Prestack migration error in transversely isotropic media

Jaramillo, Herman; Larner, Ken

doi:10.1190/1.1887612

Cited by 1 publication

(3 citation statements)

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“…If the reflector is dipping, η contributes to small-offset traveltimes as well because it governs the dip dependence of NMO velocity (Alkhalifah and Tsvankin, 1995;Tsvankin, 2001). Figure 2c); this behavior of residual moveout agrees with the prediction of Jaramillo and Larner (1995). Although the contribution of η to the NMO velocity becomes more significant with dip, the magnitude of reflection moveout decreases for steeper reflectors, which explains this dependence of residual moveout on errors in η.…”

Section: Homogeneous Vti Mediumsupporting

confidence: 69%

“…Common problems caused by ignoring anisotropy in seismic imaging include mis-ties in time-to-depth conversion, failure to preserve dipping energy during dip moveout (DMO) correction, and mispositioning of migrated dipping events (e.g., Banik, 1984;Alkhalifah et al, 1996;Tsvankin, 2001). Jaramillo and Larner (1995) study anisotropy-induced errors in prestack depth migration and show that isotropic migration algorithms fail to flatten image gathers for a wide range of transversely isotropic (TI) models. Recent field-data observations by Peng and Steenson (2001) of image gathers with residual moveout that cannot be removed by conventional isotropic methods corroborate Jaramillo and Larner's (1995) conclusions.…”

Section: Introductionmentioning

confidence: 99%

“…Jaramillo and Larner (1995) study anisotropy-induced errors in prestack depth migration and show that isotropic migration algorithms fail to flatten image gathers for a wide range of transversely isotropic (TI) models. Recent field-data observations by Peng and Steenson (2001) of image gathers with residual moveout that cannot be removed by conventional isotropic methods corroborate Jaramillo and Larner's (1995) conclusions.…”

Section: Introductionmentioning

confidence: 99%

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Analysis of image gathers in factorized VTI media

Sarkar

Tsvankin

2003

GEOPHYSICS

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Because events in image gathers generated after prestack depth migration are sensitive to the velocity field, they are often used in migration velocity analysis for isotropic media. Here, we present an analytic and numerical study of P-wave image gathers in transversely isotropic media with a vertical symmetry axis (VTI) and establish the conditions for flattening such events and positioning them at the true reflector depth. Application of the weak-anisotropy approximation leads to concise expressions for reflections in image gathers from homogeneous and factorized v(z) media in terms of the VTI parameters and the vertical velocity gradient k z. Flattening events in image gathers for any reflector dip requires accurate values of the zero-dip NMO velocity at the surface [V nmo (z = 0)], the gradient k z , and the anellipticity coefficient η. For a fixed error in V nmo and k z , the magnitude of residual moveout of events in image gathers decreases with dip, while the moveout caused by an error in η initially increases for moderate dips but then decreases as dips approach 90 •. Flat events in image gathers in VTI media, however, do not guarantee the correct depth scale of the model because reflector depth depends on the vertical migration velocity. For factorized v(x, z) media with a linear velocity variation in both the x-and z-directions, the moveout on image gathers is controlled by V nmo (x = z = 0), k z , η, and a combination of the horizontal velocity gradient k x and the Thomsen parameter δ (specifically, k x √ 1 + 2δ). If too large a value of any of these four quantities is used in migration, reflections in the image gathers curve downward (i.e., they are undercorrected; the inferred depth increases with offset), while a negative error results in overcorrection. Lateral heterogeneity tends to increase the sensitivity of moveout of events in image gathers to the parameter η, and errors in η may lead to measurable residual moveout of horizontal events in v(x, z) media even for offset-to-depth ratios close to unity. These results provide a basis for extending to VTI media conventional velocity analysis methods operating with image gathers. Although P-wave traveltimes alone cannot be used to separate anisotropy from lateral heterogeneity (i.e., k x is coupled to δ), moveout of events in image gathers does constrain the vertical gradient k z. Hence, it may be possible to build VTI velocity models in depth by supplementing reflection data with minimal a priori information, such as the vertical velocity at the top of the factorized VTI layer.

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Section: Homogeneous Vti Mediumsupporting

confidence: 69%