Possibilities of Full Waveform Inversion as an imaging method in heterogeneous solids

Abstract

Ultrasonic imaging of single objects in heterogeneous solids such as concrete is mainly performed using linearized methods that are limited to one wave type and neglect geometric reflections and mode conversions. This limits the reconstruction accuracy and can induce artifacts. By considering more information of the complete resulting wavefield, full waveform inversion (FWI) promises a more accurate imaging. The method compares the measured signals with the results of the forward calculation of the scattered field of the source signals. The distribution of all three isotropic material parameters is iteratively adjusted to minimize the difference between the calculated and measured signals. By using a viable forward model, no limiting assumptions about wave propagation are necessary. Since the problem is ill-posed, regularization techniques are used for stabilization. The paper explains the operation and possibilities of the model-based approach considering both synthetic and experimental measurement data. First results on idealized scatterer geometries in homogeneous and heterogeneous solids are presented. Beginning with synthetic measurement results for the identification of anomalies in the solid body, the method is then applied to laboratory experimental data in the ultrasonic frequency regime. In both cases the same concrete structure is adressed where a known inclusion made of extruded polystyrene foam (XPS) is representing the anomaly to be detected.