Effectiveness of magnetotelluric method forgeothermal exploration
Toshihiro Uchida
Geological Survey of Japan (GSJ),
National Institute of Advanced Industrial Science and Technology (AIST)JOGMEC – GNS Science Geothermal Workshop
2 June 2016
Tokyo, JapanEffect of clay minerals on electrical resistivity

Montmorillonite
reduces electricalresistivity more
intensively thanother clay minerals.

This work was for
exploration ofhydrothermal
mineral deposits.

2Electrical resistivity: temperature dependence

Resistivity decreases by morethan one decade whentemperature increases from15 to 200 degrees C.

Yokoyama et al. (1983)

32D MT in Sumikawa geothermal field

Uchida et al. (1994)4

SN-7D Well Logs, Sumikawa

Uchida et al. (1994)

5

Typical Resistivity of Geothermal Reservoir
Alteration minerals

Schlumberger 2D model in IcelandFranzson (1994)
Arnason et al. (1987)
6

3D MT in Ogiri Geothermal Field
Uchida (2005)

x=150 deg.
y= 60 deg.
7

3D Resistivity Model, depth sections
Uchida (2005)

AB8

3D MT Responses: Line-B
Uchida (2005)

StaticShift

Low resistivity
9Interpretation (1)Uchida (2005)

Shiramizugoe

Uchida (2005)10

Interpretation (2)Uchida (2005)

Ogiri

11

Comparison with
2D Models (1)(TM-mode)

Uchida (2005)

12

Comparison with
2D Models (2)(TM-mode)

Uchida (2005)

13

Geological Interpretation
Clay Cap

Shiramizugoe Field
4 wells（300 t/h steam）

Ogiri
Reservoir

from NEDO (2000)

Uchida (2005)

14

3D MT survey is now a must for geothermal
3D MT survey has rapidly expanded since late 2000s:
ü USA
ü Japanü New Zealandü Icelandü East Africa
ü 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
15

3D MT in Yanaizu-Nishiyama
Yanaizu-Nishiyama
geothermal field
65 MWe Power Plant in operation
since 1995

16

MT Stations
Uchida et al. (2015)

Red dots in 2000/2001
39 stations(26 are in the figure)Green dots in 2010
30 stations

DC railway at 25 km
distance17

Time Series Data
Site 630(x=N32°E)Ex
EyHx

DC train

HyHzEx

Reference site(x=true north)

EyHx
Hy

10 minutes
21:00 – 21:10
23 Oct 2010

HzAuto-scale
18

Data Example
Site 630

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3D Inversion: Area-1
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
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Area-1:
3D ModelDepth-slice
sections

Artifact?

Uchida et al. (2015)21

Area-1:
DataFitting

Area-1: Cross section (x=1.4km, y=1.0km)
Uchida et al. (2015)

Looking from east23

3D Inversion: Area-2
Uchida et al. (2015)

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
24

Depth-slice sections
Rot = 55 deg.

Uchida et al. (2015)

Rot = 90 deg.

25

Production and injection wells

DA’AD’26

Profile A-A’: Comparison with drilling dataRot = 55 deg.

Rot = 90 deg.

Uchida et al. (2015)

A’A

Average elevation 400m assumed

27

Profile D-D’: Comparison with drilling data
Rot = 55 deg.

Rot = 90 deg.

Uchida et al. (2015)

DD’
Average elevation 400m assumed

28

Clay minerals

Upper limit ofChlorite

Uchida et al. (2015)29

First full MT in NZ in 1998 by GSJ & GNS
Taupo Volcanic Zone (TVZ)

Ogawa et al. (1999)

3D MT in southern TVZ by GNS (1)Deep heat supply?

Wide MT coverage to obtain deep
resistivity image and an overall
geothermal model.
Bertrand et al. (2015)

3D MT in southern TVZ by GNS (2)2D

3D+ Dense local MT coverage toobtain detailed resistivity image ofa specific geothermal field:
Ohaaki.
Bertrand et al. (2013)

3D MT in Rotorua by GNS

Heise et al. (2016)

Summary
The role of MT for geothermal exploration
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.

Message from Yanaizu-Nishiyama MT dataü The inversion result that only utilized the dense MT stations over thegeothermal field (Area-1) produced artifacts along the edge of theinterpretation zone, probably caused by anomalous structure thatexists at the edge or just outside of the target area.
ü Inclusion of outside MT stations (Area-2) in the 3D inversion canreduce such artifacts and improve the model reliability at a deeper partof the target area.
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Summary (cont’d)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 thevertical transfer function should be considered.
ü Static shift and galvanic distortion should be included.

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