Interpretation of Surface Wave Signals following Multiple Specular Reflections in an Acoustic Microscope

Acoustic microscopes are powerful tools in determining the velocity of a leaky surface or Rayleigh wave. In the most common implementation, the velocity is inferred from a narrow band measurement of the variation of signal with lift-off [1]. However we are concerned with a time-domain implementation [2], in which the velocity is found from the time difference between the front surface reflection and the trailing leaky Rayleigh wave signal. This signal is created by a complex wave path involving propagation of a longitudinal wave through the coupling fluid, conversion to a Rayleigh wave, and reradiation back to the transducer. However, when the velocity of the Rayleigh wave is too low, or the aperture of the lens is too small, this second signal is not observed because of the absence of energy at the incident critical angle [3]. Nevertheless, in this case, a trailing signal is sometimes seen after the second or third specular reflection. In this paper, we describe a series of experiments in which such signals are seen. Based on a combination of theory and experiment, we establish the origins of these signals and identify situations in which they extend the capability of a given lens