Acoustic Blind Deconvolution and Frequency-Difference Beamforming in Shallow Ocean Environments

Synthetic time reversal (STR) is a method for blind deconvolution that is based on the time-reversal symmetry of acoustic fields. It can be used to estimate an unknown acoustic source’s broadcast waveform in an unknown multipath environment from remote transducer array recordings. This presentation will review the formulation of STR, and its use in ocean acoustics and non-destructive evaluation. In particular, experimental results are presented for the remote acoustic detection of mechanical changes to a vibrating 0.3-m-square by 3-mm-thick aluminum plate in a reverberant environment. The plate has nominally clamped edges and is subject to swept-frequency base excitation from 100 to 2,000 Hz. Sound radiated from the plate with and without a mechanical change is recorded remotely with a 15-microphone linear array and processed using STR to reconstruct a single radiated-sound signal that is deconvolved from the environment’s unknown reverberation response at signal-to-reverberation ratio levels of -7 to -13 dB. The corrected signals are processed to detect boundary clamping defects and various length cuts in the plate via standard statistical baseline comparison techniques. Detection results using STR are superior and more robust to geometrical uncertainties than equivalent results from conventional approaches. [Sponsored by ONR and NAVSEA through the NEEC]

Two high resolution beamformers, maximum entropy method (MEM) and Wagstaff‐Berrou broadband (WB2) [R. A. Wagstaff and J. Berrou, J. Acoust. Soc. Am. 75, 1133–1141 (1984)], were compared with each other and the standard FFT beamformer to determine performance degradation when the number of line array elements and the signal‐to‐noise ratio (SNR) were decreased. Experimental data from a multi‐tonal CW source recorded on an equally spaced horizontal line array were used to investigate the main beam broadening resulting from decreased SNR for SNR values ranging from 30 dB to near zero dB. Additional CW data from an equally spaced vertical line array were used to investigate the main beam broadening which resulted when the number of array elements was decreased successively from 31 to 16, 8, 4, and finally 2. Preliminary results indicate that the main beam broadening due to the decrease in array elements for both MEM and WB2 are similar and that both of these methods offer an order of magnitude decrease in the −3 dB beamwidth with respect to the standard FTT beamformer. [Work supported by Naval Electronic Systems Command, Code 612.]

Acoustic time reversal is a promising technique for spatial and temporal focusing of sound in unknown environments. Acoustic time reversal can be implemented with an array of transducers that listens to a remote sound source and then transmits a time-reversed version of what was heard. In a noisy environment, the performance of such a time-reversing array (TRA) will be degraded because the array will receive and transmit noise, and the intended signal may be masked by ambient noise at the retrofocus location. This article presents formal results for the signal-to-noise ratio at the intended focus (SNRf) for TRAs that receive and send finite-duration broadband signals in noisy environments. When the noise is homogeneous and uncorrelated, and a broadcast power limitation sets the TRA's electronic amplification, the formal results can be simplified to an algebraic formula that includes the characteristics of the signal, the remote source, the TRA, and the noisy environment. Here, SNRf is found to be proportional to the product of the signal bandwidth and the duration of the signal pulse after propagation through the environment. Using parabolic-equation propagation simulations, the formal results for SNRf are illustrated for a shallow water environment at source-array ranges of 1 to 40 km and bandwidths from several tens of Hz to more than 500 Hz for a signal center frequency of 500 Hz. Shallow-water TRA noise rejection is predicted to be superior to that possible in free space because TRAs successfully exploit multipath-propagation.

The environmental parameters are usually uncertain in complex shallow ocean environment and restrict the performance of the matching model-like method. Therefore, we need a more tolerant detection method for detecting underwater targets in the uncertain shallow ocean environment. The previous mode-subspace detection method has the characteristics of both high performance and robustness. However, the robust mode-subspace detector is suitable for vertical arrays and its performance is limited by shallow ocean environment. Therefore, we propose the tolerant detection method for estimating the robust signal subspace with horizontal arrays. We estimate the robust signal subspace by bringing uncertain parameters into the observation matrix of a horizontal array. Combined with the robust signal subspace estimation, we propose a subspace detector that tolerates uncertain parameters. The results on simulation in a uncertain shallow ocean environment show that the detector we proposed has a high average detection capability and a certain tolerance for uncertain parameters.

Acoustic diffraction allows sound to travel around opaque objects and therefore may allow beyond-line-of-sight sensing of remote sound sources. This paper reports simulated and experimental results for localizing sound sources based on fully shadowed microphone array measurements. The generic geometry includes a point source, a solid 90° wedge, and a receiving array that lies entirely in the shadow defined by the source location and the wedge. Source localization performance is assessed via matched-field (MF) ambiguity surfaces as a function of receiving array configuration, and received signal-to-noise ratio for the Bartlett and minimum variance distortionless (MVD) MF processors. Here, the sound propagation model is developed from a Green's function integral treatment. A simple 16 element line array of microphones is tested in three mutually orthogonal orientations. The experiments were conducted using an approximate 50-to-1-scaled tabletop model of a blind city-street intersection and produced ambiguity surfaces from source frequencies between 17.5 and 19 kHz that were incoherently summed. The experimental results suggest that a sound source may be localized by the MVD processor when using fully shadowed arrays that have significant aperture parallel to the edge of the wedge. However, this performance is reduced significantly for signal-to-noise ratios below 40 dB.

The Environmental Risk Management Capability - Sonar (ERMC(S)) has now been accepted into service by the UK Royal Navy. ERMC(S) is the first operational software package that provides a comprehensive risk assessment of the common concern of the potential impact of sonar usage on marine mammals. The algorithms include allowance for extended periods of use and advice on achievable mitigation options. Now that the system has achieved this significant milestone, this paper reviews the elements of the system that have made progress to this stage so successful: -The techniques and data used by the algorithms. -The inclusive development approach that allowed all the concerned stakeholders to be satisfied with the processes utilised. -The annual review cycle that will enable the tool to maintain its links with forefront research in this area. -The application of the system to non-military sound sources as the benchmark approach. This paper will show how ERMC(S) can be develop into a tool for both commercial and military sound sources using proving and established techniques.

The number of marine watercraft is on the rise-from private boats in coastal areas to commercial ships crossing oceans. A concomitant increase in underwater noise has been reported in several regions around the globe. Given the important role sound plays in the life functions of marine mammals, research on the potential effects of vessel noise has grown-in particular since the year 2000. We provide an overview of this literature, showing that studies have been patchy in terms of their coverage of species, habitats, vessel types, and types of impact investigated. The documented effects include behavioral and acoustic responses, auditory masking, and stress. We identify knowledge gaps: There appears a bias to more easily accessible species (i.e., bottlenose dolphins and humpback whales), whereas there is a paucity of literature addressing vessel noise impacts on river dolphins, even though some of these species experience chronic noise from boats. Similarly, little is known about the potential effects of ship noise on pelagic and deep-diving marine mammals, even though ship noise is focused in a downward direction, reaching great depth at little acoustic loss and potentially coupling into sound propagation channels in which sound may transmit over long ranges. We explain the fundamental concepts involved in the generation and propagation of vessel noise and point out common problems with both physics and biology: Recordings of ship noise might be affected by unidentified artifacts, and noise exposure can be both under-and over-estimated by tens of decibel if the local sound propagation conditions are not considered. The lack of anthropogenic (e.g., different vessel types), environmental (e.g., different sea states or presence/absence of prey), and biological (e.g., different demographics) controls is a common problem, as is a lack of understanding what constitutes the 'normal' range of behaviors. Last but not least, the biological significance of observed responses is mostly unknown. Moving forward, standards on study design, data analysis, and reporting are badly needed so that results are comparable (across space and time) and so that data can be synthesized to address the grand unknowns: the role of context and the consequences of chronic exposures.

Simple SummaryMarine mammal welfare research of professionally managed species has primarily focused on enrichment, habitat usage, and activity, as well as the impacts of human-oriented training sessions. However, the importance of sound in the welfare of marine mammals has rarely been mentioned. In this review, methods for acoustic welfare monitoring are discussed, including hearing tests, the incorporation of listening systems to monitor noise and communication, and cataloguing vocalizations in various health contexts. Examples from the US Navy Marine Mammal program are provided, as well as opportunities for facilities to initiate acoustic welfare monitoring. Suggested future directions of study, such as research examining the impact of sound on cognition, are also discussed.Research evaluating marine mammal welfare and opportunities for advancements in the care of species housed in a professional facility have rapidly increased in the past decade. While topics, such as comfortable housing, adequate social opportunities, stimulating enrichment, and a high standard of medical care, have continued to receive attention from managers and scientists, there is a lack of established acoustic consideration for monitoring the welfare of these animals. Marine mammals rely on sound production and reception for navigation and communication. Regulations governing anthropogenic sound production in our oceans have been put in place by many countries around the world, largely based on the results of research with managed and trained animals, due to the potential negative impacts that unrestricted noise can have on marine mammals. However, there has not been an established best practice for the acoustic welfare monitoring of marine mammals in professional care. By monitoring animal hearing and vocal behavior, a more holistic view of animal welfare can be achieved through the early detection of anthropogenic sound sources, the acoustic behavior of the animals, and even the features of the calls. In this review, the practice of monitoring cetacean acoustic welfare through behavioral hearing tests and auditory evoked potentials (AEPs), passive acoustic monitoring, such as the Welfare Acoustic Monitoring System (WAMS), as well as ideas for using advanced technologies for utilizing vocal biomarkers of health are introduced and reviewed as opportunities for integration into marine mammal welfare plans.

Minimizing and mitigating the potential effect of sound upon the environment is an increasing concern for many activities. Naval operations, seismic exploration, vessel and aircraft operations, and scientific investigations now need to consider the potential effect of underwater acoustic sources. Marine mammals are usually the primary concern, due to their widespread distribution and excellent hearing. Predicting the exposure of marine mammals is complicated by their diving behavior, which causes them to 'sample' many depth strata within the water column. Acoustic propagation and sound received levels are a function of depth as well as range. The Acoustic Integration Model (AIM) addresses this specific complication. A principal component of the central engine of AIM is a movement simulator. Both sound sources and animals, collectively addressed as 'ANIMATS', are programmed to move in location and depth over time in a realistic function. Animal movement is based on documented regional and seasonal behavioral data for each species generated. Acoustic sources and receivers are programmed to move through a virtual acoustic environment based on external environmental databases and radiated sound fields created from a choice of several propagation models. The integration component of the AIM engine then predicts the exposure level of each simulated animal at successive operator-selected time steps. Furthermore, each animat can evaluate its environment at each time step, and can be programmed to alter direction or diving behavior in response to any variable, such as sound level or sea depth. The model therefore allows the user to predict the effects of different operational scenarios and animal response levels, thereby allowing the selection of the alternative that produces the least impact and still meets operation requirements.

Formulation of the problem. Many electronic applications (for example, satellite communications) impose significant restrictions on the weight and dimensions of the antenna devices used. In addition, the antennas used should have the lowest possible cost and simplicity of design. To solve these problems, it is proposed to use a method for reducing the number of antenna elements in phased antenna arrays based on the method of point sources. Purpose. The purpose of this work is to develop a method for reducing the number of antenna elements in phased antenna arrays while maintaining the parameters of their radiation patterns. Results. As a solution, it is proposed to place the antenna elements above the metal sheet at a distance 1.5–2 times smaller than the size of the antenna array itself, which will significantly reduce the number of elements of the latter without deteriorating its radiation pattern during wide-angle two-coordinate scanning of space with the main lobe. Practical significance. Obtained results can be used to create phased antenna arrays with a reduced number of elements, which will reduce the size, weight and cost of the device.

The aperture of the towed horizontal line array is limited and the received signal is unstable in a complex ocean environment, making it difficult to distinguish the location of the sound source. To address this challenge, this paper presents a MoELocNet (Mixture of Experts Localization Network) for deep-sea sound source localization, leveraging interference structures in range-frequency domain signals from a towed horizontal line array. Unlike traditional correlation-based methods constrained by time-varying ocean environments and low signal-to-noise ratios, the model employs multi-expert and multi-task learning to extract interference periods from single-frame data, enabling robust estimation of source range and depth. Simulation results demonstrate its superior performance in the deep-sea shadow zone, achieving a range localization error of 0.029 km and a depth error of 0.072 m. The method exhibits strong noise robustness and delivers satisfactory results across diverse deep-sea zones, with optimal performance in shadow zones and secondary effectiveness in the direct arrival zone.

The oil and gas industry uses some exploration and production technologies that produce high levels of underwater sound, such as seismic surveys, underwater blasting for demolition and construction, and offshore piling. These underwater noise sources have the potential to impact marine species, which are usually reliant on sound instead of light as their primary sense for communication and sensing their environment. Regulatory interest in minimising the impacts of underwater noise on marine fauna is increasing. This paper presents a methodology for assessing these environmental impacts, with particular focus on cetaceans (whales and dolphins) and pinnipeds (seals and sea lions), although it can easily be adapted to other marine mammal species and fishes. It requires input from a variety of fields, such as: underwater acoustics for sound propagation modelling and source noise characterisation; marine bio-acoustics for determining the effects of sound on marine species’ hearing and communication; and marine ecology for identifying the marine species that may be affected and assessing the biological importance of noise-affected marine areas. These inputs are used in a risk assessment to assess the likely impacts of underwater noise on marine species, which is a collaborative effort by specialists in the fields of underwater acoustics, marine bio-acoustics and marine ecology.

Human activities in the marine environment coupled with the efficient propagation of acoustic waves, have significantly increased ocean noise levels. This rise poses a threat to marine biodiversity, especially impacting vocal-dependent marine mammals. Therefore, obtaining a more comprehensive understanding of the complex ocean soundscape, shaped by both natural and anthropogenic sources, is essential.Ocean-bottom seismometer (OBS) datasets present an opportunity to explore the marine acoustic noise field. OBS hydrophones can be used to effectively study regional marine soundscapes. Additionally, the three-component OBS seismometers capture acoustic-to-seismic conversions from the water column, enabling the detection and localization of acoustic events.Through the analysis of OBS hydrophone recordings from the NE Atlantic offshore Ireland, we develop a frequency-dependent empirical sound propagation model for the area. The model provides information on (1) the spectral amplitude decay of sound sources in the region and (2) the distance between OBSs and acoustic sources.To construct the model, we consider ship noise from individual vessels recorded by OBSs at known distances from the source, using Automatic Identification System (AIS) ship-tracking data. As a ship moves away from its closest point of approach (CPA) to an OBS, the hydrophone records the evolution of the frequency-dependent ship's acoustic signal spectral amplitude with distance. This enables us to compute amplitude decay curves for narrow frequency bands from different vessel signals recorded at various OBSs. We apply a signal-to-noise ratio (SNR) quality control criterion and consider ships sailing along 'straight' paths at a constant speed (< 2 kn variation). After normalizing and scaling the CPAs of the different curves to a reference, we stack them for each frequency band and compute polynomial fits for the stacked amplitude decay curves. We analyse the number of vessel tracks needed to build a reliable empirical model. Numerical simulations are computed to further understand the acoustic propagation properties from our empirical model.The goal is to investigate (1) how spectral amplitudes at different frequencies behave with distance from the sound sources, (2) the efficiency of frequency-dependent sound propagation in the ocean in regions with variable seafloor morphologies, and (3) the potential use of the empirical model to obtain distances to other sound sources with unknown locations, such as Baleen whales. To achieve this, we compute ratios of spectral amplitudes for pairs of different frequency bands using the fitted amplitude decay curves. We test the model's ability to locate other vessels with known AIS positions before using it to locate different sound sources with unknown positions. Locating a sound source employing the distance provided by the empirical model is valuable when OBSs are too far apart for 'traditional' multi-sensor methods. In such cases, the incoming direction of the sound source is determined by rotating the horizontal-component OBS seismic data to point to the source back-azimuth. Also, the amplitude decay information provided by the model has multiple applications in other areas where noise pollution may be a concern (e.g., marine infrastructure construction).

The SCORE site, located near San Clemente Island, is known to be abundant in marine mammals, and is the site of frequent naval operations, making it an ideal site to study what impact man-made activities may have upon these creatures. In August 2004, an experiment was performed to collect both visual and acoustic observations of odontocetes at the SCORE site. Odontocete vocalizations consist of clicks and whistles. The clicks are very wideband (often 100 kHz), so times of arrival can be measured with extremely high resolution, which makes clicks attractive waveforms for multipath ranging and depth estimation. Although the whistles are narrowband, their fundamental frequency often ranges over tens of kilohertz, so these too can be processed as if they were wideband. We will present results of processing excerpts from several weeks of data recorded on a high-frequency horizontal line array deployed from the FLIP research vessel. Bearing tracks of individuals and groups from a number of different species will be correlated and compared with visual observations. In addition, we will discuss the possibilities for range and depth estimates derived from the multipath arrivals observed in the clicks and whistles.

The impact of anthropogenic sound on the marine ecosystem has become an area of concern and subsequently increased research. As part of the Atlantic Deep Water Ecosystem Observatory Network (ADEON, adeon.unh.edu), a five-year collection effort of acoustic, oceanographic, marine ecosystem, and remote sensing satellite observations is underway. The centerpiece of the network is 6 long-term landers deployed off the coast of Jacksonville up past the mouth of the Chesapeake Bay. In this paper we present results from the first (of four) horizontal line array measurement cruises (deployed from a sailboat) between two of the moorings. The scientific objective is to evaluate the horizontal directionality of the ambient sound field and to measure the spatial correlation of the noise between the lander off of Wilmington NC and the lander off of the Virginia. A soundscape model, including environmental and source level variability will be presented. The acoustic sources will include ships, wind, rain, and marine mammals.