Multi-beam laser Doppler vibrometry for acoustic landmine detection using airborne and mechanically-coupled vibration

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1 Multi-beam laser Doppler vibrometry for acoustic landmine detection using airborne and mechanically-coupled vibration Vyacheslav Aranchuk *a,james M. Sabatier b, Amit K. Lal a, Cecil F. Hess a, Richard. D. Burgett c, and Michael O Neill d a MetroLaser, Inc., 57 White Road, Irvine, CA 964 b National Center for Physical Acoustics, University of Mississippi, University, MS 8677 c Planning Systems Inc., d LD Consulting, LLC ABSTRACT Acoustic-to-seismic coupling-based technology using a multi-beam laser Doppler vibrometer (LDV) as a vibration sensor has proved itself as a potential confirmatory sensor for buried landmine detection. The multi-beam LDV simultaneously measures the vibration of the ground at 6 points spread over a -meter line. The multi-beam LDV was used in two modes of operation: stop-and-stare, and continuously scanning beams. The noise floor of measurements in the continuously scanning mode increased with increasing scanning speed. This increase in the velocity noise floor is caused by dynamic speckles. The influence of amplitude and phase fluctuations of the Doppler signal due to dynamic speckles on the phase locked loop (PLL) demodulated output is discussed in the paper. Either airborne sound or mechanical shakers can be used as a source to excite vibration of the ground. A specially-designed loudspeaker array and mechanical shakers were used in the frequency range from 85- Hz to excite vibrations in the ground and elicit resonances in the mine. The efficiency of these two methods of excitation has been investigated and is discussed in the paper. This research is supported by the U. S. Army Research, Development, and Engineering Command, Night, Vision and Electronic Sensors Directorate under Contract DAAB5--C-4. Keywords: land mine detection, laser-acoustic sensing, laser Doppler vibrometer, LDV. INTRODUCTION Acoustic landmine detection using a multi-beam laser Doppler vibrometer (MB-LDV) as a vibration sensor has demonstrated promising results in laboratory and field experiments,. The technique uses airborne sound or mechanical shakers to excite vibration in the ground and a scanning LDV is then used to measure the ground vibration at multiple points. The presence of a buried landmine can be detected by studying the spatial distribution of the ground velocity spectra. A critical issue in landmine detection is operational speed. Use of a multi-beam LDV developed by MetroLaser significantly reduces the time of the measurements. This vibrometer illuminates the ground with a linear array of 6 beams, and simultaneously measures the ground velocity at all 6 points. The 6 beams are spread uniformly across a -meter line, and the velocity sensitivity of each beam is approximately micrometer/second. The vibrometer can work in a continuously scanning mode when all 6 beams move in the transverse direction across the interrogated area. A two-dimensional velocity map of the ground over one square meter can be obtained in a time less then seconds. A continuous scanning beam introduces additional noise to the measurements due to dynamic speckles, which increase the velocity noise floor of the vibrometer. In this paper an experimental investigation of the velocity noise floor of the LDV with a PLL demodulator caused by dynamic speckles is presented. The MB-LDV working in continuously scanning mode was tested in the field at an Army eastern temperate site. Two methods of excitation, loudspeakers and mechanical shakers, were used. The efficiency of these two methods of excitation has been investigated and is discussed in the paper. Some results of field experiments are presented and discussed.. VIBRATION MEASUREMENTS WITH MULTI-BEAM LDV A schematic of the multi-beam LDV and its principles of operation were described in detail in references,4. The principle of measurement is based on detection of the Doppler shift of laser light scattered from a vibrating object. The multi-beam LDV is a multi-channel laser heterodyne interferometer. The laser beam is split into 6 object * aranchuk@metrolaserinc.com; Phone: ; Web:

2 beams and 6 reference beams. The reference beams are frequency shifted by khz. The 6 object beams are focused onto a target along a line. The light backscattered from the target is optically mixed with the reference beams, producing 6 frequency modulated signals having a khz carrier frequency. The frequency deviation (Doppler shift) of each signal is proportional to the velocity of the target in the point of measurement. The functional layout of the system is shown in Figure. The output signals of the multi-beam LDV are demodulated by means of a 6 channel PLL. The PLL output is proportional to the velocity of the target. Each PLL output signal is then digitized with a 6 channel A-D card in a computer, and the target velocity spectrum of each beam is calculated in software. MB LDV PLL Laser Beams Scanning Direction PC/Signal Processor Object Figure. Layout of the vibration measurement system with scanning multi-beam LDV. All of the beams can be moved in the direction perpendicular to the line formed by the beams by using a rotating mirror. The vibrometer can work in two modes: stop-and-stare mode, and continuously scanning mode. In the stop-and-stare mode the beams are moved a specified angle, stopped, the data are collected, and the beams are moved to the next location. In the continuously scanning mode, the beams can be continuously scanned across the target at variable speeds. In order to produce a velocity image, the time domain data for each beam is divided into time segments, typically of length from.sec to second. Over each time segment, the velocity vs. time is Fourier analyzed to generate the velocity vs. frequency data over each time segment. When the velocity spectra over each time segment and each beam has been computed, a velocity image over the entire scanned area can be generated at any selected frequency band. In the stop-and-stare mode, each time segment corresponds to a specific location of beams, while each time segment in the continuous scanning method is an average over a finite length. Figure (see the color image on the last page) shows an example of the segmented velocity image over a buried landmine.. NOISE INDUCED BY DYNAMIC SPECKLES Scanning a laser beam across a target introduces noise at the vibrometer output due to dynamic speckles 5. Coherent light scattering from an optically rough surface creates a speckle field. Figure shows an example of a speckle pattern. The statistical characteristics of laser speckles are well studied 6. The phase of speckles is uniformly distributed in the range from π to π, and the intensity of speckles has a negative exponential probability distribution. When the laser beam moves across the target, the intensity and phase of speckles change in a random way. This results in random fluctuations of the amplitude and phase of the Doppler signal. The Doppler signal at the output of a photodetector can be written: i d [ π ( f + f ) + Φ] = I t cos () R D

3 .5 carrier signal vibration Figure. Speckle pattern. Figure 4. Doppler signal and demodulated vibration velocity for a stationary beam. f and f are the where I is the amplitude of the Doppler signal, Φ is the phase of the Doppler signal, R D frequency shift of the reference radiation and the Doppler shift of the object radiation respectively. Since the Doppler signal results from coherent addition of the reference beam and the speckles, the amplitude I and the phase Φ of the Doppler signal are random quantities. For a laser beam stationary relative to the target, the amplitude and phase of the Doppler signal do not change with time. Figure 4 shows a khz carrier Doppler signal and a demodulated velocity signal when the laser beam is stationary. When a beam moves across the target the speckles at the photodetector vary, resulting in random variation of the Doppler signal amplitude and phase. The amplitude of the Doppler signal varies randomly, and can occasionally drop down to a very small value below the photodetector noise level. In that case, because of insufficient signal, the PLL can lose lock, which results in a spike in the PLL output. When the Doppler signal reappears the PLL locks in again. Figure 5 shows the khz carrier Doppler signal and a demodulated velocity signal when the laser beam scans the target. The demodulated velocity signal (PLL output) contains spikes caused by Doppler signal drop-outs. The spikes in the demodulated signal have a broadband spectrum and increase the velocity noise floor of the vibrometer. Figure 5(b) shows an expanded view of the spike near.76 seconds, and the corresponding drop in the carrier signal amplitude..5 carrier signal carrier signal vibration - vibration (a) (b) Figure 5. (a) Doppler signal and demodulated vibration velocity for a moving beam. (b) Expanded view of the data between.7 and.8 seconds, showing the velocity spike and the drop in carrier signal amplitude.

4 The phase fluctuation of the Doppler signal due to the motion of the laser beam is another effect that increases the velocity noise floor. The frequency f C of the carrier Doppler signal at the LDV output is given by the time derivative of the argument in the cosine function in expression (): f C = f R + When the laser beam is stationary, the speckles on the photodetector do not change with time and dφ/dt = making the frequency of the carrier Doppler signal equal to: f = f + f () C When the beam moves across the target, the noise corresponding to the frequency content of dφ/dt appears in the signal 5. Figure 6 shows the dependence of the velocity noise floor of the PLL demodulated signal on the speed of the scanning beam. The velocity noise floor increases with the increase in the speed of the moving beam. As said above, two effects are responsible for the increase in the velocity noise floor: signal dropouts causing spikes in the PLL output and signal phase fluctuations. The velocity noise floor for the data points in Figure 6 was calculated by averaging the noise floor over all the time segments of. seconds of the demodulated signal. f D R + π D dφ dt () t () Velocity (microns/sec) Beam Speed (cm/sec) Figure 6. Velocity noise floor versus scanning speed of the beam. Figure 7 again shows the average noise floor vs. beam speed (dashed line), and also shows the average noise floor over time segments without spikes (solid line). The dashed line in Figure 7 is caused by both spikes in the PLL output due to the carrier signal dropout, as well as the phase fluctuations due to dynamic speckles. The solid line shows the part of the noise floor caused by the phase fluctuations of dynamic speckles. There were no time segments of length. seconds without spikes at beam speeds higher than 6 cm/second in our experiments. Velocity (microns/sec) Beam Speed (cm/sec) Figure 7. The dashed line is the average noise floor over all time segments of length. seconds. The solid line is the average noise floor over time segments of. seconds without spikes.

5 4. FIELD EXPERIMENTS The layout of the experimental setup is shown in Figure 8. The MB-LDV was mounted on a forklift at a height of. meters above the ground. The ground vibration was excited by using loudspeakers or mechanical shakers. Pseudo-random noise was used as an excitation signal. The beams were oriented in the down-track direction, and were scanned in the across-track direction. The speed of beams on the ground could be set from cm/second to cm/second. The data were taken during the time when the beams scan the ground. The PLL output signals were digitized with a 6 channel A-D card in a computer, and the data file was stored in the computer memory. A photograph of the multi-beam system mounted on the forklift is shown in Figure 9. A specially designed loudspeaker array and a mechanical shaker array were used in the frequency range from 85- Hz to excite vibration in the ground. Due to the different nature of the vibration excitation of the ground the frequency response of the ground to shakers and loudspeakers is different. Figure shows the acceleration produced by the shakers and the loudspeakers on the ground and on the vibrometer platform. The shakers have limited frequency bandwidth; they produce high excitation at low frequencies but exhibit a high frequency roll-off. The loudspeakers excite vibration of the ground over a wide bandwidth up to Hz. The shakers have an advantage over the speakers in lower excitation of the vibrometer platform. Since shakers radiate less sound in the air, they create db lower vibration of the LDV platform when they are used as an excitation source. 4 Laser beams 6 5 Scanning direction Figure 8. Schematic of experimental setup. - scanning multi-beam LDV, - loudspeaker, -signal generator, 4 - shakers, 5 - landmine, 6 PC/signal processor. Figure 9. Multi-beam LDV mounted on the forklift. ) MB-LDV, ) loudspeaker array, ) shakers. Data was obtained with the MB-LDV in continuous scanning mode on different landmines buried at depths from flush to 5 cm, and on different types of soil. Both loudspeakers and mechanical shakers were used for excitation. The pseudo-random noise was used as an excitation signal. Mines were scanned at different scanning rates from 5 cm/second to cm/second. The time of scanning a one-meter by one -meter spot was from second to seconds. A velocity image of a mine can be obtained at any of these speeds. Figures and show velocity images of mines obtained at different scanning speed. However, an increase in scanning speed increases the velocity noise floor due to dynamic speckles. A higher excitation level is required to resolve the difference in velocity over the target and off the target due to the higher velocity noise floor. Figure shows velocity images of a buried landmine obtained at a scanning speed of 5 cm/second at different excitation levels. The image in Figure a was obtained with a low excitation level while the image in Figure b was obtained with the excitation db higher. One can see that an increase in excitation level allows one to overcome the limitations imposed by the higher velocity noise floor, and improve the on target/off target contrast of the velocity image.

6 - -4 Releative level (db) Frequency (Hz) Figure. Acceleration produced by shakers and a loudspeaker on the ground and on the vibrometer. acceleration produced by shakers, - acceleration produced by a loudspeaker, - acceleration of the vibrometer produced by a speaker, 4- acceleration of the vibrometer produced by shakers. 5. CONCLUSIONS Field experiments show that buried landmines can be detected within one square meter in several seconds using a multi-beam laser Doppler vibrometer in a continuously scanning mode, with loudspeakers or shakers as the excitation source. Experiments show that shakers have better performance at low frequencies, but exhibit a high frequency roll-off. The speakers excite vibration of the ground over a wide bandwidth up to Hz. The shakers have an advantage over the speakers in lower excitation of the vibrometer platform. Increasing the scanning speed increases the velocity noise floor due to dynamic speckles. This increase in the velocity noise floor is caused by Doppler signal dropouts resulting in spikes in the PLL output, and phase fluctuation of speckles. Operation at higher scanning speeds require a higher excitation level to overcome the limitations imposed by the increasing velocity noise floor. Development of a new demodulation technique that can provide a lower noise floor in continuously scanning mode is a high-priority task of the future work. ACKNOWLEDGEMENTS This work is sponsored by U.S. Army Research, Development, and Engineering Command, Night Vision Electronic Sensors Directorate, under Contract DAAB5--C-4. The content of the information does not necessarily reflect the position or the policy of the Government and no official endorsement should be inferred. REFERENCES. Sabatier, J.M., and Xiang, N., Laser-Doppler Based Acoustic-to-Seismic Detection of Buried Mines, Proceedings of the SPIE, Vol. 7, p. 5, Orlando, March Xiang, N. and Sabatier, J.M., Land Mine Detection Measurements using Acoustic-to-Seismic Coupling, Proceedings of the SPIE, Vol. 48, p. 645, Orlando, April.. Lal, A.K., Zhang, H., Aranchuk, V., Hurtado, E., Hess, C.F., Burgett, R.D, Sabatier, J.M., Multi-beam LDV system for buried landmine detection, Proceedings of the SPIE, Vol. 589, p. 579, Orlando, April. 4. Lal, A.K., Hess, C.F., Zhang, H., Hurtado, E., Aranchuk, V., Markov, V.B., and Mayo, W.T., Whole-field laser vibrometer for buried landmine detection, Proceedings of the SPIE, Vol. 474, p. 64, Orlando, April. 5. Rothberg S. G., Barker J.F. and Halliwell N.A., Laser vibrometry: pseudo-vibrations, Journal of Sound and Vibration, 5 (), p J.W.Goodman, Statistical Properties of Laser Speckle patterns, Laser Speckle and Related Phenomena, J.C. Dainty, pp. 9-75, 975.

7 Figure. Segmented velocity image above a buried antitank mine. a). second time segment, b).5 second time segment. The scan rate was 5cm/second. Figure. Antitank mine VS. buried 5 cm deep at different scanning speeds. a) cm/second; b) cm/second; c) 5cm/second; d) cm/second. Figure. Antitank mine M5 buried 5 cm deep at different scanning speeds. a) cm/second; b) cm/second; c) cm/second; d) 5cm/second. Figure. Image of an antitank mine VS.6 buried cm deep at different levels of excitation. a) low excitation, b) db higher excitation.