MagnetoTelluric in combination with seismic data for geothermal exploration

1
MagnetoTelluricin combination with seismic data
for geothermal exploration

• A. Manzella1
• V. Spichak2

1National Research Council Institute of
Geosciences and Earth Resources(CNR-IGG), Pisa,
Italy 2GEMRC IPE RAS, Troitsk, Russia
2
Why resistivity?
Geothermal waters have high concentrations of
dissolved salts which provide conducting
electrolytes within a rock matrix
The conductivities of both the electrolytes and
the rock matrix are temperature dependent in a
manner that causes a large reduction of the bulk
resistivity with increasing temperature.
3
The resulting resistivity is also related to the
presence of clay minerals, and can be reduced
considerably when the clay minerals are broadly
distributed.
From Anderson et al., WGC2000
From Pellerin et al., 1996
4
Resistivity should be always considered with
care. Experience has shown that the apparent
one-to-one correlation between low resistivity
and the presence of fluids is not correct, since
alteration minerals produce comparable, and often
higher reduction of resistivity with respect to
fluid flow.
From Flovenz et al., WGC2005
Moreover, although the geothermal systems have an
associated low-resistivity signature, the
converse is not true.
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Why MT?

• Easy, light (now) field layout with respect to
  geolectrical soundings
• obtains a MT transfer function, from which not
  only resistivity as a function of depth may be
  computed, but also the maximum and minimum
  resistance (anisotropy) as f.d.
• allows estimation of electromagnetic strike
• may penetrate at any depth, provided the
  necessary frequency
• Disadvantage being based on a weak natural
  signal it cannot be used everywhere (EM noise
  problem). Modern data processing is required

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Various targets can be imaged by MT and seismic
geophysical methods

• Regional structure (geothermal system)
• Fracture detection
• Monitoring

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Regional exploration

• Seismic (reflection more often used)
• Advantages
• good geometrical resolution of main lithological
  units
• Disadvantages
• expensive
• small response from more permeable zones

• Magnetotelluric
• Advantages
• cheap
• recognize fluid filled volumes
• Disadvantages
• difficulty to distinguish alteration clays from
  actual fluid circulation (frozen condition)
• poor geometrical resolution (volume sounding).
  Improved with dense spacing

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Regional explorationMT examples
Minamikayabe Geothermal field, Japan
Takigami Geothermal Area, Japan
From Spichak 2003 Highly conductive areas with
apparent resistivity values not exceeding 6 Ohmm
From Ushijima et al., WGC 2005 the low
resistivity zone in the northeastern part is
intensive and shallower than that in the
southwestern par, in good agreement with the
geological feature
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Las Tres Virgenes Geothermal Area, Mexico
Mt. Amiata Geothermal Area, Italy
From Volpi et al., 2003 The interpretation
revealed a good correlation between the feature
of the geothermal field and the resistivity
distribution at depth
From Romo et al., WGC 2000 The results suggest
the presence of a highly attenuating and
conductive zone along El Azufre Canyon, which
corresponds with the production interval of wells
LV-2 and LV-3/4. A graben structure is outlined.
Ogiri geothermal zone, Japan
From Uchida, 2005 3-D view of the resistivity
model, from south. Shallow blocks to a 200m depth
are stripped out and approximate locations of
three faults are overlaid.
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Fracture/fault detection

• Seismic
• (2D and 3D reflection more often used)
• Advantages
• good geometrical resolution
• advanced techniques developed for oil exploration
• Disadvantages
• very expensive
• small response from productive fracture
• high cost/effective

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Fracture/fault detection

• Seismic
• (advanced methodology)
• Amplitude Versus Offset (AVO)
• Amplitude Variation with Azimuth and offset
  (AVAZ)
• shear wave splitting

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Fracture/fault detection

• Magnetotelluric
• Advantages
• cheap
• resistivity changes are sensible
• EM strike direction may define azimuth
• Disadvantages
• low geometrical resolution (lateral resolution
  improved when using short site spacing)

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Fracture/fault detectionMT examples
Takigami Gothermal Area, Japan
Mt. Amiata Geothermal Area, Italy
From Tagomori et al., WGC 2005 the large lost
circulation occurred at the depth of 1300 m BSL
for the well TT-14R (90 t/h) when the well
crossed through the electrical discontinuity Fb
From Fiordelisi et al., WGC 2000 Note the very
steep conductor and its correspondence in
location to the fault defined by seismic
reflection data.
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Monitoring

• Seismic
• It is very effective for gas or for oil
  investigation (water flood). Very expensive
• Not so easy to manage for geothermal since
  resolution is lower (VP and VS change is smaller
  than for oil)

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Monitoring

• Magnetotelluric
• Phase change of pore fluid (boiling/condensing)
  in fractured rocks can result in resistivity
  changes that are more than an order of magnitude
  greater than those measured in intact rocks
• Production-induced changes in resistivity can
  provide valuable insights into the evolution of
  the host rock and resident fluids.
• No examples or applications found in literature
• Some examples from SP (electric field) showing
  interesting results is it possible to use the
  same kind of information in MT? To be defined

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Monitoring

• SP monitoring

From Marquis et al., 2002 the correspondence
between the start (and the end) of the
stimulation and the increase (and decrease) in ?V
suggests a casual relationship between the two
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Integration of seismic and MT data

• It can be done
• quantitatively (joint inversion)
• qualitatively (by comparison and separated
  inversion constraining the a priori conditions)
• semi-quantitative (joint interpretation)

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Example of joint inversion
When resistivity and VP changes depends on the
same effect (e.g., permeability/porosity change)
a resistivity-velocity cross-gradients
relationship can be established and incorporated
in a joint inversion scheme.
This approach requires a strong assumption could
be valid only for limited volumes and depths
From Gallardo and Meju, 2004 Evolution of the
joint inversion process. Shown are the resultant
resistivity and velocity models for each
iteration. Note the gradual development of common
structural features in both sets of models during
the process.
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Example from comparison of results
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Example of using constrained a priori model in
MT inversion
Travale Geothermal Area, Italy
Quality of inversion results improves when
external data are used. Here we show inversion
results using an homogeneous a priori model
(above) or a detailed a priori model where
shallow lithological units have been identified
from a resistivity point of view. The resulting
models appear like out-of-focus in the first
case, whereas it provides useful information for
comparison with known geological structure in the
second case.
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Joint interpretation by post-processing simulation

• Needs
• geological data
• seismic inversion data (VP, VS)
• MT inversion data (true resistivity)
• rock physics data joining VP, VS, resistivity to
  lithology, temperature, pressure, permeability...

The key element in the joint interpretation is
the use of geothermal reservoir simulators to
obtain a final model complying with all available
data, both geophysical and thermo-hydraulic. To
be evolved!
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Conclusions
MT provides a useful contribution to geothermal
exploration and exploitation, through careful
data acquisition, processing, modeling and
interpretation.
Its integration with other geological and
geophysical data, in particular seismic, will
improve the imaging of static and dynamic
processes of geothermal systems.