Time-reversal techniques in seismic detection of buried objects.

摘要

This dissertation presents an investigation of the behavior of time-reversal focusing in soils. A basic study of time-reversal using a linear finite-difference timedomain (FDTD) model of the soil is presented. Initial numerical models show timereversal focusing to be effective in elastic media, including when a large number of scattering objects were present in the medium. When scattering objects are present, time-reversal focusing demonstrates superior focusing ability when compared to other excitation methods such as uniform excitation or time-delay focusing.; The numerical studies are followed by multiple investigations of experimental time-reversal focusing performed in sand. Time-reversal focusing effectiveness is evaluated first for experimental conditions similar to the numerical model, with multiple nearsurface scattering objects present in the medium. Time-reversal focusing is shown to be effective in the experimental context as well. Further experiments examine timereversal focusing in the more extreme case where the entire ballistic wave is blocked, and the only energy reaching the focus point is reflected from scattering objects in the medium. Time-reversal focusing is effective in focusing the remaining energy to the focus location which was in a shadowed region behind the large barrier that blocked the ballistic wave. A comparison to other focusing methods demonstrates that under these conditions, most focusing attempts with traditional methods will fail completely while time-reversal focusing does not. Additional configurations of time-reversal focusing examine its effectiveness when scattering is caused by an asymmetrical surface layers. The impact of an asymmetrical or non-uniform excitation array is also examined for time-reversal focusing in the presence of scattering objects.; An investigation of the effects of scattering object geometry is undertaken to examine the effect scattering object shape, orientation, and number of scattering objects on focusing resolution in time-reversal focusing. Scattering object field density is found to have a strong, but diminishing effect on focusing resolution as the scattering object field density increased. Loss of surface wave energy available for focusing due to mode-conversion is found to be correlated with the density of the scattering object field. Soil is a complicated non-linear medium which normally behaves in a quasi-linear fashion for the range of amplitudes in which the earlier experiments were performed.; The impact of the weak non-linear nature of the soil on time-reversal focusing is examined through a study of time-reversal focusing behavior for a variety of amplitudes that generate different levels of non-linearity in the soil. This study of nonlinearity is coupled with a study of the impact of noise on time-reversal focusing. It appears that both non-linearity and noise have an impact on time-reversal focusing effectiveness. Further, the loss from these mechanisms seems to be interrelated. Noise seems to enhance non-linear loss in the soil.