Inversion of P‐wave nonhyperbolic moveout in azimuthally anisotropic media: Methodology and field‐data application

Abstract

The azimuthally varying nonhyperbolic moveout of P‐ waves in orthorhombic media can provide valuable information for characterization of fractured reservoirs and seismic processing. Here, we present a technique to invert long‐spread, wide‐azimuth P‐wave data for the orientation of the vertical symmetry planes and five key moveout parameters — the symmetry‐plane NMO velocities and and the anellipticity parameters η(1), η(2), and η(3). The inversion algorithm is based on a 3D semblance operator applied to the full range of offsets and azimuths using a generalized version of the Alkhalifah‐Tsvankin nonhyperbolic moveout equation. Numerical tests on noise‐contaminated data show that the inversion yields satisfactory results if the offset‐to‐depth ratio reaches at least 2.5. The algorithm was successfully tested on wide‐azimuth P‐wave reflections recorded at Weyburn Field in Canada. Taking azimuthal anisotropy into account increased the semblance values for most long‐offset reflection events in the overburden, which indicates that fracturing is not limited to the reservoir level. The estimated symmetry‐plane directions are close to the azimuths of the off‐trend fracture sets determined from borehole data and shear‐wave splitting analysis. The effective moveout parameters estimated by our algorithm also provide input for P‐wave time imaging and geometrical‐spreading correction in layered orthorhombic media.