Several experiments have demonstrated the potential of Laser Doppler Vibrometry, in conjunction with acoustic-toseismic coupling or mechanical shakers, for the detection of buried landmines. For example, experiments conducted by The University Of Mississippi and MetroLaser, Inc. have shown the ability to scan a one square meter area in less than 20 seconds with a 16-beam multi-beam LDV (MB-LDV), and find the landmines under a variety of soil conditions. Some critical requirements for this technology are to reduce the measurement time, increase the spatial resolution, and reduce the size of the systems. In this paper, MetroLaser presents data from three optical systems that help achieve these requirements: 1) A Compact MB-LDV, 2) A two dimensional, or Matrix Laser Doppler Vibrometer (MX-LDV), and 3) A Whole-field Digital Vibrometer (WDV). The compact MB-LDV produces a 1-D array of beams, which may be scanned over the target surface with a scanning mirror. The size of the new, compact MB-LDV system has been reduced to approximately 17" x 11" x 9", thus enhancing its capability for field applications. The MX-LDV, to be developed in 2006, produces a 16x16 array of beams over a one meter area, allowing the ground velocity of the entire area to be measured in a single measurement. The WDV uses a camera-based interferometry system to take a snapshot of the ground vibration over a one meter square area with very high spatial resolution. Field tests for this system are scheduled for mid-2006.
This paper discusses the performance and experimental results of a multiple beam laser Doppler vibrometer designed to locate buried landmines with the laser-acoustic technique. The device increases the speed of landmine detection by simultaneously probing 16 positions on the ground over a span of 1 meter, and measuring the ground velocity at each of these positions. Experimental results are presented from controlled laboratory experiments as well as from landmine test lanes at the University of Mississippi. In the mine lanes, the multiple beam system is raised to a height of 2.5 meters with a forklift, with the 16 beams spread over a 1 meter line along the mine lane. A motor system then allows the 16 beams to be translated across the mine lane, enabling the system to scan a 1 x 1 meter area in a much shorter time than with previous scanning techniques. The effects of experimental parameters such as platform motion, angle of incidence, speckle dropout, and system depth-of-field will be presented and discussed.
This paper discusses the development and performance of a multi-beam laser Doppler vibrometer specifically designed to locate buried landmines with a laser-acoustic technique. The device aims at increasing the speed of landmine detection with this technique by at least one order of magnitude. The present system is capable of simultaneously probing sixteen positions on the ground over a span of one meter, and of measuring the ground velocity at each of these positions with a velocity resolution of about 1 micrometers /s. This architecture could also be scaled to a larger number of beams or into two dimensions. The present system uses a low (100 kHz) carrier frequency, which enables digital signal processing in a simple architecture. This paper also discusses a numerical model to simulate and predict the performance of the multi-beam vibrometer. In particular, the model attempts to address issues associated with speckle dropout, signal/noise, and maximum scanning velocity.