4. Eddy Currents, Techniques and Phenomena
A technique known as Time Delay Spectrometry (TDS), which has been used at JPL for ultrasonic tissue characterization, has promise for similar application in materials characterization as well. This technique differs from the approaches based on pulse-echo techniques which are used by other workers. Time Delay Spectrometry operates in the frequency domain directly. The transducer is excited by a rapidly swept frequency source and a tracking receiver is used to select signals arriving during a narrow time interval. In the reflection mode this time interval represents the range of the reflecting surface. In the transmission mode this time interval is adjusted to the desired acoustic delay, causing rejection of signals which follow extraneous paths. This swept frequency implementation makes coherent processing of the full analytic signal possible, which in turn allows more representative signatures to be obtained. In the reflection mode, for example, a better indication is obtained of the true strength of an interface or scatterer because the response can be made less dependent on the interference effects that so greatly alter the amplitude peaks of the conventional echo. This technique also permits an enhanced dynamic range to be obtained by applying frequency compensation directly to the transmitted signal. An added bonus is the ability to use data logging systems at rates commensurate with microprocessor operation in place of more expensive high speed transient recorders with limited memory capacity. Attenuation spectra taken on tissue specimens and on a few material samples will be presented. These data will demonstrate the ability of Time Delay Spectrometry to either minimize reverberation artifacts or to make use of the information contained in the artifact.
Gammel, P. M. and Leipold, M. H., "Materials Characterization by Time Delay Spectrometry Ultrasound" (1980). Proceedings of the DARPA/AFML Review of Progress in Quantitative NDE, July 1978–September 1979. 6.