Adverse Effects of an Edge Diffractor in Seismic Reflection Interferometry
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Pure and Applied Geophysics
Adverse Effects of an Edge Diffractor in Seismic Reflection Interferometry YOUNGSEOK SONG,1 KI YOUNG KIM,2 Abstract—To understand steeply dipping events in seismic reflection interferometry (SRI), we derived an expression that describes the difference in travel time (Ds) from a diffractor to two receivers in two dimensions. For a fixed receiver interval, the expression shows that Ds is zero when the diffractor is at the midpoint of the paired receivers, increases with an apparent velocity of half the medium velocity as the diffractor moves toward either receiver, and remains constant for a diffractor located on the same side of both receivers. The horizontal portion of Ds is slightly skewed during the normal moveout correction, yielding a maximum peak of the horizontally stacked trace at a slightly smaller time than Ds. Accordingly, the diffracted waves have an apparent velocity slightly higher than half of the medium velocity in a horizontally stacked image. This conformed to virtual data for an elastic two-layer model with a vertical boundary. We then generalized the expression to three dimensions, in which listric travel time curves were predicted for an oblique edge diffractor, a vertex diffractor offline from the receiver pair, or a buried diffractor. Based on both two- and three-dimensional analyses of the edge diffractor, we tentatively interpreted the linear and listric dipping events observed in the passive SRI image across the Korean Peninsula to have been caused by diffractors near the intersection of the profile and geologic boundaries. Keywords: Dipping event, Seismic reflection interferometry, Diffractor, Apparent velocity, Model.
1. Introduction Claerbout (1968) showed that the Green’s function for seismic waves on a surface could be obtained by autocorrelation of a seismic signal generated at a buried source. Using this concept, seismic interferometry (SI), introduced by Schuster (2001), has been
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School of Earth and Environmental Sciences, Hanyang University, Seoul, South Korea. 2 Division of Geology and Geophysics, Kangwon National University, Chuncheon, South Korea. E-mail: kykim@ kangwon.ac.kr 3 Rene´ Geophysics, Bloomington, IN, USA.
JOONGMOO BYUN,1 and RAYMOND M. RENE´3 used to image subsurface structures by cross-correlation of traces recorded at pairs of receivers. The result is a virtual record, wherein one of the receivers acts as a virtual source for seismic waves recorded at the other receiver. Wapenaar et al. (2010) described the mathematical basis for the SI method. SI can use body waves (Claerbout 1968; Draganov et al. 2009), surface waves (Campillo and Paul 2003; Kang and Shin 2006), direct waves (Snieder 2004; Lin et al. 2009), refractions (Bharadwaj et al. 2012; Mikesell et al. 2009), or reflections (Schuster 2009; Draganov et al. 2004). The seismic source can be either active (Schuster 2001; Bakulin and Calvert 2004) or passive. A passive SI uses ambient noise, which is typically dominated by persistent and strong surface waves. Examples of passive
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