The laser-acoustic detection of buried objects, such as landmines, is based on excitation of elastic waves in the ground and creating a vibration image of the ground surface by using a laser Doppler vibrometer (LDV). The technique provides high probability of detection and low false alarm rate. However, traditional LDVs require operation from a stable stationary platform due to their sensitivity to the motion of the vibrometer itself. Recently developed laser Doppler multi-beam differential vibration sensor has low sensitivity to the motion of the sensor itself, while measuring vibration velocity difference between points on the object with interferometric sensitivity. Low sensitivity to the sensor motion allows for vibration measurements from a moving vehicle. Two configurations of the developed sensor: the linear array and the 2D array sensors, are discussed in the paper. The linear array sensor measures velocity difference between points on the object illuminated with a linear array of 30 laser beams, and creates a vibration image of the object by scanning the array of beams in a transverse direction. The 2D array sensor employs an array of 34 x 23 laser beams and measures velocity difference between corresponding points on the object over the whole illuminated area simultaneously. Simultaneous measurements at all points allow for the fast recording of the vibration image of the area of interest, and makes possible calculation of the vibration phase and instantaneous velocity images. Description of the sensors and the experimental results are presented in the paper.
A laser multibeam differential interferometric sensor (LAMBDIS) was developed that provides measurement of vibration fields of objects with high sensitivity, while having low sensitivity to the whole-body motion of the object, or the sensor itself. The principle of operation of the LAMBDIS is based on the interference of light reflected from different points on the object surface illuminated with a linear array of laser beams. The Doppler shift induced by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. The performance of the sensor for laser-acoustic detection of a buried object was experimentally investigated. The ability of LAMBDIS to detect buried objects from a moving vehicle has been demonstrated in field experiments.
Laser Doppler vibrometers (LDVs) have been successfully used for ground vibration imaging in acoustic detection of buried objects. LDVs operating from a stationary platform or from a moving platform with a beam looking down can provide high sensitivity vibration measurement of the ground. However, operation from a moving vehicle with laser beams looking forward induces Doppler shift in the LDV beam. This shift can be much greater than the modulation bandwidth of the LDV. The demodulation must allow for the shift either by increasing the processing bandwidth, or by tracking the Doppler shift. The former increases the LDV noise while the latter can result in complex LDV design and signal processing. We developed a novel Laser Multi Beam Differential Interferometric Sensor (LAMBDIS) which provides measurement of vibration fields of objects with high sensitivity, while having low sensitivity to the whole body motion of the object, or sensor itself. The principle of operation of the LAMBDIS is based on the interference of light reflected from different points on the object surface illuminated with a linear array of laser beams. The Doppler shift induced by the sensor motion is approximately the same for all beams and is automatically subtracted from the measurements. Scanning the linear array of laser beams in the transverse direction provides a vibration image of the surface. Performance of the sensor for vibration imaging of a buried object was experimentally investigated. The experimental results and description of the sensor are presented in the paper.
The National Center for Physical Acoustics (NCPA) at the University of Mississippi is working on the application of
ultrasonic Doppler sonars in air for personnel motion detection. Two traditional Doppler sonar configurations, a
monostatic and a bistatic, are being studied. In the monostatic configuration, the distance between the transmitter and
the receiver is small. The proximity of the source to the receiver places a limitation on the system associated with the
overloading of the receivers' input due to acoustic energy leakage from the transmitters' output. The maximum range
of detection is therefore limited by the dynamic range of the acquisition system. In a bistatic Doppler ultrasonic sonar,
the source and receiver are spaced apart and the acoustic energy along the direct path does not constrain the maximum
acoustic power level output of the transmitter. In a monostatic configuration the acoustic signal suffers from beam
spreading and natural absorption during propagation from the transmitter to the target and from the target back to the
receiver. In a bistatic configuration the acoustic propagation is in one direction only and theoretically the detection
distance can be twice the monostatic distance. For comparison the experiments of a human walking in a building
hallway using the bistatic and monostaic Doppler sonars in air were conducted. The experimental results for human
signatures from these Doppler sonars are presented and discussed.
KEYWORDS: Acoustics, Receivers, Particles, Interfaces, Scattering, Land mines, Signal attenuation, Critical dimension metrology, Wave propagation, Chemical elements
Land mines buried a few inches below the surface of the ground can be found by acoustic excitation of the porous ground surface and measuring the particle velocity at the surface. There are various theoretical models describing the ground: from a rigid porous frame model to a compete layered poroelastic description. The goal of this paper is to use the approach of Berry et al. to calculate the acoustic field at points on the ground surface in the vicinity of an object buried in a rigid, porous soil. The excitation is point sound source placed in the air above the ground, which is modeled a rigid, porous frame. A boundary element method is used for numerical integration to calculate the scattered acoustic field due to the presence of the object. This study represents the first step towards developing a complete model of acoustic scattering from near-surface objects embedded in a layered poroelastic material. The predicted disturbance associated with the buried object is much smaller than observed in field measurements.
Measurements of a soil surface using a laser Doppler vibrometer are associated with the vibration velocity of the solid particles. Therefore, to model these measurements the deformation of the solid granular frame must be described. To properly account for the coupling of sound into the earth's surface it must be modeled as a porous medium. One model described wave propagation through porous materials with a deformable framework was developed by Biot. Poro- elastic material, described by Biot, can support two dilatational waves and one rotational wave. The dilatational waves are usually referred to as fast, or type I, waves and slow, or type II, waves. These waves deform both the solid and fluid components as they propagation. An overview of the Biot poro-elastic model is presented. Laboratory measurements on an air-filled unconsolidated packing of sand, are discussed to illustrate the predicted behavior of poro-elastic materials. The sand was excited using an acoustic wave from an air-borne source. The transmitted waves were detected using geophones and microphones buried within the sand. These measurements are compared to those using a mechanical shaker in contact with the surface.
The ratio of the surface soil particle velocity to the surface acoustic pressure is termed the acoustic to seismic transfer function. Measurements of this transfer function typically show several maximum and minimum in the frequency range between 50-500 Hz. The magnitude of this transfer function can be explained in light of the porous nature of the ground surface .The ground is modeled as a poro-elastic layer overlying a non-porous substrate. The boundary conditions at the air/porous soil and the porous soil/non- porous substrate interfaces are applied to setup the acoustic-to-seismic coupling problem. In the porous layer, up an downing going Biot Type I, II compressional and shear plane waves are allowed. In the non-porous elastic substrate down going compressional and shear plane waves are allowed. Using the Biot characteristics equations and these boundary conditions the steady state frequency dependent acoustic to seismic transfer function is calculated. Layer depths, Type I, and shear wave speeds are determined from a shallow seismic refraction survey. Soil density, air porosity and permeability are determined from other measurements. The calculated transfer functions are compared to that measured on several outdoor grounds.
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