Precision Gravity Signals at The Geysers Geothermal System

1
Precision Gravity Signals at The Geysers
Geothermal System

• P. Gettings1, R. Harris1,
• R. Allis2, D. Chapman1
• 1Dept of Geology Geophysics, University of
  Utah
• 2Utah Geological Survey

2
Outline

• Project Background
• Introduction
• Data collection
• Surveys
• Station network
• Elevation changes, seasonal annual
• Gravity changes, seasonal annual
• Conclusions future work

3
Project Introduction

• Study to use repeated gravity GPS to monitor
  mass change and deformation
• Image changes from enhanced injection
• Funded by DOE
• Investigate seasonal annual signals already
  occuring at The Geysers
• Use data from first year (Sept. 2000 - Sept. 2001)

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Data Collection

• Gravity data collected with CG-3M
• Corrected for
• Instrument drift
• Solid Earth tides
• Elevation change (from GPS)
• GPS data collected with Trimble 4700
• Post-processed static method with 30 min
  occupations
• Referenced to HOPB continuous station

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Gravity Data Acquisition
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Surveys

• Three surveys to date September 2000, April
  2001, September 2001
• Gravity and GPS data simultaneously collected
  each campaign.
• Use these data to establish a baseline for future
  monitoring of injection, and investigate seasonal
  (Sept. to April) and annual cycles already
  present.

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Station Network

• 150 stations currently installed

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Elevation ChangeSep 2000 to Apr 2001
9
Elevation ChangeApr 2001 to Sep 2001
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Production History
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Gravity ChangesSep 2000 to Apr 2001
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Gravity ChangesApr 2001 to Sep 2001
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Predicted Gravity EffectsSep 00 to Apr 01

• 1-D Bouguer Slab DG 40 DM / A
• Assume A 40 km2, DM in Mt, DG in mGal
• Net mass loss of 20.55 Mt ? -20 mGal
• Max/min mass changes
• 42.68 Mt loss ? -43 mGal signal
• 22.13 Mt gain ? 22 mGal signal
• Ground water change
• Estimate DZ 1.8m ? 15 mGal
• Assume F20
• Observed average DG -38 mGal

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Predicted Gravity EffectsApr 01 to Sep 01

• Net mass loss of 12.31 Mt ? -12 mGal
• Max/min mass changes
• 20.21 Mt loss ? -20 mGal signal
• 7.9 Mt gain ? 8 mGal signal
• Ground water change
• H41 DZ -2.8m ? -23 mGal
• Assume F20
• ABSG DZ -2.5m ? -20 mGal
• Observed average DG -25 mGal

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Conclusions

• Seasonal subsidence rates appear slightly larger
  than the average annual rate of 4 cm/yr reported
  by Mossop 1997
• Gravity changes show an overall decrease across
  the field
• Sep 00 to Apr 01 ? -38 mGal
• Apr 01 to Sep 01 ? -25 mGal
• Ground water and production changes explain
  average signal and most variability
• May have captured effect of power crunch

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Future Work

• Survey twice per year until at least 2004
• Investigate reservoir properties via 3-D models
  with gravity deformation data
• Attempt to determine seismicity mechanism
• Publish results techniques, to allow industry
  to apply to other reservoirs