Green function based ultrasonic super resolution imaging for defect location and characterization

Abstract

Green function is the important component for ultrasonic time reversal based super resolution imaging methods. Time reversal with multiple signal classification (TR-MUSIC) and its phase-coherent form (PC-MUSIC) are two typical imaging methods. In this paper, the effect of Green function on the TR-MUSIC and PC-MUSIC has been investigated. And TFM has also been introduced as a comparison. Point spread function modelling has been implemented to compare the performance of imaging methods under different forms of Green function via simulation. Several experiments have been implemented on metallic samples with defects. The defects can be divided into two categories: point-like targets and extended targets, depending on their relationship with the ultrasonic central wavelength. The perform of imaging methods under different forms of Green function has been compared in terms of lateral resolution, axial resolution, robustness to the noise and imaging for extended targets. A series of parameters have been defined to achieve quantitative comparison. The experimental results show that when compared with TR-MUSIC, PC-MUSIC is sensitive to the Green function. When the proper combination including Green function, the frequency bandwidth as well as the dimension of signal subspace has been setup, the defect can be correctly located and accurately quantitative evaluated based on the PC-MUSIC image.