Time-Reversal Techniques in Seismic Detection of Buried Objects

Land mines;Elastic waves;Time-reversal

Norville, Pelham D. ; Electrical and Computer Engineering

University:Georgia Institute of Technology

Department:Electrical and Computer Engineering

关键词: Land mines;    Elastic waves;    Time-reversal;   

Others  :  https://smartech.gatech.edu/bitstream/1853/14475/1/norville_pelham_d_200705_phd.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

An investigation is presented of the behavior of time-reversal focusing in soils.Initial numerical models demonstrate time-reversal 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.Multiple experimental investigations of experimental time-reversal focusing performed in sand evaluate time-reversal focusing effectiveness when multiple near-surface scattering objects are present in the medium.Experimental results demonstrate that time-reversal focusing is effective in the experimental context as well as the numerical models.Further experiments examine time-reversal focusing in more extreme cases where the entireballistic wave is blocked, and the only energy reaching the focus point is reflected from scattering objects in the medium.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 on focusing resolution in time-reversal focusing is also presented.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.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.