Using seismic refraction to assess geothermal potential: an updated view of crustal thickness within

1
Using seismic refraction to assess geothermal
potential an updated view of crustal thickness
within the Great Basin

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

2
Overview

• Goals of research
• Overview of seismic refraction experiments
• Discuss new crustal thickness/velocity results
• Discuss correlation of crustal thickness with
  geology geothermal occurrences

3
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

4
Refraction profiles
5
Results in Heimgartners M.S. Thesis

• 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)

6
Seismic refraction
Receivers
Source
Cross-over distance from the source, the
distance at which refracted rays arrive before
direct rays
7
Northern Walker Lane (NWL) transect
8
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
9
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

10
Idaho-Nevada-California (INC) transect
11
INC continuous crossing over the Sierra
12
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

13
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

14
Revised INC velocity-depth model

• Sierra crustal root, approx. 50 km depth
• 30 km crust southeast of Battle Mtn., agrees
  with PASSCAL 1986
• Lose resolution north of GoldStrike, but
  cross-over distances of less than 90 km suggest
  thinner crust

15
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

16
Revised crustal thickness map of the western
Great Basin100-km long area of 20-km crust SW
of Battle Mtn.- isolated but corroboratedCrustal
root under northern and central Sierra Nevada
17
Gravity map with crustal thickness data
18
Temperature Gradient map with crustal thickness
data
19
Geothermal favorability vs. Crustal thickness
20
Conclusions

• Showed that large mine blasts are effective
• Can collect data in regions not previously
  surveyed
• Thin crust southwest of Battle Mountain, NV
• Within a limited region 100 km long, 19-23
  km-thick crust
• Thin crust limited by the INC transect (30 km
  crust) - Moho dips at least 15
• 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

21
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.
22
Back-up slides
23
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)
24
INC-Barrick GoldStrike blast, reduced time
SW
NE
25
Crustal thickness map
26
Temperature Gradient map
After David Blackwell Southern Methodist
University (Coolbaugh et al., 2005)