A 3D Illumination Study to Investigate Fault Shadow Effects over the Hoop Fault Complex

Fault shadows represent zones of unreliable seismic imaging in the footwall of major extensional faults. They occur due to rapid lateral velocity variations and high velocity overburden which distort raypaths, manifesting themselves as sags on a time image. Whilst interpretation driven techniques in the depth domain have been developed to address this, the problem is fundamentally one of poor illumination which can be a function of the survey design. In this study fault sags within the Hoop Fault Complex of the Norwegian Barents Sea are investigated to gain understanding of why conventional travel-time tomography fails. Through 3D ray-trace modelling it is demonstrated the fault sags can be traced on illumination maps produced for the originally acquired azimuth. An additional azimuth is modelled, simulating acquisition in the orthogonal direction which demonstrates better illumination in these regions. Ray attributes allow the generation of synthetic gathers which are time migrated and converted to depth. These demonstrate the sags can be eliminated by the acquisition of an additional azimuth. Whilst the original narrow azimuth 3D survey design is optimal for the shallow targets, if full illumination of the fault complex is a future objective, multi-azimuth or wide azimuth surveys should be considered.