Common image gathers in the offset domain are used extensively in velocity analysis and amplitude versus offset (AVO) studies. Imaged with the correct background velocity model, the events will appear horizontal in the seismic offset gathers. Any curvature or moveout in these gathers can be used as a criterion for updating migration velocities. If the geology is complex and the ray field becomes multi-pathed, then the assumptions made for imaging data in the offset domain are violated. This will especially influence the quality of common-image gathers and, as a sequence, make it difficult to perform any form of AVO or velocity analysis. Such complicated problems typically arise in seismic imaging beneath gas, salt domes, and basalt structures. Angle-domain common image gathers (ADCIGs) uniquely define ray couples for each point in the subsurface. Therefore, each event in the data will be associated with only one subsurface location. It is possible to generate the ADCIGs with both Kirchhoff and wave-equation migration methods, these ADCIGs may be used for velocity analysis and amplitude-versus-angle (AVA) analysis. Common-angle migration creates seismic images for different reflection angles at the reflector, thus generating ADCIGs. The ADCIGs may be used for velocity analysis, and amplitude versus angle (AVA) analysis with the specific application in a fracture study. AVA can provide information about fractures at existing wells, in reservoir characterization specially in predicting the production rates of new wells, and in structural interpretation. Since AVA records the fracture information between the wells, it adds significant information to the interpretation of fractured reservoirs that cannot be easily obtained in other ways.
Discussing common-angle migration, a summary of the Kirchhoff-based method as based on the work of Bleistein et al. (2001) is presented. In addition, the wave-equationmigration based method, which is discussed in the work by Sava(2001), is examined. Further, a brief review of the application of ADCIGs in amplitude-versus-angle and azimuth (AVAZ) analysis by Gray et al. (2002) is presented.
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