Non-linear algorithms based on SAFT ideas for reconstruction of flaws

The main idea and the assumptions of time domain synthetic aperture focusin g technique (SAFT) are discussed. It is underlined that the major SAFT assu mption is that the flaw can be represented by a number of single non-intera cting scalar point scatterers, which act as point sources after illuminatio n with the incident wavefield. As a result some important classes of flaws are incorrectly reconstructed by means of SAFT. Among them are cracks from which reflected signals are similar to those from mirror-shaped flaws. Diff erent non-linear SAFT algorithms proposed earlier by the author are discuss ed and compared. Among them are SAFT with non-linear normalization (NSAFT), second order SAFT, and the radial tomography technique. The principles of these algorithms are discussed. Then their advantages and disadvantages are analysed. Unfortunately each algorithm cannot be considered universal as a classical SAFT, but for many important cases these algorithms are useful f or solving inverse problems in ultrasonics. The ideas of non-linear SAFT al gorithms are illustrated by physical experiments. (C) 2000 Elsevier Science B.V. All rights reserved.