VSP: zero-offset, walkaway, walkaround

A Vertical Seismic Profile (VSP) puts the receivers down a borehole and fires the source at the surface. The downhole geometry beats surface seismic in two ways: the receivers are physically closer to deep reflectors (less attenuation, broader bandwidth), and the source–receiver raypaths sample the Earth in ways a surface geometry can’t. Three classical flavours:

Zero-offset VSP

Source directly above the well. Every receiver depth records the same downgoing direct wave plus upgoing reflections from everything below it. Main uses: (1) absolute time–depth function for tying the well to surface seismic; (2) cross-check of the sonic log — if they disagree, the sonic has a problem; (3) vertical seismic profile of the formation directly, at frequencies 2–4× higher than surface seismic because attenuation is half the round-trip. Typical zero-offset VSP: 30 min–2 h, fired once per bit run during drilling or once at end of well.

Walkaway VSP

Source walks along a line up to 3–5 km from the well. Each source offset illuminates a different subsurface bounce-point; the full set stacked produces a 2D image strip centred on the well. Uses: (1) imaging laterally away from the well where surface seismic is poor (near salt, complex overburden); (2) AVO analysis with direct offsets; (3) velocity-model refinement via ray-traced traveltime tomography. Typical acquisition: 2–4 h per line, done at wireline tool speed (100–300 m/hr).

Walkaround VSP

Source moves in a closed circle around the well, at constant radius (0.5–2 km). Full azimuthal coverage around the well. Primary uses: (1) shear-wave splitting analysis for fracture azimuth — the two horizontal receiver components see S waves split into fast (fracture-parallel) and slow (fracture-perpendicular); (2) azimuthal velocity anisotropy from the P wave. 24–48 source positions per loop, 6–12 h total.

DAS-fibre VSP

Modern VSP work is increasingly done on Distributed Acoustic Sensing (DAS) fibre. A single fibre-optic cable cemented behind casing becomes thousands of receiver channels at ≈1 m spacing over the entire well length — no wireline tool trips, no moving parts. Zero-offset, walkaway, walkaround can all be shot on the same permanent fibre. The downside: strain-rate sensing is less sensitive than a geophone, so strong-offset work requires more source effort.

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.
• Aki, K., Richards, P. G. (2002). Quantitative Seismology (2nd ed.). University Science Books.