Imaging challenging media by full waveform inversion of ultrasonic signals
Résumé
Austenitic welds are important parts of the cooling system in nuclear power plants, which undergo extreme temperature and pressure variations that may cause defects to appear in the welded zone. If not attended, these may lead to a leakage of radioactive liquids. It is therefore crucial to assess the structural integrity of austenitic welds by detecting and imaging such defects. Ultrasonic methods are one of the reference methods in this matter. Current imaging techniques rely on phased array technology to focus the ultrasonic energy based on beamforming approaches such as the total focusing method. Because the elastic properties and exact geometry of austenitic welds are unknown, these methods fail to produce a reliable image of the inspected area. We introduce the full waveform inversion (FWI) of ultrasonic signals as a promising alternative imaging method. Adapted from Geophysics, the FWI uses a numerical model to simulate the experiment, and iterates by updating the model until the error between experimental and synthetic data is minimized. The model inputs are elastic properties in the inspected area. Eventually, it converges towards a reliable representation of the actual weld, including its geometry, elastic properties, and defects.