Richard D. Burgett

Mobile mounted laser Doppler vibrometer array for acoustic landmine detection

The use of a laser Doppler vibrometer (LDV) to sense the acoustic-to-seismic coupling ratio for buried landmine detection has previously been demonstrated. During these experiments, the LDV is mounted on a fixed platform and the beam moves continuously across the ground. Experiments show that fixed mounted LDV can achieve scanning speeds up to 3.6 km/h for successful detection of buried landmines in outdoor ground. The problems associated with taking a fixed-mount, scanning LDV and transitioning to a mobile system involve such issues as vehicle vibration, additional Doppler bandwidth due to vehicle speed, speckle noise, and sample time vs. spatial averaging. This paper presents the results of field tests with the moving platform on U.S. Army mine lanes showing that many of these issues can be overcome with an appropriately designed moving platform. The testing involved scanning different types of mines at varying depths and different speeds. Different aspects of the experiment are also discussed.

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

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.

Speckle noise in a continuously scanning multibeam laser Doppler vibrometer for acoustic landmine detection

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.

High-frequency A/S coupling for AP buried-landmine detection using laser Doppler vibrometers

The coupling of airborne sound into roadways and desert soils has been significantly investigated for the purposes of locating buried antitank (AT) landmines. However, there has been relatively little acoustic-to-seismic (A/S) coupling data collected for the purpose of buried antipersonnel (AP) landmine detection. A/S coupling landmine research has typically been accomplished with a low frequency sound source radiating pseudo-random noise in the frequency range 0f 80-300 Hz and a scanning single beam laser Doppler vibrometer (LDV) with a 10 cm beam spacing. The single beam LDV is operated in a serial data collection procedure resulting in long scan times. Recently, a data collection platform that uses 16 LDVs has been used to detect buried AT landmines. In the work reported here, this data collection platform is used to scan a significant number of AP landmines. For this purpose, the LDV beam spacing is reduced to 3 cm and the upper frequency of the sound source is increased to 2 KHz.

Multiple-beam LDV system for buried landmine detection

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.

Demultiplexing multiple-beam laser Doppler vibrometry for continuous scanning

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. Previous work 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.

Experimental investigation of buried landmine detection using time division multiplexing of multibeam laser Doppler vibrometer channels

Producing vibration images of buried landmines using a multi-beam laser Doppler vibrometer (MB-LDV) operating from a stationary platform have been accomplished in the past. Detection from a continuously moving platform can reduce the time of detection compared to stop-and-stare measurement. However, there is a speed limitation, imposed by the required spatial and frequency resolution. NCPA proposed a concept of time division multiplexing (TDM) of laser beams of a MB-LDV to overcome that speed limitation. The system, based on 16-beam MB-LDV, has been built and experimentally tested at an Army test facility. Vibration velocity profiles of buried mines have been obtained at different system speeds. Algorithms for speckle noise reduction in continuously moving MB-LDV signals have been developed and explored. The results of the current data collection, recent past data collection as well as the results of the effectiveness of speckle noise reduction techniques are presented.

Fast acoustic landmine detection using multiple beam laser Doppler vibrometry

An acoustic method of buried landmine detection consisting of excitation of the ground using airborne sound or seismic waves in the frequency range from about 50 Hz to 1000 Hz, and obtaining a velocity image of the ground surface with a laser Doppler vibrometer (LDV) has been developed and successfully tested in the laboratory and field experiments. The presence of a buried landmine can be detected by an abnormality in the velocity image. Initially, the scanning single‐beam LDV created a velocity image of the ground through point‐by‐point measurements, which resulted in a long measurement time. To reduce this measurement time, a multiple beam LDV having 16 beams configured in a linear array was developed and successfully used in field experiments. To further reduce the time of measurement, a full‐field LDV capable of making 256 vibration measurements in parallel on the target, configured as 16x16 square array of points equally separated in x‐ and y‐directions has been recently developed. The system is capable of creating a two‐dimensional vibrational velocity image of the ground surface in the time defined by the required frequency resolution. The parallel measurement makes the system a unique tool to measure transient vibrations.

Laser sensing techniques for acoustic detection of buried mines.

Acoustic detection of buried mines is based on excitation of vibrations in the ground with airborne sound and measuring vibration of the ground surface by using a laser interferometric sensor. The principle of detection uses the fact that buried mines have mechanical impedances that are smaller than that of soils. That makes the soil above a mine vibrating with magnitude higher than the surrounding soil. A laser sensor creates a vibration image of the ground by measuring vibration in many points on the surface. The presence of a buried mine can be detected by an abnormality in the vibration image. The method has shown excellent performance in field tests. One limiting factor of the method is a long measurement time due to low frequency of vibrations. Different laser interferometric techniques have been investigated to reduce the time of detection. The paper discusses application of single‐beam scanning laser Doppler vibrometry (LDV), multiple‐beam LDV, and speckle‐interferometry based methods for acoustic...

Carrier tracking and tunable passband filters for TDM-LDV mine detection

Using laser Doppler vibrometers (LDVs) to find buried land mines has been shown to have a high probability of detection coupled with a low probability of false alarms. Equally good results have been achieved using a 16-beam LDV. Time division multiplexing (TDM) of this multiple-beam LDV has also been investigated as a means of increasing the scanning speed and potentially allowing the sensor to move down the road at speeds faster than that allowed using stop-and-stare LDVs. A moving platform induces Doppler shifts in the LDV beams that are not perpendicular to the motion vector. This shift can be much greater than the modulation bandwidth of a stationary LDV signal; therefore, the demodulation must allow for the shift either by increasing the processing bandwidth, which increases the system noise or by tracking the Doppler offset and adjusting a band pass filters center frequency. A method has been developed to track the carrier frequency to compensate for the Doppler offset for each of the 16 channels caused by the moving platform and then adjusting the center frequency of a digital band pass filter. This paper will present the basic filter structure and compare the noise statistics from two different carrier tracking methods that were investigated.