James S. Martin

Experimental model for a seismic landmine detection system

A laboratory-scale experimental model has been developed and tested for a system that uses artificially generated high-frequency seismic waves in conjunction with a radar-based noncontact displacement sensor to detect buried landmines. The principle of operation of the system is to measure the transient displacement field very close to a mine location. In this way, the absorption and the geometrical spreading of the seismic waves have not reduced the effects of the mine. By using a seismic excitation, the system exploits the large difference between the elastic properties of a mine and the surrounding soil. This difference causes seismic wave interactions in the vicinity of a mine to be quite distinctive and provides a method for imaging mines and distinguishing them from typical buried clutter. Images of a variety of simulated and inert anti-tank and anti-personnel mines have been formed using this system. Burial scenarios involving natural clutter (rocks and sticks), light surface vegetation, localized burial effects, and multiple mines in close proximity have been studied. None of these scenarios appears to pose serious problems for detection performance.

Acousto-electromagnetic sensor for locating land mines

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.

Experimental investigation of the acousto-electromagnetic sensor for locating land mines

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 acoustic/electromagnetic technique for locating land mines

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. An experimental and numerical model for the system has been developed.

Field testing and development of a seismic landmine detection system

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 systems 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.

Ultrasonic displacement sensor for the seismic detection of buried land mines

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.

An investigation of surface-contacting sensors for the seismic detection of buried landmines

Techniques have been studied for the detection of buried landmines with acoustic/seismic interrogation signals. Much of this work has involved full wave-field imaging from local measurements of ground motion using noncontact sensors. These offer inherent safety for the system operator and accommodate the need to make measurements over rough ground surfaces. The system requirement is, however, only that a sensor does not intrude on the measurement rather than that it not contact the ground. An experimental investigation was conducted into the feasibility of an array of ground-contacting sensors for use in a seismic landmine-detection system that exploits full wave-field imaging. The main considerations in the design of the array sensor were safety, sensitivity, fidelity, reproducibility, and sensor-to-sensor interaction. A relatively simple and inexpensive sensor was demonstrated in an experimental simulation of a landmine-detection system. The sensor, which is suitable for inclusion in a large planar array that could be used for detection confirmation, exerts a safe normal force at the point of contact and enables detection performance comparable to that which could be achieved using noncontact techniques.

Technical issues associated with the detection of buried land mines with high-frequency seismic waves

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.

Surface-contacting vibrometers for seismic landmine detection

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.

Use of high-frequency seismic waves for the detection of buried landmines

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.