Experimental analysis of surface detection methods for two-medium imaging with a linear ultrasonic array

In ultrasonic nondestructive testing, when using an ultrasonic linear array transducer for imaging objects with nonplanar surfaces, a coupling medium must be used. To compensate for the refraction at the coupler-object interface, its shape must be known. Two methods for surface detection of convex objects in immersion are proposed, using the same linear array transducer for surface detection and for SAFT imaging. One is based on imaging technique and the other is based on the time-of-flight of the echoes on the captured ultrasonic signals. The accuracy and performance of the two methods are compared experimentally with an existing fast method called pitch-catch. The proposed methods produced smaller errors in part of the tested configurations, with a slower performance compared with the pitch-catch method. After the surface detection, in the SAFT image formation phase, the delays are calculated through a simple and fast proposed technique to determine the fastest paths, following the Fermat's principle. Images are formed for nine distinct array element groups, and then combined using the effective aperture technique. The results show that the developed methods allow interactive two-medium image formation on a general-purpose CPU.