Ultrasonic imaging in solids using time reversal operator and coherent matched field processing

This paper presents a new approach in ultrasonic imaging of solids using ultrasonic Synthetic Aperture Focus Technique (SAFT) with primary applications to the Non-Destructive Evaluation (NDE) of materials and structures. Specifically, the study utilizes Matched-Field Processing (MFP) techniques that exploit the spatial structures of the ultrasonic fields for detection and localization of scatters in the medium. Coherent-broadband MFP is compared to narrowband processing using both conventional time-delay beamformers and the Minimum Variance Distortionless Response (MVDR) beamformers. The decomposition of the time reversal operator (D.O.R.T.) method for active sensing is applied in the eigenmode decomposition of the MFP covariance matrix. The correlation between the scatters and the eigenvectors is studied in both narrowband and broadband MFP beamforming. This study also proposes an improvement in modeling the propagating waves in MFP beamforming. Specifically, the displacement structure of the longitudinal mode is considered in the array reception. Experimental testing demonstrates the ability of the DORT method to successfully separate multiple scatters in the test medium.