Estimating the ultrasonic reflectivity of naturally occurring reflectors - Flaws and microstructure

A method is presented for estimating the ultrasonic reflectivity of both artificial and naturally occurring subsurface reflectors. It is assumed that focused beams are used to interrogate the discontinuities being evaluated and that the scanned area, combined with the specified depth of focus of those beams, defines the material inspection volume. A calibration method is described that uses multiple flat-bottomed hole reflectors (FBH). Beginning with the smallest FBH specified for that inspection, additional sizes are included so that the system is calibrated over at least a factor of 10 in reflector area, where all FBH reflectors are smaller than the ultrasonic beam. A back-surface reflector (back-wall) is also included for each calibration depth to estimate the reflectivity of flaws larger than the beam. This back surface reflector, placed at normal incidence and at the aplanatic beam focus, presents an estimate of the highest probable signal amplitude that could be expected for that inspection sensitivity and at the reflector/focal depth. (Author)