Evaluation of inversion approaches for guided wave thickness mapping

Accurate inversion is vital for quantitative imaging, including ultrasonic guided wave tomography, where thickness maps of plate-like structures are reconstructed to quantify corrosion damage. The dispersive properties of guided waves are often exploited to enable thickness maps to be produced from wave speed reconstructions. Ray tomography, diffraction tomography and a hybrid algorithm combining their features were investigated to reconstruct wave speed. Test data produced from simple defects of different sizes using a realistic full elastic guided wave model and the equivalent idealized acoustic model were passed to the imaging algorithms, generating wave speed maps, and, from these, thickness maps. For both datasets, ray tomography exhibited poor resolution. Diffraction tomography performed better, but was limited to shallow, small defects. The hybrid algorithm achieved the best results, giving a resolution around 1.5-2 wavelengths from the realistic test data compared to half wavelength from the idealized case. These results were validated with experimental data, and also extended to a realistic corrosion patch confirming the trends demonstrated with simple defects. The resolution loss with realistic data compared with idealized data indicates the acoustic model cannot accurately capture guided wave scattering and an alternative approach is necessary for better resolution reconstructions.