Abstract
Imaging of 3D structural interfaces through reflected rays shooting from common-shot gathers is presented in this paper. First, by fitting the reflected arrivals picked from common-shot gathers, we calculate apparent clips and the shortest distances between sources and reflectors along two profiles. Then, based on the geometry of the profiles and a planar reflector, a unit normal vector of the reflector is determined from the apparent clips and the azimuths of two oblique profiles. A special case, when apparent clips are zero along two parallel profiles, for determining the reflector normal is also investigated. We propose three criteria to ensure the selected travel-times along two profiles resulting from the same planar reflector. These are that firstly, the same shortest distance from sources to the reflector is utilized; and secondly, we want to ensure the same normal of the reflector, and finally, the same ray distance. Prestack inverse-rays developed in this paper are applied to image the bathymetry of the Hoping Basin in the southernmost Ryukyu subduction zone and the fourth layer of the SEG/EAEG over-thrust model. Based on common-shot gathers along seven oblique profiles in the Hoping Basin, most of the reflection points are well imaged through inverse rays except when variation of the interface depth exceeds 300m across profiles with spacing greater than about 20km. Inverse-ray imaging of the over-thrust model also provides good agreement to its fourth interface except that imaging errors of about 1km in depth are found near the thrust faults. Inverse-ray imaging of 3D structures from 2D multi-channel seismic profiles is demonstrated if a pseudo-3D structure (a planar reflector) exists between profiles or if at least two profiles are within a Fresnel zone. Although this technique deals with a single-layered problem currently, it is fundamentally important when we extend it to image inhomogeneous multilayered media.
Highlights
Prestack depth migration has been widely used in the multi-channel seismic (MCS) data processing to image 3D complex structures such as faults, salt domes and gas sags (Gray et al 2001)
Based on migration velocity analysis and horizon picks over MCS stacked sections, four major sedimentary layers of the Hoping Basin are imaged through 3D inverse reflected rays (Wang 2005a)
According to the smoothness and continuity of the travel-time curves resulting from the same planar reflector, any two travel-time sets of common-shot gathers along oblique profiles are selected for determining the reflector normal from equation (A4)
Summary
Prestack depth migration has been widely used in the multi-channel seismic (MCS) data processing to image 3D complex structures such as faults, salt domes and gas sags (Gray et al 2001). Ray-based techniques (Operto et al 2000; Hua and McMechan 2003) were incorporated into the prestack depth migration for enhancing computation speed, but its applications in migration velocity analysis (MVA) and real-time monitoring are still limited (Donihoo et al 2001). May and Covey (1981) used inverse reflected and diffracted rays to image complex faults They suggested that the error in inverse-ray imaging results mainly from the travel-time gradient which may accumulate large errors in imaging lower layers. Wang (2005a) developed a 3D inverse-ray imaging technique based on common-midpoint (CMP) triangulation of oblique and sparse profiles and incorporated MVA for enhancing the velocity determination
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