Migration velocity analysis for tilted transversely isotropic media

Combining PP-waves with mode-converted PS reflections in migration velocity analysis (MVA) can help build more accurate VTI (transversely isotropic with a vertical symmetry axis) velocity models. To avoid problems caused by the moveout asymmetry of PS-waves and take advantage of efficient MVA algorithms designed for pure modes, here we generate pure SS-reflections from PP and PS data using the [Formula: see text] method. Then the residual moveout in both PP and SS common-image gathers is minimized during iterative velocity updates. The model is divided into square cells, and the VTI parameters [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] are defined at each grid point. The objective function also includes the differences between the migrated depths of the same reflectors on the PP and SS sections. Synthetic examples confirm that 2D MVA of PP- and PS-waves may be able to resolve all four relevant parameters of VTI media if reflectors with at least two distinct dips are available. The algorithm is also successfully applied to a 2D line from 3D ocean-bottom seismic data acquired at Volve field in the North Sea. After the anisotropic velocity model has been estimated, accurate depth images can be obtained by migrating the recorded PP and PS data.

Download Full-text

3‐D tomographic velocity analysis in transversely isotropic media

10.1190/1.1818014 ◽

2003 ◽

Cited By ~ 5

Author(s):

Hongbo Zhou ◽

Don Pham ◽

Samuel Gray ◽

Bin Wang

Keyword(s):

Transversely Isotropic ◽

Velocity Analysis ◽

Transversely Isotropic Media ◽

Isotropic Media

Download Full-text

Common-reflection-point migration velocity analysis of 2D P-wave data from TTI media

Geophysics ◽

10.1190/geo2013-0295.1 ◽

2014 ◽

Vol 79 (3) ◽

pp. C65-C79 ◽

Cited By ~ 3

Author(s):

Ernesto V. Oropeza ◽

George A. McMechan

Keyword(s):

Symmetry Axis ◽

Transversely Isotropic ◽

Migration Velocity ◽

P Wave ◽

Velocity Analysis ◽

Reflection Point ◽

Axis Orientation ◽

Velocity Models ◽

Migration Velocity Analysis ◽

Free Data

We have developed a common-reflection-point (CRP)-based kinematic migration velocity analysis for 2D P-wave reflection data to estimate the four transversely isotropic (TI) parameters [Formula: see text], [Formula: see text], and [Formula: see text], and the tilt angle [Formula: see text] of the symmetry axis in a TI medium. In each iteration, the tomographic parameter was updated alternately with prestack anisotropic ray-based migration. Iterations initially used layer stripping to reduce the number of degrees of freedom; after convergence was reached, a couple of more iterations over all parameters and all CRPs ensured global interlayer coupling and parameter interaction. The TI symmetry axis orientation was constrained to be locally perpendicular to the reflectors. The [Formula: see text] dominated the inversion, and so it was weighted less than [Formula: see text] and [Formula: see text] in the parameter updates. Estimates of [Formula: see text] and [Formula: see text] were influenced if the error in [Formula: see text] was [Formula: see text]; estimates of [Formula: see text] were also influenced if the error in [Formula: see text] was [Formula: see text]. Examples included data for a simple model with a homogeneous TI layer whose dips allowed recovery of all anisotropy parameters from noise-free data, and a more realistic model (the BP tilted transversely isotropic (TTI) model) for which only [Formula: see text], [Formula: see text], and [Formula: see text] were recoverable. The adequacy of the traveltimes predicted by the inverted anisotropic models was tested by comparing migrated images and common image gathers, with those produced using the known velocity models.

Download Full-text

Anisotropic migration velocity analysis using reverse-time migration

Geophysics ◽

10.1190/geo2013-0108.1 ◽

2014 ◽

Vol 79 (1) ◽

pp. R13-R25 ◽

Cited By ~ 25

Author(s):

Wiktor Waldemar Weibull ◽

Børge Arntsen

Keyword(s):

Wave Equation ◽

Seismic Anisotropy ◽

Migration Velocity ◽

Image Point ◽

Velocity Analysis ◽

Reverse Time ◽

Reverse Time Migration ◽

Time Migration ◽

Migration Velocity Analysis ◽

Isotropic Media

Seismic anisotropy, if not accounted for, can cause significant mispositioning of the reflectors in depth-migrated images. Accounting for anisotropy in depth migration requires velocity analysis tools that can estimate the anisotropic background velocity field. We extended wave equation migration velocity analysis to deal with 2D tilted transverse isotropic media. The velocities were obtained automatically by nonlinear optimization of the focusing and stack power of common-image point gathers constructed using an extended imaging condition. We used the elastic two-way wave equation to reconstruct the wavefields needed for the image and gradient computations. This led to an anisotropic migration velocity analysis algorithm based on reverse-time migration. We illustrated the method with synthetic and field data examples based on marine surface seismic acquisition. The results showed that the method significantly improves the quality of the depth-migrated image. However, as is common in the case of velocity analysis using surface seismic data, the estimation of anisotropic parameters seems to be strongly nonunique.

Download Full-text

Velocity analysis in transversely isotropic media

Geophysics ◽

10.1190/1.1441110 ◽

1980 ◽

Vol 45 (6) ◽

pp. 1094-1095 ◽

Cited By ~ 2

Author(s):

A. L. Lucas ◽

P. N. S. O’Brien ◽

J. H. Thomas

Keyword(s):

Transversely Isotropic ◽

Vertical Axis ◽

Two Dimensions ◽

P Wave ◽

Velocity Analysis ◽

Wave Surface ◽

Common Depth Point ◽

Transversely Isotropic Media ◽

Isotropic Media ◽

Ellipsoid Of Revolution

In transversely isotropic media, the moveout velocity obtained from common‐depth‐point (CDP) analysis may be significantly different from the horizontal velocity of the pseudo‐P wave. In Levin’s (1978) paper, he discusses, among other things, the problem of velocity determination in a medium in which the pseudo‐P wave surface produced by a point source is an ellipsoid of revolution. He points out that one would expect many sedimentary rocks to be transversely isotropic with a vertical axis of symmetry. In his Appendix he proves that an ellipse (using two dimensions for convenience) is one possible shape for the wave surface in such a medium. He also shows, as have others, that in this case CDP velocity analysis measures the velocity of horizontal propagation.

Download Full-text