George S. McCall

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.

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.

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.

Surface-wave-based inversions of shallow seismic structure

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.

Probing signal design for seismic landmine detection

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.

Evaluation of seismic noise for landmine detection system development

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 sensors 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 sensors 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).

Characterization of elastic wave propagation in soil

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 Institutes 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.

Environmental factors that impact the performance of a seismic landmine detection system

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 earths 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.

Test chamber for determining damage thresholds for high‐amplitude underwater sound exposure in animal models

In order to determine the effects of low‐frequency underwater sound on small animal models it is desirable to expose them to well‐characterized fields which closely simulate the (locally) plane‐wave open ocean stimulus. It is also desirable to produce pure pressure and acceleration stimuli to isolate the effects of the individual components of the acoustic plane wave. It is necessary to produce sufficiently strong signals to enable damage thresholds to be determined. The range of frequencies over which such stimuli must be produced is quite broad since it must include the actual band of interest (100–500 Hz) to examine damage on a cellular and tissue level, as well as at scaled frequencies (∼1–5 kHz) to examine the effects of organ structure. The problem is subtler than one might imagine because of the high compliance of the test animals due to the air in their lungs. The lungs will drastically alter the acoustic field in the chamber and change the loading on the sources and the chamber could alter the vi...