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.

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.

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The available seismic imaging methods are challenged by the presence of massive and complicated salt bodies. This complex overburden affects seismic wave propagation in many ways including attenuation. As a consequence, the amplitude and frequency of the recorded seismic data decay differentially with time and raypath. The differential attenuation lowers the seismic resolution and distorts the phase.