JOHNSON, JEFFREY D., BP-Amoco

Abstract: The Challenge of Subsalt Imaging - Past, Present and Future Capabilities

Exploration for economic reservoirs beneath salt structures has been a frontier activity throughout the 1990's, particularly in the Gulf of Mexico.The major challenge for explorers has been obtaining reliable structural and stratigraphic interpretations from seismic data. Contorted salt bodies often act as complex lenses that refract, distort and scatter propagating seismic energy. These effects require extraordinary, interpretive seismic processing efforts, typically 3D prestack depth migration, to image subsalt geology. Today many oil companies and seismic contractors have implemented computational and velocity model building systems that effectively produce mappable depth images. However, project costs and cycle times are high, and image data quality remains a problem in many situations.

Challenges and problems faced in current seismic subsalt imaging include:
- Velocity model resolution and tomographic updating
- Algorithm amplitude fidelity and resolution using standard Kirchhoff or other travel time methods
- Subsurface illumination due to refraction or critical angle effects
- Multiple reflection and converted wave interference with primary subsalt reflections

Despite current economic conditions R&D progress within oil companies and seismic contractors, coupled with steady increases in computer price/performance, show promise of meeting these challenges and producing reliable subsalt predictions of fluid and lithology as well as structural images. Future subsalt depth imaging developments or applications should include:
- Practical implementation of wavefield extrapolation/processing approaches including multiple and converted wave subtraction or imaging.
- Increased use of multi-component sea bottom recording, vertical cables, and long/wide offsets to improve illumination and wavefield sampling, particularly in exploitation projects.
- Advanced visualization/virtual environments for interpretive velocity modeling and processing as well as for interpretive integration with other subsurface data.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England