effectiveness of magnetotelluric method for geothermal...
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Effectiveness of magnetotelluric method for geothermal exploration
Toshihiro UchidaGeological Survey of Japan (GSJ),
National Institute of Advanced Industrial Science and Technology (AIST)
JOGMEC – GNS Science Geothermal Workshop2 June 2016Tokyo, Japan
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This work was for exploration of hydrothermal mineral deposits.
Effect of clay minerals on electrical resistivity
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Montmorillonite reduces electrical resistivity more intensively than other clay minerals.
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Yokoyama et al. (1983)
Resistivity decreases by more than one decade when temperature increases from 15 to 200 degrees C.
Electrical resistivity: temperature dependence
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Uchida et al. (1994)
2D MT in Sumikawa geothermal field
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Uchida et al. (1994)
SN-7D Well Logs, Sumikawa
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Arnason et al. (1987)
Franzson (1994)
Schlumberger 2D model in IcelandAlteration minerals
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Typical Resistivity of Geothermal Reservoir
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3D MT in Ogiri Geothermal Field
x=150 deg.y= 60 deg.
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Uchida (2005)
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3D Resistivity Model, depth sections
B
A
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Uchida (2005)
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Low resistivity
3D MT Responses: Line-B
StaticShift
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Uchida (2005)
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Shiramizugoe
Interpretation (1)
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Uchida (2005)
Uchida (2005)
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Ogiri
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Uchida (2005)
Interpretation (2)
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(TM-mode)
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Comparison with 2D Models (1)
Uchida (2005)
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(TM-mode)
13Uchida (2005)
Comparison with 2D Models (2)
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Geological Interpretation
from NEDO (2000)
Shiramizugoe Field4 wells(300 t/h steam)
Ogiri Reservoir
Clay Cap
14Uchida (2005)
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3D MT survey is now a must for geothermal
3D MT survey has rapidly expanded since late 2000s:üUSAü JapanüNew Zealandü IcelandüEast Africaü Indonesia, …
3D inversion codes in WGC2015 papers:v Newman and Alumbaugh (2000)v Sasaki (2001), Uchida and Sasaki (2003)v Siripunvaraporn et al. (2005, 2009): WSINV3DMTv Hautot et al. (2007)v Egbert and Kelbert (2012): ModEM
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Yanaizu-Nishiyamageothermal field
65 MWe Power Plant in operation since 1995
3D MT in Yanaizu-Nishiyama
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Reddots in2000/200139stations(26areinthefigure)
Greendots in201030stations
DC railway at 25 km distance
MT Stations
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Uchida et al. (2015)
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Time Series Data
Site 630(x=N32°E)
Reference site(x=true north)
10 minutes21:00 – 21:1023 Oct 2010
Ex
Hx
Hy
Ey
Hz
Ex
Hx
Hy
Ey
Hz
DC train
Auto-scale18
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Data Example
Site 630
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Rotation= 55 deg.
51 stations,15 freq (0.0134 - 229Hz)
- Zxy / Zyx- Finite difference- Static shift - No topography
(Uchida&Sasaki, 2006)
- Forward cells: 59(x) x 57(y) x 40(z)- Cell size (surface): 100m(x) x 100m(y) x 25m(z) - Blocks: 17(x) x 16(y) x 18(z) = 4896
3D Inversion: Area-1
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Area-1:
3D Model
Depth-slice sections
Artifact?
21Uchida et al. (2015)
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Area-1:
DataFitting
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Looking from east
Area-1: Cross section (x=1.4km, y=1.0km)
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Uchida et al. (2015)
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Rotation= 90 deg.
59 stations,14 freq (0.0134 - 115Hz)
- Zxy / Zyx- Finite difference- Static shift- No topography
- Forward cells: 83(x) x 77(y) x 40(z)- Cell size (surface): 100m(x) x 100m(y) x 25m(z) - Blocks: 29(x) x 26(y) x 18(z) = 13,572
3D Inversion: Area-2
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Uchida et al. (2015)
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Rot = 55 deg. Rot = 90 deg.
Depth-slice sections
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Uchida et al. (2015)
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Production and injection wells
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A
A’D
D’
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A
A’
Averageelevation400massumed
Rot = 55 deg. Rot = 90 deg.
Profile A-A’: Comparison with drilling data
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Uchida et al. (2015)
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D
D’
Averageelevation400massumed
Rot = 55 deg. Rot = 90 deg.
Profile D-D’: Comparison with drilling data
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Uchida et al. (2015)
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UpperlimitofChlorite
Clay minerals
29Uchida et al. (2015)
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First full MT in NZ in 1998 by GSJ & GNS
Ogawa et al. (1999)
Taupo Volcanic Zone (TVZ)
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3D MT in southern TVZ by GNS (1)
Bertrand et al. (2015)
Wide MT coverage to obtain deep resistivity image and an overall geothermal model.
Deep heat supply?
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3D MT in southern TVZ by GNS (2)
Bertrand et al. (2013)
+ Dense local MT coverage to obtain detailed resistivity image of a specific geothermal field: Ohaaki.
2D
3D
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3D MT in Rotorua by GNS
Heise et al. (2016)
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ü The inversion result that only utilized the dense MT stations over the geothermal field (Area-1) produced artifacts along the edge of the interpretation zone, probably caused by anomalous structure that exists at the edge or just outside of the target area.
ü Inclusion of outside MT stations (Area-2) in the 3D inversion can reduce such artifacts and improve the model reliability at a deeper part of the target area.
Summary
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Message from Yanaizu-Nishiyama MT data
ü MT is the most important geophysical tool for geothermal exploration.
ü 2D model is useful only when real geologic structure is close to 2D; but often gives false anomalies for deep structure.
ü 3D model is needed in all exploration for reliable interpretation, although it requires longer field measurement and intensive computation time for interpretation.
ü Many trials of inversion are necessary to obtain satisfactory model.
The role of MT for geothermal exploration
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Summary (cont’d)
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Issues of 3D MT inversion to be solved for geothermal application soon.
ü Topography must be included.ü Four components of the impedance and two components of the
vertical transfer function should be considered.ü Static shift and galvanic distortion should be included.