Determination of Physical Property Gradients from Measured Surface Wave Dispersion

A technique for estimating the depth-dependent profiles of a physical property, such as case hardness, multilayers over a substrate, or surface corrosion, is presented. The technique uses surface wave pulses with sufficient bandwidth to allow the observation of the frequency spectrum over the range of frequencies required for the estimation procedure. Typically, this range must include those frequencies corresponding to wavelengths smaller and larger than the characteristic dimensions or depths of the physical property gradients in question. The technique uses pulse data acquisition at two locations of the receiving transducer and, through the use of one of the pulses as a reference, calculates the deconvolved Fourier transform of the other pulse and displays the dispersion diagram of the phase velocity. With the use of an inversion algorithm previously reported, the phase velocity variation with depth is obtained and is correlated to variations of the physical property gradient. The significance of this work is the ease and speed with which the data are obtained and processed to get the desired result. Another important consequence is that this technique provides a dense data set, thereby providing much greater precision than that obtainable by previously reported tone-burst measurements.