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

TLDR: In this article, 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.

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 16 points spread over a 1-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 85-2000 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 DAAB15-02-C-0024.