Anisotropic velocity analysis using mode-converted S-waves

Sedimentary rocks are usually anisotropic because of fine scale layering an d the preferred orientation of nonspherical grains and anisotropic minerals . In order to extend seismic processing to anisotropic media, an anisotropi c velocity model is required. For transversely isotropic media with a verti cal axis of symmetry, Alkhalifah and Tsvankin (1995) have shown that a sing le anisotropy parameter eta together with the small-offset normal moveout ( NMO) velocity for PP-reflections is sufficient to perform all time-related processing of P-wave data. Although P-waves form the basis of most commerci al seismic surveys, shear waves provide unique information concerning subsu rface lithology and pore fill. Recent developments allow multi-component se ismic data to be acquired at the seafloor. By using measurements of the sma ll-offset travel-times of PP and PSV reflections to determine the parameter s of ANNIE, a simple three-parameter model proposed by Schoenberg et al. (1 996) for shales, reliable estimates of eta can be made in the absence of an y depth or vertical velocity information. The long-offset (non-hyperbolic) traveltime moveout of both PP and PSV reflections from a horizontal reflect or in a laterally homogeneous medium is accurately predicted by this approa ch, despite the fact that only small-offset data is used in the analysis. T hus the wide-offset moveout curves for both PP and PSV reflections from a h orizontal reflector in a laterally homogeneous medium can be adequately des cribed using only the small-offset travel-times of PP and PSV reflections.