The synthetic aperture focusing technique is a method for focusing ultrasonic scans used in nondestructive testing. Traditionally, the technique has mainly been used for contact testing, where the speed of sound is constant throughout the whole medium, but a number of recently proposed algorithms have extended the technique to multilayered media. One important application for such multilayer methods is immersion testing, where an object is immersed in water and the transducer is scanned within the water layer. Similarly, a multilayer technique is useful for inline ultrasound inspection of pipelines filled with liquid, for example water or oil. The multilayer methods have so far only been applied to the 2-dimensional line scan case, but in this paper we extend the the focusing procedure to the 3-dimensional surface scan case. The proposed method is based on the frequency-domain Phase Shift Migration algorithm, which is both conceptually simpler and more computationally efficient than similar time-domain methods. The performance of the focusing algorithm is tested on a three-layer structure, consisting of acrylic glass and aluminum blocks immersed in water. Several small flat-bottom holes in each block are used to simulate point scatterers, and the scatterer responses are studied using both B- and C-scan presentation. The lateral resolution of the focused image is shown to be approximately equal to half the transducer diameter, independent of both depth and layer.
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