Reflection Moveout Inversion For Horizontal Transverse Isotropy: Accuracy And Limitation

Abstract

Horizontal transverse isotropy (HTI) is the simplest azimuthally anisotropic model used to describe vertical fracturing in an isotropic matrix. Using the elliptical variation of P-wave normal-moveout (NMO) velocity with azimuth, measured in three different source-to-receiver orientations, we can obtain the vertical velocity V[subscript Pvert], anisotropy parameter δ[superscript (V)], and the azimuth a of the symmetry-axis plane. Parameter estimation from variations in the moveout velocity in azimuthally anisotropic media is quite sensitive to the angular separation between the survey lines in 2D, or equivalently source-to-receiver azimuths in 3D, and to the set of azimuths used in the inversion procedure. The accuracy in estimating the parameter α, in particular, is also sensitive to the strength of anisotropy. The accuracy in resolving δ[superscript (V)] and [subscript Pvert] is about the same for any strength of anisotropy. 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. In land seismic data acquisition having wide azimuthal coverage is quite feasible. In marine seismic data acquisition, however, where the azimuthal data coverage is limited, multiple survey directions are necessary to achieve such wide azimuthal coverage. Having more than three distinct source-to-receiver azimuths (e.g., full azimuthal coverage) provides useful data redundancy that enhances the quality of the estimates, and sets the stage for a least-square type of inversion in which the errors in the parameters estimates are minimized in a least-square sense. In layered azimuthally anisotropic media, applying Dix differentiation to obtain interval moveout velocity provides sufficient accuracy in the inversion for the medium parameters, especially where the direction of the symmetry planes is uniform. In order to obtain acceptable parameter estimates, an HTI layer overlain by an azimuthally isotropic overburden (as might happen for fractured reservoirs) should have a thickness (in time) relative to the total thickness. The total thickness should be equal to or greater than the ratio of the error in the NMO (stacking) velocity to the interval anisotropy strength of the fractured layer.