Time-reversal focusing is studied in the context of an
elastic-wave landmine-detection system. Time-reversal focusing
has been previously applied to the system and proven to be useful
in focusing energy to targets in inhomogeneous media with discrete
non-uniform scattering objects. In earlier experiments, these
scattering objects took the form of multiple rocks buried throughout the region of interest. In this study, the performance of time-reversal focusing is evaluated for the case of uniform discrete scattering objects. Cylindrical and spherical scatterers are buried below the surface to provide uniform scattering. In other media, high concentrations of uniform scatterers have been observed to produce super-resolution of the time-reversal focus point. In this paper, the super-resolution phenomenon is examined in the context of the elastic wave landmine detection system operating in a soil medium.
Time Reversal is based on the fact that most physical laws of nature are invariant for time reversal, i.e., when time t is replaced by -t, most physical laws remain unchanged. Physically this means that by time reversing, a particle will retrace its original path or trajectory. Based on this fact, systems were built which receive reflections or scattering from targets. If this reflected data is recorded, time reversed and launched into the medium again, it will focus back on the targets. This is the basis for experimental time reversal. Time reverse imaging is somewhat different in the sense that scattering from targets are recorded on the sensors, but then back propagated numerically. Narrow-band or single frequency MUSIC based time-reverse imaging algorithms have been proposed in literature for point-like targets. When this algorithm is applied to scattering from an extended target, such as a landmine, the image has good cross-range resolution, but rather poor range resolution. We propose the use of 2-D MUSIC-based algorithm to improve the near-field range resolution, which can then be used in conjunction with single frequency MUSIC to produce a final high-resolution image. A FDTD elastic-wave simulation is used to verify the results using mines and mine-like targets embedded in a heterogenous soil.
A system is under development at the Georgia Institute of Technology that utilizes a seismic source to propagate Rayleigh waves through a medium such as soil. Non-surface-contacting electromagnetic sensors are used to detect the displacement of the medium created by interaction of the Rayleigh waves with a target, such as a landmine. The system has been tested in a relatively uncluttered medium and has yielded encouraging results, demonstrating that the system is effective for the detection of targets buried just below the surface.
The system performs well in an uncluttered medium. However, when the medium is filled with a large number of scattering objects, the Rayleigh wave will be broken up by the scatterers in the medium to the point that the wave front no longer interacts with the target as it would in an uncluttered medium. This causes detection of a target to be uncertain or impossible. In an effort to extend the application of this system to a highly cluttered medium, the time reversal method is applied to the seismic system, and evaluated for focusing Rayleigh wave fronts at a desired location. Numerical and experimental results are presented for a propagation medium with no scatterers present, and with multiple scatterers present. Time-reverse focusing results are also compared to uniform excitation and time-delay beamforming methods.
An investigation of the feasibility of detecting structures buried underground through passive listening techniques will be presented. Passive detection of structures will be analyzed using elastic wave sources originating inside the structure and from sources exterior to the structure and on the surface. The primary method of investigation will be numerical models using the finite-difference time-domain method (FDTD).
A source inside the structure excites elastic waves in the structure, a portion of which travel upward along the walls of the structure and onward to the surface. An alternate form of excitation is a source such as a train, large vehicle, or an explosion located on the surface, away from the structure. Waves from this source interact with the structure and a portion of them travel up from the structure to the surface.
An array of sensors is constructed to map the field at the surface and to determine the location and basic characteristics of the structure. Generally, structures examined will be on the order of the size of an underground tunnel complex or buried room and elastic wave sources will be in the low frequency range of large machinery or vehicles.
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.