We consider ultrasonic imaging for the visualization of flaws in a material. Ultrasonic imaging is a powerful nondestructive testing (NDT) tool which assesses material conditions via the detection, localization, and classification of flaws inside a structure. We utilize reflections of ultrasonic signals which occur when encountering different media and interior boundaries. These reflections can be cast as direct paths to the target corresponding to the virtual sensors appearing on the top and bottom side of the target. Some of these virtual sensors constitute a virtual aperture, whereas in others, the aperture changes with the transmitter position. Exploitations of multipath extended virtual array apertures provide enhanced imaging capability beyond the limitation of traditional multisensor approaches. The waveforms observed at the physical as well as the virtual sensors yield additional measurements corresponding to different aspect angles, thus allowing proper multiview imaging of flaws. We derive the wideband point spread functions for dominant multipaths and show that fusion of physical and virtual sensor data improves the flaw perimeter detection and localization performance. The effectiveness of the proposed multipath exploitation approach is demonstrated using real data. 1. Introduction Ultrasonic nondestructive evaluation (NDE) has traditionally used single-element sensors for material testing. Most flaw detectors utilize A-scan measurements obtained with monolithic transducers externally placed at different positions on or close to the surface of the material. The synthesized ultrasound array aperture, generated through scanning, provides a series of A-scan data whose intensity profile is used to generate a B-scan cross-section image. Two-dimensional (2D) sensor scanning (e.g., raster scan) over the material generates a collection of B-scan images to obtain a C-scan volume image. These 1D and 2D scanning processes require dedicated hardware to control precise sensor positioning and synchronized data collection. This scanning and imaging process has been typically conducted in laboratory conditions or in industrial material testing facilities using immersion testing techniques. We note, however, that this imaging process is not practical for field testing conditions. Sensor arrays are more practical for field testing due to their increased coverage area, rapid data collection, and direct imaging capability. Although sensor arrays and beamforming techniques have been used in medical ultrasound for decades , their use in ultrasonic NDE has not
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