Using Laser Doppler vibrometry (LDV) to find buried land mines has been shown to have a high probability of detection coupled with a low probability of false alarms 1, 2 . Previous work 3, 4 has shown that is it possible to scan a square meter in 20 seconds, but this method requires that discrete areas be scanned. This limits the use of LDVs for land mine detection to a confirmation role. The current work at the University of Mississippi has been to explore ways to increase the speed of scanning to allow the sensor to move down the road at speed. One approach has been to look at the feasibility of using multiple beams to look at the same spot, time division multiplexing, in order to build a time history over small ground segments as each beam passes over the spot. The composite velocity signature built from each beam will provide a long enough time series to obtain the necessary frequency resolution.
The multi-beam laser Doppler vibrometer (MB-LDV) has been successfully used for acoustic landmine detection in field experiments at an Army test site. Using the MB-LDV in a continuously scanning mode significantly reduces the time of the measurement. However, continuous motion of a laser beam across the ground surface generates noise at the vibrometer output due to dynamic speckles. This speckle noise defines the noise floor and the probability of detection of the system. This paper studies the origins of speckle noise for a continuously scanning LDV. The structure of the speckle field exhibits points of phase singularity that normally coincide with signal dropouts. The signal dropouts and phase singularities can cause spikes in the demodulated velocity signal, which increase the noise in the velocity signal. The response of FM demodulators to input signals causing spikes in the LDV output are investigated in this paper. Methods of spike reduction in the LDV signals have been developed and experimentally investigated.
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