Summary
Time Reversal (TR) is a method applicable to classical wave systems of physics to locate and reconstruct unknown wave sources or to locate and image target scatterers (using specific signal processing enhancement and selection methods). TR relies on the principle of spatial reciprocity, i.e. the ray paths that a wave pulse will travel from a source to a receiver will be the same ray paths that an identical wave pulse will travel if the source and receiver positions are interchanged. Dispersion, multiple scattering, mode conversion, anisotropy, refraction, and weak attenuation do not limit the ability of using TR to focus on a source. For example, multiple scattering can be exploited in order to enhance the effective aperture of a Time Reversal Mirror if the forward propagation signals are collected within appropriately long time windows thereby improving TR focusing abilities. The only major limitation of TR is that TR relies on the invariance of the wave equation to the transform of t to -t for lossless media, thus the system (propagation medium and measurement apparatus) must not change in time. TR applications in elastic media include earthquake source localization and characterization in space and time, ground-based nuclear explosion monitoring, non-destructive evaluation of mechanical parts, and biomedical imaging.
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