Subsurface Imaging with Reverse Vertical Seismic Profiles
by
Mary L. Krasovec
Submitted
to the Department of Earth, Atmospheric, and Planetary Sciences on May 16, 2001 in partial fulfillment of the requirements for the
degree of Doctor of Philosophy
ABSTRACT
This thesis presents imaging results from a 3D reverse vertical seismic profile (RVSP)
dataset measured at a hydrocarbon bearing pinnacle reef in northern Michigan. The
study presented many challenges in seismic data processing and imaging, as the survey
geometry was unique in several ways.
Reverse VSP, which uses seismic sources in a borehole and receivers on the earth's
surface, is fairly rare. RVSP in 3D with a random distribution of surface geophones
is unprecedented. At the time this data was collected, no commercially available
processing tools existed to address this geometry, so a processing scheme had to be
developed.
The data processing sequence presented in this thesis, which includes amplitude
corrections, first break picking, deconvolution, wavefield separation, and application
of statics, takes advantage of the repeatible signature of the new downhole source
(Paulsson et al., 1998). Since the data can be handled in common-receiver gathers
instead of the usual common-source gathers, it can be treated like several single
offset VSPs during the processing sequence. Issues related to the 3D geometry and
the random distribution of the receiver array need not be addressed until the imaging
step.
The generalized Radon transform (GRT) migration method of Miller et al. (1987)
provides a high resolution image of a portion of the target reef at 4600 feet (1400
meters) depth. The high resolution of the image is largely due to the downhole source,
which generated a high powered signal at frequencies up to several hundred Hertz.
Another factor in the high resolution of the image is the success of receiver consistent
model-based Wiener deconvolution (Haldorsen et al., 1994), possible because the
source signature was repeatable.
Due to adverse conditions and power system failure, a large portion of the surface
array did not record data. The reduced spatial coverage limits the extent of the
migrated image, precluding an evaluation of the effectiveness of the random receiver
spread.
The limited nature of the receiver array also caused artifacts resembling migration
smiles in the image. These artifacts are partially suppressed by limiting the aperture
of the migration, but this also removes dipping reflectors from the image.
To maximize the imaging capibilities of the data, a second approach complimenting
the GRT method is developed. This approach, termed vector image isochron
(VII) migration, removes array artifacts from the image without losing energy from
dipping reflectors. This allows artifacts in the conventional image to be identified,
aiding interpretation of the GRT images. VII images also show more even illumination
than conventional images, although an effect similar to NMO stretching reduces
the resolution of the VII image as compared to the GRT image.
The VII scheme is an extension of the GRT migration process of Miller et al.
(1987), but involves forming an image which depends on the imaged plane orientation,
transforming the image based on the array geometry, then finishing the GRT
summation over plane orientations. The VII imaging method is derived in both 2D
and 3D with the assumption that the ray paths are straight and that at least one of
the arrays, source or receiver, is horizontally oriented. The surface array can have
any distribution, regular or random. The other array can have any orientation in
general, although this thesis assumes that it will be either another surface array or a
vertically oriented borehole array. Borehole surveys in deviated wells, or in multiple
wells, can be imaged with VII migration, at the cost of more computation time.
In this thesis, the VII imaging method is tested on synthetic examples as well
as the Michigan RVSP data. Is is found that, when used to compliments eachother,
GRT and VII images provide 3D information about the subsurface structure which
far surpasses surface seismics in terms of resolution. These images are directly tied
to depth, but are not limited to a slice as are crosswell studies.
The combination of the new downhole source with the processing and imaging
schemes in this thesis provide a valuable new tool for the task of reservoir delineation.
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