Time reversal acoustic (TRA) focusing allows concentration of elastic energy at a location in the soil being investigated to detect landmines. The TRA process is conducted by broadcasting a wide bandwidth signal and recording the surface vibration by a Laser Doppler Vibrometer (LDV). The system impulse response from speaker to the LDV output can then be computed by cross correlating the original and recorded signals for each channel. Each transducer re-radiates the time reversal impulse response. This provides efficient focusing of the seismic wave in both space and time, thus enhancing the nonlinear effects associated with soil and landmine vibrations. Using orthogonal initial signals the suggested TRA procedure can be implemented simultaneously with multiple transmitters to increase the scanning speed. The nonlinear effects were investigated using a phase inversion method where the TRA signal is broadcast a second time with an opposite sign and the two received signals are added in post processing. The summed signal contains mainly the results of nonlinear wave interaction and tends to cancel the linear response. Small scale land mine detection experiments were conducted using a laser Doppler vibrometer and an array of speakers in the frequency band 50-500Hz. They demonstrate that the TRA system provides high concentration of elastic wave energy in the tested area. The measurements of spectral density of the TRA focused signal reveal increased spectral density in the vicinity of mine resonance frequencies. The nonlinear TRA phase inversion method shows higher contrast between mine and no mine than the linear TRA method.
Buried in soil, landmines exhibit distinguishable nonlinear dynamic characteristics. These characteristics have been successfully used for nonlinear acoustic/seismic detection of both antipersonnel and antitank landmines. Despite a high potential of the nonlinear acoustic landmine detection technique, its utility is currently limited by a relatively high noise level of the LDV at frequencies typically used for landmine detection. To mitigate this limitation, we propose a modulation approach that exploits a nonlinear interaction of the low frequency resonance vibrations and higher frequency sound waves. The result of the modulation is manifested in a high frequency range as additional spectral components at the combination frequencies. The nonlinear response of the soil-mine dynamic system measured at the combination frequencies is used for the detection of the buried landmine. Exploring the higher frequency range has another benefit of using a directional high frequency sound source.
The detection of land mines using acoustic and seismic excitation is problematic due to the small amplitude of vibration that can be induced in the soil. Increasing this level reduces the requirement on a sensor’s noise floor and may be useful for nonlinear detection. For these experiments, an array of loudspeakers broadcast orthogonal noise signals to excite ground vibrations. A contacting geophone measures the system’s vibration response to all signals. We then correlate an excitation signal with the measured vibration response to approximate the system impulse response between a loudspeaker and the geophone. Time reversing the impulse response generates a pre-filter for each loudspeaker. Subsequent signals transmitted through the pre-filter and loudspeaker tend to be temporally focused at the receive location as well as greater in amplitude. Results compare vibration amplitude with and without the time reversal process for spatial locations near the mine.
Researchers in academia have successfully demonstrated acoustic landmine detection techniques. These typically employ acoustic or seismic sources to induce vibration in the mine/soil system, and use vibration sensors such as laser vibrometers or geophones to measure the resultant surface motion. These techniques exploit the unique mechanical properties of landmines to discriminate the vibration response of a buried mine from an off-target measurement. The Army requires the ability to rapidly and reliably scan an area for landmines and is developing a mobile platform at NVESD to meet this requirement. The platform represents an initial step toward the implementation of acoustic mine detection technology on a representative field vehicle. The effort relies heavily on the acoustic mine detection cart system developed by researchers at the University of Mississippi and Planning Systems, Inc. The NVESD platform consists of a John Deere E-gator configured with a robotic control system to accurately position the vehicle. In its present design, the E-gator has been outfitted with an array of laser vibrometers and a bank of loudspeakers. Care has been taken to ensure that the vehicle’s mounting hardware and data acquisition algorithms are sufficiently robust to accommodate the implementation of other sensor modalities. A thorough discussion of the mobile platform from its inception to its present configuration will be provided. Specific topics to be addressed include the vehicle’s control and data acquisition systems. Preliminary results from acoustic mine detection experiments will also be presented.
The resonant behavior of landmines has been exploited by an acoustic detection technique to find buried mines. The resonance of the buried mine is induced by broadcasting an acoustic wave, which couples into the ground. The resonating mine causes the soil above it to vibrate and this vibration is measured with either a laser Doppler vibrometer (LDV) or a geophone. A set of resonance frequencies, which can be attributed to the design, material, and dimensions of the mine, is exhibited when the mine, sitting on a rigid surface above the ground, is excited by an acoustic wave. These resonance frequencies shift when the mine is buried. Acoustic models have been developed to predict these burial effects on mine resonant frequency behavior. This paper will discuss measurements made of several mines of the same type buried at various depths and will compare these measurements to predictions made by a lumped element model.
A system is under development that uses seismic surface waves to detect and image buried landmines. The system, which has been previously reported in the literature, requires a sensor that does not contact the soil surface. Thus, the seismic signal can be evaluated directly above a candidate mine location. The system can then utilize small amplitude and non-propagating components of the seismic wave field to form an image. Currently, a radar-based sensor is being used in this system. A less expensive alternative to this is an ultrasonic sensor that works on similar principles to the radar but exploits a much slower acoustic wave speed to achieve comparable performance at an operating frequency 5 to 6 decades below the radar frequency. The prototype ultrasonic sensor interrogates the soil with a 50 kHz acoustic signal. This signal is reflected from the soil surface and phase modulated by the surface motion. The displacement can be extracted from this modulation using either analog or digital electronics. The analog scheme appears to offer both the lowest cost and the best performance in initial testing. The sensor has been tested using damp compacted sand as a soil surrogate and has demonstrated a spatial resolution and signal-to-noise ratio comparable to those that have been achieved with the radar sensor. In addition to being low-cost, the ultrasonic sensor also offers the potential advantage of penetrating different forms of ground cover than those that are permeable to the radar signal. This is because density and stiffness contrasts mediate ultrasonic reflections whereas electromagnetic reflection is governed by dielectric contrast.