Phased arrays technologies are used in widely different fields like radar, radiotelescope, sonar, also in NDT applications.
The phased array technique controls the delays applied to the excitation of each element in order to emit an ultrasonic pulse which propagates in a certain direction focused at specific depth.
On the other hand, the received signals of each element are delayed following a focal law for compounding the echoes generated by reflectors located in the direction of the beam propagation.
After performing the sum of N-delayed signals (aperture data), the A-scan signal is obtained and represents the material reflectivity in the propagation direction. The direction of propagation can be changed by modifying the focusing delays, which allows sweeping the area of interest with successive shots obtaining a sectorial image or S-scan.
Just a focus in emission can be created because once the array elements have been excited the wavefront is spread. However, delays in reception can be modified during the acquisition to make the focus follows the wavefront over its propagation (dynamic focusing).
This was very informative, thank you! This video shows how Phased Array Ultrasonic Testing is performed
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for the detection of defects.