MagnetoTelluric in combination with seismic data for geothermal exploration

MagnetoTelluricMagnetoTelluricin combination in combination

with seismic data with seismic data for geothermal explorationfor geothermal exploration

A. ManzellaA. Manzella11

V. SpichakV. Spichak22

11National Research Council – Institute of Geosciences and Earth National Research Council – Institute of Geosciences and Earth Resources(CNR-IGG), Pisa, ItalyResources(CNR-IGG), Pisa, Italy

22GEMRC IPE RAS, Troitsk, RussiaGEMRC IPE RAS, Troitsk, Russia

Why resistivity?Why resistivity?Geothermal Geothermal

waters have high waters have high concentrations of concentrations of

dissolved salts dissolved salts which provide which provide

conducting conducting electrolytes within electrolytes within

a rock matrix a rock matrix

The conductivities of both the electrolytes and the rock The conductivities of both the electrolytes and the rock matrix are temperature dependent in a manner that matrix are temperature dependent in a manner that causes a large reduction of the bulk resistivity with causes a large reduction of the bulk resistivity with increasing temperature.increasing temperature.

The resulting The resulting resistivity is also resistivity is also

related to the related to the presence of clay presence of clay

minerals, and can minerals, and can be reduced be reduced

considerably when considerably when the clay minerals the clay minerals

are broadly are broadly distributed.distributed.

From Pellerin et al., 1996

From Anderson et al., WGC2000

Resistivity should be Resistivity should be always considered with always considered with care. Experience has care. Experience has shown that the apparent shown that the apparent one-to-one correlation one-to-one correlation between low resistivity between low resistivity and the presence of fluids and the presence of fluids is not correct, since is not correct, since alteration minerals alteration minerals produce comparable, and produce comparable, and often higher reduction of often higher reduction of resistivity with respect to resistivity with respect to fluid flow. fluid flow.

Moreover, although the Moreover, although the geothermal systems geothermal systems have an associated low-have an associated low-resistivity signature, the resistivity signature, the converse is not true.converse is not true.

From Flovenz et al., WGC2005

Why MT?

• Easy, light (now) field layout with respect to geolectrical Easy, light (now) field layout with respect to geolectrical soundingssoundings

• obtains a MT transfer function, from which not only obtains a MT transfer function, from which not only resistivity as a function of depth may be computed, but also resistivity as a function of depth may be computed, but also the maximum and minimum resistance (anisotropy) as f.d.the maximum and minimum resistance (anisotropy) as f.d.

• allows estimation of electromagnetic strikeallows estimation of electromagnetic strike• may penetrate at any depth, provided the necessary frequencymay penetrate at any depth, provided the necessary frequency• Disadvantage: being based on a weak natural signal it Disadvantage: being based on a weak natural signal it

cannot be used everywhere (EM noise problem). Modern cannot be used everywhere (EM noise problem). Modern data processing is requireddata processing is required

Various targets can be imaged by Various targets can be imaged by MT and seismic geophysical MT and seismic geophysical

methodsmethods

• Regional structure (geothermal system)Regional structure (geothermal system)

• Fracture detectionFracture detection

• MonitoringMonitoring

Regional explorationRegional explorationSeismic (reflection Seismic (reflection

more often used)more often used)AdvantagesAdvantages• good geometrical good geometrical

resolution of main resolution of main lithological unitslithological units

DisadvantagesDisadvantages• expensiveexpensive• small response from small response from

more permeable zonesmore permeable zones

MagnetotelluricMagnetotelluricAdvantagesAdvantages• cheapcheap• recognize fluid filled volumesrecognize fluid filled volumesDisadvantagesDisadvantages• difficulty to distinguish difficulty to distinguish

alteration clays from actual alteration clays from actual fluid circulation (frozen fluid circulation (frozen condition)condition)

• poor geometrical resolution poor geometrical resolution (volume sounding). Improved (volume sounding). Improved with dense spacingwith dense spacing

Regional exploration:Regional exploration:MT examplesMT examples

From Ushijima et al., WGC From Ushijima et al., WGC 20052005““the low resistivity zone in the northeastern the low resistivity zone in the northeastern part is intensive and shallower than that in part is intensive and shallower than that in the southwestern par, in good agreement the southwestern par, in good agreement with the geological feature”with the geological feature”

Takigami Geothermal Area, JapanTakigami Geothermal Area, Japan

MinamikayabeMinamikayabe Geothermal field, JapanGeothermal field, Japan

From Spichak 2003 Highly conductive areas with apparent

resistivity values not exceeding 6 Ohm m⋅

From Romo et al., WGC 2000From Romo et al., WGC 2000 The results suggest the presence of a highly The results suggest the presence of a highly attenuating and conductive zone along El Azufre attenuating and conductive zone along El Azufre Canyon, which corresponds with the production Canyon, which corresponds with the production interval of wells LV-2 and LV-3/4. A graben interval of wells LV-2 and LV-3/4. A graben structure is outlined.structure is outlined.

Las Tres Virgenes Geothermal Area, Las Tres Virgenes Geothermal Area, MexicoMexico

From Volpi et al., 2003From Volpi et al., 2003 The interpretation revealed a good correlation The interpretation revealed a good correlation between the feature of the geothermal field and between the feature of the geothermal field and

the resistivity distribution at depththe resistivity distribution at depth

Mt. Amiata Geothermal Area, ItalyMt. Amiata Geothermal Area, Italy

From Uchida, 2005 From Uchida, 2005

3-D view of the resistivity model, from south. 3-D view of the resistivity model, from south. Shallow blocks to a 200m depth are stripped out Shallow blocks to a 200m depth are stripped out and approximate locations of three faults are and approximate locations of three faults are overlaid. overlaid.

Ogiri geothermal zone, JapanOgiri geothermal zone, Japan

Fracture/fault detectionFracture/fault detection

Seismic Seismic

(2D and 3D reflection more often used)(2D and 3D reflection more often used)AdvantagesAdvantages

• good geometrical resolutiongood geometrical resolution

• advanced techniques developed for oil explorationadvanced techniques developed for oil exploration

DisadvantagesDisadvantages

• very expensivevery expensive

• small response from productive fracturesmall response from productive fracture

• high cost/effectivehigh cost/effective

Fracture/fault detectionFracture/fault detection

Seismic Seismic

(advanced methodology)(advanced methodology)

• Amplitude Versus Offset (AVO)Amplitude Versus Offset (AVO)• Amplitude Variation with Azimuth and offset Amplitude Variation with Azimuth and offset

(AVAZ)(AVAZ)• shear wave splittingshear wave splitting

Fracture/fault detectionFracture/fault detection

MagnetotelluricMagnetotelluric

AdvantagesAdvantages• cheapcheap• resistivity changes are sensibleresistivity changes are sensible• EM strike direction may define azimuthEM strike direction may define azimuth

DisadvantagesDisadvantages• low geometrical resolution (lateral resolution improved low geometrical resolution (lateral resolution improved

when using short site spacing)when using short site spacing)

Fracture/fault detection:Fracture/fault detection:MT examplesMT examples

From Tagomori et al., WGC 2005From 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””

Takigami Gothermal Area, JapanTakigami Gothermal Area, JapanMt. Amiata Geothermal Area, ItalyMt. Amiata Geothermal Area, Italy

From Fiordelisi et al., WGC 2000From Fiordelisi et al., WGC 2000Note the very steep conductor and its Note the very steep conductor and its correspondence in location to the fault defined correspondence in location to the fault defined by seismic reflection data.by seismic reflection data.

MonitoringMonitoring

SeismicSeismic• It is very effective for gas or for oil investigation It is very effective for gas or for oil investigation

(water flood). Very expensive(water flood). Very expensive• Not so easy to manage for geothermal since Not so easy to manage for geothermal since

resolution is lower (Vresolution is lower (VPP and V and VSS change is smaller change is smaller

than for oil)than for oil)

MonitoringMonitoring

MagnetotelluricMagnetotelluric• Phase change of pore fluid (boiling/condensing) in Phase change of pore fluid (boiling/condensing) in

fractured rocks can result in resistivity changes that fractured rocks can result in resistivity changes that are more than an order of magnitude greater than are more than an order of magnitude greater than those measured in intact rocksthose measured in intact rocks

• Production-induced changes in resistivity can provide Production-induced changes in resistivity can provide valuable insights into the evolution of the host rock valuable insights into the evolution of the host rock and resident fluids.and resident fluids.

• No examples or applications found in literatureNo examples or applications found in literature• Some examples from SP (electric field) showing Some examples from SP (electric field) showing

interesting results: is it possible to use the same kind interesting results: is it possible to use the same kind of information in MT? To be definedof information in MT? To be defined

MonitoringMonitoring

SP monitoringSP monitoring

From Marquis et al., 2002From Marquis et al., 2002““the correspondence between the start (and the end) of the stimulation and the the correspondence between the start (and the end) of the stimulation and the increase (and decrease) in ΔV suggests a casual relationship between the two”increase (and decrease) in ΔV suggests a casual relationship between the two”

Integration of seismic and MT Integration of seismic and MT datadata

It can be done It can be done • quantitatively (joint inversion)quantitatively (joint inversion)• qualitatively (by comparison and separated qualitatively (by comparison and separated

inversion constraining the a priori conditions)inversion constraining the a priori conditions)• semi-quantitative (joint interpretation)semi-quantitative (joint interpretation)

Example of joint inversionExample of joint inversionWhen resistivity and VWhen resistivity and VPP changes depends on the same changes depends on the same

effect (e.g., permeability/porosity change) a resistivity-effect (e.g., permeability/porosity change) a resistivity-velocity cross-gradients relationship can be established velocity cross-gradients relationship can be established and incorporated in a joint inversion scheme. and incorporated in a joint inversion scheme.

This approach requires a This approach requires a strong assumption: could be strong assumption: could be valid only for limited valid only for limited volumes and depthsvolumes and depths

From Gallardo and Meju, 2004From Gallardo and Meju, 2004““Evolution of the joint inversion process. Shown are the Evolution of the joint inversion process. Shown are the resultant resistivity and velocity models for each iteration. resultant resistivity and velocity models for each iteration. Note the gradual development of common structural Note the gradual development of common structural features in both sets of models during the process.”features in both sets of models during the process.”

Example from comparison of Example from comparison of resultsresults

Example of using constrained Example of using constrained a priori model in MT inversiona priori model in MT inversion

Travale Geothermal Area, ItalyTravale Geothermal Area, Italy

Quality of inversion results improves Quality of inversion results improves when external data are used. Here we when external data are used. Here we show inversion results using an show inversion results using an homogeneous a priori model (above) or a homogeneous a priori model (above) or a detailed a priori model where shallow detailed a priori model where shallow lithological units have been identified lithological units have been identified from a resistivity point of view. The from a resistivity point of view. The resulting models appear like out-of-focus resulting models appear like out-of-focus in the first case, whereas it provides useful in the first case, whereas it provides useful information for comparison with known information for comparison with known geological structure in the second case.geological structure in the second case.

Joint interpretation by post-Joint interpretation by post-processing simulationprocessing simulation

Needs:Needs:• geological datageological data• seismic inversion data (Vseismic inversion data (VPP, V, VSS))• MT inversion data (true resistivity)MT inversion data (true resistivity)• rock physics data joining Vrock physics data joining VPP, V, VSS, resistivity to , resistivity to

lithology, temperature, pressure, permeability...lithology, temperature, pressure, permeability...

The key element in the joint interpretation is the use The key element in the joint interpretation is the use of geothermal reservoir simulators to obtain a final of geothermal reservoir simulators to obtain a final model complying with all available data, both model complying with all available data, both geophysical and thermo-hydraulic. To be evolved!geophysical and thermo-hydraulic. To be evolved!

ConclusionsConclusionsMT provides a useful contribution to geothermal MT provides a useful contribution to geothermal exploration and exploitation, through careful data exploration and exploitation, through careful data acquisition, processing, modeling and interpretation.acquisition, processing, modeling and interpretation.

Its integration with Its integration with other geological and other geological and geophysical data, in geophysical data, in particular seismic, particular seismic, will improve the will improve the imaging of static imaging of static and dynamic and dynamic processes of processes of geothermal systems.geothermal systems.