Acoustic Observatory Research Articles

Application of automatic identification system information to ocean soundscape modeling.

The impact of anthropogenic noise on marine life is an important issue to both the scientific community and public policy makers. Human‐generated noise has potential to disrupt critical marine mammal biological functions such as foraging, communication, and navigation. Commercial shipping contributes significantly to the ocean soundscape, typically dominating the noise field at frequencies less than 500 Hz. Market conditions, trends in vessel design and propulsion, use of more economical ship routes, operational efficiency, and environmental factors are all important variables that help shape the changing soundscape. To reliably model the temporal and spatial variability of a regional soundscape, accurate characterization of the sources of noise is needed. Acoustic recordings taken at the Point Sur Ocean Acoustic Observatory (OAO) and Automatic Identification System (AIS) reports broadcast by ships passing the OAO site have been used to determine ship source levels over the 25–600 Hz band, categorized by ship class and speed. Source levels are then applied to a model used to evaluate temporal variability of the noise field at several sites along the central California coast based on AIS‐reported shipping traffic transiting the region. Results of our calculations are presented and discussed. [Research supported by US Navy CNO(N45).]

Passive acoustic monitoring near Pt. Sur, California, in 2008–2009.

How sounds from human activities affect behavior of marine mammals is in focus of many ongoing research projects. An important step in this direction is to develop regional databases of marine mammal vocalizations and sounds generated by human activities and natural sources. Analysis of passive acoustic recordings collected on top of Sur Ridge, near the Point Sur Ocean Acoustic Observatory (OAO), in 2008–2009 is presented. The data have been acquired with high‐frequency acoustic recording package in the 10 Hz–100 kHz frequency band at a 200 kHz sampling frequency. These recordings have been scanned to detect and identify signals from various underwater acoustic sources. Pressure spectrum level has been analyzed in time/frequency space to reveal interannual, seasonal, and diel variabilities, as well as possible correlations of different sounds, with primary focus on anthropogenic sounds and marine mammal vocalizations. [Research supported by US Navy CNO(N45).]

Acoustic ecology of Antarctic pinnipeds

In aquatic-mating pinnipeds, acoustic communication plays an important role in male competition and mate attraction. Vocal repertoire size and composition during the breeding season varies between species and is presumed to be a product of interspecific differences in sexual selec- tion. In this study, we examine seasonal and diel patterns in acoustic repertoire size, composition and call activity of 4 Antarctic pinniped species: Weddell seal Leptonychotes weddellii, leopard seal Hydrurga leptonyx, Ross seal Ommatophoca rossii and crabeater seal Lobodon carcinophaga. An 11 mo (Jan 2006 - Jan 2007, no recordings Jul and Nov 2006) near-continuous dataset was collected from the Perennial Acoustic Observatory in the Antarctic Ocean (PALAOA) located on the Ekstrom Iceshelf. The Weddell seal vocal repertoire consisted of 14 call types. Calls were present throughout the year except in February (11 mo), while repertoire composition varied considerably between months. The leopard seal vocal repertoire consisted of 7 call types. Calls were present between Octo- ber and January (4 mo). All call types were used in a uniform manner throughout the entire call period. The Ross seal vocal repertoire consisted of 5 call types. Ross seal vocalizations were present from December until February (3 mo). Repertoire composition varied little between months. Crab- eater seals produced one vocalization type, present from August to December (5 mo). Vocalizations in these species are likely produced in a breeding context. Inter-specific differences in behavioral ecology and interactions with abiotic and biotic environmental factors shape vocal behavior resulting in each species filling its own acoustic ecological niche.

Screening large data sets and real‐time data streams for bioacoustic signals.

This presentation will discuss a major challenge of passive‐acoustic monitoring systems: the analysis of large data sets to identify the occurrence of bioacoustic signals of interest. Triggered by the rapid development of digital audio technology and the increasing capacity of memory devices, it has become easier than ever to produce large, long‐term acoustic data sets. However, analyzing these data sets is challenging, as suitable automated detection and classification systems are needed to perform the analysis in a reasonable amount of time. One such large data set is currently produced by the Perennial Acoustic Observatory in the Antarctic Ocean. This passive acoustic observatory has been operated by the Alfred Wegener Institute since January 2006 and has generated more than 10,000 hours of data to date. The observatory features a data link via satellite, which allows analysis of the recorded hydroacoustic data in real time in Germany. However, to be able to run custom algorithms over the entire data set in several times real‐time speed, a distributed computing system was developed and applied. Here we provide a detailed description of this system and discuss further possible applications.

Effects of offshore mesoscale eddies and fronts on inshore shallow water acoustic propagation

Shallow water shelf areas inside of western boundary currents have two distinctly different ocean acoustic environments determined by the type of front that separates deep offshore from shallow inshore. Prograde fronts allow for the ducting of offshore internal waves up onto the shelf as the main source the sound speed fluctuations over the internal wave band. Retrograde fronts block the propagation of offshore internal waves setting up stability conditions that allow for propagation of locally generated, large amplitude non‐linear solitary waves as the major source of sound speed variability. Here, acoustic propagation is examined for both environments with data from two similar fixed system propagation experiments, one for the prograde environment off the coast of south Florida near the site off the Acoustic Observatory, and the second for the retrograde front environment of the Mid‐Atlantic Bight the SWO6 site. Offshore mesoscale features of fronts and eddies are shown to determine mean sound speed profiles and the energy of the internal wave fields. In turn, intensity fluctuations and temporal coherencies of broadband acoustic signals over several octaves are observed to vary with variations of the sound speed. For some locations, observations of mesoscale features alone can predict sonar performance.

From acoustic observatories to robust passive sonar

The evolution of the DARPA Robust Passive Sonar (RPS) as well as the ONR Shallow Water Acoustic Testbed (SWAT) programs can be traced from concept of an acoustic observatory posed by Munk in 1980 through several assessment and feasibility studies to their current implementations. During this, the thinking on several key hypotheses matured. (i) Are noise fields directional enough to sustain high array gains? (ii) What are the tradeoffs among nonstationarity caused by ship motion, array configuration (geometry and the number of sensors), and ‘‘snapshots’’ needed for stable adaptive processing? (iii) What is the interaction between gains from vertical and horizontal apertures? (iv) How much signal gain degradation is acceptable? (v) What methods of post-processing can be done for normalization, tracking, and 3-D localization? This presentation will give a brief summary of the history of RPS and SWAT and pose the question of how well we can answer some of hypotheses which motivated them.

Noise model beamforming

Adaptive beamformers are usually based on the premise that noise is stationary and Gaussian, and hence completely characterized by a covariance matrix. The standard approach then attempts to use measured data to estimate that covariance and form a set of beamformer weights that optimally rejects the estimated noise. This standard method may not be robust to mismatch between real noise and the noise estimate. In particular, low frequency passive sonar noise is usually nonstationary, owing primarily to the motion of the shipping noise sources. We investigate alternative robust noise models that allow for nonstationarity. One such model [W. A. Kuperman and F. Ingenito, J. Acoust. Soc. Am. 67, 1988–1996 (1980)] assumes a uniform probability distribution for surface noise sources. This produces an optimal beamformer that is robust in uniformly rejecting surface noise, no matter where sources occur on the ocean surface or how they move. The noise performance of this beamformer will be compared to standard adaptive approaches for array configurations proposed by ONR for the Acoustic Observatory Program. The high fidelity simulation model used for noise prediction includes nonstationary source motion and realistic range-dependent multipath propagation effects. [Work supported by DARPA.]

Some approaches to robust, snapshot-deficient, adaptive processing

We typically assume that longer arrays and more sensors are always beneficial. However, one of the issues identified by the ONR Ocean Acoustic Observatory Panel was ‘‘snapshot-deficient adaptive processing’’ arising, for example, from the motion of strong interferers (e.g., ships) through the small resolution cells of large arrays. This paper discusses two different but related concepts to reject the moving interferers: (1) null broadening for individual nulling and (2) model-based projection for collective nulling. The plane wave null broadening approach is combined with the robust white noise constraint and extended to matched field processing. The model-based projection exploits the fact that most interferers are close to the surface while targets of interest are usually in the middle of the water column. The projection matrix is constructed from the most significant eigenvectors of a replica covariance matrix from many independent sources within a surface strip. Simulations and experimental data illustrate the techniques.

1998–99 blue whale call abundance off the central California coast using a bottom-mounted hydrophone

Blue whale vocalizations were monitored off the central California coast during 1998–99 using the Naval Postgraduate Schools (NPS) Ocean Acoustic Observatory (OAO), a former US SOSUS array. The density of B calls were estimated using a matched filter detector based on a deverberated blue whale B call recorded from a towed array [T. C. Moore, Naval Postgraduate School, thesis (1999)]. The detectors’ performance characteristic was factored into the estimation to come up with the associated errors. Diurnal, seasonal, and inter-annular trends in call density were investigated. Also, the inter-call interval of the B call was measured. An unexpected large call density was observed during the first week of January in 1999. Historically, blue whales are sighted in Monterey Bay from approximately late July to October. Results of the blue whale call density estimation are discussed.

Detection and censusing of blue whale vocalizations along the central California coast using a former SOSUS array

To investigate the feasibility of automating the detection and censusing of blue whale vocalizations over a large coastal region using the Naval Postgraduate School (NPS) Ocean Acoustic Observatory (OAO), a four-day experiment was conducted along Central California in the summer of 1997. During the experiment, array data were archived continuously at the NPS OAO, a former SOSUS array. In addition to shore-based acoustic monitoring, an aircraft was assigned to locate blue whales in the Monterey Bay National Marine Sanctuary, and a research vessel, manned with observers and instrumented with a towed hydrophone array, was used to confirm locations of the blue whales and classify the vocalized near-field signals. The shipboard measurements were required to provide a means to separate the source signal characteristics from their multipath signatures. The towed array data were deverberated, source level and characteristics were estimated, and call-to-call variability was studied. Based on robust signal parameters, two simple autodetection correlation filters for the ‘‘A’’ and ‘‘B’’ blue whale calls were constructed, respectively. By applying the autodetection filters to the OAO data, the number of blue whale calls for the summer of 1998 was tallied. [Research supported by SERDP/ONR.]

A feasibility field study of monitoring blue whales using the Pt. Sur Ocean Acoustic Observatory

In the summer of 1997, two three-day experiments were conducted to test the feasibility of acoustically detecting, classifying, localizing, and tracking blue whales at long ranges using a former SOSUS listening array located at the Naval Postgraduate School Ocean Acoustic Observatory (OAO) at Pt. Sur, California. During each experiment, full-array data were archived continuously at the OAO. In concert with the shore-based acoustic monitoring, an aircraft was assigned to locate blue whales in the Monterey Bay National Marine Sanctuary and to direct a research vessel to a whale site. The research vessel was manned with observers and instrumented with a towed hydrophone array to ground-truth the locations of the blue whales and classify the vocalized near-field signals. These shipboard measurements were required to provide a means to separate the source signal characteristics from the multipath signatures for the calibration and validation of broadband, model-based localization methods. In this presentation, an overview of this feasibility field study is provided. Initial experimental as well as modeling results are presented, which include assessments of the predictability (i.e., variability) of the vocalized sound and the uniqueness of the location-dependent multipath structure, both are fundamental to the applicability of model-based algorithms. [Research supported by ONR.]

The nation’s fixed undersea surveillance assets—A national resource for the future

Following the Cold War, the Navy’s extensive undersea surveillance tracking net, SOSUS, provides an unparalleled opportunity for converting defense infrastructure into a civilian national asset. Designed to track submarines by their faint acoustic signals, SOSUS enjoyed tremendous success during the Cold War and was a major factor in the United States’ dominance in antisubmarine warfare. The SOSUS hydrophone arrays in both the Atlantic and Pacific face an uncertain future of shutdowns and closings. Yet those arrays and associated signal processing provide a unique acoustic ‘‘window’’ into the deep ocean—the volume of data collected is enormous. Over the last two years some limited experiments in tracking marine mammals, detecting seismic activity, and law enforcement have confirmed a tremendous civilian potential for this data. This paper reviews the history and characteristics of SOSUS, its potential for civilian use, and calls for the conversion of unused SOSUS assets into a National Acoustic Observatory. Using modern computers and networking technology, such an observatory could provide data from SOSUS arrays for research, law enforcement, and unparalleled synoptic pictures of marine mammal and seismic activity. However, to make this a reality will take the timely and cooperative efforts of defense, civilian agencies, and universities.