3D VSP and salt-proximity imaging

3D VSP extends walkaway and walkaround into a full 2D shot grid at the surface, recorded into downhole receivers. The result is a local 3D illumination volume centred on the well. Its most important use in practice is imaging steep salt flanks.

Why salt flanks are hard

Salt has a P-velocity of about 4500 m/s in a matrix of clastic sediment at 2000–3500 m/s. Rays from surface shots are refracted strongly as they cross the salt top — they bend down into the faster salt and can miss the flank entirely. Flanks with dips above ≈60° are typically un-imaged by surface seismic, leaving a “shadow” tens to hundreds of metres wide on each side of the salt.

How 3D VSP fills it

Drill a well in the sediment alongside the salt. Put receivers down the well. Fire shots on a surface grid. Waves travelling from the shots to the downhole receivers include reflections off the salt flank from inside the sediment — a geometry surface seismic simply can’t reproduce. The illumination cone from each well receiver covers a flank segment the surface geometry misses.

Real-world uses

Gulf of Mexico sub-salt plays — Thunder Horse, Atlantis, Mad Dog — were de-risked by 3D VSPs before development. North Sea chalk fields adjacent to salt diapirs (Tyra, Dan) used 3D VSP for flank delineation. Campos Basin pre-salt carbonates (Lula, Sapinhoá) rely on 3D VSP for layer-cake salt-top imaging. A typical program runs 20–45 days, costs $3–8 M, and de-risks development wells worth$100–200 M each.

Practical constraints

Drill-cost risk rises steeply as the well approaches the salt: within 100 m of salt, drilling becomes dangerous (kick risk, lost circulation), so 3D VSP wells are typically 200–500 m off the salt. Receivers must be in the correct depth range — too shallow and they don’t see the deep flank; too deep and the reflection geometry doesn’t work. Wireline-deployed receiver strings have time limits; DAS fibre is now standard for long-duration 3D VSP.

References

• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.
• Vermeer, G. J. O. (2002). 3-D Seismic Survey Design. SEG Geophysical References 12.