3d mt imaging for conventional and workshop ......outline quick review of the mt method challenges...
TRANSCRIPT
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Workshop Workshop From science to praxis: Experiences employing Geophysical methods
to characterize Geothermal anomalies
From science to praxis: Experiences employing Geophysical methods
to characterize Geothermal anomalies
26 mayo - mai - May 2021 9:00 - 13:30
3D MT Imaging for Conventional
and Unconventional
Geothermal Exploration
Sophie Hautot,
IMAGIR
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Outline
● Quick review of the MT method● Challenges in MT when applied to geothermal exploration
- Data acquisition and processing
- High resolution 3D imaging tools for complex geothermal targets● 3D inversion of MT data: Two case studies
- Conventional geothermal system : Carribean Sea
- Unconventional geothermal system : Massif Central, France● Conclusion
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A quick review of the Magnetotelluric method● A passive imaging technique● Data collected are time series : Fluctuations of the natural electric and magnetic
fields on Earth’s surface● Data processing in the frequency domain to derive the electrical resistivity
structure of the subsurface
Impedance tensor Z=f(ρa,Φ,f)
(E x (ω )
E y (ω ))=(
Zxx (ω )
Zxy (ω )
Z yx (ω )Z yy (ω )
)(B x (ω )
B y (ω ))
Ex
Ey
Bx
Ey
By
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A quick review of the Magnetotelluric method
● Penetration depth is a function of frequency (1/period)● In geothermal exploration : ~1000 Hz – 1000 seconds →
Penetration down to several kilometers● Z
xy= -Z
yx and Z
xx= Z
yy= 0 in 1D
● Zxy
≠ Zyx
≠ Zxx
≠Zyy
in 3D
● MT is particularly sensitive to highly conductive materials, hence a standard technique in geothermal exploration (fluids, clay cap, heat source, ..)
● Quality dependent on the energy source and anthropic noise.
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A quick review of the Magnetotelluric method
● Geothermal prospects close to inhabited areas (power lines, road traffic,..)
● Signal/Noise low when small resistivity in 0.1-1 second range (weak natural signals)
● Special care for data acquisition and processing are essential before interpretation
- Set up of sensors
- Length of time series
- Noise correction
Power line
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MT data interpretation
● Commercial 3D MT inversion is young (~15 years)● Until recently, 3D inversion was performed only by experts in academics and
Oil&Gas major companies
● In geothermal exploration :- 1D inversion was popular to image conventional geothermal systems
(target = clay cap) : Fast and cheap interpretation tools.- But 1D should be interpreted carefully.- 3D MT imaging becomes a standard : Easier access to 3D inversion codes.
Also, the complexity of potential geothermal targets increases.
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ConventionalClassical convective hydrothermal systemMT well adapted to image the clay cap, hydrothermal fluids, in some cases magma,..
after Cumming et al. 2000
I s o t h e r m s
1 K M1 : 1
P r o p y l i t i cA l t e r a t i o n i n
F r a c t u r e dG e o t h e r m a l
R e s e r v o i r
H e a t a n d G a sf r o m M a g m a
B a s i nC l a y s
C h l o r i d eS p r i n g
A c i d S u l p h a t eF u m a r o l e
Conventional and unconventional geothermal systems
Modified after Moeck, 2014
UnconventionalAny other geothermal target different from the classical convective hydrothermal system- Can be in sedimentary or cristalline context- Geothermal fluids circulate through fracture networks.- System more complex to image
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MT data collected in conventional geothermal prospects around the world
conventional geothermal systems
Carribean Sea South America Kenya Republic of Djibouti
Clear signature from short to longer periods :Volcanics – Clay cap – Reservoir – potentially, heat source
Diagonals terms (Zxx and Zyy) frequently ignored but amplitude not negligible
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MT data collected in one unconventional geothermal prospect
Unconventional geothermal systems
MT soundings are a few km apart
Strong heterogeneity between sites.
Only 3D imaging can allow to interpret such data sets.
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3D inversion of MT data
Most available 3D inversion codes are based on model space inversion• Number of unknown (model parameters) >> number of data.• The problem is solved with a strong smoothness constraint.
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3D inversion of MT data
We propose a different approach :• Data driven inversion• Number of data >> Model parameters• Block parametrisation adjusted to the resolution of data• Result is more « blocky » but no strong smoothness constraint• Allows to image strong resistivity contrasts, even at depth• The full tensor is always inverted
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Case Study 1: 3D MT imaging of a conventional system (Carribean Sea)
● Project with Teranov● Acquisition of 68 MT sites● Irregular distribution and quality (low energy, anthropic noise)
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The inversion grid is adapted to the data resolution and distribution : 2931 unknown for 17390 data (full tensor, all periods).
Case Study 1: 3D MT imaging of a conventional system (Carribean Sea)
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Results : Image of the « clay cap » and structures at depth
Case Study 1: 3D MT imaging of a conventional system (Carribean Sea)
Plan view
Vertical cross section
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Case Study 2: 3D MT imaging of a unconventional system (Massif Central, France)
From Ars et al., 2017
Project with TLS GeothermicsComplex geological context. The geothermal targets are deep crustal faults within a cristalline and volcanic region
MT survey (48 sites)
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Case Study 2: 3D MT imaging of a unconventional system (Massif Central, France)
The adapted grid allows to model very heterogeneous data : Large diagonal terms, out of quadrant phases (Zyx),..
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Case Study 2: 3D MT imaging of a unconventional system (Massif Central, France)
Results show the extension of faults at depth and crustal heterogeneities
Vertical cross section
Plan view
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Conclusion● Progress in equipment, processing and inversion tools allow to obtain more and
more trustable images of geothermal systems● With MT alone, there is still some ambiguity in the interpretation of the features
imaged.● For example, a low resistivity cannot always be only explained in term of clay cap● Integration with other geophysical methods and joint inversion are the way forward
to better characterise geothermal systems● Promising results with joint inversion :
- Case study 1 : Joint MT/Gravity (Tarits et al., 2017)- Case study 2 : Joint MT/Gravity/Ambient Noise (Ars et al., 2017)
Merci ! Contact : [email protected]
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