Analysis of Higher Order Moveout in Terms of Vertical Velocity Variation and VTI Anisotropy

Abstract

The standard NMO equation for a seismic reflection from a flat interface within a homogeneous, isotropic media is exact. Whenthe subsurface is layered or has TI anisotropy, this is no longer true. The accuracy of the NMO equation decreases with increasingoffset, preventing gathers from being flattened at long offset. Theaddition of a fourth order term to the NMO equation provides abetter physical model for reflection moveout and is a necessaryrequirement to flatten gathers at long offsets. Flat events arerequired for a good stack response, multiple suppression, amplitudeversus offset analysis, etc.Accurate measurement of both the moveout velocity and fourthorder moveout coefficients is time consuming and prone to errors; any measurement of moveout velocity is sensitive to the fourthorder coefficient and vice versa. A semi-automated method is usedto obtain both coefficients simultaneously from an initial estimate. This is more accurate than attempting to measure or refine bothcoefficients separately.Once known, the reflection moveout can be inverted forproperties of the subsurface media. The NMO equation iscommonly used to obtain velocity information. Both the verticalvelocity variation and the TI anisotropy make a contribution to thefourth order moveout coefficient. These two effects can beunwrapped using the moveout velocity. Practical analysis ofsynthetic and real seismic data exhibiting fourth order moveouteffects has been undertaken. This has allowed estimates for the TIanisotropy parameters to be made. This inversion for anisotropy issensitive to uncertainties in both the second and fourth ordermoveout coefficients, but accurate measurement and somesmoothing of velocity information can allow a good solution.