| dc.description.abstract | Parameter estimation from elliptical variations in the normal-moveout (NMO) velocity
in azimuthally anisotropic media is sensitive to the angular separation between the
survey lines in 2D, or equivalently source-to-receiver azimuth in 3D, and to the set of
azimuths used in the inversion procedure. The accuracy in estimating the orientation
of the NMO ellipse, the parameter cr, in particular, is also sensitive to the strength of
anisotropy.
To invert for the parameters the NMO ellipse, at least three NMO-velocity measurements
along distinct azimuth directions are needed. In order to maximize the accuracy
and stability in parameter estimation, it is best to have the azimuths for the
three source-to-receiver directions 60° apart. Having more than three distinct source-to-receiver azimuths (e.g., full azimuthal coverage) provides a useful data redundancy
that enhances the quality of the estimates.
In orthorhombic media, inverting for the semi-axes of the NMO-ellipse allows the
computation of the difference in the anisotropic parameters δ[superscript (1)] and δ[superscript (2)]. Additional information such as well data, is necessary in order to determine δ[superscript (1)] and δ[superscript (2)]. Furthermore, the accuracy in estimating the semi-axes of the NMO-velocity ellipse is about the same for any strength of anisotropy.
To maximize quality in the inversion process, it is recommended that at the design
stage of seismic data acquisition to have small sector sizes (≤ 10°) with adequate fold
and offset distribution.
For three azimuth directions, 60° apart, to perform the inversion, an azimuthally
anisotropic layer overlain by an azimuthally isotropic overburden (as might happen for
fractured reservoirs) should have a time thickness, relative to the total time, of at least the ratio of the error in the NMO (stacking) velocity to the interval anisotropy strength of the fractured layer. Coverage along more than three azimuths, however, improves this limitation, which is imposed by Dix differentiation, by at most 50% depending on the number of observations (NMO Velocities) that enter the inversion procedure. | en_US |
