© 2005 Paulsson Geophysical Characterization of the San Andreas Fault at Parkfield Using a Massive...

© 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

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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

© 2005 Paulsson Geophysical

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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© 2005 Paulsson Geophysical

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

1000

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3000

4000

5000

6000

7000

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)