored locations yet remain undetected. The study highlighted the challenges of passive acoustic monitoring of rare species, particularly of small del- phinids in an environment which is both physically and acoustically challenging. Nonetheless, we demonstrated that T-PODs are effective in studies of Maui's dolphin distribution, that Maui's dolphins are found in North Island harbours and remain at risk from gillnet bycatch. We make a number of rec- ommendations concerning acoustic monitoring studies of rare cetaceans, principally that a thorough understanding of the target signals and the acoustic environment being monitored is essential for maximising acoustic detection rates.

Research of the Two-Vehicle Joint Operation Control System Based on Joint-Movement Navigation

This article detailedly elaborated the design principles of the two-vehicle joint operating control system based on joint-movement navigation，meanwhile it presented the frame of hardware circuit and software design. Adopting passive acoustic detection triangulation technique and two- vehicle navigation by the same sound frequency and time-sharing operation, together with dynamic route optimization strategy, so as to make sure that the two-vehicle has a higher joint-movement speed and positioning accuracy, and it automatically avoids obstacles and site boundary.

Target detection in a shallow water environment using a two-dimensional array.

The detection and tracking of targets in cluttered, shallow water locations have become increasingly important for both maritime security and coastal environmental monitoring. Ambient noise contributions from snapping shrimp, as well as recreational and commercial ship traffic, can limit target detection performance. Few studies have investigated the possibility of enhanced passive acoustic detection in these locations using multiple arrays. Two synchronized 24 element arrays with 300 Hz–50 kHz bandwidth were deployed at the Kilo Nalu Nearshore Reef Observatory (University of Hawaii, Honolulu, HI) in an “L” shape configuration with a vertical array within the 12 m water column and horizontal array orthogonally aligned on the seafloor. This subtropical site is located between Honolulu Harbor and a recreational boating and diving area. Broadband snapping shrimp noise, which dominates the background levels, has a diurnal variation. A series of experiments examined the feasibility of passive acoustic monitoring with array beamforming of a variety of targets including small craft, open circuit scuba divers, and a REMUS autonomous underwater vehicle. The relative performance of narrowband versus broadband beamforming techniques for detection and tracking was quantified for these targets. Moreover, the applicability of nearfield techniques, such as wavefront curvature, was also investigated. [Work supported by the DHS.]

A wavelet packet adaptive filtering algorithm for enhancing manatee vocalizations

Approximately a quarter of all West Indian manatee (Trichechus manatus latirostris) mortalities are attributed to collisions with watercraft. A boater warning system based on the passive acoustic detection of manatee vocalizations is one possible solution to reduce manatee-watercraft collisions. The success of such a warning system depends on effective enhancement of the vocalization signals in the presence of high levels of background noise, in particular, noise emitted from watercraft. Recent research has indicated that wavelet domain pre-processing of the noisy vocalizations is capable of significantly improving the detection ranges of passive acoustic vocalization detectors. In this paper, an adaptive denoising procedure, implemented on the wavelet packet transform coefficients obtained from the noisy vocalization signals, is investigated. The proposed denoising algorithm is shown to improve the manatee detection ranges by a factor ranging from two (minimum) to sixteen (maximum) compared to high-pass filtering alone, when evaluated using real manatee vocalization and background noise signals of varying signal-to-noise ratios (SNR). Furthermore, the proposed method is also shown to outperform a previously suggested feedback adaptive line enhancer (FALE) filter on average 3.4 dB in terms of noise suppression and 0.6 dB in terms of waveform preservation.

Beaked whale (Mesoplodon densirostris) passive acoustic detection in increasing ambient noise

Passive acoustic detection is being increasingly used to monitor visually cryptic cetaceans such as Blainville's beaked whales (Mesoplodon densirostris) that may be especially sensitive to underwater sound. The efficacy of passive acoustic detection is traditionally characterized by the probability of detecting the animal's sound emissions as a function of signal-to-noise ratio. The probability of detection can be predicted using accepted, but not necessarily accurate, models of the underwater acoustic environment. Recent field studies combining far-field hydrophone arrays with on-animal acoustic recording tags have yielded the location and time of each sound emission from tagged animals, enabling in-situ measurements of the probability of detection. However, tagging studies can only take place in calm seas and so do not reflect the full range of ambient noise conditions under which passive acoustic detection may be used. Increased surface-generated noise from wind and wave interaction degrades the signal-to-noise ratio of animal sound receptions at a given distance leading to a reduction in probability of detection. This paper presents a case study simulating the effect of increasing ambient noise on detection of M. densirostris foraging clicks recorded from a tagged whale swimming in the vicinity of a deep-water, bottom-mounted hydrophone array.

Rare detections of North Pacific right whales in the Gulf of Alaska, with observations of their potential prey

The North Pacific right whale Eubalaena japonica was heavily exploited throughout the Gulf of Alaska by both historical whaling and 1960s illegal Soviet catches. It is now extremely rare in this region (2 sightings between 1966 and 2003 and passive acoustic detections on 6 days out of 80 months of recordings at 7 locations). From 2004 to 2006, 4 sightings of right whales occurred in the Barnabus Trough region on Albatross Bank, south of Kodiak Island, Alaska, USA. Sightings of right whales occurred at locations within the trough with the highest density of zooplankton, as measured by active acoustic backscatter. Net trawls through a high-density demersal layer (~150 to 175 m) revealed large numbers of euphausiids and oil-rich C5-stage copepods. Photo-identification and genotyping of 2 whales failed to reveal a match to Bering Sea right whales. Fecal hormone metabo- lite analysis from 1 whale estimated levels consistent with an immature male, indicating either recent reproduction in the Gulf of Alaska or movements between the Bering Sea and the Gulf of Alaska. Large numbers of historic catches of right whales occurred in pelagic waters of the Gulf of Alaska, but there have been few recent detections in deep water. Given that there is no other location in the Gulf of Alaska where right whales have been repeatedly seen post-exploitation, the Barnabus Trough/Albatross Bank area represents important habitat for the relict population of North Pacific right whales in the Gulf of Alaska, and a portion of this area was designated as critical habitat under the US Endangered Species Act in 2006.

Risk mapping for sensitive species to underwater anthropogenic sound emissions: Model development and validation in two Mediterranean areas

Recent observations of cetacean mass strandings, coincident with anthropogenic sounds emissions, have raised concerns on the potential environmental impact of underwater noise. Cuvier’s beaked whale (Ziphius cavirostris) was reported in all the cited stranding events. Within the NATO Marine Mammal Risk Mitigation project (MMRM), multiple interdisciplinary sea trials have been conducted in the Mediterranean Sea with the objective of developing tools and procedures to mitigate the impact of underwater sound emissions. During these cruises, visual observations, passive acoustic detections and environmental data were collected. The aim of this study was to evaluate “a priori” predictions of Cuvier’s beaked whale presence in the Alboran Sea, using models developed in the Ligurian Sea that employ bathymetric and chlorophyll features as predictors. The accuracy of these predictions was found adequate and elements are given to account for the uncertainties associated to the use of models developed in areas different from their calibration site.

Time reversal acoustic approach for non‐lethal swimmer deterrent.

Protection against surface and underwater threats from swimmers constitutes one of the most challenging aspects of port security. The envisioned risk mitigation consist of passive acoustic detection and localization and active diver deterrent, which can be done by focusing high intensity acoustic waves. The main goal of this study is to explore the possibility of using the time reversal acoustics (TRA) sound focusing system for non‐lethal swimmer neutralization. This breakthrough technology enables the precision targeting of a hostile diver with minimum impact to the marine environments. The acoustic noise radiated by the diver is used to focus the acoustic energy, while the moving diver acts as an active self‐disclosing acoustic beacon. In a shallow water environment the radiated noise, direct and multi‐path interference, is rich and is exploited through the use of TRA. The effectiveness of the TRA focusing and the diver deterrence zone are modeled taking into consideration the frequency range, harbour depth, and geometry of the transducer array. The real diver noise and hydroacoustic signature parameters collected by Stevens Institute of Technology are used in the theoretical analysis. It is demonstrated that outside the focal region, the TRA system produces acoustic intensities that are not harmful for marine life.

In vitro characterization of perfluorocarbon droplets for focused ultrasound therapy

Focused ultrasound therapy can be enhanced with microbubbles by thermal and cavitation effects. However, localization of treatment is difficult as bioeffects can occur outside of the target region. Spatial control of bubbles can be achieved by ultrasound-induced conversion of liquid perfluorocarbon droplets to gas bubbles. This study was undertaken to determine the acoustic parameters for bubble production by droplet conversion and how it depends on the acoustic conditions and droplet physical parameters. Lipid-encapsulated droplets containing dodecafluoropentane were manufactured with sizes ranging from 1.9 to 7.2 µm in diameter and diluted to a concentration of 8 × 106 droplets mL−1. The droplets were sonicated in vitro with a focused ultrasound transducer and varying frequency and exposure under flow conditions through an acoustically transparent vessel. The sonications were 10 ms in duration at frequencies of 0.578, 1.736 and 2.855 MHz. The pressure threshold for droplet conversion was measured with an active transducer operating in pulse-echo mode and simultaneous measurements of broadband acoustic emissions were performed with passive acoustic detection. The results show that droplets cannot be converted at low frequency without broadband emissions occurring. However, the pressure threshold for droplet conversion decreased with increasing frequency, exposure and droplet size. The pressure threshold for broadband emissions was independent of the droplet size and was 2.9, 4.4 and 5.3 MPa for 0.578, 1736 and 2.855 MHz, respectively. In summary, we have demonstrated that droplet conversion is feasible for clinically relevant sized droplets and acoustic exposures.

Autonomous underwater glider based embedded real-time marine mammal detection and classification.

Autonomous marine vehicles offer the potential to provide low-cost data suitable for passive acoustic monitoring applications of marine mammals. Due to their extremely low-power consumption and long range, gliders are an attractive option for long-term deployments. Challenges related to power availability, payload size, and weight have previously restricted the viability of marine mammal monitoring. As an example, the wide bandwidth of odontocete echolocation clicks requires a high sampling rate and poses challenges with respect to limitations in power, size, and weight of the deployed system. Recent developments in commercial off-the-shelf hardware driven by the mobile phone industry’s need for multimedia-rich smart phones have resulted in low-power architectures capable of performing computationally demanding signal processing and stochastic recognition tasks in real time. We describe our work on a small form-factor, light-weight package used to perform real-time passive acoustic detection and classification. The system detects echolocation clicks using Teager energy. Echolocation clicks are then classified using cepstral features processed by a Gaussian mixture model. [Work sponsored by ONR.]

Frequency domain tracking of passive vessel harmonics.

This paper presents a method for passive acoustic detection and tracking of small vessels in noisy, shallow water marine environments. Passive spectra of boats include broadband noise as well as tones that are harmonics of the engine speed and shaft/propeller rotation. Past work suggests that the location in frequency and the relative amplitudes of these harmonics can be used to determine specific characteristics of the vessel such as the number of blades on the propeller and engine type/speed. However, the low signal to noise ratio of quiet targets and Doppler shifts incurred because of source and receiver motion complicate the identification of these tones in the lofargram. To address this issue, a combined detection and tracking approach is proposed in which intermittent and wandering harmonic content is tracked with a multi-dimensional Kalman filter. Results from recorded passive signatures from several classes of vessels in marine and freshwater environments in the Pacific Northwest are presented and discussed.

Passive acoustic detection of schools of herring

Herring (Clupea pallasii and C. harengus) have been observed to release gas from their bladders during vertical migration likely to adjust buoyancy and also when under strong predation pressure. Based on recently measured and modeled sound for individual fish, spectral levels are estimated for entire herring schools in the ocean for both scenarios, and the feasibility of passive detection is explored. For a typical school of migrating herring near-surface spectral levels of about 50 dB rel., 1 microPa radical Hz at 3-7 kHz are predicted. If wind conditions are calm where migrating herring are found, such as for Pacific herring in Prince William Sound, Alaska, passive detection is very likely. For an exemplary 10 metric ton compact school, peak spectral source levels of about 80-90 dB rel. 1 microPa radical Hz ref. 1 m are predicted, yielding a range of detection against calm wind background of about 1000 m. Field measurements of potential gas-release events agree with the predictions for the compact school scenario with regard to levels and spectral shape and indicate that passive acoustic monitoring is feasible and could be a prime tool to study predator-prey interactions.

Detecting humpback whale sounds in the Bering Sea: Confounding sounds in a cacophony of noise.

Humpback whales (Megaptera novaeangliae) are top predators of large zooplankton and forage fish, and one of the most common large whales in the Bering Sea. While present on feeding grounds, humpback whales produce nonsong sounds probably associated with feeding or social contacts. However, little is known about these highly variable sounds, and their detection is challenging. Recordings were collected during 2006–2007 at the long‐term oceanographic moorings M2, M4, and M5 in the eastern Bering Sea. Passive acoustic detection of humpback whale calls in these recordings was confounded by a variety of other sounds, which fall within the same parameters as nonsong humpback vocalizations. An automatic algorithm that detects tonal sounds in the 300–950 Hz frequency band was used to find humpback calls. Raw detections were visually examined to verify the accuracy of the detections. This algorithm resulted in a significant number of wrong detections (false positives), especially sounds of bearded seals (Erignathus barbatus) and bowhead whales (Balaena mysticetus). An error rate was calculated to correct the high number of false detections, and further refinements were made to the automatic detection algorithm. The problems and possible solutions to detecting humpback whale sounds in complex acoustic recordings will be presented.

Two-Stage Selection of the Best Signal-To-Noise Ratio with Related Approximations

We consider the problem of calibrating k (≥ 2) channels or signal processors and select the best one, that is the one having the largest signal-to-noise ratio (SNR). Assuming a Gaussian distribution for the back-ground white noise or clutter, the problem reduces to selecting from k (≥ 2) normal populations the one with the largest value of the absolute mean. Assuming unknown means and a common unknown variance, we proceed under Bechhofer's 1954 indifference-zone formulation. Since the common variance is unknown, a single-stage procedure guaranteeing a preassigned level of the minimum probability of correct selection (PCS) would not exist. We propose a two-stage selection methodology whose minimum PCS meets the minimum PCS requirement up to the second-order term with respect to the difference of the absolute means. The roles of the proposed approximation are examined both via exact computations and simulations. A realistic example on passive acoustic sonar detection of marine mammals (Abraham 2004) is included with data analyses.

Probability of Detection and Optimal Sensor Placement for Threshold Based Detection Systems

This paper provides a probabilistic analysis of simple detection systems which are based on thresholding feature values extracted from a sensor signal. For such systems, this paper explains how to calculate the probability of detection as a function of range from the sensor to the object of interest. This function is important in that it enables optimal positioning of a group of sensors, either maximizing detection rates for a given number of sensors or informing the minimum number of sensors necessary to achieve a desired probability of detection throughout an area. An example case study is presented, based on a novel approach to passive acoustic diver detection in noisy environments.