NMO, Velocity Picking and Anisotropy
- Primary reflectors in CDP gathers are initially corrected for moveout using the hyperbolic 2 term equation which is exact if the overburden is uniform and isotropic within the region sampled by a gather.
- Stacking velocity is initially picked from gathers muted to have maximum offset limited to reflector depth. The resultant velocity approximates (but is usually higher than) the RMS velocity of the layered overburden. From it the average interval velocity between picked horizons is estimated using the DIX formula and used as a guide to avoid picking multiple energy which is often associated with unrealistic interval velocity inversions.
- Stacking velocities are checked for consistency at line intersections and corrected if agreement is unsatisfactory.
- If data is available with offsets significantly greater than reflector depth the offset muting is relaxed and a 3 term moveout equation appropriate to either a laterally uniform layered isotropic or anisotropic overburden is used. The equation derived by Al-Chalabi (1973) addresses the former and that due to Tsvankin and Thomsen (1994) the latter. Both require that the velocity function in the overburden is correct if they are to correct the long offset moveout.
- If VTI isotropy is known or suspected to occur, use of the anisotropic correction requires specification of the η parameter which is related to Thomsen’s parameters ε and δ. Scans of η values with a velocity function based on two term velocity picks are made using a relaxed mute. The derived optimum value represents the effective η to be applied throughout the gather.
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