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.
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.
A torsional and longitudinal waveguide was introduced several years ago and shown to be effective for measuring various properties of liquids, including viscosity, density and temperature. The instrument is simply a specially constructed, thin rod (waveguide), one end of which is inserted into the liquid slurry. Torsional or extensional waves are generated in the rod, via a magnetostrictive mechanism, by passing a current through a coil which fits over the dry end of the rod. Liquid properties are correlated to different attributes of the waves (e.g., speed and amplitude) that travel down the rod and reflect off the end that is inserted into the liquid. Different properties of the material can be determined using waveguides of different cross-section. Noncircular rods are used to measure density, while viscosity is measured with circular rods. Since temperature affects these same wave attributes it would be desirable to have an independent measure of the temperature. This is accomplished by using a thermocouple sheath as the sensor part of the waveguide. In this way, the influence of temperature can be decoupled from the other properties of interest. In addition, the temperature is measured at the same point where the other properties of the liquid are being measured. The basic design of the sensor will be presented along with experimental results.
The determination of concrete integrity, especially in concrete bridge decks, is of extreme importance. Current systems for testing concrete structures are expensive, slow, or tedious. State of the art systems use ground penetrating radar, but they have inherent problems especially with ghosting and signal signature overlap. The older method of locating delaminations in bridge decks involves either tapping on the surface with a hammer or metal rod, or dragging a chain-bar across the bridge deck. Both methods require a `calibrated' ear to determine the difference between good sections and bad sections of concrete. As a consequence, the method is highly subjective, different from person to person and even day to day for a given person. In addition, archival of such data is impractical, or at least improbable, in most situations. The Diagnostic Instrumentation and Analysis Laboratory has constructed an instrument that implements the chain-drag method of concrete inspection. The system is capable of real-time analysis of recorded signals, archival of processed data, and high-speed data acquisition so that post-processing of the data is possible for either research purposes or for listening to the recorded signals.
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