High Bandwidth Acoustic Detection System (HBADS) is an emerging active acoustic sensor technology undergoing study by the US Army’s Night Vision and Electronic Sensors Directorate. Mounted on a commercial all-terrain type vehicle, it uses a single source pulse chirp while moving and a new array (two rows each containing eight microphones) mounted horizontally and oriented in a side scan mode. Experiments are performed with this synthetic aperture air acoustic (SAA) array to image canonical ground targets in clutter or foliage. A commercial audio speaker transmits a linear FM chirp having an effective frequency range of 2 kHz to 15 kHz. The system includes an inertial navigation system using two differential GPS antennas, an inertial measurement unit and a wheel coder. A web camera is mounted midway between the two horizontal microphone arrays and a meteorological unit acquires ambient, temperature, pressure and humidity information. A data acquisition system is central to the system’s operation, which is controlled by a laptop computer. Recent experiments include imaging canonical targets located on the ground in a grassy field and similar targets camouflaged by natural vegetation along the side of a road. A recent modification involves implementing SAA stripmap mode interferometry for computing the reflectance of targets placed along the ground. Typical strip map SAA parameters are chirp pulse = 10 or 40 ms, slant range resolution c/(2*BW) = 0.013 m, microphone diameter D = 0.022 m, azimuthal resolution (D/2) = 0.01, air sound speed c ≈ 340 m/s and maximum vehicle speed ≈ 2 m/s.
There is interest in imaging ground target threats that are hidden in vegetation, straw grass, and foliage. In some instances, radar signals cannot penetrate through the “clutter.” Long wavelength sound waves might be capable of penetrating through the clutter so that the target can be acoustically detected and imaged. We study sound scattering by a canonical target “model disk” (aluminum, 4 inches diameter x 0.75 inch thick) in the presence of clutter. The clutter is modeled by a vertical hexagonal array of slender stainless steel rods that have an overall diameter D = 8 cm. The individual rods (diameter d = 0.089 cm, length L = 91 cm) are aligned and supported by two perforated thin (thickness = 1.8 mm) circular aluminum plates of diameter = 10 cm. Results for received backscattered tone burst echoes (at a frequency of 25.8 kHz) show that it is possible to detect the details of the target disk in the presence of a twodimensional circularly shaped cluster of rigid cylinders in air (representing clutter). The disk and “model” clutter targets were then taken out of the laboratory environment to an outdoor test site where the High Bandwidth Acoustic Detection System (HBADS) [developed by the US Army’s Night Vision and Electronic Sensors Directorate] performed an acoustic imaging experiment on the target-clutter and mechanical rigging apparatus. Using a linear frequency modulated LFM chirp pulse signal (2-15 kHz) driving a single speaker, echoes are detected by a 16 element microphone array while the HBADS vehicle is traveling ~ 1 m/s along a road. The strip-mapped synthetic aperture acoustic array SAA can image certain features of the apparatus.
When airborne sound at two primary tones, f1, f2 (closely spaced near a resonance) excites the soil surface over a buried landmine, soil wave motion interacts with the landmine generating a scattered surface profile which can be measured over the "target." Profiles at the primaries f1, f2, and nonlinearly generated combination frequencies f1-(f2-f1) and f2+(f2-f1) , 2f1-(f2-f1), f1+f2 and 2f2+(f2-f1) (among others) have been measured for a VS 2.2 plastic, inert, anti-tank landmine, buried at 3.6 cm in sifted loess soil and in a gravel road bed. [M.S. Korman and J.M. Sabatier, J. Acoust. Soc. Am. 116, 3354-3369 (2004)]. It is observed that the "on target" to "off target" contrast ratio for the sum frequency component can be ~20 dB higher than for either primary. The vibration interaction between the top-plate interface of a buried plastic landmine and the soil above it appears to exhibit many characteristics of the mesoscopic/nanoscale nonlinear effects that are observed in geomaterials like sandstone. Near resonance, the bending (softening) of a family of increasing amplitude tuning curves, involving the vibration over the landmine, exhibits a linear relationship between the peak particle velocity and corresponding frequency. Tuning curve experiments are performed both on and off the mine in an effort to understand the nonlinearities in each case.
The vibration interaction between the top-plate of buried VS 1.6 and VS 2.2 plastic, anti-tank landmines and the soil above it appears to exhibit similar characteristics to the nonlinear mesoscopic/nanoscale effects that are observed in geomaterials like rocks or granular materials. In nonlinear detection schemes, airborne sound at two primary frequencies f1 and f2 (chosen several Hz apart on either side of resonance) undergo acoustic-to-seismic coupling. Interactions with the compliant mine and soil generate combination frequencies that, through scattering, can effect the vibration velocity at the surface. Profiles at f1, f2, f1-(f2-f1) and f2+(f2-f1) exhibit a single peak while profiles at 2f1-(f2-f1), f1+f2 and 2f2+(f2-f1) are attributed to higher order mode shapes. Near resonance the bending (softening) of a family of increasing amplitude tuning curves (involving the surface vibration over the landmine), exhibits a linear relationship between the peak particle velocity and corresponding frequency. Subsequent decreasing amplitude tuning curves exhibit hysteresis effects. New tuning curve results for buried M 14 and VS 50 plastic anti-personal landmines along with experiments with a buried “plastic drum head” mine simulant behave similarly. Slow dynamics explains the amplitude difference in tuning curves for first sweeping upward and then downward through resonance, provided the soil modulus drops after periods of high strain.
In nonlinear acoustic detection experiments involving a buried inert VS 2.2 anti-tank landmine, airborne sound at two closely spaced primary frequencies f1 and f2 couple into the ground and interact nonlinearly with the soil-top pressure plate interface. Scattering generates soil vibration at the surface at the combination frequencies | m f1 +- n f2 | , where m and n are integers. The normal component of the particle velocity at the soil surface has been measured with a laser Doppler velocimeter (LDV) and with a geophone by Sabatier et. al. [SPIE Proceedings Vol. 4742, (695-700), 2002; Vol. 5089, (476-486), 2003] at the gravel lane test site. Spatial profiles of the particle velocity measured for both primary components and for various combination frequencies indicate that the modal structure of the mine is playing an important role. Here, an experimental modal analysis is performed on a VS 1.6 inert anti-tank mine that is resting on sand but is not buried. Five top-plate mode shapes are described. The mine is then buried in dry finely sifted natural loess soil and excited at f1 = 120 Hz and f2 = 130 Hz. Spatial profiles at the primary components and the nonlinearly generated f1 - (f2 - f1) component are characterized by a single peak. For the 2f1+f2 and 2f2 + f1 components, the doubly peaked profiles can be attributed to the familiar mode shape of a timpani drum (that is shifted lower in frequency due to soil mass loading). Other nonlinear profiles appear to be due to a mixture of modes. This material is based upon work supported by the U. S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.
Measurements of the acoustic impedance of a VS 2.2 anti-tank plastic landmine reveal significant resonances in the frequency range between 80 and 650 Hz. The top surface resonances are due to its complicated mechanical structure vibrating in air. The lowest mode of the landmine results in a high Q simple harmonic oscillator resonance of the top surface, which behaves like a rigid mass. At higher frequencies the top surface behaves like thin circular plat acoustic modes. When these landmines are buried in soils, the modes are mass loaded. Resonances from measurements of the normal component of the acoustically induced soil surface particle velocity (due to sufficient acoustic-to-seismic coupling) are used for detection schemes. Since the interface between the top plate and the soil responds to pressure fluctuations nonlinearly, characteristics of landmines, the soil, and the interface are rich in nonlinear physics and allow for new methods of buried landmine detection not previously exploited. Here, the structure of a family of resonant tuning curves for relatively low amplitude, but nonlinear drive levels, reveals the “nonclassical” nonlinear resonant behavior of the soil-landmine oscillator.
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.
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