Nonreflection seismic and inversion of surface and guided waves

S 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 shearwave 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). Surfacewave 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). Surveys based on surface waves provide a low-cost, noninvasive means of probing the shallow subsurface using either active sources (Xia et al., 1999) or in passive mode using microtremors (Aki, 1957, 1965; Louie, 2001; Okada, 2003). In fact, the past decade has witnessed a revival in the microtremor method because of the realization that ambient noise correlations are closely related to the surface-wave Green’s function (Campillo and Paul, 2003). Recent work has shown that other surface-wave observables besides dispersion possess sensitivity to density in addition to shear-wave velocity (Lin, et al., 2012). When velocity decreases with depth, the existence of leaky waves (Ryden and Park, 2004) attests to the richness of nonreflected-wave phenomena. In a way, it is too bad that these topics must be collectively identified by what they are not (i.e., nonreflection); however, the negative terminology emphasizes the gaping hole in our understanding of the subsurface if only reflections are taken into account. It is with this background that the special section has come together. Surface waves feature prominently in many of the outstanding articles that follow. Guided, leaky, and refracted waves round out the cast of seismic wave types that appear in this special section. The articles are grouped into four subsections: