We design a wave-equation based demigration-remigration workflow that computes a measure of uneven seismic illumination effects on angle-gather amplitudes. We then use the workflow to generate a set of illumination weights, which, when applied to migrated gathers, improves the structural imaging in subsalt areas. We demonstrate the effectiveness of our method with narrow-azimuth 3D subsalt examples from Gulf of Mexico.

Subsalt imaging at the Thunder Horse Field in the Gulf of Mexico is challenging primarily because the salt canopy, overlying roughly 75% of the structure, greatly distorts subsalt illumination and causes imaging and resolution problems. Since the Thunder Horse discovery, advancements in seismic acquisition techniques and imaging technologies have significantly improved subsalt images. The latest successful application is from a tilted transverse isotropy (TTI) reverse time migration (RTM) project combining two wide azimuth towed streamer (WATS) data sets and three narrow azimuth towed streamer (NATS) data sets. The addition of an extra WATS data set and the application of the recent imaging technologies are key contributors to the dramatic structural image improvements with better defined three-way events and a higher signal-to-noise ratio (S/N).

A methodology has been developed for improving seismic depth images generated in areas of complex structure, such as subsalt. The main goal of the wave-equation angle-based illumination workflow is to calculate a set of illumination weight gathers and apply them to migrated field angle gathers. To obtain meaningful illumination weight gathers, it is important to use the field-acquisition geometry information, migration velocity models, and interpreted horizons. The workflow is model based and requires horizons conformable to real structure, preferably picked on the field seismic depth images. The workflow is implemented using one-way and two-way wave-equation methods and is tested on narrow-azimuth towed-streamer (NATS) and merged NATS with wide-azimuth towed-streamer (XWATS) data sets. The method weights down poorly illuminated areas while preserving strongly illuminated areas, which is opposite to the compensation methods. As a result, an overall improvement in seismic depth images can be observed, especially in subsalt areas. The impact of the workflow on improving well-tie estimation at the Thunder Horse subsalt development in the Gulf of Mexico is demonstrated.

No abstract

No abstract