Building, Owning & Operating an Independent Power Producer Business in Turkey by Implementing an Innovative Power Cycle Technology.
1. Effect of Magnetotellurics Data
Density in Geothermal Resource
Exploration
Mehran Gharibi
Quantec Geoscience
GeoPower, Turkey 2011
2. Outline:
• Objectives
• Introduction to Magnetotelluric (MT) Method
• Titan-24 and Spartan MT measurements
• MT in Geothermal Exploration
• 3D Inversion and Interpretation
• Summary
3. Objectives:
Investigate resolution and reliability of the 3D MT
resistivity models in terms of the data density used
in inversion; i.e. Resolution vs. Frequency density.
4. Magneto-Telluric (MT) Method
• The MT is a frequency-domain natural-field EM geophysical
method
– frequency range 10000 Hz - 0.001 Hz
• The method uses the earth’s EM natural time variations
- Lower Frequencies ( < 1 Hz): Ionospheric; initiated by interaction
of the solar wind with the earth’s magnetic field.
- Higher Frequencies ( > 1 Hz): Atmospheric; initiated by global
lightning activities.
• Image the earth's electrical resistivity structure
– from nearly the surface to several tens of km.
5. MT measurements
Schematic of simultaneous measurement of plane-wave electromagnetic
field fluctuations in five components: Ex, Ey, Hx, Hy and Hz
Ground surface
Electric field dipoles Ex
Ey
Hz Hy
Hx
Magnetic field sensors
⎛ E x ⎞ ⎛ Z xx Z xy ⎞⎛ H x ⎞ Apparent Resistivity of
⎜ ⎟=⎜ ⎟⎜ ⎟ Calculate Inversion
⎜E ⎟ ⎜Z Z yy ⎟⎜ H y ⎟ Resistivity Subsurface
⎝ y ⎠ ⎝ yx ⎠⎝ ⎠
6. Calibration Chamber for Magnetic Sensors
3-layer passive shielded and active
Active-Field
field cancellation calibration room Cancellation
Frame
Solenoid Magnetic Passive Active
Frequency Sheilding Sheilding
coil sensor (Hz) Factor Factor
4m
10K 100,000 100,000
1K 50,000 50,000
100 2,000 2,000
10 100 1,000
1 50 1,000
0.1 20 1,000
3-layer Passive
Magnetically
0.01 10 1,000
Shielded Room
7. Spartan MT acquisition
Acquisition system
• Full tensor MT
• 24-bit resolution
• Automatic data acquisition
• Remote reference processing
Field setup
• Man portable
• Coils buried
• Flexible spacing
• Low environmental footprint
8. Titan-24 MT Acquisition
Typical 2 Channel
Station AM
Set-up
• Full tensor MT
• 24-bit resolution
Battery
• High spatial site density
• DC-IP acquisition capability
2400m
current electrode (mobile) cross-line potential electrode (fixed)
cross-
50m in-line potential electrode (fixed)
in-
100m
Bx infinity current electrode (fixed)
By 100m
2 channel AM >10 km
Base magnetometer Site
1 channel AM
LAN Link to Logging Truck Line Length - 2400 m
24 E x 100m dipoles
12 E y 100m dipoles
25 current stations
2 Bx/B y magnetometer sites
>20 km Bx
By
Remote
Magnetometer
site 8
9. MT Method in Geothermal Explorations:
• MT data are used to produce electrical resistivity distribution
of the subsurface
• Electrical resistivity is a function of;
• solid matrix - geological formation and alteration
• pore fluids - chemistry and salinity
• porosity – geological fissure and fracture
• temperature
• A geothermal system or a hydrothermal reservoir is defined
and controlled by a combination of the above factors.
• Resistivity signature associated with the geothermal system is
used to detect/map/characterize the reservoir
10. Interpretation of the MT Data
⎛ 0 Z⎞ ⎛ 0 Z xy ⎞ ⎛ Z xx Z xy ⎞
• Data ⎜
⎜− Z ⎟ ⎜ ⎟ ⎜ ⎟
⎝ 0⎟
⎠
⎜Z
⎝ yx 0 ⎟ ⎠
⎜Z
⎝ yx Z yy ⎟
⎠
ρ ρxy, ρyx
• Inversion
1-D 2-D 3-D
• Resistivity
Model
11. Spartan MT Field Survey
• Large scale MT survey over a geothermal resource area
• More than 160 MT sites
• Site spacing between 500 m and 1000 m
• Frequency range 250 Hz – 0.001 Hz
• Objective is to identify an exploration drilling location
• Geothermal reservoir target at >1500 m
• Interpretation based on 3-D with different number of frequencies.
13. 3-D Inversions
The original frequency band is decimated for the 3-D inversions
5 Frequencies (1 per decade)
Original Decimated
14. 3-D Inversion – 5 Frequencies
• # of frequency per site: 5
• # of sites: 162
• # of Mesh: 25 x 41 x 18
• Inversion time: 602 hours
• Hardware: 2 x CPU processor with 4 GB RAM
4 km
15. 3-D Inversion – 5 Frequencies
Surface = 0 m
Conductive
Cap
Resistive
Core
Interpreted
Geothermal
System
16. 3-D Inversion – 5 Frequencies
Surface = -1300 m
Conductive
Cap
Resistive
Core
Interpreted
Geothermal
System
17. 3-D Inversion – 5 Frequencies
Surface = -2000 m
Anomaly or
Artifact !?
Interpreted
Geothermal
System
18. 3-D Inversion – 5 Frequencies
Surface = -3000 m
Anomaly or
Artifact !?
Interpreted
Geothermal
System
19. 3-D Inversion – 5 Frequencies
Selected Cross-sections
Conductive
Resistive Cap
Core
Interpreted
Geothermal
System
20. 3-D Inversions
The original frequency band is decimated for the 3-D inversions
18 Frequencies (3 per decade)
Original Decimated
21. 3-D Inversion – 18 Frequencies
• # of frequency per site: 18
• # of sites: 162
• # of Mesh: 25 x 41 x 18
• Inversion time: 552 hours
• Hardware: 2 x 4 CPU processor with 24 GB RAM
4 km
22. 3-D Inversion – 18 Frequencies
Surface = 0 m
Conductive
Cap
Resistive
Core
Interpreted
Geothermal
System
23. 3-D Inversion – 18 Frequencies
Surface = -1300 m
Conductive
Cap
Resistive
Core
Interpreted
Geothermal
System
24. 3-D Inversion – 18 Frequencies
Surface = -2000 m
No Artifact !
Geothermal reservoir!?
25. 18 Frequencies Selected Cross-sections 5 Frequencies
Interpreted Interpreted
Geothermal Geothermal
System System
26. Summary
• Geothermal fields and resources can efficiently be mapped
and characterized using MT measurements.
• 3-D inversions are computationally expensive;
- a subset of the dataset with 3 frequencies/decade
would be sufficient to produce the resistivity
distribution of the subsurface while minimizing the
artifacts
• Development in 3D inversion;
- Parallel computing of the 3-D MT inversion using
cluster of high performance CPUs (e.g. 48 CPUs and 64
GB RAM)
- It speeds up the inversion process several times (5-10
times).
27. Thank You
Toronto, Canada
Reno, USA
Hermosillo, Mexico
Mumbai, India
Arequipa, Peru Goiania, Brazil
Lobatse, Botswana
Santiago, Chile
Mendoza, Argentina Brisbane, Australia
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