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|Title: ||Anisotropic Parameter Estimation from PP and PS Waves in 4-Component Data|
|Authors: ||Traub, Barbel M|
|Supervisor(s): ||Li, Xiang-Yang|
|Issue Date: ||Jun-2005|
|Publisher: ||University of Edinburgh; College of Science and Engineering; School of GeoScience|
|Abstract: ||The estimation of anisotropic parameters in the shallow subsurface becomes increasingly
important for 4C seismic data processing in order to obtain accurate
images in both time and depth domain. I focus on two approaches to evaluate
anisotropy in seismic data: using P-wave data and PS-converted (C-wave) data.
To gain better insight into the accuracy and sensitivity of anisotropic parameters
to for instance layering and compaction gradients, I undertake numerical modelling
studies and verify the results with full-wave modelling as well as findings
from the real data from a 4C data set from the Alba field.
The focus of this thesis is on vertical transverse isotropy (VTI) which widely occurs
in marine sediments and cannot be neglected in seismic processing. P-wave
data alone cannot constrain the vertical velocity and the depth scale of the earth
model for a VTI medium. Therefore, the joint inversion of non-hyperbolic P- and
converted wave (C-wave) or S-wave data from long offsets has been suggested. I
carried out a detailed analysis of the resolution and accuracy of non-hyperbolic
moveout inversion for P-, S- and C-waves for a single VTI layer in two parts.
First, I introduce the concept of the inherited error delta inh as a measure of the possible
resolution of the moveout approximations for the different wave types. The
range of this error stays constant regardless of the magnitude of the anisotropic
parameter for each wave type. Second, I analyse the accuracy of non-hyperbolic
moveout inversion. I find that for anisotropy parameter eta the error of estimation
from C-wave data is in most cases about half that from P-wave data. Inversion of
non-hyperbolic S-wave moveout data does not resolve the anisotropy parameter
due to the presence of cusps in the data.
The study is then extended to a multilayered medium considering only P- and
C-waves. The results confirm the findings from the single layer case. Furthermore,
I investigate phase effects on parameter estimation for P- and C-waves. It
is suggested that eta estimated from C-wave data gives a better description of the
anisotropy found in a medium than the eta values picked from P-wave data.
To verify the above findings near-surface effects are studied on the 4C data from
the Alba field and accompanied by a full-waveform modelling study. I find that
the picked eta values from P-wave data are distinctly larger than the eta values from
C-wave data and also larger than the eta values from VSP data. The full-wave
modelling study shows that picked eta values from P-wave data may account for
influence of structure such as velocity gradients in the near-surface and are influenced
by high velocity ratios and phase reversals.
Finally, I have carried out an integrated analysis of the Alba 4C data to demonstrate
how seismic anisotropy can be estimated from 4C seismic data and how
such information can be used to improve subsurface imaging. The results are presented
in two parts. The first part deals with non-hyperbolic moveout analysis for
estimating anisotropic parameters to gain improved stacked sections. The second
part describes migration model building and final imaging. The models are evaluated
by comparison with VSP data results and with a synthetic modelling study
for three events of the overburden. The evaluation confirms that the anisotropy
parameter obtained from C-wave moveout corresponds better with the VSP data
than the values directly estimated from P-wave data.|
|Appears in Collections:||Earth and Planetary Science Research Institute thesis and dissertation collection|
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