In this paper, we consider the problem of detecting and locating buried land mines and subsurface objects by using seismic waves. We demonstrate an adaptive seismic system that maneuvers an array of receivers, according to an optimal positioning algorithm based on the theory of optimal experiments, to minimize the number of distinct measurements to localize the mine. The adaptive localization algorithm is tested using numerical model data as well as laboratory measurements performed in a facility at Georgia Tech. Cases with one and two targets are presented. It is envisioned that future systems should be able to incorporate this new method into portable mobile mine-location systems.
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.
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.