Time-reversal focusing is experimentally applied in an elastic medium in the presence of multiple scattering objects on the order of a wavelength in size. The effectiveness of time-reversal is compared to time-delayed focusing and uniform excitation of the transducer array for focusing energy to a desired location within the medium. A filter is also designed to improve the bandwidth of the excitation signal. Time-reversal focusing is investigated in the context of an elastic-wave landmine detection system. Results are presented demonstrating the advantages and limitations of time-reversal in excitation of a resonance in a TS-50 landmine buried in the medium. A special case is presented for a landmine buried in shadow regions where uniform excitation fails to illuminate the target while time-reversal focusing yields improved target illumination.
The effectiveness of time-reversal focusing is evaluated in the presence of an asymmetric surface layer that changes the direction of the propagating waves, but does not continually scatter or block the propagating wave front. Interactions between the wave front and the surface layer are dependent on the depth and material properties of the asymmetric surface layer and its orientation in the medium with respect to the incident wave. Time-reversal focusing is shown to perform significantly better than other excitation methods for the purpose of delivering energy to the location of a buried land mine.
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.
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