© 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 u Introduction u Zero Offset Shot l Wavefield separation and lithology log matching l Forward modeling of reflections from vertical structures u Far Offset Shot l Wavefield separation and forward modeling of horizontal reflections u Depth migration u 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

© 2005 Paulsson Geophysical Zero Offset Shot Recorded at depths between 46m and ~2400m BGL

© 2005 Paulsson Geophysical Zero Offset VSP shot: Reflected Wavefield Used for Migration Boness and Zoback, 2005

© 2005 Paulsson Geophysical Zero Offset Synthetic Modeling Reflection from a reflector under the array Reflection from near vertical SAF

© 2005 Paulsson Geophysical TANK Shot Middle Mountain

© 2005 Paulsson Geophysical Strong reflected phases in raw data after first arrival Vertical ComponentRadial Component

© 2005 Paulsson Geophysical

Reflected wavefield obtained with 3D wavefield separation and downgoing deconvolution

© 2005 Paulsson Geophysical Synthetic modeling of a horizontal reflector for TANK

© 2005 Paulsson Geophysical Depth migrated image from two shots with two strong reflections Tertiary sediments-Granite Interface Sandstone-Granite Interface?

© 2005 Paulsson Geophysical 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...

© 2005 Paulsson Geophysical 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 u 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. u Filling the gaps in the image and a better determination of structures’ dips requires the use of many more sources. u 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. u For the first time, 1D borehole observations could be extrapolated to the 3D volume surrounding the borehole

© 2005 Paulsson Geophysical Acknowledgements u 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 u USGS and EarthScope for access to the SAFOD well, and for funding the workover rig and dynamite shots u Geometrics for providing the recording system u Velocity model provided by Eylon Shalev and Peter Malin at Duke University

© 2005 Paulsson Geophysical Velocity profiles at the SAFOD site in California