Document Type: Case Study

Authors

1 School of Mining, Petroleum & Geophysics Engineering, Shahrood University of Technology, Shahrood, Iran

2 School of Mining Engineering, College of Engineering, University of Tehran, Tehran, Iran

3 Department of geothermal energy, Renewable Energy Organization of Iran, Ministry of Energy, Tehran, Iran

Abstract

The Ardabil geothermal area is located in the northwest of Iran, which hosts several hot springs. It is situated mostly around the Sabalan Mountain. The Sabalan geothermal area is now under investigation for the geothermal electric power generation. It is characterized by its high thermal gradient and high heat flow. In this study, our aim is to determine the fractal parameter and top and bottom depths of the magnetic sources. A modified spectral analysis technique named “de-fractal spectral depth method” is developed and used to estimate the top and bottom depths of the magnetized layer. A mathematical relationship is used between the observed power spectrum (due to fractal magnetization) and an equivalent random magnetization power spectrum. The de-fractal approach removes the effect of fractal magnetization from the observed power spectrum, and estimates the parameters of the depth to top and depth to bottom of the magnetized layer using the iterative forward modelling of the power spectrum. This approach is applied to the aeromagnetic data of the Ardebil province. The results obtained indicated variable magnetic bottom depths ranging from 10.4 km in the northwest of Sabalan to about 21.1 km in the north of the studied area. In addition, the fractal parameter was found to vary from 3.7 to 4.5 within the studied area.

Keywords

[1]. Mandelbrot, B. B. (1983). The Fractal Geometry of Nature, W. H. Freeman, San Francisco.

[2]. Bansal, A.R., Gabriel, G, and Dimri, V.P. (2010). Power law distribution of susceptibility and density and its relation to seismic properties: An example from the German Continental Deep Drilling Program: Journal of Applied Geophysics. 72 (2): 123-128.

[3]. Fedi, M., Quarta, T. and De Santis, A. (1997). Improvements to the Spector and Grant method of source depth estimation using the power law decay of magnetic field power spectra, Geophysics. 62: 1143-1150.

[4]. Spector, A. and Grant, F.S., (1970). Statistical Models for Interpreting Aeromagnetic Data, Geophysics. 35: 293-302.

[5]. Pilkington, M. and Todoeschuck, J.P. (1993). Fractal magnetization of continental crust, Geophys. Res. Lett. 20: 627-630.

[6]. Pilkington, M., Todoeschuck, J.P. and Gregotski, M.E. (1994). Using fractal crustal magnetization models in magnetic interpretation, Geophysical Prospecting. 42: 677-692.

[7]. Maus, S. and Dimri, V.P. (1995). Potential field power spectrum inversion for scaling geology, J. Geophys. Res. 100: 12605-12616.

[8]. Maus, S., Gordon, D. and Fairhead, D. (1997). Curie temperature depth estimation using a self-similar magnetization model, Geophys. J. Int. 129: 163-168.

[9]. Ravat, D., Pignatelli, A., Nicolosi, I. and Chiappini, M. (2007). A study of spectral methods of estimating the depth to the bottom of magnetic sources from near-surface magnetic anomaly data, Geophys. J. Int. 169: 421-434.

[10]. Bouligand, C., Jonathan, M., Glen, G. and Blakely, J.R. (2009). Mapping Curie temperature depth in the western United States with a fractal model for crustal magnetization, J. Geophys. Res. 114: 1-25.

[11]. Bhattacharyya, B.K. and Leu, L.K. (1975). Spectral Analysis of Gravity and Magnetic Anomalies due to Two-dimensional Structures, Geophysics. 40: 993-1013.

[12]. Okubo, Y., Graf, R.J., Hansent, R.O., Ogawa, K. and Tsu, H. (1985). Curie point depths of the island of Kyushu and surrounding areas Japan, Geophysics.53: 481-494.

[13]. Shuey, R.T., Schellinger, D.K., Tripp, A.C. and Alley, L.B. (1977). Curie depth determination from aeromagnetic spectra, Geophys. J. Roy. Astr. Soc. 50: 75-101.

[14]. Blakely, R. (1988). Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada, J. Geophys. Res. 93: 11817-11832.

[15]. Salem, A., Ushijima, K., Elsirafi, A. and Mizunaga, H. (2000). Spectral Analysis of Aeromagnetic Data for Geothermal Reconnaissance of Quseir area, Northern Red Sea, Egypt, Proceeding World Geothermal Congress, Kyushu, Japan. 1669-1674.

[16]. Bhattacharyya, B.K. and Leu, L.K. (1977). Spectral analysis of gravity and magnetic anomalies due to rectangular prismatic bodies, Geophysics. 41: 41-50.

[17]. Tanaka, A., Okubo,Y. and Matsubayashi, O. (1999). Curie point depth based on spectrum analysis of magnetic anomaly data in East and Southeast Asia, Tectonophysics. 306: 461-470.

[18]. Blakely, R.J. (1995). Potential theory in gravity and magnetic applications, Cambridge Univ. Press, Cambridge.

[19]. Bansal, A.R., Gabriel, G., Dimri, V.P. and Krawczyk, C.M. (2011). Estimation of depth to the bottom of magnetic sources by a modified centroid method for fractal distribution of sources: An application to aeromagnetic data in Germany, Geophysics. 76: 3: 11-22.

[20]. Salem, A., Green, C., Ravat, D., Singh, K.H., East, P., Fairhead, J.D., Mogren, S. and Biegert, E. (2014). Depth to Curie temperature across the central Red Sea from magnetic data using the de-fractal method. Tectonophysics. 75-86.

[21]. Ravat, D. (2004). Constructing full spectrum potential-field anomalies for enhanced geodynamical analysis through integration of surveys from different platforms (INVITED), EOS, Trans. Am. geophys. Un. 85 (47), Fall Meet. Suppl., Abstract G44A-03.

[22]. Finn, C.A. and Ravat, D. (2004). Magnetic Depth Estimates and Their Potential for Constraining Crustal Composition and Heat Flow in Antarctica, EOS, Trans. Am. geophys. Un, 85(47), Fall Meet. Suppl., Abstract T11A-1236.

[23]. Ross, H.E., Blakely, R.J. and Zoback, M.D. (2004). Testing the Utilization of Aeromagnetic Data for the Determination of Curie-Isotherm Depth, EOS, Trans. Am. geophys. Un., 85 (47), Fall Meet. Suppl., Abstract T31A-1287.

[24]. Connard, G., Couch, R., and Gemperle, M. (1983). Analysis of Aeromagnetic Measurements from Cascade Range in Central Oregon, Geophysics. 48: 376-390.

[25]. Ross, H.E., Blakely, R.J. and Zoback, M.D. (2006). Testing the use of aeromagnetic data for the determination of Curie depth in California: Geophysics. 71 (5): L51-L59.

[26]. Pilkington, M. and Todoeschuck, J.P. (1995). Scaling nature of crustal susceptibilities. Geophys. Res. Lett. 22: 779-782.

[27]. Maus, S. and Dimri, V.P. (1994). Scaling properties of potential fields due to scaling sources. Geophys. Res. Lett. 21: 891-894.

[28]. SKM (Sinclair Knight Merz). (2005). Resource review of the Northwest Sabalan geothermal project. Report submitted to SUNA. 61P.

[29]. 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, Tehran. 83 P.

[30]. Okubo, Y., Matsushima, J. and Correia, A. (2003). Magnetic spectral analysis in Portugal and its adjacent seas, Phys. Chem. Earth. 28: 511-519.

[31]. Hisarli, Z.M., Dolmaz, M.N., Okyar, M., Etiz, A. and Orbay, N. (2011). Investigation into regional thermal structure of the Thrace Region, NW Turkey, from aeromagnetic and borehole data, Stud. Geophys. Geod. 56: 269-291.

[32]. Ghaedrahmati, R., Moradzadeh, A., Fathianpour, N., Lee, S.K. and Porkhial, S. (2013). 3-D inversion of MT data from the Sabalan geothermal field, Ardabil, Iran, Journal of Applied Geophysics. 93: 12-24.