Recently, seismic landmine detection techniques have been investigated using ground-contacting sensors to measure ground motion generated by propagating surface waves and their interactions with buried objects such as landmines and clutter. Seismic waves have been generated using both ground-coupled and airborne sources, while non-contact sensors such as radar and laser-Doppler vibrometers have been preferred due to safety concerns. However, ground-contacting sensors can be effectively used provided that the contact with the ground does not adversely affect the propagation of seismic waves, that the sensor to ground coupling is repeatable, and that the sensors have low enough contact force to preclude triggering buried landmines. A groundcontacting sensor has been built with a low-cost commercially available accelerometer in a small lightweight package that ensures consistent coupling to the ground. Design and development of the sensor included experimental testing of several prototypes in a laboratory model, as well as analytical modeling of sensor response. A thirty-two-element line array capable of adjusting to surface contours of up to eight inches was tested at a U.S. Government facility in a temperate climate. The array enabled high-contrast detections of several AT landmines in both dirt and gravel roadbed sites.
KEYWORDS: Sensors, Land mines, Data acquisition, Field programmable gate arrays, Safety, Interfaces, Computer aided design, Prototyping, Time metrology, Wave propagation
Inexpensive ground-contacting accelerometers have been demonstrated in field experiments as appropriate vibrometers for a seismic landmine detection system. A thirty-two-element line array of these has been used to detect a variety of anti-tank (AT) landmines under realistic field conditions. Images of data measured by scanning this line array to synthesize a larger plane array have shown that the two-dimensional array offers potential advantages in terms of both measurement speed and landmine image contrast. The simultaneity of measurements with a physical array, as compared to synthetic array measurements that have been performed in the past, presents opportunities for improved landmine detection algorithms. Issues pertaining to the implementation of large arrays of vibrometers include sensor fidelity, array fidelity, scalability, and safety. Experimental measurements with prototype sensors in the laboratory and at a field test site have demonstrated the robust and repeatable ground coupling of the sensor in sand, dirt, gravel, and grass. Ground loading has been investigated with multiple array configurations with the dominant effect being an increase in the wave speeds of the surface waves. While the field experiments with the line array were conducted using commercially available data acquisition hardware and software, a custom data acquisition and processing system has been developed to meet the requirements of a large array of sensors. A lightweight sensor ensures the safety of touching the ground over buried landmines as the contact force is significantly less than the force required to detonate typical anti-personnel (AP) landmines and AT landmines.
Several non-contact vibrometers have been investigated for use in a seismic landmine-detection system developed at Georgia Tech; however, these non-contact vibrometers are relatively complex and expensive compared to commercially available microphones. This makes the commercial microphones an appealing alternative in applications where reduced surface-standoff distances are permissible (such as small autonomous systems or hand-held mine detectors that exploit seismic techniques). The seismic wave field involves multiple modes of propagation. Among these, the Rayleigh wave has been found to be particularly effective for the interrogation of near-surface soil where landmines are likely to be found. Thus the seismic system currently under development preferentially excites this wave type. The acoustic pressure in the air that results from a Rayleigh wave’s surface displacement can only be sensed close to the ground because Rayleigh waves are subsonic in most soils and produce evanescent acoustic fields in the air. Experimental measurements in a laboratory model have shown that buried pressure-fused landmines can be detected by measurement of the acoustic pressure within about five centimeters of the ground’s surface. Signal processing efforts including planar near-field acoustic holography, k-space filtering, and mode extraction have been used to amplify the effects of the Rayleigh wave. The signal-to-noise ratio of microphone measurements can also be improved by decreasing the microphone’s height above the soil surface or by improving the coupling of the microphone to the evanescent field with a waveguide or a horn. Experimental measurements made with the microphone compare well with direct measurements of surface displacement made using a radar-based non-contact vibrometer that has been described in previous papers.
A technique has been developed that exploits remote seismic sources and local measurement of the surface displacement of the ground for the detection of buried landmines. Most of the previously reported investigation of this technique has focused on non-contact displacement sensors in order to ensure the safety of the operators of both handheld and vehicle-based systems. This is not inherently a constraint that requires a non-contact sensor, but rather one requiring a sensor that is non-intrusive (i.e. its presence does not alter the measured quantity). Current research is directed toward the development of autonomous and semi-autonomous robotic systems based on this technique. Here both unit cost and power consumption are issues of comparable importance to the survival of the sensor platform. Non-intrusive surface-contacting vibrometers are therefore a reasonable alternative. Several configurations have been studied for suitable vibrometers. The configuration that has shown the most promise is based on a commercial accelerometer coupled to the ground with a small normal force and isolated from the backing structure that is used to reposition it between measurements. It is a relatively simple matter to detect seismic motion with an accelerometer. The major issue in an effective implementation of the technique is to combine reproducibility with fidelity in the measurement. These are competing goals in that reproducibility is easily achieved with large normal forces, but fidelity requires that these be small. Sufficient reproducibility for imaging purposes has been achieved with normal forces that pose no danger of landmine detonation. Unlike reproducibility, fidelity is linked to both the nature of the imposed force and to its magnitude through the nonlinearity of the soil’s elasticity. Both continuous and incremental motions of the sensor platform have been studied, although incremental movement shows the most promise for the intended application.
KEYWORDS: Land mines, Wave propagation, Sensors, Time metrology, Soil science, Radar sensor technology, Radar, Signal to noise ratio, Particles, Electrodynamics
Experimental and numerical models have been utilized at Georgia Tech in the research and development of a seismic landmine detection technique which generates seismic waves in the soil using a surface-coupled electrodynamic transducer and detects normal surface displacements with a non-contact radar sensor. As the numerical models have shown a strong dependence upon material properties of the soil as a function of depth, experiments have been conducted at six field sites and in the experimental model to quantify the effect of different soil conditions upon the operation of the seismic landmine detection system and to measure depth-dependent material properties. Measurements have been made with and without buried anti-personnel and anti-tank mines to determine the effects that landmines have upon the propagation of seismic waves. Surface waves have been measured using the non-contact radar sensor as well as triaxial accelerometers and geophones. Post-processing has included the examination of particle motion in three dimensions, the identification of individual wave types through polarity tracking and dispersion curves, and the extraction of individual propagating waves. The field sites include wet and dry sand at a beach, a roadbed at a U. S. Government facility in a temperate climate, frozen ground, clayey soil with and without rocks, and a silt-sand mixture in a coastal region.
KEYWORDS: Signal to noise ratio, Land mines, Interference (communication), Sensors, Signal detection, Complex systems, Time metrology, Electronic filtering, Binary data, Nonlinear optics
This paper addresses the design of time-domain signals for use as seismic excitations in a system that images buried landmines. The goal of the design is the selection of a signal that provides sufficient contrast for the post-processed landmine image in the shortest possible measurement time. Although the goal is relatively straightforward and the problem appears similar to one of system identification for a linear time invariant (LTI) system, practical implementation of many commonly accepted approaches to the system-identification problem has proven difficult. The reason for this is that the system under consideration exhibits observable nonlinearity over the entire range of drive levels that are of interest. The problem is therefore constrained by the requirement that nonlinear effects be tolerable rather than imperceptible (i.e. that the nonlinearity be sufficiently weak that the system can be reasonably characterized as linear). Several candidate signal types that have been shown to offer good noise immunity for the LTI system identification problem were considered. These included circular chirps, binary-sequence-based (BSB) signals, and numerically optimized randomly seeded multisines. Based on purely experimental figures of merit, circular chirps with flat amplitude and linearly swept frequency offered the best performance among the signals that were tested.
KEYWORDS: Land mines, Mining, Sensors, Time metrology, Radar, Prototyping, Data acquisition, Signal processing, Signal detection, Radar sensor technology
A technique for the detection of buried landmines, which uses a seismic probing signal in conjunction with a non-contact radar-based surface displacement sensor, has been studied for several years at Georgia Tech. Laboratory experiments and numerical models have indicated that this technique shows great promise for imaging a large variety of mine types and burial scenarios. In order to develop a detection system based on this technique, recent studies have focused on transitioning the experimental work from laboratory models to realistic field environments, which poses several challenges for system development. Unknown soil properties at field sites as well as the presence of local inhomogeneities, vertical stratification, and surface variations make the propagation and the modal content of the seismic probing signal more difficult to predict. This complicates the processing required to image buried mines. The small-scale surface topography and naturally-occurring ground cover impede the function of the system's non-contact sensor, which must be capable of looking through the ground cover and spatially averaging its measurement over the irregular surface. A prototype detection system has been tested at several field sites with widely disparate soil properties. Problems were encountered that required modifications to the system sensor, scanning technique, and signal processing algorithms. Following these changes, system performance comparable to that observed in laboratory models was demonstrated during field testing.
For several years a system has been under development at Georgia Tech that uses seismic surface waves to detect and image buried landmines. The details of this system have been previously reported in the literature. Current work involves the transition from a laboratory experimental system to a field-operable experimental system with the ultimate goal of creating an integrated field-operable prototype. Several issues have arisen in the transition to field testing. One of these is the nature and magnitude of the noise levels that limit system performance at field sites and the relevance of these for predicting noise that might be encountered in a realistic demining scenario. Noise introduced to the system sensor (a radar-based, non-contact displacement sensor) can arise from many sources (both natural and manmade). It may be received through a variety of mechanisms in addition to the sensor's primary transduction mechanism. Moreover, even noise which is received through the primary transduction mechanism need not involve purely seismic motion of the ground that is being interrogated. It might instead represent motion of the sensor's support structure or the purely local coupling of airborne noise into surface motion. To understand these effects, measurements have been made using ground contacting sensors at four field sites where other system-related measurements have also been made. The nature of the noise measurements has required that refinements be made to both the sensors themselves (triaxial geophones) and to the data acquisition system used for the measurement of the system’s seismic interrogation signals (a 12-bit, PC-based digitizer).
The inversion of surface wave propagation measurements to determine soil properties within a few meters of the surface is being investigated to facilitate the development and simulation of seismic landmine detection techniques. Knowledge of soil types, soil material properties, inhomogeneities, stratification, water content, and nonlinear mechanisms in both the propagation path and the source-to-surface coupling can be used to validate and improve both numerical and experimental models. The determination of the material properties at field test sites is crucial for the continued development of numerical models, which have shown a strong dependency on the assumed soil parameter variations in elastic moduli and density as a function of depth. Field experiments have been conducted at several test sites using both surface and sub-surface sensors to measure the propagation of elastic waves in situ with minimal disruption of the existing soil structure. Material properties have been determined from inversion of surface wave measurements using existing spectral analysis of surface waves (SASW) techniques. While SASW techniques are computer-intensive, they do not disturb the existing soil structure during testing as do borehole and trench techniques. Experimental data have been compared to results from 3-D finite-difference time-domain (FDTD) modeling of similar soil structures and measurement methods.
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.
To optimize a landmine detection system currently being developed at Georgia Tech that uses both electromagnetic and elastic waves, wave propagation in soils has been studied to evaluate propagation characteristics and to identify nonlinear mechanisms. The system under development generates elastic waves in the soil using a surface-contacting transducer designed to preferentially excite Rayleigh waves, thus interrogating the surface layers of the soil. These waves propagate through the region of interest and interact with buried landmines and typical clutter objects (i.e., rocks, sticks, and man-made objects). Surface displacements are measured using a non-contact radar sensor that is scanned over the region of interest. To characterize the wave propagation effects as a function of drive amplitude and as a function of input signal type, a series of experiments was conducted using the radar sensor, accelerometers, and geophones at two test sites, the experimental model at Georgia Tech and a field test site at the Georgia Tech Research Institute's Cobb County Research Facility in suburban Atlanta. The two test sites presented different soils as the experimental model uses damp, compacted sand as a soil surrogate while the field test site has a well-weathered mixture of sand, silt, and clay. Surface displacement measurements were made using the radar sensor while both surface and subsurface measurements were made using triaxial accelerometers and geophones. Linear and nonlinear dispersion, wave speed changes, and nonlinear saturation were observed in the measured data.
An array of radars is developed as a stand off sensor for use in elastic/seismic mine detection systems. The array consists of N radar sensors which operate independently to sense the displacement of the surface of the earth due to elastic waves propagating in the earth. Each of the sensors consists of a lens-focused, conical, corrugated, horn antenna and a homodyne radar. The focused antenna allows the sensor to have greater standoff than with the previous unfocused antenna while maintaining the spatial resolution required for a mine detection system. By using an array of N sensors instead of a single sensor, the scan rate of the array is improved by a factor of N. A theoretical model for the focused antenna is developed and an array of two radars is developed and used to validate the theoretical model. This array is tested in both the experimental and the field models for the elastic mine detection system. Results from both systems are presented.
KEYWORDS: Land mines, Wave propagation, Sensors, Data modeling, Environmental sensing, Signal to noise ratio, Mining, Dispersion, Radar sensor technology, Radar
A system has been developed that uses high frequency seismic waves and non- contacting displacement sensors for the detection of land mines. The system consists of a moving displacement sensor and a stationary elastic-wave source. The source generates elastic waves in the earth. These waves propagate across the minefield where they interact with buried mines. The sensor measures the displacements at the earth's surface due to the passage of the waves and the interactions of the waves with mines. Because the mechanical properties of the mine are different from those of the earth, the surface displacements caused by the interaction are distinct form those associated with the free-field propagation of the waves. This provides the necessary cue for mine detection. The system has been demonstrated in a controlled laboratory environment, and efforts are currently underway to transition this work into field tests. Moving the experimental effort into the outdoor environment is a critical milestone toward the ultimate goal of this research effort, which is the design of a field-operable mine detection and classification system. There are many issues associated with this transition. Foremost among these is the propagation characteristics of seismic waves in the field environment and, particularly, the mechanisms that limit the energy which can be coupled into the seismic signal that is used to search for mines. To investigate this, a measurements was undertaken to determine the effects of environmental factors at both sites on the generation and propagation of seismic waves. At both sites, strong non-linearity was observed which limited the energy content of the incident signal.
Over the past three years a system has been under development at Georgia Tech that utilizes a seismic interrogation signal in combination with a non-surface- contacting, radar-based displacement sensor for the detection of buried landmines. Initial work on this system investigated the workability of the system concept. Pragmatic issues regarding the refinement of the current experimental laboratory system into a system which is suitable for field testing and, in turn, one which would be suited to field operations have been largely ignored until recently. Both field operations and realistic field testing require a system that is different from the original laboratory system in two crucial ways. One of these is that a field system needs a sensor standoff from the ground surface larger than the original 1 to 2 cm. This is necessary in order to account for small-scale topography, to avoid ground cover such as grass, and to minimize the risk to the operator. A second difference is that the scanning speed of a field system must be substantially greater than that of the original laboratory system, which takes several hours to image 1 m2 of ground surface. From an operational standpoint, the reason for this is obvious. From an experimental standpoint, it is also important because ambient conditions are difficult to control on long time scales outdoors. Both of these new requirements must be met within the design parameters that were established empirically during the development of the laboratory system.
A hybrid technique has been developed that uses both electromagnetic and elastic waves in a synergistic manner to detect buried land mines. The system consists of a moving electromagnetic radar and a stationary elastic-wave source. The source generates elastic waves in the earth. These waves interact with the buried mine and cause both the mine and the earth to be displaced. Because the mechanical properties of the mine are different from those of the earth, the displacements in the region of interaction are distinct from those associated with the free-field propagation of the waves. The radar is used to detect displacement and, thus, the mine. Initial investigations have demonstrated the feasibility of this scheme under controlled conditions. The current experimental effort if focused on understanding and overcoming the issues associated with using the system in field conditions.
A hybrid technique is presented that simultaneously uses both electromagnetic and acoustic waves in a synergistic manner to detect buried land mines. The system consists of an electromagnetic radar and an acoustic source. The acoustic source causes both the mine and the surface of the earth to be displaced. The electromagnetic radar is used to detect these displacements and, thus, the mine. To demonstrate the viability of this technique, experimental models have been constructed. The models use an electrodynamic transducer to generate an acoustic surface wave, a tank filled with damp sand to simulate the earth, simulated mines, and a radar to measure the vibrations. The technique looks promising; we have been able to measure the interactions of the acoustic waves with both simulated antipersonnel mines and antitank miens buried in damp sand. We have measured strong resonance in some of the mines; these resonances are shown to help differentiate the mine from clutter.
A hybrid technique is presented that simultaneously uses both electromagnetic and acoustic waves in a synergistic manner to detect buried land mines. The system consists of an electromagnetic radar and an acoustic source. The acoustic source causes both the mine and the surface of the earth to be displaced. The electromagnetic radar is used to detect these displacements and, thus, the mine. To demonstrate the viability of this technique, an experimental system has been constructed. The system uses an electrodynamic transducer to induce an acoustic surface wave, a tank filled with damp sand to simulate the earth, a simulated mine, and a radar to measure the vibrations. The technique looks promising; we have been able to detect both simulated antipersonnel mines and antitank mines buried in damp sand from the experimental results obtained with the system.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.