© 2005 Paulsson Geophysical
Characterization of the San Andreas Fault at Parkfield Using a Massive 3D VSP
J. Andres Chavarria, Alex Goertz, Martin Karrenbach, Paul Milligan, Bjorn Paulsson
December 2005
Paulsson Geophysical Services, Inc. (P/GSI)Brea, CA
© 2005 Paulsson Geophysical
Introduction Zero Offset Shot
Wavefield separation and lithology log matching Forward modeling of reflections from vertical structures
Far Offset Shot Wavefield separation and forward modeling of horizontal reflections
Depth migration Conclusions
© 2005 Paulsson Geophysical
80 Level array of 3C seismometers
Data recorded at 0.25 ms included:
Numerous microearthquakes of magnitudes up to 2.7.
Some of the smallest events were not recorded along the entire array and are not in the catalog
13 Far offset shots recorded on 80 levels at the bottom portion of the well
Zero offset shot recorded at 180 levels spanning the entire portion of the drilled well
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Zero Offset Shot Recorded at depths between 46m and ~2400m BGL
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Zero Offset VSP shot: Reflected Wavefield Used for MigrationBoness and Zoback, 2005
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Zero Offset Synthetic Modeling
Reflection from a reflector under the array
Reflection from near vertical SAF
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TANK Shot
Located @ Middle Mountain
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Strong reflected phases in raw data after first arrival
Vertical Component Radial Component
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Reflected wavefield obtained with 3D wavefield separation and downgoing deconvolution
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Synthetic modeling of a horizontal reflector for TANK
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Depth migrated image from two shots with two strong reflections
Tertiary sediments-Granite Interface
Sandstone-Granite Interface?
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Depth migrated image with structural guidance to enhance vertical reflections
Vertical structure, observed previously from PH observations (Chavarria et al., 2003; Chavarria, 2004)
San Andreas Fault structures...
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Tertiary Seds-Granite Interface
Sandstone-Granite Interface ???
Near vertical faults
Possible SAF?
Composite of Kirchhoff Migrated Images for Zero Offset and Tank Shots, with and without structural guidance for enhancement of both near vertical and horizontal structures
© 2005 Paulsson Geophysical
Conclusions
The long aperture and fine spacing between seismometers provided partial images of the fault zone without strong artifacts. Sediment-Granite interfaces were imaged as well.
Filling the gaps in the image and a better determination of structures’ dips requires the use of many more sources.
The use of a long borehole array within the SAFOD well provided important information about the structure of the Parkfield segment of the San Andreas Fault.
For the first time, 1D borehole observations could be extrapolated to the 3D volume surrounding the borehole
© 2005 Paulsson Geophysical
Acknowledgements
U.S Department of Energy for financial support to build the 400 level array and developing the borehole seismic software through cooperative agreement with the U.S. Department of Energy’s Office of Fossil Energy, contract DE-FC26-01NT41234; DE-FC26-03NT15418
USGS and EarthScope for access to the SAFOD well, and for funding the workover rig and dynamite shots
Geometrics for providing the recording system
Velocity model provided by Eylon Shalev and Peter Malin at Duke University
© 2005 Paulsson Geophysical
Velocity profiles at the SAFOD site in California
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0 500 1000 1500 2000 2500 3000
Depth from datum of 670 (m)
Inte
rval
Vel
ocit
y (
m/s
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P/GSI VSP (75 m smoothing)
Sonic (75 m smoothing)