Inverse-ray imaging from triangulation of zero-offset reflection times

• Published in: Geophysical journal international 2005-11, Vol.163 (2), p.599-610
• Main Author: Wang, Tan K.
• Format: Article
• Language: English
• Subjects: Basins; forearc basin; Imaging; Inverse; migration; normal incidence; ray tracing; Reflection; Seismic engineering; Seismic phenomena; subduction; Three dimensional; Triangulation
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1111/j.1365-246X.2005.02747.x

Imaging of subsurfaces based on 3-D inverse rays shooting from coincident sources and receivers along oblique profiles is presented. Two take-off angles of a 3-D inverse ray, one along the seismic profile and another orthogonal to the profile, are determined based on the common direction of inverse rays shooting from three non-collinear sources/receivers. Inverse-ray tracing from the triangulation of sources/receivers to the reflection points provides more objective imaging than does map migration, for which traveltimes along the oblique and/or sparse profiles must be interpolated into rectangular grids. Error analysis of 3-D inverse-ray imaging over a 2-D subduction model, with variation in spacing and orientation of profiles, indicates that the variation of the structural interface can be well resolved when spacing of the traveltime grids is less than the lateral scale of structural variation. All possible triangulations of reflection times along two neighbouring profiles are also implemented in inverse-ray imaging of a SEG/EAEG over-thrust model. The result shows that the coverage of inverse rays from triangulation of all reflection times is much denser than that from triangulation based on the three closest-spaced stations. Furthermore, imaging errors of the interface depth from most of the SEG/EAEG over-thrust model, where the imaging density is higher than 2000 points km−2, are less than 0.1 km even when the profile spacing is relatively large. In this study, migration velocity analysis of 2-D multi-channel seismic data and all possible triangulations of picked reflection times along oblique seismic profiles in the southernmost Ryukyu subduction zone are conducted. Application of 3-D inverse rays to a forearc basin in this subduction zone shows that the sediment thickness near the western end of the Ryukyu forearc region varies strongly with a maximum thickness greater than 6 km. Furthermore, a sediment subsidence greater than 1 km is found in the SE portion of the basin. Slab buckling and basement rise due to the oblique subduction of the Philippine Sea Plate may have resulted in this subsidence and induced strong and shallow earthquakes below the forearc basin.