Moveout-based geometrical-spreading correction for PS-waves in layered
anisotropic media
Journal of Geophysics and Engineering, 5 (2):
195--202 (June 2008)DOI: 10.1088/1742-2132/5/2/006
Abstract
This paper is devoted to pre-stack amplitude analysis of reflection
seismic data from anisotropic (e.g., fractured) media. Geometrical-spreading
correction is an important component of amplitude-variation-with-offset
(AVO) analysis, which provides high-resolution information for anisotropic
parameter estimation and fracture characterization. Here, we extend
the algorithm of moveout-based anisotropic spreading correction (MASC)
to mode-converted PSV-waves in VTI (transversely isotropic with a
vertical symmetry axis) media and symmetry planes of orthorhombic
media. While the geometrical-spreading equation in terms of reflection
traveltime has the same form for all wave modes in laterally homogeneous
media, reflection moveout of PS-waves is more complicated than that
of P-waves (e.g., it can become asymmetric in common-midpoint geometry).
Still, for models with a horizontal symmetry plane, long-spread reflection
traveltimes of PS waves can be well approximated by the Tsvankin-Thomsen
and Alkhalifah-Tsvankin moveout equations, which are widely used
for P-waves. Although the accuracy of the Alkhalifah-Tsvankin equation
is somewhat lower, it includes fewer moveout parameters and helps
to maintain the uniformity of the MASC algorithm for P- and PS-waves.
The parameters of both moveout equations are obtained by least-squares
traveltime fitting or semblance analysis and are different from those
for P-waves. Testing on full-waveform synthetic data generated by
the reflectivity method for layered VTI media confirms that MASC
accurately reconstructs the plane-wave conversion coefficient from
conventional-spread PS data. Errors in the estimated conversion coefficient,
which become noticeable at moderate and large offsets, are mostly
caused by the offset-dependent transmission loss of PS-waves.