Reflection Moveout Inversion In Azimuthally Anisotropic Media: Accuracy, Limitation, And Acquisition

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.