A single beam laser Doppler vibrometer (LDV) has been used in acoustic-to-seismic mode [Sabatier, J.M. and Xiang, N. IEEE Trans. <i>Geoscience and Remote Sensing </i><b>39</b>, 2001, pp. 1146-1154; Xiang, N. and Sabatier, J.M., <i>J. Acoust. Soc. Am. </i><b>113</b> Mar 2003]. One of the major requirements is the operational scanning speed in the acoustic detection methods. To increase the operational speed, the LDV must move continuously along the ground. An initial effort has demonstrated the feasibility of continuously scanning the ground by controlling the mirrors in a scanning laser vibrometer [Valeau et al., Development of a time-frequency representation for acoustic detection of buried objects, <i>J. Acoust. Soc. Am</i>., 2003 (submitted).]. A continuously scanning LDV on a stationary platform has been employed. This work will discuss systematic investigations using a continuously scanning LDV to obtain field data in Army test lanes.
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.
A model has been developed to allow the scanned data obtained using a laser Doppler vibrometer-based acoustic-to- seismic landmine detection system to be analyzed without operator interaction. The ground vibration data from the LDV are pre-processed to form images in a 2-D data format. A parametric model was established to describe the amplified magnitude velocity phenomena induced by buried landmines. This model incorporates amplitude, size, position and background amplitude parameters into an automatic analysis process. An iterative regression approach is described which can be used as a major part of the automatic landmine recognition. The estimated parameters, such as the amplitude relative to the background, the size, and the shape of a target are used to make the decision regarding the presence of a mine. Once a positive decision is made, the estimated position parameters are used to localize the target location.
Acousto-to-seismic coupling has proven to be an extremely accurate technology for locating buried landmines. Most of the research to date has focused on linear acoustic techniques in which sound couples into the ground, interacts with the buried mine, and causes increased vibration of the ground above the mine. However, Donskoy has suggested that nonlinear acoustic techniques may be applicable to acoustic mine detection. This technique has recently been used with success in field tests at the University of Mississippi and US Army mine lanes. In the nonlinear acoustic technique, airborne sound is produced at two primary frequencies which couple in to the ground and a superimposed compressional wave interacts with the mine and the soil. Because the mine is compliant, contact between the soil and the mine is maintained during the compression phase of the wave, but they are separate during the tensile phase. This creates a bimodular oscillator that is inherently non-linear. This effect has been demonstrated on inert landmines at the University of Mississippi and at US Army test lanes. Results of these tests indicate that nonlinear measurements over buried landmines have more sensitivity than linear measurements. Non-compliant objects such as concrete disks do not exhibit nonlinear phenomena but can be located using linear techniques.
A desirable characteristic for a landmine detection system is the ability of the detector to 'look' out in front of the vehicle a significant distance. The obvious reason for this is to reduce the risk to the vehicle and its operators and to allow a safe stopping distance for the vehicle. Several experiments were conducted at Fort A. P. Hill to investigated the feasibility of a forward-looking system based on acoustic-to-seismic coupling. The system, developed at the National Center for Physical Acoustics, insonifies the ground with high amplitude (120 dB), broadband (80-300 Hz) sound and measures the resulting ground vibration with a scanning Laser Doppler Vibrometer (LDV). Images produced by these scans show a distinct contrast in several frequency bands between ground vibrations over a buried mine and those not over a buried mine. In a forward-looking system, both the sound source and the LDV are moved farther from the scanned area. This configuration both reduces the sound pressure level at the scanned area and decreases the angle at which the LDV beam strikes the ground. These effects reduce the contrast between the over-mine and off-mine signals. In addition, the image is distorted at the shallower LDV-ground angles. However, the results from the experiments demonstrate that the acoustic-to-seismic forward-looking approach is feasible once these technical hurdles are overcome.
Acoustic-to-seismic (A/S) coupling has been used successfully to locate anti-personnel (AP) mines with a high probability of detection (P<SUB>d</SUB>). This work builds on previous efforts that have demonstrated the high P<SUB>d</SUB> and low false alarm rate capabilities of A/S coupling in finding ant-tank (AT) mines. This paper discusses the initial results obtained from applying A/S coupling mine detection on anti-personnel mines. Due to the smaller size of AP mines, AP mine detection is more challenging than AT mine detection. The analysis results in this paper are based on A/S coupling mine detection data fro AP mines collected using a laser Doppler vibrometer-based mine detection system. The primary challenge in AP mien detection is to maintain a low false alarm rate while retaining this high probability of detection.
Visible vertical-cavity surface-emitting lasers (VCSELs) are potential light sources for polymer optical fibre (POF) data transmission systems. Minimum attenuation of light in standard PMMA-POFs occurs at about 650 nm. For POFs of a few tens of meters in length VCSELs at slightly longer wavelengths (670 - 690 nm) are also acceptable. So far, the visible VCSELs have been grown by metal organic chemical vapour deposition (MOCVD). They may also be grown by a novel variant of molecular beam epitaxy (MBE), a so-called all-solid-source MBE or SSMBE. In this paper, we describe growth of the first visible-light VCSELs by SSMBE and present the main results obtained. In particular, we have achieved lasing action at a sub-milliamp cw drive current for a VCSEL having the emission window of 8um in diameter, while a 10um device exhibited an external quantum efficiency of 6.65% in CW operation at room temperature. The lasing action up to temperature of 45°C has been demonstrated.
The use of acoustic-to-seismic coupling to detect buried landmines has been successfully demonstrated over the past year. The technique uses a laser Doppler vibrometer (LDV) to measure the velocity of the ground vibration as it is being sonified. As it is currently implemented, the LDV scans individual points on the ground. The technique shows much promise, but it is slow when compared to some other techniques. This work investigates the feasibility of acquiring data with the LDV as the beam moves continuously across the ground. Simple models were developed and experiments were performed to explain the cause of this noises. These result are presented and the feasibility of the approach is discussed. It has been shown that this approach is possible, but that the continuous scanning process introduces noise into the data.
During the early 1980s, the phenomenon of acoustic-to- seismic coupling was used to detect buried objects or mines. In these early measurements, large 2 Hz geophones measured the low frequency normal component of the soil particle velocity over buried targets. Several different, naturally- occurring ground types were studied in these measurement, including grass-covered ground; bare, sandy soil surfaces; and 'dirt' roads. Since the large geophone averages the particle velocity over the area of the sensor case, acoustic-to-seismic transfer function measurements were made with new, smaller-sized geophones. Higher frequency measurements were made using accelerometers. 3D maps of the surface particle velocity were made using measured seismic/acoustic transfer function data. Recognizing the need for a non-contact sensor and the need to investigate the geophone/soil coupling effect in the acoustic-to-seismic transfer function, additional measurements were made using a laser Doppler vibrometer (LDV). This paper explains the acoustic-to-seismic coupling mine detection measurement technique using both geophones and an LDV. The early measurements of the acoustic-to-seismic coupling transfer function for mine-like targets are discussed as well as some recent measurements using a LDV.
Probability as logic is used to estimate the surface velocity of a patch of soil driven by an incident acoustic wave. The data used by the estimation procedure is obtained from a laser Doppler vibrometer (LDV). The output of the LDV is an intermediate-frequency carrier that is frequency- modulated by the soil surface velocity. Additionally, the LDV output is amplitude modulated by an undesirable variation in the returned laser signal due to dynamic optical speckle. The effect of the amplitude modulated by an undesirable variation in the returned laser signal due to dynamical optical speckle. The effect of the amplitude modulation on the estimate of the soil surface velocity is illustrated with results obtained using the Markov chain Monte Carlo method.
Airborne acoustic waves coupled into the surface of the ground excite Biot Type I and II compressional and shear waves. This coupling of airborne sound into the ground is termed acoustic-to-seismic coupling. If a land mine or other inhomogeneity is presented below the surface, the ground surface vibrational velocity or S/A ratio will increase due to reflection and scattering of the Type II compressional wave. The dispersion characteristics of this wave in solids determines the mine detection limits. The S/A ratio is read with a laser doppler vibrometer (LDV). The loud speaker and LDV were mounted onto a large forklift at Fort AP Hill. This system was used to scan patches of ground at the Fort AP Hill calibration mine lanes. An investigation on the variability of surface velocity over different background types and mine types is described. The results of these initial field exercises are described.
Low detector signals, acoustic coupling and speckle noise are challenging problems in the laser Doppler-based acoustic-to-seismic detection of land mines. Scanning insonified patches over buried targets with the spatial resolution required in minefield applications demands processing a large quantity of detection data. To achieve an efficient and robust detection, acoustic-to-seismic coupling on the ground is considered as a system under test (SUT), number-theoretical maximum-length sequences (M-sequences) have been applied as the acoustic excitation to the SUT. Exploiting their excellent auto-correlation property and high noise immunity due to high signal energy and noise suppression, a fast algorithm (so-called fast M-sequence transform) is implemented in the cross-correlation procedure to extract the impulse response of the SUT directly from laser Doppler vibrometer signals with a high signal-to-noise ratio. The advantage of directly obtaining impulse responses is also exploited in featuring a time windowing technique to isolate the acoustic coupling into the laser Doppler-based system.