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Assessment of geothermal energy potential by geophysical methods: Nevşehir Region, Central Anatolia
Journal of Volcanology and Geothermal Research 295 (2015) 55–64
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Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
Assessment of geothermal energy potential by geophysical methods: Nevşehir Region, Central Anatolia Alper Kıyak a,b,⁎, Can Karavul b, Levent Gülen b, Ertan Pekşen c, A. Rıza Kılıç a a b c
General Directorate of Mineral Research and Exploration, Department of Geophysical Research, Çankaya, 06810 Ankara, Turkey Sakarya University, Department of Geophysical Engineering, Serdivan, 54187 Sakarya, Turkey Kocaeli University, Department of Geophysical Engineering, Umuttepe, 41380 Kocaeli, Turkey
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Article history: Received 20 May 2014 Accepted 3 March 2015 Available online 9 March 2015 Keywords: Geothermal VES MT 3D Euler deconvolution Calderas Central Anatolia
a b s t r a c t In this study, geothermal potential of the Nevşehir region (Central Anatolia) was assessed by using vertical electrical sounding (VES), self-potential (SP), magnetotelluric (MT), gravity and gravity 3D Euler deconvolution structure analysis methods. Extensive volcanic activity occurred in this region from Upper Miocene to Holocene time. Due to the young volcanic activity Nevşehir region can be viewed as a potential geothermal area. We collected data from 54 VES points along 5 profiles, from 28 MT measurement points along 2 profiles (at frequency range between 320 and 0.0001 Hz), and from 4 SP profiles (total 19 km long). The obtained results based on different geophysical methods are consistent with each other. Joint interpretation of all geological and geophysical data suggests that this region has geothermal potential and an exploration well validated this assessment beyond doubt. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Turkey is the seventh in the world ranking of geothermal energy potential (Jennejohn et al., 2012). Turkey has four main geothermal energy regions; the West, Central and East Anatolian regions, and the North Anatolian Fault Zone where there is young volcanic activity (Drahor and Berge, 2006). Among these especially the west Anatolia region has the highest geothermal potential. Some of the geothermal areas and their temperatures are: Kızıldere (242 °C), Germencik (232 °C), Simav (135 °C), Afyon (95 °C), Kozaklı (90 °C), Kızılcahamam (80 °C), Gönen (80 °C), and Kırşehir (57 °C) (Serpen et al., 2008). The first time to generate electricity by using geothermal resources in Turkey was attempted by the General Directorate of Mineral and Research Institute (MTA) at Kızıldere-Denizli in 1968. A power plant which is generating 0.5 MWe electric energy from geothermal resources was put into service by MTA at Kızıldere-Denizli region in 1974. In this study, we have presented the results of geothermal research around the town of Göre which is located at the Central Anatolian region of Turkey (Fig. 1).
⁎ Corresponding author at: General Directorate of Mineral Research and Exploration, Department of Geophysical Research, Çankaya, 06810 Ankara, Turkey. Tel.: + 90 312 2011407; fax: +90 312 2878749. E-mail address: [email protected] (A. Kıyak).
http://dx.doi.org/10.1016/j.jvolgeores.2015.03.002 0377-0273/© 2015 Elsevier B.V. All rights reserved.
Geophysical methods such as electric and electromagnetic are often used for geothermal exploration. Hydrothermal fluids modify the electrical resistivity based on the fluid content of surrounding geological formations. Generally, ionic conduction increases with temperature, salinity and porosity in the rocks (Özürlan and Şahin, 2006). According to previous geological studies a caldera exists in the Nevşehir area and this area could be a potential geothermal resource (Le Pennec et al., 1994; Froger et al., 1998). The volcanism in the Central Anatolia is continental arc type (Pasquare et al., 1988). There is a low velocity upper mantle region beneath this area obtained by seismological studies (Gans et al., 2009). In a Curie point study of Turkey Ateş et al., 2005 suggested that the 580 °C isotherm contour around the Cappadocia Volcanic Province (CVP) could be a potential reservoir for a geothermal system. According to this study, Curie point depths near CVP are estimated to be around 8–12 km. Based on the previous geological and geophysical studies, one can infer that the CVP may be a good place for geothermal conduits. Therefore, we applied several geophysical methods such as vertical electrical sounding (VES), self-potential (SP), gravity and magnetotelluric (MT) methods in order to delineate the geothermal area around the town of Göre. 2. Geological settings The geological map of the study area is presented in Fig. 2. The geology of the Central Anatolia associated with the Nevşehir–Acıgöl Caldera
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Fig. 1. Location map of the Nevşehir region, Central Anatolia, Turkey (Başarsoft/Google Earth, 2012).
Fig. 2. The geology map of the Nevşehir region, Central Anatolia. The geophysical profiles are also shown.
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Fig. 3. The DEM image (http://www.geomapapp.org/) of the study area and the location of VES, MT and SP profiles. The probable faults (dashed black and red lines), fault (black lines) and the Nevşehir Caldera Boundary (circle with tick marks) are also shown.
Complex developed since the Late Miocene (Froger et al., 1998). The result of the convergence of African-Arabian and Eurasian plates gave rise to a volcanic belt that extends from Greece to Turkey to Iran (Şengör and Yılmaz, 1981; Innocenti et al., 1982). The continental arc volcanic products cover extensive areas in Central Anatolia. The basement is represented by the Paleozoic rocks of the Kırşehir metamorphic massif. The metamorphic basement mainly consists of marble, mica schist, and quartzite. The Eocene and Oligocene age rocks cover this Paleozoic basement with an unconformity and they consist of the Upper Cretaceous gabbro, diabase, andesite, dacite, granite, and granodiorites. They are overlain by the Upper Miocene Ürgüp Ignimbrite Formation. The Ürgüp Formation is overlain by basaltic volcanics. The Quaternary volcanic rocks of Acıgöl, the Quaternary travertine and alluvial deposits are the youngest formations at the top of the sequence (Türkecan et al., 2004). The volcanic activity continued from the Miocene to present time in the region. The Miocene volcanism was created by the subduction of the Neotethyan oceanic slab under the Anatolian plate (Türkecan et al., 2004). The volcanism was acidic in the Upper Miocene (Froger et al., 1998). On the contrary, the volcanism was more alkaline during the Pliocene time. The Quaternary volcanism was bimodal as indicated by the presence of both acidic and alkaline volcanic products. In the CVP, the Quaternary volcanism ended with two stratovolcanoes: Hasan and Erciyes Volcanoes. This volcanic activity produced basaltic, andesitic, dasidic lava flows and pyroclastics (Pasquare et al., 1988). The geophysical survey area is located within the Nevşehir Caldera Complex. The caldera complex is in the middle of Acıgöl, Nevşehir and Çardak triangle (Fig. 1). The caldera complex was delineated by a gravity anomaly with − 35 mgal. The shape of the anomaly is circular and its radius is approximately 15 km. Young volcanic activity from the Miocene to Holocene time produced the Nevşehir and Acıgöl Caldera Complex (Froger et al., 1998). The Nevşehir Caldera region is cut by NW–SE and NE–SW trending conjugate set of faults (Figs. 2, 3). In this region, there are many existing boreholes for irrigation purposes that reach depths between 150 and 350 m. Measured temperatures of water in these boreholes are between 24 and 38 °C. The
electrical conductivity (EC) of the water in these boreholes is also measured. EC values of water in the wells vary between 30 and 9000 mS/cm. 3. Geophysical methods in geothermal exploration In geothermal resource exploration various geophysical methods are used. Among these are VES (Olhoeft, 1981; Telford et al., 1990; Reynolds, 1997), MT (Cagniard, 1953, Groom and Bailey, 1989; Berdichevsky et al., 1998; McNeice and Jones, 2001), SP (Revil and Jardani, 2013; Marcos et al., 2014), and gravity (Blakely, 1996; Khalil et al., 2014).
Fig. 4. The Bouguer gravity anomaly map of the Nevşehir region. The locations of VES, MT and SP profiles are shown within the Nevşehir caldera marked with dark solid circle.
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Different geophysical methods are sensitive to different physical parameters so that geothermal resources can be delineated by an integrated interpretation. For example VES methods for measuring electrical resistivity are based on primarily temperature effects. In rocks electrical conduction takes place by passage of current through the fluid in the pores. Temperature is one of the factors that modify the rock resistivity. Also, water content and chemistry effect the conductivity of rocks (Manzella, 2009). In this study, a classical Schlumberger electrode array was used for VES application. The distances between two current electrodes were varied between 5 and 8 km (Fig. 3). A DC generator with a maximum 2A was used as a source. The VES data was processed by the 1D inversion module of Geosystem Winglink (Ver 2.2) software (Geosystems, 2008). Then we combined all 1D inversion results to generate an inverted cross-section of the corresponding profile. In magnetotelluric (MT) method natural electromagnetic field of earth is used as an energy source to measure electrical resistivity of earth. This method gives useful information about deep reservoirs. MT measurements give important hints about characteristics of the studied geological formations, because of the conductivity change between rock units. The depth of geological structures and the limits of big stratigraphic units can be obtained by the interpretation of MT data (Manzella, 2009). The observed data were transferred from time to frequency domain by using Phoenix SSMT 2000 software package in order to plot curves at different frequencies (Phoenix Geophysics, 2005). After that, apparent resistivity and phase (Cagniard, 1953) were calculated and decomposed in two modes (TM, TE) and the data saved as Electronical Data International (EDI) files. In the field, MT equipment was set up with respect to the magnetic north. To create a 2D section, the observed data were rotated to the geologic direction to make the MT profiles perpendicular to each other. We used Strike code developed by McNeice and Jones (2001) based on Groom and Bailey's (1989) method to find an angle for the profile. MT signals were collected at 320 Hz–0.001 Hz frequency range (Fig. 3). For 2D inversion, we used apparent resistivity and phase at 13 MT stations which measured frequencies between 10,000 and 0.001 Hz. The inversion routine started with an initial value of 100 Ω-m homogenous medium. After 55 iterations, the inversion routine reached 2D resistivity cross-section with 2.75% RMS values. In the SP method only the naturally existing voltage changes in the earth are measured. In geothermal areas, very large SP anomalies are observed and these are caused by a combination of thermoelectric effects This thermoelectric effects are caused by hydrothermal fluid flow in porous and fracture zones (Manzella, 2009). The movement of water in porous media creates measurable amount of SP anomaly and this is a well-known fact (Telford et al., 1990; Reynolds, 1997). In the field, a pair of non-polarizing Cu–CuSO4 electrodes and a digital voltmeter with a high input-impedance were used to measure the potential differences. The SP data collected with a gradient array and the data plotted in the middle of the electrode pairs. In addition, the stacked SP anomaly was calculated and plotted as well. In this study the distance between two electrodes was 50 m (Fig. 3). Gravity surveys are used in geothermal exploration to map lateral density variations, because there is a density difference between continental crust and an intruding magmatic body. Gravity surveys help to reveal the fracture zones that are located in the geothermal field (Manzella, 2009). 3.1. Gravity and 3D Euler deconvolution interpretation Nevşehir Caldera Complex has been studied using gravity by MTA (Ekingen and Güvün, 1978). We reprocessed the MTA Bouguer gravity data acquired in 1978. We used Oasis Montage software package to
Fig. 5. a. 3D Euler deconvolution structure and depth analysis results are shown on the Bouguer gravity anomaly map, SI = 0 for a sill or dyke or step. The Nevşehir caldera marked with dark solid circle. b. 3D Euler deconvolution structure and depth analysis results are shown on the Bouguer gravity anomaly map, SI = 1 for a pipe. The Nevşehir caldera marked with dark solid circle. c. 3D Euler deconvolution structure and depth analysis results are shown on Bouguer gravity anomaly map, SI = 2 for a sphere. The Nevşehir Caldera marked with dark solid circle.
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Fig. 6. 1D VES inversion results of the Profile-D. The SND-1 well location is shown, The SP Profile-D is indicated with red arrows.
Fig. 7. 2D MT inversion result of the Profile N8. The SND-1 well location is shown, The SP Profile-D is indicated with red arrows.
employ 3D Euler deconvolution. Euler 3D deconvolution was employed to estimate the position of the structural lineaments in the study area from Bouguer data. The main objective of the Euler 3D deconvolution is to represent the geological structures that cause the gravity or magnetic anomalies using a 2D grid (Oruç and Selim, 2011). Euler 3D process is based on Euler's homogeneity equation, an equation that relates the field and its gradient components to the location of the source with the degree of homogeneity, which may be interpreted as a structural index (Thompson, 1982). This system uses a least square method to solve Euler's equation simultaneously for each grid position with sub-grid (window). The structure index (SI) is an exponential factor corresponding to the rate at which the field falls off with distance, for a source of a given geometry. The value of SI parameter depends on the type of the source body and the potential field (gravity or magnetic). For example in gravity field the SI value is equal to 0 for a sill or dyke or step, 1 for a pipe, and 2 for a sphere (Khalil et al., 2014).
Fig. 8. The SP anomaly curves obtained along the Profile-D. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
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Fig. 9. 1D VES inversion results of the Profile-B. The SP Profile-B is indicated with red arrows.
exists in the west and increases westwards (shown in yellow to red to purple color in Fig. 4). This anomaly has a −30 mgal amplitude and it has nearly 150 km2 surface area This Bouguer gravity variation indicates that the basement becomes shallower towards the west. The Nevşehir Caldera is possibly filled with low density materials such as volcanic ash and/or Neogene sediments. Hence the observed anomaly between Boğazköy and Güvercinlik reflects the actual caldera collapse area (Fig. 4). While employing the 3D Euler deconvolution of Bouguer gravity data, we used different structural index (SI) parameters (SI = 0, SI = 1, SI = 2). Fig. 5a,b,c shows different SI parameter results. The 3D Euler deconvolution results that used SI = 2 parameter are more compatible with our caldera depth model inferred from geoelectrical (MT, VES) and geological study results. Fig. 10. The SP anomaly curves obtained along the Profile-B. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
3.2. Interpretation of geoelectrical measurements (VES, MT, SP)
One of the findings from these studies is that most of the Nevşehir Caldera Complex is characterized by a remarkable, circular negative Bouguer gravity anomaly that has approximately 10 km diameter (Fig. 4). A local less pronounced negative Bouguer gravity anomaly
Geoelectrical measurements were acquired along 5 VES, 4 SP and 2 MT profiles (Figs. 2,3). The VES and SP profiles labeled as A and E are N–S profiles and the rest are E–W profiles. In MT and VES sections, different structures are classified based on their resistivity values as follows: very low resistivity (VLR) (~ 3–10 Ω-m), low resistivity (LR) (~10–40 Ω-m), medium resistivity (MR) (~ 40–150 Ω-m), high resistivity (HR) (~ 150–800 Ω-m) and very high resistivity (VHR) (~800–5000 Ω-m).
Fig. 11. 1D VES inversion results of the Profile-C.
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Fig. 12. 2D MT inversion result of the Profile N5.
Fig. 13. 1D VES inversion results of the Profile-A. The SP Profile-A is indicated with red arrows.
The structures labeled as HR and MR, that are located between approximately 1500 and 1000 m elevations, are surface crustal units based on VES and MT measurements.
Fig. 14. The SP anomaly curves obtained along the Profile-A. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
3.3. D and N8 profiles (E–W) The VES measurements obtained along the D section indicate the presence of a VLR structure between approximately 700 and − 1500 m elevations (Fig. 6). There are also LR and MR structures between approximately − 700 and − 1500 m elevations and beneath D7 and D11 stations (Fig. 6). Therefore the edge on the MR structure appears to be a hydrothermal convection zone. That is why the location of the SND-1 well was chosen to reach this zone (DS station). The same VLR structure described above is also observed in the N8-MT profile between approximately 1000 and −1500 m elevations, beneath the N806 and N812 stations (Fig. 7). The MR structure is surrounded in the west and east with VHR and HR structures, respectively. The SP data that acquired alone the D profile is given in Fig. 8. There are positive and negative SP anomaly variations throughout the profile. It is well-known that facts with regard to positive SP anomalies indicate presence of ascending hot fluids and negative SP anomalies indicate descending in relatively cold fluids (Türker et al., 1991; Marcos et al., 2014). These observed SP anomaly variations and also the MT and VES measurements along the D profile indicate hydrothermal activity within an intensely deformed and fractured zone.
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Fig. 15. 1D VES inversion results of the Profile-E. The SP Profile-E is indicated with red arrows.
In Fig. 8 stacked SP trend becomes negative towards the east of the DS station which may be interpreted as ascending and descending hydrothermal fluid boundary zone.
3.4. B profile (E–W)
Fig. 16. The SP anomaly curves obtained along the Profile-E. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
The B profile is for checking southward continuation of the VLR conductive structure seen in the D profile (Fig. 9). The VLR structure continues southward, however it becomes deeper along the B profile. SP measurements were also acquired between B11 and B29 VES stations to check areas considered to be fractured (Fig. 10). Although there are small SP anomaly variations until the B19 station, afterwards large amplitude changes are present towards the east. The stacked SP trend also becomes positive to the east of station B19. The location of the observed changes coincides with the northwest southeast trending G1 fault (Figs. 2, 3 and 9).
Fig. 17. The plot of apparent resistivity versus AB/2 distance measured at the drilling location (left panel). 1D inversion result of the VES data (right panel). The location of the drilling was proposed based on the VES curve.
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3.5. C and N5 profiles (E–W) The VLR structure continues towards the south, however it becomes shallower and confined between approximately 700 and − 300 m elevations (Fig. 11). Under this VLR structure a distinct MR structure is present (C5–C11, Fig. 11). This MR structure is better constrained in the N5-MT profile and it continues downwards until approximately − 6000 m elevation beneath N507–N514T stations (Fig. 12).
3.6. A and E profiles (N–S) The N–S trending A and E profiles pass through the middle of the Nevşehir Caldera (Figs. 2,3,13, and 15). The VLR structure identified along the E–W trending D, B, C, N8 and N5 profiles is also clearly observed along the A and E profiles, however it passes into LR and MR structures towards the south. These profiles cut through the G1, G2, G3, and G4 faults (Figs. 2,3). Observed lateral discontinuities correspond to these faults (Fig. 13). The positive and negative behavior of observed SP anomalies can be associated with fluid circulation in the deformed and fractured zone along the N–S trending A and E profiles (Figs. 14 and 16). These probable faults are also observed in the VES results. Fig. 18. The plot of apparent resistivity versus period measured at the drilling location (upper panel). The plot of apparent phase versus period measured at the drilling location (lower panel). The static shift corrected results are shown with solid lines.
Fig. 19. The lithostratigraphic and temperature logs of the SND-1 well. Geological period (right panel). Soil symbols and descriptions with depth and leak info (middle panel). Graph of log (right panel).
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4. Drilling results A suggested well location based on the inferred geophysical model is chosen and it is planned to reach a depth of 3 km. Fig. 17 shows apparent resistivity variations of VES data and 1D inversion result at the drilling location. Similarly, apparent resistivity and phase behavior of MT result at the drilling location are displayed in Fig. 18. The vertical geological section is shown in Fig. 19. At the bottom (2900 m depth) of the well-log of Thermic-1 an instantaneous temperature of 185 °C was measured and at 2485 m depth of Thermic-2 137 °C temperature was measured. At 2485 m depth, when both Thermic-1 and Thermic-2 are compared it is observed that an increase in temperature inside the well was correlated with waiting time. This increase in temperature begins with dominance of the units like sandstone, conglomerate around 1300 m depth below volcano sedimentary units. Especially in Thermic-2 between 1800 and 2500 m depth, temperature increase caused by a fracture system or conglomerate. It is difficult to distinguish the source of hot fluid horizons except between 2450 and 2550 m depths. Probably a granite-altered andesite contact is the hot fluid conduit at 2450–2550 m depths. We can also infer the presence of a fracture zone based on water leak values at these levels. 5. Conclusion A potential geothermal system was delineated around the Nevşehir Caldera Complex, Göre, Nevşehir region based on geophysical investigations. Evaluation of previous geological and geophysical studies combined with Gravity, SP, VES, and MT measurements provided important data and also processing the Bouguer data using Euler 3D analysis was useful to reveal the detailed structure of the region and generation of a geothermal model. Based on the synthesis of all our results, a drill location was determined to validate our geothermal model. This exploration well was successfully reached to the hot fluid at a depth of 2.9 km with a temperature of 185 °C. For the first time this finding conclusively validated the geothermal potential of the Göre, Nevşehir region. Acknowledgments We would like to thank the General Directorate of Mineral Research and Exploration (12.02.00/929-888) for funding the field studies and geophysical surveys and permitting the publication of the results. We thank anonymous reviewers and the editor Prof. Jürgen W. Neuberg for valuable contributions. References Ateş, A., Bilim, F., Büyüksaraç, A., 2005. Curie point depth investigation of Central Anatolia, Turkey. Pure Appl. Geophys. 162, 357–371. Berdichevsky, M.N., Dmitriev, V.I., Pozdnjakova, E.E., 1998. On two-dimensional interpretation of magnetotelluric soundings. Geophys. J. Int. 133 (3), 585–606. Blakely, R.J., 1996. Potential Theory in Gravity and Magnetic Applications. Cambridge University Press (ISBN-13:978-0521575478, 437 pp.).
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Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
Assessment of geothermal energy potential by geophysical methods: Nevşehir Region, Central Anatolia Alper Kıyak a,b,⁎, Can Karavul b, Levent Gülen b, Ertan Pekşen c, A. Rıza Kılıç a a b c
General Directorate of Mineral Research and Exploration, Department of Geophysical Research, Çankaya, 06810 Ankara, Turkey Sakarya University, Department of Geophysical Engineering, Serdivan, 54187 Sakarya, Turkey Kocaeli University, Department of Geophysical Engineering, Umuttepe, 41380 Kocaeli, Turkey
a r t i c l e
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Article history: Received 20 May 2014 Accepted 3 March 2015 Available online 9 March 2015 Keywords: Geothermal VES MT 3D Euler deconvolution Calderas Central Anatolia
a b s t r a c t In this study, geothermal potential of the Nevşehir region (Central Anatolia) was assessed by using vertical electrical sounding (VES), self-potential (SP), magnetotelluric (MT), gravity and gravity 3D Euler deconvolution structure analysis methods. Extensive volcanic activity occurred in this region from Upper Miocene to Holocene time. Due to the young volcanic activity Nevşehir region can be viewed as a potential geothermal area. We collected data from 54 VES points along 5 profiles, from 28 MT measurement points along 2 profiles (at frequency range between 320 and 0.0001 Hz), and from 4 SP profiles (total 19 km long). The obtained results based on different geophysical methods are consistent with each other. Joint interpretation of all geological and geophysical data suggests that this region has geothermal potential and an exploration well validated this assessment beyond doubt. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Turkey is the seventh in the world ranking of geothermal energy potential (Jennejohn et al., 2012). Turkey has four main geothermal energy regions; the West, Central and East Anatolian regions, and the North Anatolian Fault Zone where there is young volcanic activity (Drahor and Berge, 2006). Among these especially the west Anatolia region has the highest geothermal potential. Some of the geothermal areas and their temperatures are: Kızıldere (242 °C), Germencik (232 °C), Simav (135 °C), Afyon (95 °C), Kozaklı (90 °C), Kızılcahamam (80 °C), Gönen (80 °C), and Kırşehir (57 °C) (Serpen et al., 2008). The first time to generate electricity by using geothermal resources in Turkey was attempted by the General Directorate of Mineral and Research Institute (MTA) at Kızıldere-Denizli in 1968. A power plant which is generating 0.5 MWe electric energy from geothermal resources was put into service by MTA at Kızıldere-Denizli region in 1974. In this study, we have presented the results of geothermal research around the town of Göre which is located at the Central Anatolian region of Turkey (Fig. 1).
⁎ Corresponding author at: General Directorate of Mineral Research and Exploration, Department of Geophysical Research, Çankaya, 06810 Ankara, Turkey. Tel.: + 90 312 2011407; fax: +90 312 2878749. E-mail address: [email protected] (A. Kıyak).
http://dx.doi.org/10.1016/j.jvolgeores.2015.03.002 0377-0273/© 2015 Elsevier B.V. All rights reserved.
Geophysical methods such as electric and electromagnetic are often used for geothermal exploration. Hydrothermal fluids modify the electrical resistivity based on the fluid content of surrounding geological formations. Generally, ionic conduction increases with temperature, salinity and porosity in the rocks (Özürlan and Şahin, 2006). According to previous geological studies a caldera exists in the Nevşehir area and this area could be a potential geothermal resource (Le Pennec et al., 1994; Froger et al., 1998). The volcanism in the Central Anatolia is continental arc type (Pasquare et al., 1988). There is a low velocity upper mantle region beneath this area obtained by seismological studies (Gans et al., 2009). In a Curie point study of Turkey Ateş et al., 2005 suggested that the 580 °C isotherm contour around the Cappadocia Volcanic Province (CVP) could be a potential reservoir for a geothermal system. According to this study, Curie point depths near CVP are estimated to be around 8–12 km. Based on the previous geological and geophysical studies, one can infer that the CVP may be a good place for geothermal conduits. Therefore, we applied several geophysical methods such as vertical electrical sounding (VES), self-potential (SP), gravity and magnetotelluric (MT) methods in order to delineate the geothermal area around the town of Göre. 2. Geological settings The geological map of the study area is presented in Fig. 2. The geology of the Central Anatolia associated with the Nevşehir–Acıgöl Caldera
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Fig. 1. Location map of the Nevşehir region, Central Anatolia, Turkey (Başarsoft/Google Earth, 2012).
Fig. 2. The geology map of the Nevşehir region, Central Anatolia. The geophysical profiles are also shown.
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Fig. 3. The DEM image (http://www.geomapapp.org/) of the study area and the location of VES, MT and SP profiles. The probable faults (dashed black and red lines), fault (black lines) and the Nevşehir Caldera Boundary (circle with tick marks) are also shown.
Complex developed since the Late Miocene (Froger et al., 1998). The result of the convergence of African-Arabian and Eurasian plates gave rise to a volcanic belt that extends from Greece to Turkey to Iran (Şengör and Yılmaz, 1981; Innocenti et al., 1982). The continental arc volcanic products cover extensive areas in Central Anatolia. The basement is represented by the Paleozoic rocks of the Kırşehir metamorphic massif. The metamorphic basement mainly consists of marble, mica schist, and quartzite. The Eocene and Oligocene age rocks cover this Paleozoic basement with an unconformity and they consist of the Upper Cretaceous gabbro, diabase, andesite, dacite, granite, and granodiorites. They are overlain by the Upper Miocene Ürgüp Ignimbrite Formation. The Ürgüp Formation is overlain by basaltic volcanics. The Quaternary volcanic rocks of Acıgöl, the Quaternary travertine and alluvial deposits are the youngest formations at the top of the sequence (Türkecan et al., 2004). The volcanic activity continued from the Miocene to present time in the region. The Miocene volcanism was created by the subduction of the Neotethyan oceanic slab under the Anatolian plate (Türkecan et al., 2004). The volcanism was acidic in the Upper Miocene (Froger et al., 1998). On the contrary, the volcanism was more alkaline during the Pliocene time. The Quaternary volcanism was bimodal as indicated by the presence of both acidic and alkaline volcanic products. In the CVP, the Quaternary volcanism ended with two stratovolcanoes: Hasan and Erciyes Volcanoes. This volcanic activity produced basaltic, andesitic, dasidic lava flows and pyroclastics (Pasquare et al., 1988). The geophysical survey area is located within the Nevşehir Caldera Complex. The caldera complex is in the middle of Acıgöl, Nevşehir and Çardak triangle (Fig. 1). The caldera complex was delineated by a gravity anomaly with − 35 mgal. The shape of the anomaly is circular and its radius is approximately 15 km. Young volcanic activity from the Miocene to Holocene time produced the Nevşehir and Acıgöl Caldera Complex (Froger et al., 1998). The Nevşehir Caldera region is cut by NW–SE and NE–SW trending conjugate set of faults (Figs. 2, 3). In this region, there are many existing boreholes for irrigation purposes that reach depths between 150 and 350 m. Measured temperatures of water in these boreholes are between 24 and 38 °C. The
electrical conductivity (EC) of the water in these boreholes is also measured. EC values of water in the wells vary between 30 and 9000 mS/cm. 3. Geophysical methods in geothermal exploration In geothermal resource exploration various geophysical methods are used. Among these are VES (Olhoeft, 1981; Telford et al., 1990; Reynolds, 1997), MT (Cagniard, 1953, Groom and Bailey, 1989; Berdichevsky et al., 1998; McNeice and Jones, 2001), SP (Revil and Jardani, 2013; Marcos et al., 2014), and gravity (Blakely, 1996; Khalil et al., 2014).
Fig. 4. The Bouguer gravity anomaly map of the Nevşehir region. The locations of VES, MT and SP profiles are shown within the Nevşehir caldera marked with dark solid circle.
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Different geophysical methods are sensitive to different physical parameters so that geothermal resources can be delineated by an integrated interpretation. For example VES methods for measuring electrical resistivity are based on primarily temperature effects. In rocks electrical conduction takes place by passage of current through the fluid in the pores. Temperature is one of the factors that modify the rock resistivity. Also, water content and chemistry effect the conductivity of rocks (Manzella, 2009). In this study, a classical Schlumberger electrode array was used for VES application. The distances between two current electrodes were varied between 5 and 8 km (Fig. 3). A DC generator with a maximum 2A was used as a source. The VES data was processed by the 1D inversion module of Geosystem Winglink (Ver 2.2) software (Geosystems, 2008). Then we combined all 1D inversion results to generate an inverted cross-section of the corresponding profile. In magnetotelluric (MT) method natural electromagnetic field of earth is used as an energy source to measure electrical resistivity of earth. This method gives useful information about deep reservoirs. MT measurements give important hints about characteristics of the studied geological formations, because of the conductivity change between rock units. The depth of geological structures and the limits of big stratigraphic units can be obtained by the interpretation of MT data (Manzella, 2009). The observed data were transferred from time to frequency domain by using Phoenix SSMT 2000 software package in order to plot curves at different frequencies (Phoenix Geophysics, 2005). After that, apparent resistivity and phase (Cagniard, 1953) were calculated and decomposed in two modes (TM, TE) and the data saved as Electronical Data International (EDI) files. In the field, MT equipment was set up with respect to the magnetic north. To create a 2D section, the observed data were rotated to the geologic direction to make the MT profiles perpendicular to each other. We used Strike code developed by McNeice and Jones (2001) based on Groom and Bailey's (1989) method to find an angle for the profile. MT signals were collected at 320 Hz–0.001 Hz frequency range (Fig. 3). For 2D inversion, we used apparent resistivity and phase at 13 MT stations which measured frequencies between 10,000 and 0.001 Hz. The inversion routine started with an initial value of 100 Ω-m homogenous medium. After 55 iterations, the inversion routine reached 2D resistivity cross-section with 2.75% RMS values. In the SP method only the naturally existing voltage changes in the earth are measured. In geothermal areas, very large SP anomalies are observed and these are caused by a combination of thermoelectric effects This thermoelectric effects are caused by hydrothermal fluid flow in porous and fracture zones (Manzella, 2009). The movement of water in porous media creates measurable amount of SP anomaly and this is a well-known fact (Telford et al., 1990; Reynolds, 1997). In the field, a pair of non-polarizing Cu–CuSO4 electrodes and a digital voltmeter with a high input-impedance were used to measure the potential differences. The SP data collected with a gradient array and the data plotted in the middle of the electrode pairs. In addition, the stacked SP anomaly was calculated and plotted as well. In this study the distance between two electrodes was 50 m (Fig. 3). Gravity surveys are used in geothermal exploration to map lateral density variations, because there is a density difference between continental crust and an intruding magmatic body. Gravity surveys help to reveal the fracture zones that are located in the geothermal field (Manzella, 2009). 3.1. Gravity and 3D Euler deconvolution interpretation Nevşehir Caldera Complex has been studied using gravity by MTA (Ekingen and Güvün, 1978). We reprocessed the MTA Bouguer gravity data acquired in 1978. We used Oasis Montage software package to
Fig. 5. a. 3D Euler deconvolution structure and depth analysis results are shown on the Bouguer gravity anomaly map, SI = 0 for a sill or dyke or step. The Nevşehir caldera marked with dark solid circle. b. 3D Euler deconvolution structure and depth analysis results are shown on the Bouguer gravity anomaly map, SI = 1 for a pipe. The Nevşehir caldera marked with dark solid circle. c. 3D Euler deconvolution structure and depth analysis results are shown on Bouguer gravity anomaly map, SI = 2 for a sphere. The Nevşehir Caldera marked with dark solid circle.
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Fig. 6. 1D VES inversion results of the Profile-D. The SND-1 well location is shown, The SP Profile-D is indicated with red arrows.
Fig. 7. 2D MT inversion result of the Profile N8. The SND-1 well location is shown, The SP Profile-D is indicated with red arrows.
employ 3D Euler deconvolution. Euler 3D deconvolution was employed to estimate the position of the structural lineaments in the study area from Bouguer data. The main objective of the Euler 3D deconvolution is to represent the geological structures that cause the gravity or magnetic anomalies using a 2D grid (Oruç and Selim, 2011). Euler 3D process is based on Euler's homogeneity equation, an equation that relates the field and its gradient components to the location of the source with the degree of homogeneity, which may be interpreted as a structural index (Thompson, 1982). This system uses a least square method to solve Euler's equation simultaneously for each grid position with sub-grid (window). The structure index (SI) is an exponential factor corresponding to the rate at which the field falls off with distance, for a source of a given geometry. The value of SI parameter depends on the type of the source body and the potential field (gravity or magnetic). For example in gravity field the SI value is equal to 0 for a sill or dyke or step, 1 for a pipe, and 2 for a sphere (Khalil et al., 2014).
Fig. 8. The SP anomaly curves obtained along the Profile-D. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
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Fig. 9. 1D VES inversion results of the Profile-B. The SP Profile-B is indicated with red arrows.
exists in the west and increases westwards (shown in yellow to red to purple color in Fig. 4). This anomaly has a −30 mgal amplitude and it has nearly 150 km2 surface area This Bouguer gravity variation indicates that the basement becomes shallower towards the west. The Nevşehir Caldera is possibly filled with low density materials such as volcanic ash and/or Neogene sediments. Hence the observed anomaly between Boğazköy and Güvercinlik reflects the actual caldera collapse area (Fig. 4). While employing the 3D Euler deconvolution of Bouguer gravity data, we used different structural index (SI) parameters (SI = 0, SI = 1, SI = 2). Fig. 5a,b,c shows different SI parameter results. The 3D Euler deconvolution results that used SI = 2 parameter are more compatible with our caldera depth model inferred from geoelectrical (MT, VES) and geological study results. Fig. 10. The SP anomaly curves obtained along the Profile-B. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
3.2. Interpretation of geoelectrical measurements (VES, MT, SP)
One of the findings from these studies is that most of the Nevşehir Caldera Complex is characterized by a remarkable, circular negative Bouguer gravity anomaly that has approximately 10 km diameter (Fig. 4). A local less pronounced negative Bouguer gravity anomaly
Geoelectrical measurements were acquired along 5 VES, 4 SP and 2 MT profiles (Figs. 2,3). The VES and SP profiles labeled as A and E are N–S profiles and the rest are E–W profiles. In MT and VES sections, different structures are classified based on their resistivity values as follows: very low resistivity (VLR) (~ 3–10 Ω-m), low resistivity (LR) (~10–40 Ω-m), medium resistivity (MR) (~ 40–150 Ω-m), high resistivity (HR) (~ 150–800 Ω-m) and very high resistivity (VHR) (~800–5000 Ω-m).
Fig. 11. 1D VES inversion results of the Profile-C.
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Fig. 12. 2D MT inversion result of the Profile N5.
Fig. 13. 1D VES inversion results of the Profile-A. The SP Profile-A is indicated with red arrows.
The structures labeled as HR and MR, that are located between approximately 1500 and 1000 m elevations, are surface crustal units based on VES and MT measurements.
Fig. 14. The SP anomaly curves obtained along the Profile-A. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
3.3. D and N8 profiles (E–W) The VES measurements obtained along the D section indicate the presence of a VLR structure between approximately 700 and − 1500 m elevations (Fig. 6). There are also LR and MR structures between approximately − 700 and − 1500 m elevations and beneath D7 and D11 stations (Fig. 6). Therefore the edge on the MR structure appears to be a hydrothermal convection zone. That is why the location of the SND-1 well was chosen to reach this zone (DS station). The same VLR structure described above is also observed in the N8-MT profile between approximately 1000 and −1500 m elevations, beneath the N806 and N812 stations (Fig. 7). The MR structure is surrounded in the west and east with VHR and HR structures, respectively. The SP data that acquired alone the D profile is given in Fig. 8. There are positive and negative SP anomaly variations throughout the profile. It is well-known that facts with regard to positive SP anomalies indicate presence of ascending hot fluids and negative SP anomalies indicate descending in relatively cold fluids (Türker et al., 1991; Marcos et al., 2014). These observed SP anomaly variations and also the MT and VES measurements along the D profile indicate hydrothermal activity within an intensely deformed and fractured zone.
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Fig. 15. 1D VES inversion results of the Profile-E. The SP Profile-E is indicated with red arrows.
In Fig. 8 stacked SP trend becomes negative towards the east of the DS station which may be interpreted as ascending and descending hydrothermal fluid boundary zone.
3.4. B profile (E–W)
Fig. 16. The SP anomaly curves obtained along the Profile-E. The VES station locations indicated with red arrows. Location of faults are also shown with black arrows.
The B profile is for checking southward continuation of the VLR conductive structure seen in the D profile (Fig. 9). The VLR structure continues southward, however it becomes deeper along the B profile. SP measurements were also acquired between B11 and B29 VES stations to check areas considered to be fractured (Fig. 10). Although there are small SP anomaly variations until the B19 station, afterwards large amplitude changes are present towards the east. The stacked SP trend also becomes positive to the east of station B19. The location of the observed changes coincides with the northwest southeast trending G1 fault (Figs. 2, 3 and 9).
Fig. 17. The plot of apparent resistivity versus AB/2 distance measured at the drilling location (left panel). 1D inversion result of the VES data (right panel). The location of the drilling was proposed based on the VES curve.
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3.5. C and N5 profiles (E–W) The VLR structure continues towards the south, however it becomes shallower and confined between approximately 700 and − 300 m elevations (Fig. 11). Under this VLR structure a distinct MR structure is present (C5–C11, Fig. 11). This MR structure is better constrained in the N5-MT profile and it continues downwards until approximately − 6000 m elevation beneath N507–N514T stations (Fig. 12).
3.6. A and E profiles (N–S) The N–S trending A and E profiles pass through the middle of the Nevşehir Caldera (Figs. 2,3,13, and 15). The VLR structure identified along the E–W trending D, B, C, N8 and N5 profiles is also clearly observed along the A and E profiles, however it passes into LR and MR structures towards the south. These profiles cut through the G1, G2, G3, and G4 faults (Figs. 2,3). Observed lateral discontinuities correspond to these faults (Fig. 13). The positive and negative behavior of observed SP anomalies can be associated with fluid circulation in the deformed and fractured zone along the N–S trending A and E profiles (Figs. 14 and 16). These probable faults are also observed in the VES results. Fig. 18. The plot of apparent resistivity versus period measured at the drilling location (upper panel). The plot of apparent phase versus period measured at the drilling location (lower panel). The static shift corrected results are shown with solid lines.
Fig. 19. The lithostratigraphic and temperature logs of the SND-1 well. Geological period (right panel). Soil symbols and descriptions with depth and leak info (middle panel). Graph of log (right panel).
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4. Drilling results A suggested well location based on the inferred geophysical model is chosen and it is planned to reach a depth of 3 km. Fig. 17 shows apparent resistivity variations of VES data and 1D inversion result at the drilling location. Similarly, apparent resistivity and phase behavior of MT result at the drilling location are displayed in Fig. 18. The vertical geological section is shown in Fig. 19. At the bottom (2900 m depth) of the well-log of Thermic-1 an instantaneous temperature of 185 °C was measured and at 2485 m depth of Thermic-2 137 °C temperature was measured. At 2485 m depth, when both Thermic-1 and Thermic-2 are compared it is observed that an increase in temperature inside the well was correlated with waiting time. This increase in temperature begins with dominance of the units like sandstone, conglomerate around 1300 m depth below volcano sedimentary units. Especially in Thermic-2 between 1800 and 2500 m depth, temperature increase caused by a fracture system or conglomerate. It is difficult to distinguish the source of hot fluid horizons except between 2450 and 2550 m depths. Probably a granite-altered andesite contact is the hot fluid conduit at 2450–2550 m depths. We can also infer the presence of a fracture zone based on water leak values at these levels. 5. Conclusion A potential geothermal system was delineated around the Nevşehir Caldera Complex, Göre, Nevşehir region based on geophysical investigations. Evaluation of previous geological and geophysical studies combined with Gravity, SP, VES, and MT measurements provided important data and also processing the Bouguer data using Euler 3D analysis was useful to reveal the detailed structure of the region and generation of a geothermal model. Based on the synthesis of all our results, a drill location was determined to validate our geothermal model. This exploration well was successfully reached to the hot fluid at a depth of 2.9 km with a temperature of 185 °C. For the first time this finding conclusively validated the geothermal potential of the Göre, Nevşehir region. Acknowledgments We would like to thank the General Directorate of Mineral Research and Exploration (12.02.00/929-888) for funding the field studies and geophysical surveys and permitting the publication of the results. We thank anonymous reviewers and the editor Prof. Jürgen W. Neuberg for valuable contributions. References Ateş, A., Bilim, F., Büyüksaraç, A., 2005. Curie point depth investigation of Central Anatolia, Turkey. Pure Appl. Geophys. 162, 357–371. Berdichevsky, M.N., Dmitriev, V.I., Pozdnjakova, E.E., 1998. On two-dimensional interpretation of magnetotelluric soundings. Geophys. J. Int. 133 (3), 585–606. Blakely, R.J., 1996. Potential Theory in Gravity and Magnetic Applications. Cambridge University Press (ISBN-13:978-0521575478, 437 pp.).
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