Seismic imaging need not be synonymous with or rely on the presence of reflectors in the Earth. Much can be gleaned from the nonreflected wavefield. For example, direct waves map out smooth velocity variations in crosswell seismic tomography, wide-angle refracted waves play a crucial role in full waveform inversion (Hole et al., 2005), and surface waves provide unmatched sensitivity to near-surface shear-wave velocity structure. Guided waves exist in both cracks (Korneev, 2008) and boreholes, the latter referred to as tube waves. The famous Biot slow wave is itself a guided-wave phenomenon akin to a tube wave (Norris, 1987). Surface-wave dispersion has a long history in seismology and was the first seismic characteristic to be subjected to an automated inversion procedure (Dorman and Ewing, 1962).

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