Click to view article in PDF format.
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])
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.
|
A VSP image and a
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
The fact that VSP data provide an independent image that can be moved up and down to find an optimal match between VSP and
An example of a VSP-based stratigraphic calibration of a
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
Note that this interpretation procedure leads to the conclusion that some thin-bed units correlate with peaks in the
|