Quantitative imaging of thickness maps using ultrasonic guided wave tomography based on full-waveform inversion

An ultrasonic guided wave tomography method based on full waveform inversion (FWI) is developed for high resolution and accurate reconstructions of the remaining wall thickness in isotropic plates. The inversion is based on local optimization of the waveform misfit function between modeled and measured data. The modeled data is computed in the frequency domain implementing a full-wave equation using Finite Difference (FD) method, accounting for higher order effects such as diffractions and multiple scattering. Experiments were carried out to measure the dispersive guided wave propagating through different types of defects. The inversion is applied iteratively to discrete frequency components from low to high frequencies. The resulting wave velocity maps are then converted to thickness maps by the dispersion characteristics of selected guided modes. The experimental results show that the FWI algorithm can achieve accurate reconstructions with the thickness error less than 1 mm on a 10 mm thick plate.