Abstract:
Interpretation and inversion of azimuthally varying nonhyperbolic reflection moveout requires accounting for both velocity anisotropy and subsurface structure. Here, our previously derived exact expression for the quartic moveout coefficient A 4 is applied to P‐wave reflections from a dipping interface overlaid by a medium of orthorhombic symmetry. The weak‐anisotropy approximaton for the coefficient A 4 in a homogeneous orthorhombic layer is controlled by the anellipticity parameters η ⁽¹⁾ , η ⁽²⁾ , and η ⁽³⁾ , which are responsible for time processing of P‐wave data. If the dip plane of the reflector coincides with the vertical symmetry plane [x 1 , x 3 ], A 4 on the dip line is proportional to the in‐plane anellipticity parameter η ⁽²⁾ and always changes sign for a dip of 30○. The quartic coefficient on the strike line is a function of all three η–parameters, but for mild dips it is mostly governed by η ⁽¹⁾ —the parameter defined in the incidence plane [x 2 , x 3 ]. Whereas the magnitude of the dip line A 4 typically becomes small for dips exceeding 45○, the nonhyperbolic moveout on the strike line may remain significant even for subvertical reflectors. The character of the azimuthal variation of A 4 depends on reflector dip and is quite sensitive to the signs and relative magnitudes of η ⁽¹⁾ , η ⁽²⁾ , and η ⁽³⁾ . The analytic results and numerical modeling show that the azimuthal pattern of the quartic coefficient can contain multiple lobes, with one or two azimuths of vanishing A 4 between the dip and strike directions. The strong influence of the anellipticity parameters on the azimuthally varying coefficient A 4 suggests that nonhyperbolic moveout recorded in wide‐azimuth surveys can help to constrain the anisotropic velocity field. Since for typical orthorhombic models that describe naturally fractured reservoirs the parameters η (1,2,3) are closely related to the fracture density and infill, the results of azimuthal nonhyperbolic moveout analysis can also be used in reservoir characterization.
