Velocity analysis and moveout corrections
Velocity analysis and moveout corrections
SEARCH
This section describes applications for velocity analysis and methods available for move out correction of data.
Automatic velocity analysis is used routinely on nearly all projects, with interactive velocity analysis tools being used to QC the results and update when required.
Moveout corrections
Linear moveout: LMO
Standard linear moveout based on a single velocity.
Hyperbolic moveout: HMO
Hyperbolic moveout based on a single velocity and application time.
Normal moveout correction: NMO
Normal moveout corrections based on the NMO equation:
t2 = t02 + (x/V)2,
where x is offset, V is the RMS velocity and t0 the zero offset time. The velocity can be specified as 2D or 3D with interpolation between control points.
Fourth order moveout
For a layered or anisotropic earth, the moveout equation contains higher order terms that become important at longer offsets. The 4th order coefficients are supplied to the moveout modules in the same manner as the RMS velocity.
Severe stretch artefacts can be caused when applying higher order moveout corrections at longer offsets. In such cases block moveout corrections (i.e. time-invariant moveout over a specified window) can be applied to minimize these effects for target events.
A high-resolution moveout correction that attempts to compensate for wavelet stretch at longer offsets can also be applied.
Ray traced moveout
Moveout corrections can be computed assuming a locally v(z) velocity function by using ray tracing and applied to the data. This moveout correction is consistent with the ray-tracing in Kirchhoff pre-stack time migration and is used primarily as a QC of velocities used in time migration. The moveout correction can also incorporate VTI anisotropy.