Towards the application of 2D elastic full-waveform inversion to ultrasonic data for non-destructive testing
All man-made buildings or bridges have a limited lifetime frame. To test these materials, non-destructive testing (NDT) methods are widely used. One these methods is ultrasonic testing. Conventional ultrasonic testing methods include traveltime tomography and migration which yield images of the material discontinuities. In contrast, Full Waveform Inversion (FWI) has the potential to infer multi-parameter models of elastic material properties for better characterization potential imperfections or material weakening. In this work, I apply elastic FWI to ultrasonic data acquired at the Fraunhofer Institute for Non-destructive Testing (IZFP) in Saarbrücken. During the acquisition, two sets of data are acquired: one with an air-filled pipe and a second with water-filled pipe in the center of the concrete block. In FWI I considered directly transmitted P-waves only. Synthetic reconstruction tests revealed that the P-wave velocity of the pipe cannot be inferred reliably when the P-wave velocity contrast is high, e.g., for the considered air filled and water filled pipe. For these same reasons also the application of elastic FWI to the acquired lab data could not retrieve the actual P-wave velocity of the pipe, even though the observed data could be fitted well.