1. MagnetoTelluric in combination with seismic data for geothermal exploration A. Manzella 1 V. Spichak 2 1 National Research Council – Institute of Geosciences and Earth Resources(CNR- IGG), Pisa, Italy 2 GEMRC 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 Pellerin et al., 1996 From Anderson et al., WGC2000

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. Moreover, although the geothermal systems have an associated low- resistivity signature, the converse is not true. From Flovenz et al., WGC2005

5. Why MT? Easy, light (now) field layout with respect to geolectrical soundingsEasy, 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.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 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 cannot be used everywhere (EM noise problem). Modern data processing is requiredDisadvantage: being based on a weak natural signal it cannot be used everywhere (EM noise problem). Modern data processing is required

6. Various targets can be imaged by MT and seismic geophysical methods Regional structure (geothermal system)Regional structure (geothermal system) Fracture detectionFracture detection MonitoringMonitoring

7. Regional exploration Seismic (reflection more often used) Advantages good geometrical resolution of main lithological unitsgood geometrical resolution of main lithological unitsDisadvantages expensiveexpensive small response from more permeable zonessmall response from more permeable zonesMagnetotelluricAdvantages cheapcheap recognize fluid filled volumesrecognize fluid filled volumesDisadvantages difficulty to distinguish alteration clays from actual fluid circulation (frozen condition)difficulty to distinguish alteration clays from actual fluid circulation (frozen condition) poor geometrical resolution (volume sounding). Improved with dense spacingpoor geometrical resolution (volume sounding). Improved with dense spacing

8. Regional exploration: MT examples 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” Takigami Geothermal Area, Japan MinamikayabeGeothermal field, Japan Minamikayabe Geothermal field, Japan From Spichak 2003 Highly conductive areas with apparent resistivity values not exceeding 6 Ohm ⋅ m

9. 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. Las Tres Virgenes Geothermal Area, Mexico From Volpi et al., 2003 The interpretation revealed a good correlation between the feature of the geothermal field and the resistivity distribution at depth Mt. Amiata Geothermal Area, Italy 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. Ogiri geothermal zone, Japan

10. Fracture/fault detection Seismic (2D and 3D reflection more often used) Advantages good geometrical resolutiongood geometrical resolution advanced techniques developed for oil explorationadvanced techniques developed for oil explorationDisadvantages very expensivevery expensive small response from productive fracturesmall response from productive fracture high cost/effectivehigh cost/effective

11. Fracture/fault detection Seismic (advanced methodology) Amplitude Versus Offset (AVO)Amplitude Versus Offset (AVO) Amplitude Variation with Azimuth and offset (AVAZ)Amplitude Variation with Azimuth and offset (AVAZ) shear wave splittingshear wave splitting

12. Fracture/fault detection MagnetotelluricAdvantages cheapcheap resistivity changes are sensibleresistivity changes are sensible EM strike direction may define azimuthEM strike direction may define azimuthDisadvantages low geometrical resolution (lateral resolution improved when using short site spacing)low geometrical resolution (lateral resolution improved when using short site spacing)

13. Fracture/fault detection: MT examples 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” Takigami Gothermal Area, Japan Mt. Amiata Geothermal Area, Italy From Fiordelisi et al., WGC 2000 Note the very steep conductor and its correspondence in location to the fault defined by seismic reflection data.

14. MonitoringSeismic It is very effective for gas or for oil investigation (water flood). Very expensiveIt is very effective for gas or for oil investigation (water flood). Very expensive Not so easy to manage for geothermal since resolution is lower (V P and V S change is smaller than for oil)Not so easy to manage for geothermal since resolution is lower (V P and V S change is smaller than for oil)

15. MonitoringMagnetotelluric 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 rocksPhase 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.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 literatureNo 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 definedSome examples from SP (electric field) showing interesting results: is it possible to use the same kind of information in MT? To be defined

16. 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 ΔV suggests a casual relationship between the two”

17. Integration of seismic and MT data It can be done quantitatively (joint inversion)quantitatively (joint inversion) qualitatively (by comparison and separated inversion constraining the a priori conditions)qualitatively (by comparison and separated inversion constraining the a priori conditions) semi-quantitative (joint interpretation)semi-quantitative (joint interpretation)

18. Example of joint inversion When resistivity and V P 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.”

19. Example from comparison of results

20. 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.

21. Joint interpretation by post- processing simulation Needs: geological data geological data seismic inversion data (V P, V S ) seismic inversion data (V P, V S ) MT inversion data (true resistivity) MT inversion data (true resistivity) rock physics data joining V P, V S, resistivity to lithology, temperature, pressure, permeability... rock physics data joining V P, V S, 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!

22. 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.