XIANG XIAO'S PHD THESIS
LOCAL REVERSE TIME MIGRATION
WITH VSP GREEN'S FUNCTIONS
(Ph.D Dissertation)
Xiang Xiao, University of Utah
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ABSTRACT
Three methods are introduced for imaging the reflectivity
distribution with natural VSP Green's functions:
interferometric datuming and imaging, local
reverse time migration for subsalt reflector imaging, and local
reverse time migration for salt flank imaging. The transmitted
arrivals in the recorded VSP Green's function are back-projected
from the well while in standard migration these transmitted
arrivals are forward modeled. This means that the overburden
velocity model is not needed for migration and so leads to a
significant improvement in imaging.
There are three main chapters in this dissertation.
Chapter 2. Surface seismic profile (SSP) data can be naturally redatumed
below salt with the natural Green's functions obtained from a
VSP shot gather.
No overburden or salt velocity model is needed and the quality of the image only depends on the accuracy
of the 
around the well and the receiver aperture width.
The subsalt image is more accurate than
that provided by migrating
the SSP or VSP data alone. The numerical images show that they inherit the illumination aperture
of SSP data, which is usually wider than the VSP data.
Chapter 3. A VSP imaging method that only requires a
target-oriented local velocity model around the well is presented
in the form of a Kirchhoff integral. Here, we call this method
`local reverse time migration'. The complex overburden and salt
body geometry are not needed, and the source-side statics are
automatically accounted for. All kinematic and dynamic effects,
including anisotropy, absorption and all other unknown/undefined
rock effects outside of this local velocity model are
automatically accounted for. The numerical tests empirically
validate this method by accurately imaging the sediments next to
the vertical well.
Chapter 4. The local reverse time migration method is
adapted for imaging salt flanks by transmitted P-to-S waves. There
are two strategies: separation of 
and 
transmitted
wavefields before acoustic backward projection and `separation
while imaging'. Here, only a localized velocity model and the
receiver wavefields are used to delineate the salt boundaries with
transmitted P-to-S waves. Numerical tests on data associated
with an elastic salt model and Gulf of Mexico VSP data demonstrate its
superiority over the traditional migration method.