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Demonstration of a High-Frequency Acoustic Barrier With a Time-Reversal Mirror H. Song, Member, IEEE, W. A. Kuperman, W. S. Hodgkiss, Member, IEEE, T. Akal, and P. Guerrini
Abstract—An acoustic barrier has been demonstrated using a time-reversal mirror. The experiments, at 3500 Hz, utilized a source-receiver array, a probe source collocated with a receive array, and an echo repeater to emulate a disturbance. The successful demonstration is based on the idea that a disturbance such as an object between a time-reversal mirror (TRM) and its focus will significantly disturb the focal region and, in particular, the quiescent region. Index Terms—Acoustic barrier, echo repeater, time-reversal mirror (TRM).
I. INTRODUCTION
R
ECENTLY, time reversal has been demonstrated to be a robust acoustic phenomenon [1]–[5]. In particular, the ocean acoustics experiments [2]–[4] have demonstrated a robustness suggestive of system implementation. Here, we present results that demonstrate the utility of a time-reversal mirror (TRM) functioning as the central component of an acoustic barrier. The classic difficulty in constructing an acoustic tripline barrier is that the scattered field must be extracted from the usually much more intense and usually fluctuating direct-arriving beam, i.e., “looking into the sunlight effect.” The shadow is the destructive interference between the unscattered field and scattered field from the object. Diffraction limits [6] and other effects make it difficult to detect this disturbance. We address the issue of the direct blast by localizing it using a TRM that focuses on a probe source position; in our case, five kilometers away in 110-m water depth. We have already shown in previous work [2]–[4] that the acoustic field above and below the focus is typically 15–20 dB less than the field at the focus. A disturbance between the TRM and the focus will fill in the quiescent region. The detection of a disturbance in the quiescent region is the diagnostic of the tripwire barrier. If the field is maintained continuously by refreshing the TRM focus, then acoustic fluctuations caused by oceanographic disturbances are eliminated [7].
Fig. 1. The geometry of the TRM barrier test trip line experiment in June 2000 (North of Elba Island, Italy).
II. KINEMATICS OF ACOUSTIC BARRIER Manuscript received October, 2002; revised December 31, 2002. This work was supported by the Office of Naval Research, grant N00014-94-1-0458. H. Song, W. A. Kuperman, and W. S. Hodgkiss are with the Marine Physical Laboratory/Scripps Institution of Oceanography, La Jolla, CA 92093-0701 USA (e-mail:
[email protected];
[email protected];
[email protected]). T. Akal was with the NATO SACLANT Undersea Research Center, La Spezia 19138, Italy. He is now with TUBITAK-MAN, Marmara Research Center, Earth and Marine Sciences Research Institute, Kocaeli 41470, Turkey (e-mail:
[email protected]). P. Guerrini is with the NATO SACLANT Undersea Research Center, La Spezia 19138, Italy (e-mail:
[email protected]). Digital Object Identifier 10.1109/JOE.2003.811900
We schematically represent the barrier in Figs. 1 and 2, which also represents the experimental configuration from which results will be presented below. A probe-source (PS) signal is received on a 29-element source-receive array (SRA), time reversed and transmitted, focusing at the original PS position. When an object is placed in the back-propagated field, the field is disturbed. This can be simulated by combining a mode model with a waveguide target formulation [8]. An example of the back-propagated field at the focus range as a function of depth
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Fig. 2. Schematic explanations of the experimental procedure for the time-reversal barrier test.
Fig. 4. Same as Fig. 3 except for a PS at 83-m depth. Note that the main blast and the echo-repeater signal arrived simultaneously in (b) and (e).
without and with the object is shown in Figs. 3 and 4. Clearly, the scattered field of the object fills in the quiescent region. We can estimate the effectiveness of such a barrier just by using some simple sonar equation arithmetic. First, we consider the non-TRM barrier of either a single source or a broadside pulse from an array, the latter being the more general case. The source array provides coherent gain (AG) over the level of an individual source. With transmission loss (TL) over the whole path and a noise level (NL), we can estimate the signal-to-noise ratio (SNR) at the other end of the barrier without an object to be SL
SNR
AG
TL
NL
(1)
Whereas, for an object of target strength (TS) at mid distance, we have SNR
SL
AG
TL
TS
TL
NL (2)
The (far-field) target strength associated with the shadow of an object of projected cross-section area can be approximately by [9] Fig. 3. Experimental results of a barrier test: (a)–(c) TRM for a PS at 43-m depth and (d)–(f) Non-TRM broadside transmission. (a) and (d) show the field without an object whereas (b) and (e) show both the unscattered (early arrival) and scattered field (late arrival) from the echo repeater. The dynamic range for the color plots is 19 dB. A target strength of 56 dB was simulated. (c) and (f) show the energy over the time window as a function of depth: without (blue) and with (red) an object. Clearly, (c) demonstrates that TRM barrier (red) can exploit the quiescent region below the focus for target detection.
TS
(3)
where is a wavelength. Typical numbers in the experiment m), 29 are a range of 5 km, frequency of 3500 Hz ( elements, each with nominal source level of 178 dB// Pa at 1
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IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 28, NO. 2, APRIL 2003
delay, the main blast and the echo-repeater signal arrived at the VRA simultaneously. Clearly, the energy distribution as a function of depth in panel (c) demonstrates that the TRM barrier (red) can exploit the quiescent region below and above the focus for target detection as described in Section II: e.g., around 50–90-m depth in Fig. 3 and 40–70-m depth in Fig. 4. Again, we can further increase the gain by array processing. On the other hand, the non-TRM broadside transmissions in panel (f) show similar random characteristics along the entire water column whether a target is present or not, making it difficult to distinguish between environmental fluctuations and a target disturbance. REFERENCES Fig. 5. Diagram illustrating the net system gain of 56 dB from the six-element echo repeater simulating scattering from an object in the forward direction.
m. At this frequency, TL is on the order of 65 dB and the NL is about 80 dB in bandwidth of the signal (100 Hz). Consider the SNR at the 5-km range without the target, which can be considered the background SNR with the object. We have dB by using (1). Now consider a SNR of dB. the TS of a 60-m 5-m object. From (3), we have TS dB We now have a SNR of by using (2). Hence, in this configuration, we would be trying to pick out a fluctuating signal whose mean is 3 dB below the fluctuating background. For a smaller object, the scattered field would be less, making the simple barrier even more intractable. Now consider using the focused field from the TRM. As shown in Figs. 3(a) and 4(a), the quiescent regions are of order 15–20 dB quieter than at the focus. Therefore, in these originally quiescent regions, a disturbance should provide a detectable signal level above the background without array processing (“deflected direct blast”), suggesting that the TRM tripwire is a viable concept. Below we present experimental results confirming this estimate. III. EXPERIMENT During the time-reversal experiment conducted in June 2000, we investigated the acoustic-barrier concept using a 6-element echo repeater at 65-m depth drifting along the R/V Alliance, which traversed the tripline between two moored vertical arrays separated by 5 km in 110-m water depth, as shown in Fig. 1. Fig. 2 shows a schematic of the experiment. A 10-ms CW pulse from the PS at the bottom of vertical receive array (VRA) is received and time reversed at the SRA. The transmitted signal is then refocused at the PS location of the VRA. This time-reversed signal is also captured by the echo repeater simulating the disturbance and is retransmitted to the VRA with various amplitudes simulating different target strengths. Fig. 5 illustrates the net system gain of 56 dB obtained from the six-element echo repeater simulating scattering from an object in the forward direction. For comparison, non-TRM (one-way) broadside transmissions were also made. Figs. 3 and 4 show examples for a probe source at 43- and 83-m depth, respectively. In Fig. 4, with an appropriate time
[1] M. Fink, “Time-reversed acoustics,” Sci. Amer., pp. 91–97, Nov. 1999. [2] W. A. Kuperman, W. S. Hodgkiss, H. C. Song, T. Akal, C. Ferla, and D. R. Jackson, “Phase conjugation in the ocean: Experimental demonstration of a time reversal mirror,” J. Acoust. Soc. Amer., vol. 103, no. 1, pp. 25–40, 1998. [3] W. S. Hodgkiss, H. C. Song, W. A. Kuperman, T. Akal, C. Ferla, and D. R. Jackson, “A long range and variable focus phase conjugation experiment in shallow water,” J. Acoust. Soc. Amer., vol. 105, no. 3, pp. 1597–1604, 1999. [4] T. Akal, G. Edelmann, S. Kim, W. S. Hodgkiss, W. A. Kuperman, and H. C. Song, “Low frequency and high frequency ocean acoustic phase conjugation experiments,” in Proc. 5th European Conf. Underwater Acoust., Lyon, France, July 2000. [5] W. A. Kuperman et al., “Ocean acoustics, matched-field processing and phase conjugation,” in Imaging of Complex Media with Acoustic and Seismic Waves, Fink et al., Eds. Berlin, Germany: Springer-Verlag, 2002. [6] N. C. Makris, “A spectral approach to 3-D object scattering in layered media applied to scattering from submerged spheres,” J. Acoust. Soc. Amer., vol. 104, no. 4, pp. 2105–2113, 1998. [7] S. Kim, W. A. Kuperman, W. A. Hodgkiss, H. C. Song, G. Edelmann, and T. Akal, “Robust time reversal focusing in the ocean,” J. Acoust. Soc. Amer., 2002, submitted for publication. [8] F. Ingenito, “Scattering from an object in a stratified medium,” J. Acoust. Soc. Amer., vol. 82, no. 6, pp. 2051–2059, 1987. [9] R. J. Urick, Principles of Underwater Sound. New York: McGrawHill, 1967, pp. 306–327.
H. Song (M’02) received the B.S. and M.S. degrees in marine engineering and naval architecture from Seoul National University, Seoul, Korea, in 1978 and 1980, respectively, and the Ph.D. degree in ocean engineering from the Massachusetts Institute of Technology, Cambridge, MA, in 1990. From 1991 to 1995, he was with Korea Ocean Research and Development Institute, Ansan. Since 1996, he has been a member of the scientists of the Marine Physical Laboratory/Scripps Institution of Oceanography, University of California, San Diego. His research interests include time-reversed acoustics, robust matched-field processing, and wave-propagation physics.
W. A. Kuperman was with the Naval Research Laboratory, Washington, DC, the SACLANT Undersea Research Centre, La Spezia, Italy, and most recently the Scripp Institution of Oceanography of the University of California, San Diego, where he is a Professor and Director of its Marine Physical Laboratory. He has conducted theoretical and experimental research in ocean acoustics and signal processing.
SONG et al.: HIGH-FREQUENCY ACOUSTIC BARRIER WITH A TIME-REVERSAL MIRROR
W. S. Hodgkiss (S’68–M’75) was born in Bellefonte, PA, on August 20, 1950. He received the B.S.E.E. degree from Bucknell University, Lewisburg, PA, in 1972, and the M.S. and Ph.D. degrees in electrical engineering from Duke University, Durham, NC, in 1973 and 1975, respectively. From 1975 to 1977, he was with the Naval Ocean Systems Center, San Diego, CA. From 1977 to 1978, he was a faculty member in the Electrical Engineering Department, Bucknell University. Since 1978, he has been a member of the faculty of the Scripps Institution of Oceanography, University of California, San Diego, and on the staff of the Marine Physical Laboratory. Currently, he is Deputy Director, Scientific Affairs, Scripps Institution of Oceanography. His present research interests include areas of signal processing, propagation modeling, and environmental inversions with applications of these to underwater acoustics and electromagnetic wave propagation. Dr. Hodgkiss is a Fellow of the Acoustical Society of America.
T. Akal was a Principal Senior Scientist at SACLANT Undersea Research Center, La Spezia, Italy, where, over the past 33 years, he has been leading research projects related to underwater acoustic and seismic propagation and marine sediment acoustics. He is currently collaborating with TUBITAK- Marmara Research Center, Earth and Marine Sciences Research Institute, Turkey, Marine Physical Laboratory of Scripps Institution of Oceanography at University of California, San Diego, and Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY.
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P. Guerrini received the M.S. degree in electronic engineering from the University of Genoa, Italy, in 1978. He was with Marconi Italiana, where he was involved with communication systems. In 1981, he joined the Engineering and Technology Department of SACLANT U.R.C., where he was mainly involved in the design of acquisition systems and acoustic arrays. He has been involved in a variety of projects and scientific sea trials.
