A reverse time migration-based multistep angular spectrum approach for ultrasonic imaging of specimens with irregular surfaces

Xiongbin Yang; Kai Wang; Yanfeng Xu; Lei Xu; Wenxiang Hu; Hao Wang; Zhongqing Su

doi:10.1016/j.ultras.2020.106233

A reverse time migration-based multistep angular spectrum approach for ultrasonic imaging of specimens with irregular surfaces

Ultrasonics. 2020 Dec:108:106233. doi: 10.1016/j.ultras.2020.106233. Epub 2020 Jul 31.

Authors

Xiongbin Yang¹, Kai Wang¹, Yanfeng Xu¹, Lei Xu¹, Wenxiang Hu², Hao Wang², Zhongqing Su³

Affiliations

¹ Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong Special Administrative Region.
² Institute of Acoustics, Tongji University, Shanghai, 200092, PR China.
³ Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong Special Administrative Region; The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, PR China. Electronic address: Zhongqing.Su@polyu.edu.hk.

PMID: 32771810
DOI: 10.1016/j.ultras.2020.106233

Abstract

We develop a new ultrasonic imaging framework for non-destructive testing of an immersed specimen featuring an irregular top surface and demonstrate its capability of accurately depicting the lower surfaces of multiple damages hidden in the specimen. Central to the framework is a multistep angular spectrum approach (ASA), via which the forward propagation wavefields of wave sources and backward propagation wavefields of the received wave signals are calculated. Upon applying a zero-lag cross-correlation imaging condition of reverse time migration (RTM) to the obtained forward and backward wavefields, the image of the specimen with an irregular surface can be reconstructed, in which hidden damages, if any and regardless of quantity, are visualized. The effectiveness and accuracy of the framework are examined using numerical simulation, followed with experiment, in both of which multiple side-drilled holes, at different locations in aluminum blocks with various irregular surfaces, are characterized. Results have proven that multiple damages in a specimen with an irregular surface can be individually localized, and the lower surface of each damage can further be imaged accurately, thanks to the RTM-based algorithm in which multiple wave reflections from the specimen bottom are taken into wavefield extrapolation. The proposed imaging approach presents higher computational efficiency, compared to conventional RTM, and enhanced imaging contrast over prevailing total focusing methods.

Keywords: Angular spectrum approach (ASA); Irregular surface; Nondestructive testing (NDT); Reverse time migration (RTM); Ultrasonic imaging.