Multi-parameter reconstruction of velocity and density using ultrasonic tomography based on full waveform inversion

In this paper, an ultrasonic tomography method based on acoustic multi-parameter full waveform inversion (FWI) is developed for high-resolution reconstructions of velocity and density in metal components. The challenging issue in multi-parameter FWI is to deal with the trade-off effects between different parameters of different natures. Moreover, different parameters have different orders of amplitudes in the wave-field which make the inversion ill-conditioned. The inverse Hessian has been shown to mitigate the coupling effects and rescale the amplitudes of different parameters, and then the reliable updates of different parameters are available. The simultaneous reconstructions of velocity and density by using the truncated Gauss–Newton method, in which the inverse Hessian can be considered through a matrix-free conjugate gradient solution of the Gauss–Newton normal equation, are investigated by simulation as well as experiment on the centred inclusions. The results show that this method can effectively alleviate the trade-off effects between velocity and density, and thus the velocity of the inclusion is accurately reconstructed and the reconstruction of the density is well achieved.