Time Reversal Acoustics

11/15/2010

In a volatile time of worldwide spread of stealth technology the risk to the homeland port security from underwater threats increases significantly. Explosives can be brought to domestic harbors by undetected scuba divers or attached to the vessel hulls of ships entering a harbor. Although underwater detection technologies exist, hostile diver identification and deterrent remains a challenge. Since the terrorist attacks of September 11, 2001, it became imperative to deploy effective systems to monitor both the surface and underwater potential threats. While detection of scuba divers can be conducted with the help of active and passive sonars, the major underwater defense still constitutes the use of explosive charges that can kill divers and all marine life around.
 
Application of lethal weapons is acceptable in the case of determined attack that can not be stopped otherwise. Less harmful, non-lethal methods of diver deterrent are based on application of the intensive sound. Air-guns become a part of integrated anti-swimmer port protection program. Typically, intensive underwater sound systems introduced for diver deterrent generate low frequency underwater sound that interferes with breathing, induces vestibular disorientation, panic, uncontrolled ascent to surface and subsequent decompression sickness. It has been shown that strong sound with acoustic pressure above 170 - 180 dB re 1Pa causes lung and liver damage.. Yet, the major deficiency of all deployed interdiction systems is that the sound is radiated in all directions and can negatively affect marine life as well stun friendly divers. To avoid collateral damage to marine life, the focusing of acoustic energy in a tight focal zone around the target becomes most critical. Today’s power of signal processing and thorough knowledge of underwater acoustical physics provide an excellent viable platform and technology to construct an intelligent, safe and effective underwater sound focusing system.
 
This paper discusses possible application of 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 acoustic noise radiated by a diver is used to focus the acoustic energy back to the diver. The diver acts as an active self-disclosing acoustic beacon.
 
Time Reversal Acoustics is one of the hottest topics in acoustics. The significance of TRA is in its ability to efficiently focus an acoustic signal regardless of the position of the initial source and regardless of the heterogeneity of the propagation medium. The TRA systems have a range of multi-discipline applications, including destruction of tumors and kidney stones in medicine, detection of defects in construction, and long-distance communication in the ocean. Impressive results were shown for kidney stone destruction, land mine detection and treatment of atrial fibrillation. Recently, the experiments in shallow sea demonstrated superior ability of the TRA system to focus the underwater sound across distances of up to 30 km
 
We explored the feasibility of deterring the diver away from critical port infrastructure by an application of the TRA principles. The major advantage of proposed approach is that the TRA system for non-lethal swimmer neutralization focuses all radiated, refracted, reflected and scattered underwater sound back to a hostile swimmer. The TRA focusing technique further takes an unprecedented advantage of spatial heterogeneities and multiple reflections present in shallow harbors that decrease the efficiency of other active insonation methods. The use of passive recordings of sound radiated by a diver enables low cost and high efficiency integrated detection and deterrent approach. Substantial research by the Stevens Institute of Technology pawed the way to TRA deterrent by demonstrating confident reception of the acoustic noise generated by scuba diver regulators at distances up to 700 m.
The concept of the TRA system for non-lethal swimmer neutralization is presented in Figure 1. The recorded diver sound is time reversed, amplified and focused back to the diver location in real-time. 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.
 
We developed a model to establish the feasibility of TRA focusing system and estimate an effective zone of the diver deterrence in a shallow sea model. Short sound bursts were numerically traced trough a generalized sea model with inclined seefloor that acted as a waveguide, where emitters were located at the shallow region. Two-dimensional maps of sound pressure level (SPL) were calculated at various distances, 50 m to 200 m from emitters to diver. Figure 2 shows the SPL in the vicinity of a diver for the TRA system with 20 underwater emitters, with acoustic power of 100W each, randomly distributed within approximately two meters from each other. Good focusing of sound waves in a direction of propagation (x) and in water depth (z) was achieved, while focusing in transverse direction (y) was less pronounced. Comparative calculations also demonstrated that noise burst of higher frequency focused better, while the lower frequency produced larger focal zone with comparable peak temporal averaged intensity of underwater sound.
 
Model calculations demonstrate that focused underwater SPL can reach 190 dB re 1Pa, which can be practical for non-lethal diver deterrent. According to the publication of the Applied Research Laboratories of the University of Texas at Austin, this is sufficient to disorient and divert any bareheaded swimmer due to vestibular effects (dizziness) and overall loudness. Another important implication is that the size of water column where high intensity sound is focused is confined within a relatively narrow volume of several cubic meters, while underwater sound intensities outside that region are substantially less and it will not be harmful for marine life. The focused underwater sound intensity decays with distance, as it is shown in
 
Based on our modeling the application TRA focusing to deter and neutralize hostile divers is feasible. Major advantages of non-lethal TRA interdiction approach constitute the precise focusing ability in arbitrary heterogeneous environment, the ability to use passive detection for aiming the impact, reduced harm to marine life, low cost and relatively small deployment size. The objective of further research will be a construction of real TRA non-lethal interdiction acoustic focusing system and verification of its efficacy in marine environment.