Tariq Alkhalifah
tariq@sep.stanford.edu
The main difficulty in extending seismic processing to anisotropic media is to recover anisotropic velocity fields from surface reflection data. Applications of true anisotropic processing and parameter extraction on field data are rare. Often, additional sources of information (i.e. well logs) were needed to resolve the anisotropic velocity fields necessary for processing.
I use two methods to invert for those velocity fields. The first one is based on the dip-dependence of P-wave normal-moveout velocity; this method requires a presence of dipping structures (i.e. faults). The second method is based on the nonhyperbolic moveout behavior of P-wave reflections from horizontal events at far offsets; this method requires a presence of large offsets in the data, large with respect to the depth of interest (i.e. offset to depth ratios > 1.5).
Time processing in transveresly isotropic (TI) media with a vertical
symmetry axis (VTI media) depends just on the zero-dip normal-moveout velocity
[
]
and the effective parameter 
that reduces to the difference between Thomsen's parameters 
and 
in the limit of weak anisotropy. As a result, both inversions are applied
to extract and
as
a function of vertical time. Once these two parameters are estimated a
full anisotropic processing sequence, including VTI dip moveout and time
migration, can be applied to the data.
Application of anisotropic processing to P-wave data from the shale-dominated region of offshore Angola demonstrates the importance of considering anisotropy, especially as it pertains to focussing and imaging of dipping events. The fault imaging improvements, in comparison with isotropically processed sections in this region, are significant.
The lithology of offshore Trinidad ,on
the other hand, is formed of alternating sequences of sand and shale dominated
layers. Average (effective) anisotropy is much lower in Trinidad compared
to the offshore Angola due to the large amount of sand in the Trinidad
subsurface. Nevertheless, accounting for anisotropy in seismic processing
results again in improved imaging of structural and stratigraphic features.
The imaging improvement is shown for two different lines. Inversion for
an interval value of the anisotropy parameter ,
suggests that low values are correlated with sands (or any other isotropic
material), while high interval
values are correlated with shales. Correlation between separate independent
measurements for
across common midpoints enhances the credibility of such estimates as a
representation of real geologic parameters. Finally, the
curve agrees well with gamma-ray well-log measurements used as a shale
estimate. This result confirms the hypothesis that anisotropy is caused
by shales in the subsurface, and the inversion for interval
can subsequently be used to estimate lithology.
Tariq is currently serving time as a post-doc with the Stanford Exploration Project (SEP) at Stanford University. He received his B.S. in geophysics from the University of Petroleum and Minerals in Saudi Arabia and his M.S. and Ph.D. in geophysics from Colorado School of Mines. His research interests include ray tracing, velocity inversion, and imaging, particularly in anisotropic media.
Stanford Exploration Project