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. When the subsurface is layered or has TI anisotropy, this is no longer true. The accuracy of the NMO equation decreases with increasing offset, preventing gathers from being flattened at long offset. The addition of a fourth order term to the NMO equation provides a better physical model for reflection moveout and is a necessary requirement to flatten gathers at long offsets. Flat events are required for a good stack response, multiple suppression, amplitude versus offset analysis, etc. Accurate measurement of both the moveout velocity and fourth order moveout coefficients is time consuming and prone to errors; any measurement of moveout velocity is sensitive to the fourth order coefficient and vice versa. A semi-automated method is used to obtain both coefficients simultaneously from an initial estimate. This is more accurate than attempting to measure or refine both coefficients separately. Once known, the reflection moveout can be inverted for properties of the subsurface media. The NMO equation is commonly used to obtain velocity information. Both the vertical velocity variation and the TI anisotropy make a contribution to the fourth order moveout coefficient. These two effects can be unwrapped using the moveout velocity. Practical analysis of synthetic and real seismic data exhibiting fourth order moveout effects has been undertaken. This has allowed estimates for the TI anisotropy parameters to be made. This inversion for anisotropy is sensitive to uncertainties in both the second and fourth order moveout coefficients, but accurate measurement and some smoothing of velocity information can allow a good solution.