Türkiye derin yeraltı sıcaklığı ve sıcaklık gradyanı dağılımının belirlenmesine yeni bir yaklaşım

A deep subsurface-temperature gradient distribution map of Turkey is generated by a never before applied approach, in which the Curie-Point-Depth (CPD) temperature data and the available deep well temperature measurements across the country are utilized. All the maps in the present work are generated using the Generic Mapping Tools (GMT) software. The CPD values are obtained by digitizing the contours of the Curie-Point Depth Map of Turkey, published by the MTA (General Directorate of Mineral Research and Exploration), Turkey. The contour digitization is performed with the 500-meter intervals between the depths from 6000 to 27500 meters, below the reference of MSL (mean sea level). In this study, the Curie isotherm of 560 ºC at CPDs, where crustal rocks lose permanent magnetization, is assumed beneath Turkey. The CPD data and its subsequent contours from the ground level (GL) are obtained by combining the digitized CPD data and the surface topography data of Turkey, the latter of which is above the reference of MSL. The exact coordinates for CPDs that correspond to those for the deep wells, for which the temperature data are available, are determined using the GMT software. Then, the depth interval and temperature difference for each pair of CPD and deep-well are used to calculate the subsurface temperature gradient at the coordinate of each pair. Subsequently, the temperature gradient data are mapped and the deep subsurface-temperature gradient distribution map of Turkey is generated. Since, the Curie isotherm of 580 ºC is also considered for the continents by some researchers in the literature, another deep subsurface gradient distribution map of Turkey is also generated based on the Curie isotherm of 580 ºC. In between the both maps a detectable or significant difference is not observed. If the deep subsurface temperatures are assumed to vary linearly with depth, the estimates of deep-subsurface-temperature gradients are found to be in between 2.28 to 11.10 C/100m, based on the Curie isotherm of 560 C, and are found to be in between 2.37 to 11.56 C/100m, based on the Curie isotherm of 580 C. The highest values of the deep-subsurface-temperature gradient are concentrated in the area between Mardin and Şırnak, in the Southeastern region of Turkey. On the other hand, the lowest values of the deep-subsurface-temperature gradient are located in the area of Şebinkarahisar, Reşadiye, Alurca and Gümüşhane, in the eastern Pontides Mountain regions. The highest value of the deep-subsurface-temperature gradient in the Western Anatolia is about 8.33 ºC/100 m in the area around Alaşehir, Baklacı, Akkeçili and Erenköy. When the variation of the deep subsurface-temperatures from the measurement point in wells to the corresponding CPDs is fit with exponential function, the resultant exponential temperature gradients yielded the estimate of subsurface temperatures that vary of from 20 to 265 ºC at 2-km depth and that vary of from 40 to 455 ºC at 5-km depth beneath Turkey. Based on the subsurface temperature gradients with exponential behavior, the extensional region of Western Anatolia has the highest subsurface temperature estimates. In Mardin-Şırnak region, however, the subsurface temperatures may be as low as has 58 ºC in average at 2-km depth and may go up to 193 ºC in average at 5-km depth. At an average depth of 7654 meters in this region, the subsurface temperature is expected to reach 560 ºC. Thus it may be said that the Mardin-Şırnak area has low temperatures to depths of about 4 km but experiences a high gradient change at depths beyond about 5 km. A very important feature of the deep subsurface temperature gradient and the subsurface temperature distribution maps is the indication of potential hot dry rock resources in the active extensional province of Western Anatolian, portions of Central Turkey, Southeastern region of Turkey, and the Central Pontic belts of the NAFZ (North Anatolian Fault Zone). These resources may also lie within reach (at about 5 km depth in Western Anatolia and Central Turkey) of the current drilling technology and could be exploited to augment the energy budget of the country if the enhanaced geothermal systems (EGS) technology is given serious attention.