Images reconstructed by use of (a) the WISE method after the 199-th iteration (1,
018 runs of the wave equation solver), (b) the sequential waveform inversion
algorithm after the 43-rd iteration (57, 088 runs of the wave equation solver),
(c) the bent-ray model-based sound speed reconstruction method, and (d) the
sequential waveform inversion algorithm after the 1-st iteration (1, 024 runs of
the wave equation solver) from the noise-free non-attenuated data. The grayscale
window is [1.46,1.58] mm/μs.