Laurent Fillinger

Stevens Passive Acoustic System for underwater surveillance

The Stevens Passive Acoustic System allows the detection, tracking and classification of various surface and underwater sources of sound including surface vessels, swimmers, various types of divers, and unmanned underwater vehicles. This system was developed by Stevens in its Maritime Security Laboratory, which was established to support research in the area of Anti-Terrorism and Force Protection. The focus of this lab has been the persistent detection and classification of threats posed by surface and subsurface intruders utilizing a multiplicity of technologies. Using these capabilities, we have investigated the set of acoustic parameters fundamental to underwater acoustic threat detection, including diver acoustic signatures, acoustic transmission loss, and acoustic environmental noise. The Stevens Passive Acoustic System has successfully demonstrated surface ship detection and classification. The system provides simultaneous acquisition and analysis of acoustical signals using 4 hydrophones. The analysis functions includes arbitrary digital filtering, spectral analysis and cross-correlation for simultaneous processing of signals from several hydrophones, acoustical source separation, and determination of bearing for different targets relative to the central underwater mooring. The system also records and stores the complete raw acoustical data set, enabling further research and analysis of the acoustic signals. Novel acoustic methods of signal processing used in the system include: 1. Method of precise hydrophone localization. 2. Cross-correlation method for target bearing determination 3. Cross-correlation methods for extraction of target signatures from numerous sources. 4. Feature-based automated diver detection algorithm. 5. Measurements of the ship noise modulation spectrum that is related to propeller and shaft rotation (Detection of Envelope Modulation on Noise — DEMON method). The Stevens Passive Acoustic System has been used in tests in the Hudson River and NY Harbor where a large library of ship acoustic signatures has been collected. Several US Navy sponsored trials demonstrated the ability of the Stevens system for effective diver detection at distances up to 700m.

Wideband nonlinear time reversal seismo-acoustic method for landmine detection

Acoustic and seismic waves provide a method to localize compliant mines by vibrating the top plate and a thin soil layer above the mine. This vibration is mostly linear, but also includes a small nonlinear deviation. The main goal of this paper is to introduce a method of processing that uses phase-inversion to observe nonlinear effects in a wide frequency band. The method extracts a nonlinear part of surface velocity from two similar broadcast signals of opposite sign by summing and cancelling the linear components and leaving the nonlinear components. This phase-inversion method is combined with time reversal focusing to provide increased seismic vibration and enhance the nonlinear effect. The experiments used six loudspeakers in a wood box placed over sand in which inert landmines were buried. The nonlinear surface velocity of the sand with a mine compared to the sand without a mine was greater as compared to a linear technique.

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.

Acoustic ship signature measurements by cross-correlation method

Cross-correlation methods were applied for the estimation of the power spectral density and modulation spectrum of underwater noise generated by moving vessels. The cross-correlation of the signal from two hydrophones allows the separation of vessel acoustic signatures in a busy estuary. Experimental data recorded in the Hudson River are used for demonstration that cross-correlation method measured the same ship noise and ship noise modulation spectra as conventional methods. The cross-correlation method was then applied for the separation of the acoustic signatures of two ships present simultaneously. Presented methods can be useful for ship traffic monitoring and small ship classification, even in noisy harbor environments.

Towards a passive acoustic underwater system for protecting harbours against intruders

The international rules for the protection of harbors against threats and intruders typically apply to threats emanating from the land side. Protection against actions and threats from the water side is much less regulated, apart from some individual cases. Potential threats from the water side may consist of intruders including divers with and without delivery vehicles, swimmers, small submarines, small vessels or unmanned underwater vehicles. To increase the level of readiness against underwater threats in harbors, the Netherlands Organization for Applied Scientific Research (TNO) and the Stevens Institute of Technology have recently combined their long-standing expertise on observation systems and sonar technology and are developing an integrated system that can be used to protect harbors from both surface and underwater intruders. In the fall of 2010, TNO and Stevens will initiate joint experiments in a harbour in The Netherlands, where passive acoustic systems from both parties will be combined. In this paper, potential performance of the experimental system is illustrated via simulations. Initial experimental results will be presented at the conference.

Cross‐correlation of ship noise for water traffic monitoring.

Various aspects of the monitoring of ship traffic using correlation of signals recorded by a pair of hydrophones are considered and demonstrated on real signals recorded in the Hudson River. The underwater acoustic noise generated by ships reaches the various hydrophones with a delay depending on their relative positions. That delay can be extracted by cross‐correlation and can serve as a basis for determination of the direction of the ship. This method allowed finding directions for several ships in heavy traffic of Hudson River. The vessel triangulation can be done using information from two or more appropriately located hydrophone pairs. Another application demonstrated is separation of the acoustic signature (noise spectra) from several ships. The last application is the estimation of the ship noise modulation spectrum that is related to propeller and shaft rotation by means of short time cross‐correlation. Comparison with the conventional detection of envelope modulation on noise method shows a close...

Automated passive acoustic intruder detection based on correlation approach.

We consider the problem of passive acoustic detection of intruders in shallow waters using the signals acquired from multiple hydrophones. The suggested approach uses correlation of the acoustic signals recorded by different hydrophones as well as other features such as periodicity. Cross correlation of signals is conducted within a sliding window after passing through a multiband filter bank. This multiband filtering allows leveraging the intrinsic spectral properties of emitted signals to discriminate intruder emission from acoustic noise sources such as ship traffic typically present in the river. Further heuristic processing exploits the periodic nature of signals such as human breathing, by detecting the beginnings and endings of the diver’s inhalation signals. The detector assesses whether the recognized features are consistent with previously documented breathing rates in order to automatically and robustly detect the presence of a diver. The developed approach was applied in diver detection tests ...

Time Reversal Acoustic focusing with random reverberator

Time Reversal Acoustic (TRA) focusing system based on an external reverberator can provide good focusing with minimum side lobes even with a few transducers. The focusing ability of such system can be increased using reverberator with rough boundaries or by adding internal random scatterers. The experiments were conducted with a reverberator made of a polyethylene bottle filled with water and a single piezoceramic disk attached externally to its wall. Experiments demonstrated that inserting in the bottle random hard scatterers or altering of the bottle surface significantly decreases the size of the focused spot and the level of side lobes. The new pseudo‐impulse method of TRA focusing allowed suppressing piezotransducer resonances and provided formation of short wide band signals. The ability of such TRA focusing system based on random reverberator to form simultaneously multiple focuses and produce focal spots of the complex shape has been demonstrated. The application of binary radiation regime led to several fold increasing of the amplitude of the focal spot while the focal structure practically was not affected. Applications of such random focusing system for medical imaging and HIFU treatment is discussed.

Detection of broadband sound sources using a randomly spaced linear array

Arrays are commonly used in relatively narrow band applications. The array design and processing are usually optimized in order to maximize the gain in the look-direction, while minimizing the interference from sources located in other directions (but possibly not far off from the look-direction). This led for instance to the definition of minimum redundancy linear arrays that maximize the resolution for a given number of sensors. This maximization is, however, effective only at the design frequency of the array. For many active applications (such as active sonar or radar), which are narrow band in essence, this limitation is not an issue. However, if the signal to be detected in its entirety has a broadband frequency content, optimizing the performance of the array at a given frequency leads to possibly poor performance at other frequencies. Since optimal performance cannot be achieved at all frequencies simultaneously, one can aim for sub-optimal but useful performance throughout the entire frequency band. One possibility for achieving this is based on the use of a random hydrophone spacing. The effectiveness of the approach is investigated using simulations and is illustrated with experimental data collected using a randomly spaced linear array.

Pile driving noise: Source level and sound generation mechanisms.

Offshore percussive pile driving generates loud impulsive sounds in the marine environment, sometimes characterized in terms of their “source level,” a measure of acoustic pressure in the far field of its source. Special difficulties with the definition of source level for this application are described. Environmental assessment of future building projects requires a proper understanding of the sound generation mechanisms and their dependence on the various parameters (e.g., hammer type, stroke energy, pile construction, and sediment properties). Progress is reported on a combined numerical and experimental approach to modeling the sound generation and propagation through the medium. The acoustic response of the pile and its near field in fluid and sediment are calculated with a finite element model. The far field is obtained by coupling the acoustic field near the pile, from the finite element model, to a horizontal wavenumber integration model. Measurements carried out on a test pile in shallow water at...