Velocity fields are a necessary consequence of seismic data processing, but rarely a key deliverable. The use of velocities to predict pore pressure in the subsurface generally represents an opportunistic leveraging of what is essentially a QC product, to be used in a process for which the product was not specifically intended. Some velocities are picked (or re-picked) specifically for pressure prediction, but this is the exceptional case.
Because the vast majority of seismic data are collected for P-wave imaging, the great majority of pressure predictions from seismic velocities will be performed on P-wave velocity cubes. P-wave velocities are less sensitive to pore pressure variations than are S-wave velocities, C-wave (mode converted P-wave) velocities, or interval Vp/Vs ratios. P-wave interval velocities (derived from P-wave moveout curves), as a saving grace, generally have much less pronounced anisotropic traveltime effects than SV or SH-wave interval velocities derived from SV- or SH-wave moveout curves.
3D/4C surveys are often acquired with an eye to utilizing C-wave images to augment co-located P-wave images in image-challenged portions of the survey area. As with 3D P-wave surveys, which require the production of a 3D P-wave velocity cube for seismic processing, a 3D/4C survey will require the production of a C-wave velocity cube. This C-wave velocity cube will have higher pore pressure sensitivity than the co-located P-wave velocity cube, but will also have higher VTI anisotropic moveout effects.
Land shear vibrators can be used to acquire pure mode S-wave data, whose velocities will have the highest pore pressure sensitivity. However, the production of true S-wave data is extremely limited. To the best of the speaker’s knowledge, only one commercial enterprise and a handful of academic consortia are currently collecting S-wave reflection data in North America. So the higher sensitivity of S-wave velocities to pore pressure and the greater magnitude of S-wave VTI anisotropy (SH or SV) is not a pressing issue at this time.
For custom (bespoke) pore pressure prediction problems, lookahead VSPs can offer the opportunity to derive interval Vp/Vs ratio ahead of the bit by registering near-vertical P-wave and C-wave (P-down, S-up) events. The interval Vp/Vs ratio can be converted to a pore pressure either by Eaton’s method (with suitable choice of exponent) or by application of a direct vertical method. As with a velocity based approach, a Vp/Vs pore pressure prediction requires establishment of a normal compaction trend.
Possibly the most interesting application of enhanced S-wave velocity sensitivity to pore pressure lies in the arena of well logging. Dipole logging returns both P-wave and S-wave velocities over a given interval. The S-wave velocity has the dual benefit of not being sensitive to fluid velocities, yet being more sensitive to changes in pore pressure. On the debit side, the S-wave velocities are generally more sensitive to hole deviation than are the P-wave velocities.
References for these applications can be found in a December 2006 TLE review article “Subsalt pressure prediction from multicomponent seismics (and more!)”, or by dropping an email to the speaker at firstname.lastname@example.org