2D elastic full-waveform inversion of ultrasonic data for non-destructive testing
Regular monitoring of buildings and other structures is a crucial task to prevent failures of building materials, accompanied by material and environmental damage or even fatalities. Inadequate fabrication, deterioration or unexpected excessive stresses can alter and weaken building components, so that its functionality or structural integrity is no longer guaranteed. A prominent example for material failure with dramatic consequences is the collapse of the Morandi Bridge in Genua on August 14, 2018, killing 43 people.
Non-destructive testing (NDT) methods can help to evaluate the current state of building materials and to avoid such failures. One popular method in NDT is the acquisition and analysis of ultrasonic waves. In the present work, I use ultrasonic measurements for NDT of concrete structures. The aim is to detect and characterize small anomalies like inclusions or flaws in concrete using a high resolution imaging technique called full-waveform inversion (FWI). First, synthetic tests are performed to evaluate the feasibility and limits of FWI in application to ultrasonic measurements. The synthetic tests show a succesfull reconstruction of even small-scale anomalys within a sub-wavelength range. In a second part, real ultrasonic data are acquired to test the applicability of FWI at real data. The thesis is done in cooperation with the Fraunhofer Institute for Nondestructive Testing (IZFP) in Saarbrücken, that specifically build a concrete block for this case and provide the expertise and equipment for the acquisition of ultrasonic data.
Towards 3D full-waveform inversion of near-surface field data
Full-waveform inversion is beside the application on exploration scale also a promising method to image targets in shallow depth. The experiment site is part of the Ettlinger Linie, an artificial trench that was built during the war of the Spanish succession and is now filled up with sediments. A three component source and three component geophones were used in the data acquisition in 2017. The data set is analysed for consistency due to the recording on several days. The WAVE-Toolbox developed within the WAVE-Project at the Applied Geophysics group is used for the forward modelling and the waveform inversion. At Jülich supercomputing centre benchmarks for the forward modelling were run to compare the code performance. First synthetic 3D elastic full-waveform inversion reconstruction tests were successful.