Penetration Depth of the Scanning Acoustic Microscope

A definition for the penetration depth of the scanning re- flection acoustic microscope is given. With this definition it is possible to calculate the penetration depth of a given lens geometry for a given material. The penetration depth depends on the elastic parameters of the object, the signal-to-noise ratio, and the operation frequency of the acoustic microscope. Calculations show that for high-impedance ma- terials, the penetration depth is limited by the wavelength of the surface waves and hence by frequency. For low-impedance materials the open- ing angle of the lens can be properly selected to make the longitudinal or shear wave penetration dominant, effectively increasing the penetra- tion well above the wavelength limit of the surface wave.