Abstract: A calibration method includes receiving data characterizing a plurality of acoustic signals reflected by a defect in a target object, and a first depth of the defect relative to a surface of the target object. A first acoustic signal and a second acoustic signal are detected by a detector at a first location and a second location, respectively, on the surface of the target object. The plurality of acoustic signal includes the first acoustic signal and the second acoustic signal. The method also includes determining an envelope function based on at least the first acoustic signal and the second acoustic signal. The method further includes identifying a target distance between the detector and the defect. The target distance is associated with a peak value of the envelop function. The method also includes calculating a detection angle based on the target distance and the first depth of the defect.