Re-examination of critical-angle reflectometry used in ultrasonic non-destructive testing for estimating material anisotropy

Seismic critical-angle reflectometry (SCR) can hardly be regarded as a reliable standalone method for anisotropic parameter estimation of strata in seismic exploration, but ultrasound critical-angle reflectometry (UCR) is useful in ultrasonic non-destructive testing (NDT) and medical ultrasound for the estimation of anisotropy in media. Therefore, there is a conflict between using critical-angle reflectometry (CR) in seismic exploration and applications of ultrasound. This study aims to find the reason as to why CR is useful in ultrasonic NDT and medical ultrasound but unreliable in seismic exploration for anisotropy estimation of media. The results of the study show that the critical angle can be accurately determined from the maximum amplitude of the reflections for a plane wave as used in ultrasonic NDT and medical ultrasound and thus the anisotropy of the medium can be successfully estimated by UCR. However, for the spherical waves used in seismic exploration, the maximum amplitude of the reflections is shifted towards post-critical offsets and the shift of the maximum amplitude relative to the critical point depends not only on the frequency and model parameters but also the azimuth. Therefore, SCR cannot currently be regarded as a reliable standalone method for the anisotropic parameter estimation of strata.