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GCHigh
Resolution
P-P Imaging of Deepwater Near-Seafloor Geology*
AND
GCHigh
Resolution
P-SV Imaging of Deepwater Near-Seafloor Geology**
By
Bob A. Hardage1 and Paul E. Murray1
Search and Discovery Article #40200 (2006)
Posted July 4, 2006 (Part 1), September 20 (Part 2)
*Adapted from the Geophysical Corner column, prepared by the authors and entitled “Technique Improves Deep Imaging,” in AAPG Explorer, July, 2006, as Part 1 of a two-part series.
**Adapted from the Geophysical Corner column, prepared by the authors and entitled “P-SV Data Most Impressive Image,” in AAPG Explorer, August, 2006, as Part 2 of a two-part series.
Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, University of Texas, Austin, Texas ([email protected] )
P-P Imaging (Part 1)
Multicomponent seismic data have unique value for studying near-seafloor geology in deepwater environments. When properly processed, P-P (compressional) and P-SV (converted-shear) images made from 4-C seismic data acquired in deep water with seafloor sensors show near-seafloor geology with amazing detail.
This is the first of two parts that describe how improved imaging of near-seafloor, deepwater strata can be achieved with conventional multicomponent seismic data.
This part focuses on P-P imaging; Part 2 focuses on P-SV imaging.
uIntroductionuFigures 1-2uAcquisition / processinguApplicationsuCommentuFigures 3-4uWave lengthuIncreasing frequencyuAppendix
uIntroductionuFigures 1-2uAcquisition / processinguApplicationsuCommentuFigures 3-4uWave lengthuIncreasing frequencyuAppendix
uIntroductionuFigures 1-2uAcquisition / processinguApplicationsuCommentuFigures 3-4uWave lengthuIncreasing frequencyuAppendix
uIntroductionuFigures 1-2uAcquisition / processinguApplicationsuCommentuFigures 3-4uWave lengthuIncreasing frequencyuAppendix
uIntroductionuFigures 1-2uAcquisition / processinguApplicationsuCommentuFigures 3-4uWave lengthuIncreasing frequencyuAppendix
uIntroductionuFigures 1-2uAcquisition / processinguApplicationsuCommentuFigures 3-4uWave lengthuIncreasing frequencyuAppendix |
In deepwater multicomponent seismic data acquisition, there is a large elevation difference between source stations (an air gun at the sea surface) and receiver stations on the seafloor. Conventional processing of deepwater 4-C seismic data involves a wave-equation datuming step that transforms the data to a domain in which sources and receivers are on the same depth plane. This step effectively removes the water layer and allows the data to be processed as if the source was on the seafloor. This adjustment of source-receiver geometry also allows deepwater multicomponent data to be processed with software already developed for shallow-water environments where marine multicomponent data acquisition technology was originally developed and applied. An example of a good-quality, deepwater P-P image of near-seafloor geology made with this wave-equation datuming approach is shown as Figure 1a. This image shows local geology associated with a fluid-gas expulsion chimney that extends to the seafloor. If a person wishes
to study near-seafloor strata, a new approach to P-P imaging of
deepwater multicomponent seismic data is to not eliminate the large
elevation difference between sources and receivers but to take advantage
of that elevation difference. The objective is to process deepwater
multicomponent data similar to the way Users of VSP
technology know VSP data provide high- The P-P processing
illustrated here can be done with either 2-C or 4-C seafloor sensors.
The fundamental requirement is to acquire data with a sensor having a
hydrophone and a
D=P+Z/cos(F) U=P--Z/cos(F)
“F” defines the incident angle at which the downgoing compressional wave arrives at the seafloor. Once this wavefield separation is done, deepwater multicomponent seismic data are defined in terms of downgoing and upgoing wavefields, just as are VSP data. Having access to downgoing (D) and upgoing (U) wavefields means sub-seafloor reflectivity can be determined by taking the ratio U/D. This reflectivity wavefield is then segregated into stacking corridors, and data inside these corridors are summed to create image traces just like VSP data have been processed for the past 20-plus years.
Figure 1b shows a P-P image made with this technique using the same
deep-water data displayed in Figure 1a. The
improvement in
Applying this VSP-style
imaging technique to deepwater multicomponent seismic data is proving to
be invaluable for gas hydrate studies, geomechanical evaluations of
deepwater seafloors and other applications where it is critical to image
near-seafloor geology with optimal Every seismic data-processing technique, however, has constraints and pitfalls. Two principal constraints of the technology described here are:
P-SV Imaging (Part 2)General Comment
In
Part 1, we considered how to improve the seismic
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