Underwater non-lethal weapon based on principles of time reversal acoustics

Transcription

1 Underwater non-lethal weapon based on principles of time reversal acoustics Alexander Sutin, and Yegor Sunelnikov Citation: Proc. Mtgs. Acoust. 13, (2011); View online: View Table of Contents: Published by the Acoustical Society of America Articles you may be interested in Partial dereverberation used to characterize open circuit scuba diver signatures The Journal of the Acoustical Society of America 136, 623 (2014); / Passive acoustic detection of closed-circuit underwater breathing apparatus in an operational port environment The Journal of the Acoustical Society of America 132, EL310 (2012); /

2 Proceedings of Meetings on Acoustics Volume 13, XVI International Conference on Nonlinear Elasticity in Materials Prague, Czech Republic 5-11 June 2011 Physical Acoustics Underwater non-lethal weapon based on principles of time reversal acoustics Alexander Sutin* and Yegor Sunelnikov *Corresponding author s address: Davidson Lab, Stevens Institute of Technology, 711 Hudson, Hoboken, NJ 07030, asutin@stevens.edu 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 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 modelled taking into consideration the frequency range, harbour depth, geometry of the transducer array. It is demonstrated that outside the focal region, the TRA system produces acoustic intensities that are not harmful for marine life. Published by the Acoustical Society of America through the American Institute of Physics 2013 Acoustical Society of America [DOI: / ] Received 11 Oct 2011; published 14 Jan 2013 Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 1

3 1. Introduction. One of the most challenging aspects of port security is to ensure protection against surface and underwater threats. In particular, significant terrorist threat might be posed to domestic harbors in the form of an explosive device delivered underwater by a scuba diver. In general, surface threats can be detected by radar and infrared/optic surveillance systems, leaving the detection of underwater targets to the application of acoustic systems. There are numerous commercial diver detection sonar systems in production [1-4] and active sonar is one of the major components of the Integrated Anti-Swimmer System (IAS) used by the U.S. Coast Guard [1]. Along with improving diver detection technology, the questions arises regarding how to respond to underwater diver threats once they are detected. Historically, grenades and small arms, as well as low frequency active sonars, have been utilized against subsurface threats. Explosives are effective but lethal. The Coast Guard response tactics are guided by the Commandant s Use of Force Policy [1]. Simply put, that as the tactical situation warrants U.S. Coast Guard units must enter the use of force continuum at the lowest level possible to compel compliance. This is similar to civil law enforcement policies, where police must first identify themselves as law enforcement officials; before firing warning shots and before applying deadly force. The underwater loudhailer provides the means to identify themselves and warn potential intruders that they are violating a security zone. The Coast Guard is working with the Joint Non-Lethal Weapons Directorate to develop additional response tools to encourage or force compliance while using less-than-lethal force measures. The most detailed research of application of intensive acoustic waves for swimmer deterrent was conducted by Applied Research Laboratory, Texas [5]. Currently the air guns and sparker sources are applied in integrated anti-swimmer detection systems [6,7]. Parameters of several such systems are presented in the Table 1. Table. 1. Acoustic source level of several impulse systems that can be used as non lethal weapon. Name of the system Source level References Bolt PAR 600B air-gun 222 db re 1 Pa at 1 m [8] Squid 2000 & Delta Sparker 222 db re 1 Pa peak to peak at 1 m [9] Seismic Sound Source AA301 Seismic Boomer 215 db re 1 Pa peak to peak at 1 m [10] Sound Source. Underwater sparker with a parabolic acoustic reflector 240 db re 1 Pa peak to peak at 1 m [11] These impulse systems can produce acoustic signal in order of 180 db re 1 Pa at a distance of 100 m that initiates diver discomfort. The main disadvantage of these systems is that they radiate acoustic impulse to all directions and disturb marine life around. They also can negatively affect friendly divers in the area of deployment. To avoid collateral damage to marine life, the focusing of acoustic energy in a tight focal zone around the target becomes most critical. This paper discusses possible application of Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 2

4 the Time Reversal Acoustic (TRA) principles to enable precision targeting of a hostile diver with intense focused sound and minimal impact to the marine life. The two possible ways for the TRA focusing is considered. The first method utilizes an acoustic noise radiated by a diver to focus acoustic energy back to the diver. The diver acts as an active self-disclosing acoustic beacon. The second method is based on application of powerful ship sonar and acoustic signal reflected from a diver is used for the TRA focusing back to the target. This paper extends the results presented in our first paper that describes the first method [12] and presents estimations for possible application of powerful ship sonar for diver deterrent using the second method. 2. Concept of the TRA Systems for Non lethal Swimmer Neutralization using diver radiation The recently developed TRA principles are among the hottest topics in modern acoustics. The TRA opens a way for focusing the acoustic signal in highly inhomogeneous media in the presence of strong reverberation and scattering. Dr. Fink, the inventor of the TRA method, recently demonstrated the excellent ability of TRA to provide spatial control and focusing of an acoustic beam in an inhomogeneous media using very simple means [13-14]. In time-reversal acoustics, a signal, radiated from a source, is recorded by an array of transducers, timereversed and then re-transmitted into the medium. The re-transmitted signal propagates back through the medium and focuses on the source of radiation. The concept of application of diver generated signal for TRA focusing of the same signal back to a diver is shown in Fig. 1. TRA electronic Unit Power Amplifier Array of hydrophones and emitters for TRA system Figure 1. Concept of the TRA system for non-lethal swimmer neutralization using diver radiation. An array of hydrophones listens to the acoustic noise. After filtering the recorded diver sound is extracted, time reversed, amplified and focused back to the diver in real time. The response time of such focusing is on the order of tenth of a second for distances up to 300 m. In this process the diver acts as an acoustic beacon allowing precise spatial focusing of underwater sound back to a moving diver. The suggested interdiction TRA system consists of several receiver- emitter units that pick up diver sound and radiate it back.. According to the paper [15] the acoustic source level of the diver can reach 179 db re 1 Pa at 1m. Stevens institute of Technology conducted numerous tests of passive acoustic diver detection [16]. These researches demonstrated that diver acoustic signals are very different from acoustic signal generated by marine life. There are no fish and Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 3

5 marine animal breathing with the similar rate at human and narrow band frequency components generated by scuba diver equipment are not typical for marine life. Diver acoustic signature recorded in a joint test with TNO (Holland) is presented in Fig. 2. [17]. Time (s) Figure 2. Spectrogram of acoustic signature generated by diver during test in Den Helder (Holland) [17]. The TRA system uses the acoustic signature radiated by the swimmer for time reversal refocusing of this signal. The system works using the following sequence of TRA steps: Step 1. Recording of the acoustic signal generated by a diver. Signals generated by a diver are recorded by all hydrophones and sent to a TRA electronic unit for further processing. Step 2. Filtering, amplification, time reversal, and normalization of the recorded signals. The signals recorded by each hydrophone are filtered in the frequency band defined by known diver signature in the frequency band of 1-10 khz. The filtered signals are normalized, time reversed and saved in the TRA electronic unit. Step 3. All prepared TRA signals are radiated simultaneously from powerful underwater transducers. The radiated signals undergo multiple reverberations and converge at the initial point of radiation (e.g. the diver). Furthermore, the TRA focused signal can be increased using binary mode of radiation. In this mode the all positive points of TRA signal prepared for radiation are changed to their maximal value and all negative points to their minimal value. The examples of the radiated and TRA focused signals in the frequency band between 1 and 3 khz are presented in Fig. 3. Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 4

6 Standard TRA Amplitude TRA radiated wave Time (ms) Amplitude Waveform in the TRA focus Time (ms) Binary mode Amplitude Time (ms) Amplitude Time (ms) Figure 3. Waveforms of radiated and TRA focused signals for standard and binary regime of radiation. We developed a computer model to investigate the feasibility of TRA focusing and estimate an effective zone of the diver deterrence in a shallow sea model. Wide band noise signals radiated by a diver were numerically traced trough a generalized sea model with inclined see floor that acted as a waveguide, where emitters were located at the shallow region. Calculations were performed in a time domain at 100 khz Nyquist frequency. The details of the model were published in [12]. The TRA system was placed near shore where the water depth was about 5 m. The inclination of the bottom was assumed about 2.5º. Figure 4. SPL in the vicinity of a diver for the TRA system with 20 underwater emitters. Focusing was constructed using in the frequency range of khz (top) and 2-4 khz (bottom). Two-dimensional maps of Sound Pressure Level (SPL) were calculated at various distances, 50 m to 200 m from emitters to diver. Fig. 4 shows the SPL in the Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 5

7 vicinity of a diver at distance of 50 m from the TRA system with 20 underwater emitters, with acoustic power of 100 W each, randomly distributed within approximately two meters from each other. The horizontal axis is in the direction the TRA system to a diver, with zero corresponding to diver location 50 m (in X axis) away from the system, vertical axis is water depth. Top and bottom panels show TRA focusing field constructed using random acoustic emission in the frequency range of khz and 2-4 khz respectively. Left panels show field structure in a transverse direction, right panels show field structure in along the propagation direction. The colorbar provides a scale in db re 1 Pa. Model calculation demonstrates that it is feasible to achieve good localization and amplification of the acoustic emission derived diver deterring signal at distances up to 200 m using multiple sonar sources. Binary TRA produces sharp and intense focusing of acoustic energy. 3. Concept of the TRA Systems for Non lethal Swimmer Neutralization based on application of powerful ship sonar Another practical way for realization of TRA acoustic method of diver deterrent is implementation of deterring equipment that is permanently installed on a ship. For example, the powerful submarine detection sonar is capable of producing sufficient acoustic energy to make a diver in proximity of that sonar uncomfortable. The TRA focusing of such sonar emission shall substantially extend the region of diver deterrence and enable precise diver targeting thus spearing the marine live around. The NANY sonar AN/SQS-56 works at frequency of 3.75 khz [18] and it is unlikely that its SPL could be less than 220 db relative to 1 Pa at 1 m. Moreover, the sonar emission frequency is close to the peak sensitivity of human ear (3 khz) thus presumably requiring less sonar power for creating a diver hearing discomfort. External emitter Figure 5. The schema of the diver TRA deterrent based on application of the ship sonar. The schema of the diver TRA deterrent based on application of the ship sonar is shown in Fig. 5. An interrogation signal is produced by an external low power emitter. Reflected from the diver signal is time reversed, filtered, amplified, and sent back to the diver. The advantage of external emitter is that it limits the need for radiation from powerful ship sonar which is activated only for diver deterrent. Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 6

8 The shallow sea model constituted a homogeneous water layer restricted by two planar reflection surfaces (surface and bottom) with the reflection coefficients close to one [12]. The ship sonars were positioned at depth of 5-10 m. Bottom inclination angle was assumed 2.5 degrees. For the considered distances below 300 m and frequency range up to 4 khz the sound attenuation was neglected. The ship sonar was positioned in the shallow portion of the sea model, while the diver was located at the deeper portion of the channel. a b c Radiated and received signals are shown in the Fig. 6. First, short 5 ms sweep signal is radiated by the external low power emitter (blue) and corresponding acoustic signal at the diver location (green) is shown in panel a. In panel b the blue color shows the signal reflected from diver and green shows the signal Time (ms) Figure 6. Modeled signals in process of diver deterrent. received back by the sonar. Second, after time reversal received signal is amplified, sent into the sea (blue) and focuses on the diver (green) as shown in panel c. Corresponding signals for binary regime of focusing are demonstrated in panel d. All signals are normalized for the ease of visualization. In order to estimate the pressure gain of TRA scheme over a conventional signal the SPL in the focal area was calculated at various distances between the sonar and diver ranging from 50 to 300 m. Attenuation of focal acoustic pressure with distance is shown in Fig. 7a for two signals: direct signal radiated by sonar, shown by red diamonds, and binary d Pressure (db re 1 Pa) Direct sonar signal TRA focused signal Distance (m) a Depth (m) Distance (m) b Figure 7. a - SPL of direct radiated signal coming to a diver and TRA focused binary signals as a function of distance to the diver; b - Peak acoustic pressure in db re 1 Pa of binary TRA deterring pulse at 100 m. time reversed focused signal, shown by blue squares. For the ease of comparison both Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 7

9 signals assumed source sonar acoustic pressure of 220 db relative to 1 Pa at 1 m. There is a substantial 15 db gain of the TRA acoustic pressure compared to direct signal coming to a diver. Within the limitation of current model, this gain remains constant over a considered range of distances from the ship, indicating that sufficient sound focusing can be achieved near the ship and relatively far from the ship. The distribution of binary TRA deterring pulse peak acoustic pressure at 100 meters is illustrated in Fig. 7b. 4. Conclusion It is feasible to use the TRA focusing for diver deterrent without significant disturbance to the marine life, which is not otherwise attainable using common systems. We estimated that the standard NAVY ship sonar has sufficient power and with sufficient knowledge about diver acoustic signature its TRA focusing in shallow waters has practical implications to be sought as non lethal underwater acoustic weapon. Stevens Institute of Technology has substantial experience in collection of acoustic signatures of divers and applications of TRA acoustics methods [16]. Future research constitutes development of TRA diver deterrent system prototype that can be built using available hardware and software for land mine detection [19]. References 1. Environmental assessment of the installation and operation of an integrated antiswimmer system. Commandant United States Coast Guard (G-OPC), May KONGSBERG Introduces New Diver Detection Sonars C12572F10053BBF8?OpenDocument. 3. Lovik, A.R.Bakken, J. Dybedal, T. Knudsen, J. Kjoll, Underwater protection system, IEEE proceeding Ocean 2007, pp Cerberus Swimmer Detection Sonar System Description 5. Non-lethal swimmer neutralization study. Technical document 3138, Applied Research Laboratories The University of Texas at Austin, NUWC Demos Swimmer Defense System. Marine Technology Reporter.10/08/ R. Walker, Underwater port security, R & D, R. D. McCauleya, J. Fewtrellb, A. N. Popperc. High intensity anthropogenic sound damages fish ears. J. Acoust. Soc. Am. 2003, 113 (1),, Squid 500, Squid 2000 & Delta Sparker Seismic Sound Source, Sparker Assemblies. Applied Acoustics AA201 and AA301 Seismic Sound Source, Boomer Plates. Applied Acoustics. Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 8

10 11. Schaefer, R., Grapperhaus, M. Non-lethal unfriendly swimmer and pipe defense combining sound and flash pulses using a new sparker (2006) Proceedings of SPIE - The International Society for Optical Engineering, 6204, art. no A. Sutin, Y.Sinelnikov. Time Reversal Acoustic Approach for Non-Lethal Swimmer Deterrent. Proceedings of the Waterside Security Conference, Marina di Carrara, Italy, November Fink, M., Cassereau, D., Derode, A., Prada,C., Roux, P., Tanter, M., Thomas J.- L., Wu F. Time-reversed acoustics, Rep. Prog. Phys. 2000, 63, Fink M. Time reversed acoustics, Scientific American, 1999, pp Radford, C.A ; Jeffs, A.G. ; Tindle, C.T. ; Cole, R.G. ; Montgomery, J.C., Bubbled waters: The noise generated by underwater breathing apparatus. Marine and freshwater behaviour and physiology (2005). Vol.38, iss.4, p A.Sutin, B. Bunin, A. Sedunov, N. Sedunov, M. Tsionskiy, M. Bruno. Stevens Passive Acoustic System for Underwater Surveillance. Proceedings of the Waterside Security Conference, Marina di Carrara, Italy, November TNO. Netherlands. SOBEK: An underwater detection network. aag2=909&item_id= AN/SQS-56(I)/DE1160(I) Sonar Systems Sutin, A., Libbey, B., Fillinger, L., Sarvazyan, A. Wideband nonlinear time reversal seismo-acoustic method for landmine detection, (2009) Journal of the Acoustical Society of America, 125 (4), pp Proceedings of Meetings on Acoustics, Vol. 13, (2013) Page 9