Extended Abstract, 22nd EM Induction Workshop Weimar, Germany, August 24-30, 2014
Interpretation of Kemaliye Geothermal Field Using 2D&3D MT Inversion Erhan Erdogan1, Ahmet Başokur2, Erdin Bozkurt3, Max Moorkamp4 and Anna Avdeeva4 1 Enerjeo Kemaliye Enerji Üretim A.S., Kavacık Mahallesi, Energy Plaza No:2-8 Beykoz-Istanbul [
[email protected]]. 2Ankara University, Faculty of Engineering, Dept. of Geophysical Engineering, Tandogan 06100 Ankara, Turkey [
[email protected]] 3 Middle East Technical University, Department of Geological Engineering, Üniversiteler Mahallesi, Dumlupınar Bulvarı No: 1, 06800 Ankara, Turkey [
[email protected]] 4University of Leicester, Department of Geology, Leicester, UK SUMMARY MT/AMT and TDEM data are acquired at 300 measurement stations during a geothermal exploration programme in the Kemaliye concession located along the northern margin of the Gediz Graben (western Turkey). The 2-D and 3-D inversion methods were applied to delineate the topography of the basement and to interpret the fault geometries. The geological interpretation of the resistivity models help to classify the concession area into two geothermally potential zones for further investigation. Keywords: Magnetotellurics, 2-D modelling, 3-D modelling, Geothermal exploration, Inversion, Normal fault. INTRODUCTION Western Anatolian extensional province forms main geothermal field in Turkey where graben-bounding normal faults are considered as the main control on hot water circulation. Fault damage zones form the main targets during exploration programmes. The marble horizons within the metasedimentary sequence of the Menderes Massif are interpreted as the main aquifers. Fine-grained graben infill overlies the basement either tectonically or unconformably. Shales and sandstones in the graben fill shows low resistivity values and they provide sufficient resistivity contrast to delineate deeper parts of the basement. The location and geometry of the graben faults can be interpreted from the distribution and extension of low resistivity zones. The graben system in western Anatolia, in this regard, provides prosperous conditions for the efficient application of magnetotelluric method (MT).
EW- and/or WNWESE-trending structures) along both margins. Apart from margin-bounding normal faults, there are several second-order synthetic and antithetic faults (Figure 1) that develop on the hanging walls of the major faults. The high-angle normal faults cut and display the detachment fault in a rather step-like morphology along the southern margin. There are also approximately ∼NNE−SSW-trending sub-vertical faults with a clear strike-slip tendency (transfer faults) that are oriented almost perpendicular to graben-bounding normal faults. The interaction zones of high-angle normal faults and the intersection areas of normal and transfer faults are considered as the target locations.
GEOLOGY The Gediz Graben forms one the best and well-studied graben in western Anatolia. The graben evolved in two stages with a short-time break in-between. The early ca. Miocene stage is manifested by a core-complex formation in the footwall of a now north-dipping low-angle normal (detachment) fault along the southern margin of the graben and Miocene fluvial sedimentation within the supradetachment basin. The detachment forms the contact between metamorphics below and the fluvial sediments above (Figure 1). The later rift-mode is described as the formation of a modern graben bounded by high-angle normal faults (ca.
Figure 1. Geological cross-sections across the Gediz Graben, showing major break-away fault along its southern margin and a set of secondary synthetic and antithetic faults in its hanging wall (Çiftçi and Bozkurt, 2010). Note the presence of high-angle normal faults along the northern margin of the graben, where the investigation area is located. Page 1 of 4
Erdogan et al., Kemaliye Geothermal Field
DATA ACQUISITION The MT/AMT and TDEM data were acquired by Phoenix MTU-5 and V8 receivers, respectively. The most of 300 stations are located over a 250 meters spacing grid. The orientation of the 2D lines is decided in view of geological strike. Figure 2 illustrates the locations of measurement stations.
Figure 2. Site location map of the survey area. Red line shows the border of the license area. The black line shows the 2-D profile directions. The long periods were recorded using MTC50 coils for 18 hours. The high frequencies between 300-10000 Hz are measured by using MTC30 coils. TDEM data were acquired at each site for static shift correction. The size of the transmitter loop selected as 100x100 m2. The TDEM measurements were recorded at only 25 Hz frequency. The remote station was located at 5.5 km far away from the survey area.
2-D Inversion Groom-Bailey (1992) and phase tensor decomposition (Caldwell et al., 2004) were applied to all profile data independently, before the application of 2-D inversion. The results of these two decompositions produced almost the same rotation angles (40) that is consistent with the geological direction of the graben. Because the data set shows one-dimensional character for the high and intermediate frequencies (see Figure 4), the inversion results obtained from the rotated and non-rotated data are not significantly different for the interested depth range for geothermal purposes. Since the geological strike is about NESW direction, the 26 lines for the 2-D inversion are constructed as being perpendicular to the geological strike. These NESW profiles are inverted by using a commercial software (WinGlink). The distance between two stations divided to three blocks and minimum block thickness selected as 15 m for all inversions. TE, TM and tipper magnitude data were used for all profiles. Zxy is selected as TM and Zyx selected as TE modes. Figure 4 illustrates an example for a 2D inverted model.
Data Processing Remote process was applied to all time series data using SSMT2000 software (Phoenix Geophysics). The impedance and tipper data were estimated for a frequency range between 10K-0.001 Hz. The outliers were edited by using MTEditor software. A good quality data set is obtained even though agricultural activity is at high level during the survey. The cultural noise only affected the data quality in a few stations. Figure 3 shows a sample apparent resistivity and phase curve.
Figure 4. An example for the resistivity model obtained from the 2D inversion of MT data. Figure 3. Apparent resistivity and phase data at station KM1-0. Page 2 of 4
Erdogan et al., Kemaliye Geothermal Field
The 2D resistivity data was geologically tuned with log of an existing wellbore drilled in the concession area. The interpretation was mainly focused on the location and geometry of the faults (major and secondary) and on the horizon boundaries (location of, and depth to, the boundary between the basement metamorphic rocks below and Mio–Pliocene sediments [cover rocks] above). High resistivity (ca. >35 ohm-m) is assigned to metamorphic basement (gneisses, schists, granites and marbles), the intermediate resistivity (ca.
