Synthetic aperture focusing technique with virtual transducer for immersion inspection of solid objects.

The conventional synthetic aperture focusing technique (SAFT) can improve the lateral resolution and increase signal-to-noise ratio (SNR) in ultrasonic imaging. However, the use of SAFT in immersed inspection is limited due to the issues related to the refraction effects at the boundary between the water and a solid object. The virtual transducer (VT) method employing a focused transducer or phased array is proposed as a solution to this problem. In the VT method, the focal point of a focused aperture is used as a virtual source of ultrasonic waves. If the virtual source is placed on or below the surface of an inspected solid object, SAFT can be applied in the same way as for contact inspection. Combining VT with the new phase-shift migration SAFT (PSM-SAFT) algorithm allows one to reconstruct images of multilayer objects with high resolution and high SNR. What is more, the computation efficiency of PSM-SAFT can significantly reduce the time needed for image reconstruction. Experimental validation of the proposed technique is presented in this paper. We present the results of experiments where the VT was created using a focused transducer and a phased array in the immersion inspection of a stainless steel block containing artificial defects. The performance of four different focused apertures used for generating a virtual transducer is evaluated in relation to the PSM-SAFT algorithm implemented in the frequency domain. [ABSTRACT FROM AUTHOR]