Using seismic refraction to assess crustal thickness within the Great Basin

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Using seismic refraction to assess crustal
thickness within the Great Basin

• Michelle Heimgartner, John Louie, James Scott,
  Weston Thelen, Christopher Lopez, Mark Coolbaugh,
  and Satish Pullammanappallil

University of Nevada, Reno Nevada Seismological
Laboratory Great Basin Center for Geothermal
Energy
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Overview

• Goals of research
• Overview of seismic refraction experiments
• Discuss current crustal thickness/velocity
  results
• Mention future work
• Discuss correlation of crustal thickness with
  geology geothermal occurrences

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Goals of refraction experiments

• 1. Compile existing crustal information
• Establish a facility for long-range crustal
  surveys
• Collect three new crustal refraction profiles
• Integrate new and prior results create a
  regional crustal model that is available to
  others
• Relate crustal model to geology

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Refraction profiles
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Results so far

• Areas of extremely thin crust (approx. 20 km
  thick, northern Nevada)
• Crustal root beneath the northern and central
  Sierra Nevada
• Crustal thickness correlates with heat flow in
  the Great Basin
• Not all geophysical data sets agree (Teleseismic
  vs. refraction/reflection)

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Seismic refraction
Receivers
Source
Cross-over distance from the source, the
distance at which refracted rays arrive before
direct rays
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Northern Walker Lane (NWL) transect
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NWL- Barrick GoldStrike blast

• Deployed 199 Texans over 450 km distance
• Seismic source 38,000 kg Barrick GoldStrike mine
  blasts
• Blast arrivals are visible over 300 km from source

Barrick GoldStrike mine blast, 8-30 Hz filtering
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NWL velocity-depth model
Louie et al., 2004

• Crustal root beneath Sierra Nevada Mtns. (gt50 km)
• Thin crust 19-23 km thick near Battle Mountain, NV

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Idaho-Nevada-California (INC) transect
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INC continuous crossing over the Sierra
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INC-Barrick GoldStrike blast

• Deployed 411 Texan instruments along a 600 km
  transect (spaced approx. 1.5 km apart)
• Recorded several 77,000 kg blasts at Barrick
  GoldStrike
• Blast arrivals are visible 400 km from the source

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INC-Toms Place earthquake

• Earthquake magnitude 1.6
• Shallow epicenter located directly beneath the
  transect line
• Provides crustal velocities along the interior of
  the transect

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INC velocity-depth model

• Sierra crustal root, approx. 50 km depth
• 30 km crust in northern NV, agrees with PASSCAL
  1986
• Lose resolution north of Barrick, but cross-over
  distances of less than 90 km suggest thinner crust

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Northern Nevada Utah transect (NNUT)

• Several large mine blasts an earthquake on the
  Wasatch Front
• Historically large blast for Simplot
• Provide information for the Great Basin-Wasatch
  transition
• Provide refraction control through northern Utah

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Crustal thickness map of the Great Basin
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Gravity map with crustal thickness data
Gravity map from Oppliger, 2003, University of
Nevada, Reno
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Temperature Gradient map with crustal thickness
data
After David Blackwell, Southern Methodist
University (Coolbaugh et al., 2005)
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Crustal thickness map with current geothermal
sites

• Correlation thinner crust does host current
  geothermal sites

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Geothermal favorability vs. Crustal thickness
NBMG Map 151
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Future work

• New 3D model for INC transect
• including more mine blasts and smaller
  earthquakes
• Continue to process NNUT data
• Update crustal thickness map
• Correlate crustal thickness with geology
• Publish results, and make available to others

3D model
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Conclusions

• Showed that large mine blasts are effective
• Can collect data in regions where not previously
  surveyed
• Thin crust at Battle Mountain, NV
• Within a limited region of 100 km, 19-23 km thick
  crust
• Southern extent of thin crust limited by the INC
  transect (30 km crust)
• Thin crust near Battle Mountain supported by
  crossover distances from the INC and NWL
  experiments
• Gravity data supports thin crust
• Deep root under the Sierra Nevada
• Evidence for deep root in northern Sierra and no
  root in southern Sierra
• Integrate old and new crustal data
• Select survey techniques for consistency

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Acknowledgements
This material is based upon work supported by
the U.S. Department of Energy under instruments
numbered DE-FG07-02ID14211 and DE-FG36-02ID14311,
managed through the DOE Golden Field Office.
The instruments used in the field program were
provided by the PASSCAL facility of the
Incorporated Research Institutions for Seismology
(IRIS) through the PASSCAL Instrument Center at
New Mexico Tech. Data collected during this
experiment will be available through the IRIS
Data Management Center. The facilities under the
IRIS Consortium are supported by the NSF under
Cooperative Agreement EAR-0004370 and the DOE
National Nuclear Security Administration. The
California Integrated Seismic Network (USGS
Cooperative Agreement 04HQAG0004) provided
earthquake locations used in the experiment. We
would like to thank Barrick GoldStrike, Round
Mountain, Kennecott Bingham Canyon, Simplot and
Cortez mines for their cooperation and
willingness to help.
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Back-up slides
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INC refraction transect

• Seismic source mine blasts (200,000 lb) and
  small local earthquakes (magnitude 1.5-3.8)
• 24-bit single channel, portable seismograms
  (Texans) connected to 4.5 Hz geophones
• Deployed 411 Texan instruments along a 600 km
  transect (spaced approx. 1.5 km apart)
• Instruments recorded for 96 hours (four 24-hour
  periods)

Barrick GoldStrike mine, Battle Mountain, NV
(above) Texan Instruments (right)
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INC-Barrick GoldStrike blast, reduced time
SW
NE
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Crustal thickness map
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Temperature Gradient map
After David Blackwell Southern Methodist
University (Coolbaugh et al., 2005)