Two-Dimensional Magnetotelluric Modeling of the Sabalan Geothermal Field, North-West Iran

نوع مقاله : مقاله پژوهشی

نویسندگان

1 Assistant Professor, Department of Mining Engineering, Birjand University of Technology, Birjand, Iran

2 Associate Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran

چکیده

During 2007, a magnetotelluric (MT) survey in the frequency range of 0.002-320 Hz was carried out on southwestern of Sabalan geothermal region (Moeil valley, Ardabil); the aim of which was modeling of the shallow and deep electrical resistivity structures related to the local geothermal reservoirs and heat system recharge at depth. Twenty eight soundings were conducted in the study area, and the collected MT data were found to be two-dimensional (2D), based on dimensionality (skew parameter) analysis. The NNW-SSE (30°W) direction was identified as the dominant electrical strike in the area. Data along a profile crossing the hot springs with seven MT stations, have been implemented for modeling and inversion. Dimensionality analysis shows that a 2D interpretation of the data is justified, although the presumed geoelectric strike direction is not consistent over the whole profile and frequencies. MT data were analyzed and modeled using MT2DInvMatlab inversion source codes and the finite elements (FEM) method for forward modeling. Inversion parameters as an input file and appropriate mesh blocks design are prepared before start of the modeling and inversion. MT2DInvMatlab software includes a topography file into a forward model for terrain effects compensation in the inversion process. After setting up the model parameter, 2D inversion of the Sabalan magnetotelluric data was performed. Smoothness–constrained least square methods with a spatially regularization parameter estimation and the ACB (Active Constraint Balancing) algorithm were employed in MT2DInvMatlab to stabilize the model. Both apparent resistivity and phase data were used to have models with minimum misfit for TM, TE and joint TE+TM mode data. The TM mode apparent resistivity and phase are better fitted than the TE mode, as a consequence of the inductive nature of the 2D TE response in a 3-D geothermal field structures. However, the apparent resistivity and phase data are also well fitted in the joint inversion of TM and TE mode data. Although the TM mode data is often used for 2-D modeling of MT data in geothermal field studies, we have shown the other two dimensional electrical resistivity models, using apparent resistivity and phase data of TM, TE and joint TE+TM mode data. These models resolved a good correlation between the features of the geothermal field and resistivity distribution at depth. The resulting models reveal the presence of a resistive cover layer (Cap-rock) underlain by an anomalous conductive layer and other geological structures such as fluid-filled faults (about 500-1000 m below the ground surface). A very low resistivity (3-5 ohm-m) feature was found at the depths below 2000 m, bounded by two more resistive (100-500 ohm-m) features that can be interpreted as the main reservoir of the geothermal system in the area. At shallow depths, the resistivity model obtained from the MT data is consistent with the general conceptual resistivity model proposed for high-temperature geothermal systems. The deeper electrical structure was found to be more resistive (100 ohm-m) due to the presence of metamorphic rock formations. According to this results, heat source of the geothermal structure and heat transition zone from deep sources to shallow reservoir, is predicted at 2~7Km at depth.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Two-Dimensional Magnetotelluric Modeling of the Sabalan Geothermal Field, North-West Iran

نویسندگان [English]

  • Gholam Abbas Fanaee Kheirabad 1
  • Behrooz Oskooi 2
1 Assistant Professor, Department of Mining Engineering, Birjand University of Technology, Birjand, Iran
2 Associate Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Tehran, Iran
چکیده [English]

During 2007, a magnetotelluric (MT) survey in the frequency range of 0.002-320 Hz was carried out on southwestern of Sabalan geothermal region (Moeil valley, Ardabil); the aim of which was modeling of the shallow and deep electrical resistivity structures related to the local geothermal reservoirs and heat system recharge at depth. Twenty eight soundings were conducted in the study area, and the collected MT data were found to be two-dimensional (2D), based on dimensionality (skew parameter) analysis. The NNW-SSE (30°W) direction was identified as the dominant electrical strike in the area. Data along a profile crossing the hot springs with seven MT stations, have been implemented for modeling and inversion. Dimensionality analysis shows that a 2D interpretation of the data is justified, although the presumed geoelectric strike direction is not consistent over the whole profile and frequencies. MT data were analyzed and modeled using MT2DInvMatlab inversion source codes and the finite elements (FEM) method for forward modeling. Inversion parameters as an input file and appropriate mesh blocks design are prepared before start of the modeling and inversion. MT2DInvMatlab software includes a topography file into a forward model for terrain effects compensation in the inversion process. After setting up the model parameter, 2D inversion of the Sabalan magnetotelluric data was performed. Smoothness–constrained least square methods with a spatially regularization parameter estimation and the ACB (Active Constraint Balancing) algorithm were employed in MT2DInvMatlab to stabilize the model. Both apparent resistivity and phase data were used to have models with minimum misfit for TM, TE and joint TE+TM mode data. The TM mode apparent resistivity and phase are better fitted than the TE mode, as a consequence of the inductive nature of the 2D TE response in a 3-D geothermal field structures. However, the apparent resistivity and phase data are also well fitted in the joint inversion of TM and TE mode data. Although the TM mode data is often used for 2-D modeling of MT data in geothermal field studies, we have shown the other two dimensional electrical resistivity models, using apparent resistivity and phase data of TM, TE and joint TE+TM mode data. These models resolved a good correlation between the features of the geothermal field and resistivity distribution at depth. The resulting models reveal the presence of a resistive cover layer (Cap-rock) underlain by an anomalous conductive layer and other geological structures such as fluid-filled faults (about 500-1000 m below the ground surface). A very low resistivity (3-5 ohm-m) feature was found at the depths below 2000 m, bounded by two more resistive (100-500 ohm-m) features that can be interpreted as the main reservoir of the geothermal system in the area. At shallow depths, the resistivity model obtained from the MT data is consistent with the general conceptual resistivity model proposed for high-temperature geothermal systems. The deeper electrical structure was found to be more resistive (100 ohm-m) due to the presence of metamorphic rock formations. According to this results, heat source of the geothermal structure and heat transition zone from deep sources to shallow reservoir, is predicted at 2~7Km at depth.

کلیدواژه‌ها [English]

  • Magnetotellurics
  • Geothermal
  • Reservoir
  • MT2DInvMatlab
  • Sabalan

مراجع

Ander, M.A., Gross, R. and Strangway, D.W., 1984, A detailed magnetotelluric / audiomagnetotellurics study of the Jemez Volcanic Zone, New Mexico. J. Geophysics. Res., 89(B5), 3335–3353.
Bahr, K., 1988, Interpretation of the Magnetotelluric impedance tensor: regional induction and local telluric distortion. Journal of Geophysics, 62, 119-127.
Bedrosian, P., Unsworth, M., Egbert, G. and Thurber, C., 2004, Geophysical images of the creeping segment of the San Andreas fault: implications for the role of crustal fluids in the earthquake process. Tectonophysics, 385, 137–158.
Berberian, M., 1981, Active faulting and tectonics of Iran. In: Gupta, H.K., Delaney, F.M. (Eds.), Zagros Hindu Kush, Himalaya Geodynamic Evolution, Geodyn. Ser., vol. 3. American Geophysical Union, Washington, DC, 33–69.
Berktold, A., 1983, Electromagnetic studies in geothermal regions. Geophysics Survey, 6, 173–200.
Bertrand, E., 2010, Magnetotelluric imaging beneath the Taiwan orogen: An arc-continent collision. PhD Thesis, Department of Physics, University of Alberta.
Bogie, I., Khosrawi, K. and Talebi, B., 2005, Geological Results from the Drilling of the Northwest Sabalan Geothermal Project, Iran, Proceedings, World Geothermal Congress.
Bogie, I., Cartwright, A.J., Khosrawi, K., Talebi, B. and Sahabi, F., 2000, The Meshkin Shahr geothermal prospect, Iran, Proceedings, World Geothermal Congress 2000, 997-1002.
Brasse, H., Lezaeta, P., Rath, V., Schwalenberg, K., Soyer, W. and Haak, V., 2002, The Bolivian Altiplano conductivity anomaly. Journal of Geophysical Research, 107(B5), doi: 10.1029/2001JB000391.
Bromley, C.J., Khosrawi, K. and Talebi, B., 2000, Geophysical Exploration of the Sabalan Geothermal Prospects in Iran. Proceedings, World Geothermal Congress 2000, 1009-1014.
Cagniard, L., 1953, Basic theory of the magnetotelluric method. Geophysics, 18, 605–635.
Emami H., 1994, Meshkinshahr 1; 100, 000 Scale Geological Map. Geological Survey of Iran.
ENEL., 1983, Geothermal power development studies in Iran, General Report on Sabalan Zone, Ministry of Energy, Islamic Republic of Iran, 120 pp.
Fanaee Kheirabad, G.A. and Oskooi, B., 2010, Magnetotelluric modeling of Sabalan geothermal field, NW Iran, Presented in 20th international electromagnetic induction workshop, Giza, Egypt.
Fanaee Kheirabad, G.A., Oskooi, B., Porkhial, S. and Rahmani, M.R., 2010, Investigation of Sabalan geothermal field structure using Magnetotelluric data. Presented in 14th Geophysics Conference of Iran, Tehran, Iran.
Fanaee Kheirabad, G.A. and Oskooi, B., 2011, Magnetotelluric interpretation of Sabalan geothermal field in northwest of Iran. Journal of the Earth and Space Physics, 37(3), 1-11
Geosystem SRL., 2003, A Guide for Using WinGLink Software, ver. 2.1.1.
Groom, R.W. and Bailey., R.C., 1989, Decomposition of the Magnetotelluric impedance tensor in the presence of local three-dimensional galvanic distortion. Journal of Geophysical Research, 94, 1913-1925.
Ghaedrahmati, R., Mradzadeh., A., Fathianpour., N. and Kon Lee, S., 2013, Investigating 2-D MT inversion codes using real field data. Arabian Journal Of Geoscience, 7, 1-16.
Ghaedrahmati, R., Moradzadeh., A., Fathianpour., N., Lee, SK. and Porkhial S., 2013, 3-D inversion of MT data from the Sabalan geothermal field, Ardabil, Iran. Journal of Applied Geophysics, 39, 12-24.
Hafizi, M.K., Aiobi, M. and Rahimi, A., 2002, The combination of 2-D and 1-D inversion for 2.5D interpretation of magnetotelluric geothermal sites. Expanded abstr. EAGE 64th Conference, Florence, Italy.
Heinson, G.S., Direen, N. and Gill, R., 2006, Magnetotelluric evidence for a deep-crustal mineralizing system beneath the Olympic Dam iron oxide copper–gold deposit, southern Australia. Geology, 34, 573–576.
Heise, W., Caldwell, T.G., Bibby, H.M. and Bannister, S.C., 2008, Three-dimensional modelling of magnetotelluric data from the Rotokawa geothermal field, Taupo Volcanic Zone, New Zealand. Geophysical Journal International, 173, 740–750.
KML, 1998, Sabalan geothermal project, Stage 1, Surface exploration, final exploration report. Kingston Morrison Limited Co. Report 2505-RPT-GE-003 for the Renewable Energy Organization of Iran (SUNA), Tehran, Iran, 83 pp.
Ledo, J., 2005, 2D versus 3D magnetotelluric data interpretation: Surveys in Geophysics, 26, 671-806.
Lee, S.K., Kim , H,J., Song, Y. and Lee, C., 2009, MT2DInvMatlab- A program in MATLAB and FORTRAN for two-dimensional magnetotelluric inversion, Computer and Geoscience, 35, 1722-1735.
Manzella, A., 2004, Resistivity and heterogeneity of Earth crust in an active tectonic region, southern Tuscany, Italy. Ann. Geophys., 47, 107–118.
McNeice, G.W. and A.G. Jones, 2001, Multisite, multifrequency tensor decomposition of Magnetotelluric data, Geophysics, 66, 158-173.
Mogi, T. and Nakama, S., 1993, Magnetotelluric interpretation of the geothermal system of the Kuju volcano, southwest Japan. J. Volcanol. Geotherm. Res., 56, 297–308.
Muller, A. and Haak, V., 2004, 3-D modeling of the deep electrical conductivity of Merapi volcano (Central Java): integrating magnetotellurics, induction vectors and the effect of steep topography. Journal of Volcanology and Geothermal Research, 138, 205-222.
Oskooi, B., Pedersen, L.B., Smirnov, M., Árnason, K., Eysteinsson, H. and Manzella, A., 2005, The deep geothermal structure of the Mid-Atlantic Ridge deduced from MT data in SW Iceland. Phys. Earth Planet. Inter., 150, 183–195.
Oskooi, B., Fanaee Kheirabad, G.A., Habibian Dehkordi, B. and Nieuwenhuis, G., 2015, Three-dimensional conductivity model of the Sabalan geothermal field, NW Iran, interpreted from magnetotelluric data. Arabian Journal Of Geosciences, 2015(8), 3149-3157.
Oskooi, B., Takalu, M., Montahaei, M. and Rahmani, M.R., 2016, A recent magnetotelluric investigation of the Sabalan geothermal field in north-western Iran. Bollettino di Geofisica Teorica ed Applicata.
Patro, B.P.K., Harinarayana, T., Sastry, R.S., Rao, M., Manoj, C., Naganjaneyulu, K. and Sarma, S.V.S., 2005, Electrical imaging of Narmada-Son lineament zone, Central India from magnetotellurics. Physics of the Earth and Planetary Interior, 148, 215–232.
Spichak, V. and Manzella, A., 2009, Electromagnetic sounding of geothermal zones. Journal of Appl. Geophysics, 68(4), 459-478.
Swift, C.M., 1967, A magnetotelluric investigation of electrical conductivity anomaly in the southwestern united states. PhD Thesis Massachusetts Institute of Technology.
Talebi, B., 2006, Numerical modeling of the NW Sabalan geothermal field, Iran. Proc. Thirty-First Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California.
Talebi, B., Khosrawi, K. and Ussher, G., 2005, Review of resistivity surveys from the NW Sabalan geothermal field, Iran. Proc. World Geothermal Congress, Antalia, Turkey.
Tikhonov, A.N., 1950, On determining electrical characteristics of the deep layers of the Earth. Doklady Akademii Sel'skohozaÃjstvennyh Nauk, 73, 295–297.
Uchida, T., Lee, T.J., Honda, M. and Andan, A., 2002, 2-D and 3-D interpretation of msgnetotelluric data in the Bajawa geothermal field, central Flores, Indonesia. Bull. Geol. Surv. Japan, 53, 265-283.
Unsworth, M.J., Jones, A.G., Wei, W., Marquis, G., Gokarn, S.G., Spratt, J.E., INDEPTH-MT TEAM, 2005, Crustal rheology of the Himalaya and southern Tibet inferred from magnetotelluric data. Nature 438, 78–81.
Volpi, G., Manzella, A. and Fiordelisi, A., 2003, Investigation of geothermal structures by magnetotellurics (MT): an example from the Mt. Amiata area, Italy. Geothermics, 32, 131–145.
Wannamaker, P.E., Jiracek, G.R., Stodt, J.A., Caldwell, T.G., Gonzales, V.M., McKnight, J.D. and Porter, A.D., 2002, Fluid generation and pathways beneath an active compressional orogen, the New Zealand southern Alps, inferred from magnetotelluric data. J. Geophys. Res., 107(B6), ETG 6-1-ETG 6-20.
Wannamaker, P.E., Hohmann, G. and Ward, S., 1984, Magnetotelluric responses of three-dimensional bodies in layered earths. Geophysics, 49, 1517–1533.
Yi, M.J., Kim, J.H. and Chung, S.H., 2003, Enhancing the resolving power of least squares inversion with active constraint balancing. Geophysics, 68, 931-941.
Yousefi, H., Noorollah, Y., Ehara, S., Itoi, R., Yousefi, A., Fujimitsu, Y., Nishijima, J. and Sasak, K., 2009, Developing the geothermal resources map of Iran. Geothermics, doi:10.1016/j.geothermics.2009.11.001.
Zhang, P., Roberts, R.G. and Pedersen, L., 1987, Magnetotelluric strike rules. Geophysics, 52, 267–278.
فیزیک زمین و فضا
دوره 46، شماره 4
بهمن 1399
صفحه 27-37

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