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GCVSP Image Adjustment to Stratigraphy and 3-D
Seismic
*
Bob Hardage1
Search and Discovery Article #40447 (2009)
Posted September 17, 2009
*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, August, 2009, and entitled
“Welding Geology to
Seismic
Images”. Editor of Geophysical Corner is Bob A. Hardage ([email protected])
. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, The University of Texas at Austin ([email protected])
General Statement
Vertical
seismic
profiling (VSP) is a measurement procedure in which a
seismic
sensor is lowered to a sequence of selected depths
in a well by wireline, and at each of the downhole receiver stations that sensor then records the downgoing and upgoing
seismic
wavefields produced
by a surface-positioned source (Figure 1). An important concept to understand regarding
VSP imaging is that VSP recording geometry causes the stratigraphy at a VSP well – where sequence
boundaries are known as a function of depth from well logs and sample/core control – to be welded to the VSP image, which is known as a function of
VSP reflection time.
This welded relationship between stratigraphy and a VSP image results because VSP receivers are distributed vertically through
geologic image space, allowing both stratigraphic depth and
seismic
travel time to be known at each downhole receiver station. This dual-coordinate
domain (depth and time) involved in a VSP measurement means that any geologic property known as a function of depth at a VSP well can be accurately
positioned on, and rigidly welded to, the time coordinate of the VSP image (Figure 1).
The reverse situation also is true: The VSP image can be accurately positioned on, and welded to, the depth coordinate of the
stratigraphic column at a VSP well. This latter option of transforming a VSP image to the stratigraphic depth domain is not done as often, because
the usual objective of prospect
interpretation
is to insert stratigraphy into 3-D
seismic
data
volumes that are defined as functions of
seismic
image
time, not as functions of stratigraphic depth.
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Concept
A VSP image and a 3-D seismic image often have different time datums, because the images were made by different contractors
who used different depth datums for the time origin, different replacement velocities to move source stations to this depth datum, and different illuminating wavelets. As a result, an interpreter often has to shift a VSP
image up or down relative to a 3-D seismic image to determine an optimal match between the two images.
The concept of a welded bond between a VSP image and the stratigraphy at a VSP well means that whenever an interpreter moves
a VSP image up, say by 20 ms, to better correlate with a 3-D seismic image, the stratigraphy moves up by the same amount (20 ms) in 3-D seismic
image space. Likewise, if the VSP image has to be moved down to create an optimal waveform character match with the 3-D data , then the straigraphy
shifts down by the same amount in the 3-D seismic volume.
The fact that VSP data provide an independent image that can be moved up and down to find an optimal match between VSP and 3-D
seismic reflection character is the fundamental property of the VSP-to- seismic calibration technique that establishes the correct time shift
between 3-D seismic image time and VSP image time. When the time shift between the 3-D seismic and VSP images is determined, then the correct
time shift between the 3-D seismic image and the stratigraphy at the VSP calibration well is also defined, because that stratigraphy is welded
to the VSP image and moves up and down in concert with the VSP image-time axis.
Example
An example of a VSP-based stratigraphic calibration of a 3-D data volume is shown as Figure 2. This VSP image is the same one displayed in Figure 1
and was produced from a large-offset VSP survey where the source was positioned 600 meters (2,000 feet) from the receiver well. The
fact that stratigraphy is welded to the VSP image, causing stratigraphic interfaces to move up and down in concert with the VSP image during
the VSP-to- seismic image calibration process, is what ensures that targeted thin-bed units are positioned in the correct time windows in the
3-D seismic volume when an optimal alignment is established between the VSP and 3-D images.
In Figure 2, this VSP-based interpretation procedure leads to the conclusion that although the tops of thin-bed units
19C and 15 are positioned at VSP image times of 1.432 s and 1.333 s, respectively, they have to be inserted into the 3-D data volume 18 ms
earlier in image time at 3-D image times of 1.414 s and 1.315, respectively.
Conclusion
Note that this interpretation procedure leads to the conclusion that some thin-bed units correlate with peaks in the 3-D volume.
Some thin-beds are associated with troughs.
Some thin-beds are positioned on zero-crossings of the 3-D wiggle-trace data . However, for each thin-bed unit we can be sure
that we have defined the proper 3-D seismic data window at the VSP calibration well, where seismic attributes can be calculated to study the
distribution of each thin-bed reservoir throughout 3-D image space.
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