Geophysical Prospecting, 2006, 54, 187–197

Deep resistivity structure of the Dikili- region, west Anatolia, revealed by two- dimensional inversion of vertical electrical sounding data Gulc¨ ¸in Oz¨ urlan,¨ 1∗ M. Emin Candansayar2 and M. Hudavendigar¨ S¸ahin3 1Istanbul Technical University, Faculty of Mines, Geophysical Engineering Department, TR-34469 Maslak, , , 2Ankara University, Faculty of Engineering, Department of Geophysics, 06100 Besevler, Ankara, Turkey, 3General Directorate of Mineral Research and Exploration (MTA), Department of Geophysics, Ankara, Turkey

Received April 2004, revision accepted September 2005

ABSTRACT Within the framework of the National Marine Geological and Geophysical Program, we re-examined deep vertical electrical sounding (VES) data. The data, measured in 1968 by the General Directorate of Mineral Research and Exploration (MTA) of Turkey with the aim of exploring the deep resistivity structure of the Dikili–Bergama region, focus on the geothermal potential. The geoelectrical resistivity survey was conducted using a Schlumberger array with a maximum electrode half-spacing of 4.5 km. The two-dimensional (2D) inversion was utilized to interpret the VES data that were collected along 15- to 30-km profiles. The 2D resistivity–depth cross-sections obtained show very low resistivity values near the Dikili and Kaynarca hot springs. The 2D inversion results also indicate the presence of fault zones striking nearly N–S and E–W, and fault-bounded graben-horst structures that show promising potential for geothermal field resources. The 2D gravity model, which is in good agreement with the density variation of the region, supports the resistivity structure revealed by 2D inversion. The lithology information obtained from the borehole near Kaynarca also confirms the results of the resistivity interpretation and the density model.

tential. The region is located in an area of western Anatolia INTRODUCTION in the that is significantly affected by exten- In this study, within the interdisciplinary National Marine sional tectonics. The area has complex magmatic and vol- Geological and Geophysical Program, the earlier large-scale canic geological structures and numerous graben-horst struc- VES data are re-interpreted with a state-of-the-art interpreta- tures (Ercan et al. 1984; Seyitoglu˘ and Scott 1992; Jeckelmann tion technique, namely the 2D smoothed damped least-squares 1996; Yılmaz et al. 2000). There are also a number of inversion algorithm. The computer program, which has been hot springs and geothermal emissions with surface temper- used successfully in different studies (El-Qady, Ushijima and atures of 40–150 ◦C. In the investigation area, i.e. Dikili– Ahmed 2000; Candansayar and Bas¸okur 2001), was devel- Bergama, the earlier geophysical studies were carried out oped by Uchida (1991). Some applications of the 2D inversion for various purposes. These studies are not comparable with have provided reliable earth models for the subsurface (see, each other since they are of different scales and from dif- e.g. Schulz and Tezkan 1988; Sasaki 1989; Loke and Barker ferent places. Most of the previous works were conducted 1996). in order to investigate the geothermal potential at several This project investigates the deep resistivity structure of small locations in the region where there are approximately the Dikili–Bergama region, focusing on the geothermal po- 80 thermal water and steam outlets. For example, a joint Japanese–Turkish pilot project was carried out to estimate

∗E-mail: [email protected] the development possibility of the geothermal resources of the

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Kaynarca thermal spring (JICA 1987). This thermal spring, rich in geothermal sources. The Bergama graben, which is which is the hottest in the region, has a temperature of 92 ◦C. one of the largest E–W grabens in the western Anatolia re- It emerged after an earthquake in 1939 (Eyidogan˘ et al. 1991). gion, is about 60 km long and 5 km wide, and the Bakırc¸ay The Kaynarca thermal spring has a reservoir temperature of River runs through it (Yılmaz et al. 2000). These authors ◦ 159–179 C, based on SiO2 geothermometers (Jeckelmann found that the topography is asymmetric, being steeper along 1996). the northern side of the valley where Mount Kozak rises It is a common expectation that the infill sediments of a steeply to over 800 m from the graben floor, which is 50– graben or fractured rock show a high resistivity contrast with 80 m above sea-level. According to Yılmaz et al. (2000), both compact basement crystallines and structures, such as dikes, sides of the Bergama graben show normal faulting. This fault lava domes, etc. This high resistivity contrast is considered to pattern is in line with an extensional regime in the N–S direc- be a good target for electrical methods. The objective of this tion. The graben floor sediments are associated with existing study is to map the extent of geothermal activity, to detect active processes controlled by the faults on the boundaries possible major fault zones leading to the emergence of hot of the graben. Furthermore, Yılmaz et al. (2000) concluded water in the form of springs, and to obtain the thickness of that the Bergama infill, which is not exposed anywhere but infill sediments in the grabens. For these purposes, the VES is calculated from gravity data (The Turkish Petroleum Co., data measured by Ozc¨ ¸ic¸ek (1968) are re-interpreted using 2D unpublished report), is thicker than 500 m Jeckelmann (1996) inversion and are jointly interpreted with the results of a 2D suggested that the geology and morphology of the Kozak in- model of gravity data. trusion is of prime importance for the development of most of the regional thermal water. This region represents the major infiltration area and provides the Na+SO 2(Ca2)(HCO3)-type GEOLOGICAL SETTING 4 waters which are the primary chemical signature of the sur- Figure 1 shows the geological map of the investigation area rounding thermal springs. (after Jeckelmann 1996). A large part of the exploration area is covered with volcanic material, known as ‘Yuntdag˘ vol- canics’ (Akyurek¨ and Soysal 1983), and some residual rock GEOPHYSICAL MEASUREMENTS formations cover a small part of the area in the east. Ac- cording to the studies carried out by Ong¨ ur¨ (1972), the ig- In this study, 11 profiles, consisting of 164 VES soundings, neous rocks show dome structures and consist of lava flows with profile lengths varying between 2 and 30 km are inter- that contain basalt, andesite, trachyandesite, trachyte and preted. The profile and station intervals are 1 km and the rhyolite compositions and tuff-agglomerate layers. In addi- maximum electrode spacing is 4–9 km. Figure 2 shows the tion, magmatic activities of all kinds and ages can be ob- profiles, the VES sounding measurement points, the well lo- served, as in the NE–SW extensional horst-graben systems, cations and the known thermal springs within the investi- which characterize the structure of the region. The region is gation area. The profiles labelled T, U and V extend about

Figure 1 Geological map of the Dikili– Bergama region (after Jeckelmann 1996). The investigation area is outlined.

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 Deep resistivity structure of Dikili-Bergama region, Turkey 189

Figure 2 Location map of the VES sta- tions around Dikili and Kaynarca ther- mal springs. The superimposed faults and grabens (solid lines) of the region are known from previous work (Yılmaz et al. 2000).

30 km in a NW–SE direction between Dikili and Zeytindag.˘ is the trace of the Zeytindag˘ horst, characterized by high re- The SW–NE-trending, 17-km long C, D and E profiles lie be- sistivity values. The basement of the Dedetepe Mountain at tween Katıralanı and Samanlı. The VES measurements were the right-hand end of profiles C, D and E, is clearly visi- taken at 18 points along each of these profiles. The 9-km- ble with high resistivity values. Furthermore, the maps show long A and B profiles extend between C¸ andarlı and Kırıklar the existence of a low-resistivity zone (= 10 m) between in a SW–NE direction and consist of 10 stations each. The the Dikili and the Kaynarca thermal springs. Small high- 6-km-long P and R profiles and the 2-km-long S profile ex- resistivity anomalies observed at various locations correspond tend around the Res¸adiye hot springs in a NW–SE direction. to lava domes. However, the surface maps, drawn to obtain The data from the VES soundings were measured with the elec- information about deeper structures (for AB/2 = 1500 and trodes arranged approximately in the same direction of each 2000 m), show clearly that the horsts characterized by high corresponding profile direction using a Schlumberger con- resistivity values extend to deeper depths. Another significant figuration. observation is that the lateral resistivity variations become more prominent as the electrode spacings increase in size. The Surface resistivity maps most significant discovery in the surface maps for AB/2 spac- The apparent-resistivity curves measured at different stations ings of 1500 and 2000 m is that the high-resistivity structure in in this wide area with its complex geology show great vari- the Zeytindag˘ horst shows a heterogeneous character and its ations. Ozc¨ ¸ic¸ek (1968) collected the VES data and produced roots appear to be split into several parts at greater depths. a high-resistivity basement map rather than interpreting the data as sections showing resistivity structure. The apparent- 2D RESISTIVITY INVERSION resistivity surface maps are shown in Fig. 3. These maps were obtained for half-electrode spacings (AB/2) of 150 m, 1000 m, The 2D inversion algorithm, developed by Uchida (1991), 1500 m and 2000 m. The most striking feature in these maps was applied to two sets of sounding-profiling data

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 190 Gulc¨ ¸in Oz¨ urlan,¨ M. Emin Candansayar and M.H. S¸ahin

Figure 3 Apparent resistivity maps for spac- ings (AB/2) of 150 m, 1000 m, 1500 m and 2000 m from the top to the bottom panel, respectively.

collected along profiles E and V, which are perpendicular to The program utilizes two kinds of mesh: a model mesh and a each other. The algorithm uses the finite-element forward- finite-element calculation mesh. The model mesh describes the modelling method and the smoothing non-linear least-squares 2D resistivity section beneath a survey line by dividing it into scheme for inversion. In this algorithm, the damping para- many rectangular blocks. The resistivity of each block repre- meter (or regularization parameter) is calculated at each itera- sents an unknown in the inversion procedure. The calculation tion, using the Akaike Bayesian Information Criterion (ABIC). mesh is formed for each sounding station by substituting the

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 Deep resistivity structure of Dikili-Bergama region, Turkey 191

resistivity value of each element by that of the correspond- The known thermal springs, geological structures and well ing block in the model mesh. In order to handle almost three locations are also shown above the model in Fig. 4(a). decades of electrode spacings, the element size is increased The resistivity model for profile E indicates anomalous re- gradually with increasing distance from the centre of the calcu- gions marked by low resistivity values, typically less than 5 lation mesh. Following the reciprocity theorem, current elec- m, between stations E2 and E 18 along the profile. The trodes are placed near the centre of the calculation mesh, and most significant feature is an extremely low resistivity zone potential electrodes are placed at the outer positions. In this (<3 m) located under Dikili and Kaynarca. These extremely way, the inversion algorithm can handle the field data ob- low resistivities are thought to be closely related to the geother- tained from a survey line consisting of a number of sounding mal structure assumed to be characterized by a vertical fault stations. Hence this feature makes this code very convenient along which the geothermal reservoir may have formed. The for interpretation of the data. Only the last 15 VES data col- hot-water reservoir or hydrothermally altered zone under Dik- lected along profile E were used for the inversion. The model ili and Kaynarca may be related to this low-resistivity zone, mesh consisted of 30 × 30 blocks for the 15-km-long profile between depths of approximately 500 m and 600 m. Be- E and 60 × 30 blocks for the 30-km-long profile V. Between tween the very low resistivity zone and the electrical basement, each station, the model mesh is divided into two blocks lat- which consists of high-resistivity Palaeozoic rocks, there is a erally so that the vertical thickness of each block is 500 m. zone of intermediate resistivity between depths of 600 m and However, the code uses the calculation mesh for the forward 900 m. Another important feature is the highly conductive solution and each block is subdivided into several elements zone detected between stations E10 and E18 along profile E. for robustness of the finite-element solution. More blocks can This location corresponds to the Dikili depression shown on be incorporated between each corresponding station in the the location map that is superimposed on the tectonic structure model mesh. However, in general the data is not adequate for (Yılmaz et al. 2000), shown in Fig. 2. The Kaynarca thermal resolving the resistivity of these blocks. spring is located close to station E16. Additionally, a high- Before starting the inversion of the field data, the quality of resistivity zone close to the surface to the west of profile E the model mesh and of the calculation mesh is tested using for- (between stations E24 and E26) corresponds to the Yuntdag˘ ward modelling with matching field layouts. The calculated volcanics III (the Dedetepe Mountain). Zones of this type are apparent resistivities were smaller than in the homogeneous important in the development of hot springs since such verti- model resistivities by about 2–4%. Apparent resistivities of the cal transition borders in volcanic areas may act as a conduit perpendicular pseudosections were inverted for the initial ho- in driving the hot water to the surface. The resistivity model mogeneous model. The initial model resistivity was set equal to for profile E is in good agreement with borehole information the average of the measured apparent resistivities of the corre- from JICA (1987). In particular, it is notable that the resistiv- sponding pseudosection data. This value is equal to 30 m for ity values near Kaynarca (around DG-1) are lower than those the data collected along profile E and 27 m for the data col- outside the geothermal activity (DG-2 and DG-3). The stratig- lected along profile V. The estimated model was obtained after raphy of the three boreholes in descending order is as follows: six iterations with an 8.75% rms error for profile E (Fig. 4a). DG-1 (683 m), alluvium, Yuntdag˘ volcanics I and III; DG-2 We inverted the same data set with two other different start- (201.5 m), composed of talus deposits of 5 m thickness and ing models with resistivities set at 10 times larger (300 m) Yuntdag˘ volcanics I; DG-3 (202.1 m), 1 m talus deposits and and 10 times smaller (3 m) than the average apparent resis- Yuntdag˘ volcanics III. The detail from another borehole K- tivity. These inversion results (not shown here) were attained 1, which is the deepest one (1500 m) in the survey area, is after six iterations with an 8.69% rms error for the 300-m given later for correlation and interpretation of resistivity and homogeneous initial model and an 8.79% rms error for the 3- gravity models. m homogeneous initial model. All the estimated models were Figures 4(b, c) shows that the measured and the calculated obtained after the same number of iterations with nearly the apparent resistivity pseudosections for the estimated model for same rms error. There were no significant differences among profile E are in good agreement. The conformity of these two the inversion results. As a result, models obtained from 2D data sets is also an important criterion of the reliability of the inversion using different starting models mainly show that the interpretation of the models. inversion algorithm is stable. Additionally, this comparison Similarly to the previous sounding-profiling data, we in- reveals that the measured data are sufficient to find reliable verted the apparent-resistivity data collected along profile V resistivity models. for three different initial models. These inversions resulted in

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 192 Gulc¨ ¸in Oz¨ urlan,¨ M. Emin Candansayar and M.H. S¸ahin

Figure 4 (a) Estimated model obtained from the 2D inversion of apparent-resistivity data acquired along profile E. (b) The measured apparent-resistivity pseudosection collected along profile E. (c) The calculated apparent- resistivity pseudosection obtained from the 2D forward-modelling solution for the model shown in the top panel of this figure. Thermal springs and well locations (DG-1, DG-2, DG-3 and K-1) are shown on the top panel model.

rms errors of 8.70%, 9.06% and 9.25% after 12 iterations for file, which corresponds to the Kaynarca thermal spring at the 270, 27 and 2.7 m homogeneous initial models, respectively. intersection of the Bergama graben and the Dikili depression. The estimated models are also very similar to each other. Cal- Figure 6 shows examples of typical VES data and theoreti- culated and measured apparent-resistivity values are very close cal apparent-resistivity curves obtained for the models shown to each other, which is additional evidence for the stability of in Fig. 4(a) and 5. It can be seen clearly that the theoretical the inversion algorithm. Only the estimated model obtained curves obtained from the models are in good agreement with for the 27 m homogeneous initial model is shown in Fig. 5. the observed VES data at all locations. Although the fit of The most significant feature of this section is that the borders measured and theoretical data for all curves is good enough, of the high-resistivity basement are identified very clearly. In we conducted a sensitivity analysis of the models which could particular,between stations V46 and V54, it can be seen clearly greatly improve the interpretation of the models. that the high-resistivity layers (ρ>120 m) correspond to the For such a sensitivity analysis, the model resolution matrix basement. Another important feature is the highly conductive can be used (e.g. Friedel 2003; Stummer, Maurer and Green zone, detected between stations V18 and V24 along the pro- 2004). Although this matrix is defined for linear problems,

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 Deep resistivity structure of Dikili-Bergama region, Turkey 193

Figure 5 Estimated models obtained from the 2D inversion of apparent- resistivity data acquired along profile V.

1000 of distortion flags, to investigate the resolution, stability and E16 efficiency of the estimated model obtained from the 2D resis- V18 V46 tivity inversion. The radius of resolution is calculated from the E26 diagonal elements of the model resolution matrix. The distor- tion flag shows whether the maximum of the spread function 100 is off-diagonal, indicating a possible geometrical distortion in

the image. We used these two quantities to estimate the res-

m) Ω

( olution quality of our models. Figures 7(a.b) summarizes the

. s

e information contained in the diagonal elements of the model

R

. p

p resolution matrix for the estimated models of profiles E and A 10 V, respectively. The colour scale corresponds to the value of the diagonal element of the model resolution matrix, scaled by the radius of resolution. The high values show that each of the corresponding blocks is well resolved. The distortion flag for all of the model parameters is zero, which means that all 1 10 100 1000 10000 diagonal elements of the model resolution matrix take on the AB/2 (m) maximum of their related non-diagonal rows. Both measures (radius of resolution and distortion flag) indicate the high Figure 6 Example of typical VES curves in the region (for locations of VES stations, see Figure 2). Symbols indicate the measured data, quality of both inversion results. Approximately the first 600 and solid lines show the calculated data. The calculated data are ob- m in both models yielded high resolution and showed little tained from the 2D forward-modelling solution for the inverted mod- or no distortion, as can be expected. Furthermore, the model els shown in Figure 4(a) and 5. quality is also acceptable, according to the magnitude of the diagonal elements of the model resolution matrix for deeper within the linearization limits it is also applicable to non-linear blocks. To summarize the results of our sensitivity analysis, problems. We can multiply the model resolution matrix by a the model parameters have been shown to be well resolved. particular test model to see how that model would be resolved by the inversion solution. However, in 2D resistivity inversion, GRAVITY MODELLING the number of model parameters is very large (for the mod- els in this paper, it is = 750), defeating the practicability of To combine the results of 2D resistivity inversion, we per- this strategy. The information content of the model resolu- formed 2D gravity modelling using the WinGLinkTM. com- tion matrix needs to be condensed to more a practical level. mercial software. The data was collected by the MTA in an In order to achieve this, Friedel (2003) introduced two space- 8km× 8 km area using 300 m intervals. The SW–NE di- dependent scalar quantities: a radius of resolution and a set rectional profile approximately overlapped the 3 km–12 km

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 194 Gulc¨ ¸in Oz¨ urlan,¨ M. Emin Candansayar and M.H. S¸ahin

Figure 7 Diagonal elements of the model resolution matrix, scaled to the radius of res- olution: (a) for profile E (b) for profile V.

of the resistivity profile E. The 2D gravity modelling was and wet conditions, respectively. Measurements on another carried out in accordance with this profile. The topography important geological feature in the survey area, the Yuntdag˘ of the region was also taken into account in the modelling. volcanics III (Tyu III), gave a mean density value between The measured and calculated gravity data are shown in Fig. 8 2.41 and 2.47 g/cm3, the values for dry and wet conditions, (top panel) and the 2D gravity density model in Fig. 8 (bot- respectively. In the absence of any information about the tom panel). The theoretical curve obtained from the model is density at these great depths, the highest density value of in good agreement with the observed gravity data. The 2D 2.90 g/cm3 is assumed for the basement rocks in the gravity gravity model in the bottom panel, constructed from different model. prism-shaped blocks of different densities, has benefited from CORRELATION WITH BOREHOLE DATA the results of 2D resistivity inversion (see Fig. 4a), borehole information and rock-property measurements (JICA 1987). The stratigraphic structure of the study area is very complex Densities of 1.74 g/cm3 (in dry conditions) and 2.10 g/cm3 (in and is a product of volcanic and magmatic activities of var- wet conditions) were measured in borehole DG-1 at a depth ious ages. The lithology obtained from borehole K-1 by the of 300 m. This result agrees with the depth of the very low MTA project in 1998 near Kaynarca provides a good repre- resistivity zone under Dikili and Kaynarca that we obtained sentation of the stratigraphic sequence of the region. How- from the inversion (see Figs 4a and 5). The mean density value ever, rock-property measurements have not been carried out of the new rock-property measurements for Yuntdag˘ volcanics for K-1; therefore we have density information only down to I (Tyu I) is between 2.59 and 2.63 g/cm3, the values for dry the same level as for DG-1. The lithology of borehole K-1 is

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 Deep resistivity structure of Dikili-Bergama region, Turkey 195

Figure 8 Results of 2D gravity modelling. Measured data (circles) and the calculated gravity data (solid line) are shown in the top panel. The 2D gravity model as a function of depth for the Bouguer gravity data collected along the profile that was approximately in the same direction as resistivity profile E is shown in the bottom panel. shown in Fig. 9. In well K-1, the top layer is an alluvial layer fluid in reservoir rocks, where intermediate resistivity results (approximately 74 m thick), composed of sand, gravel and of VES inversion are observed (see Figs 4a and 5). The inte- fragments of biotite-hornblende and andesite. Under the allu- grated assessment report by JICA (1987) suggests that there vium, Yuntdag˘ volcanics III (TyuIII) can be subdivided into six are presumably two different reservoirs: a shallow reservoir lavas based on their petrographic and stratigraphic features. and a deep reservoir. The former is formed mainly in allu- For example, biotite-hornblende-andesite, known as ‘Dikili vial deposits, and the latter is formed in the fracture zones of Bi-Ho-Andesite’, is found in most of the survey area and also Tertiary volcanic rocks. In addition, hydrothermal veins that in the area surrounding Kaynarca. Several forms of hydrother- coincide with fractures are seen. Furthermore, hydrothermal mal alteration, which led to the chemical and mineralogical al- minerals (quartz, calcite and pyrite) are reported in the frac- teration of rock by geothermal fluids, are observed. Between ture zones. A limestone layer (1262–1346 m) is overlaid on the depths of 267 m and 520 m in borehole K-1, an acidic al- volcanic material containing the basement. At the bottom of teration zone characterized by kaolin is observed. This zone the geological column of borehole K-1 is the basement, which seems to represent the caprock in the geothermal system. Be- consists of andesite. Important information obtained from this tween depths of 590 m and 720 m, strong alterations are de- borehole is that the temperature at the bottom is 125 ◦C. JICA tected. This type of alteration is a good indicator of geothermal (1987) suggested much higher temperatures (220–230 ◦C), in- structures, formed around the upward conduit of geothermal dicating a potentially significant geothermal energy source for

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 196 Gulc¨ ¸in Oz¨ urlan,¨ M. Emin Candansayar and M.H. S¸ahin

consist of various kinds of andesite varying from pyroxene andesite to dacite. At depths between 150 m and 200 m, hy- drothermal veins are observed in borehole DG-1 in Kaynarca, coinciding with the fractures inferred from the low-resistivity zone obtained from the inversion. However, in the rocks of DG-2 and DG-3, hydrothermal veins are sparse. From the resistivity model of profile E, we cannot distinguish the ef- fect of the first reservoir, formed in alluvial deposits, from the altered zone at the caprock of the second reservoir, marked by very low resistivity. This interpretation is in good agree- ment with the borehole information from borehole K-1, where an alteration zone between depths of 267 m and 520 m is observed. The resistivity inversion results have been incorporated with Figure 9 The lithology of the region, deduced from well K-1 (after the 2D model of the gravity data in order to achieve an over- Eris¸en et al. 1996). all understanding of the region. Furthermore, the lithological data from the borehole has been taken into account in or- the reservoir at greater depth, which is inferred from the iso- der to define the geothermal structure relating to the 2D re- tope thermometer results. sults. Based on the results, very low conductivity zones, which correspond to the geothermal activity of the Dikili and the Kaynarca hot springs, are identified. Also, major fault zones DISCUSSION AND CONCLUSIONS bounding the two main graben structures, i.e. the Dikili de- In this study, deep resistivity structure is modelled to delineate pression in a NW–SE direction and the Bergama graben in an the structural features of the Dikili–Bergama region, using 2D E–W direction, have been detected. This confirmed the earlier inversion of VES data. The observed apparent-resistivity sur- geological studies that identified these fault zones. Further- face maps are drawn to determine the vertical and lateral re- more, the topography of the high-resistivity basement, show- sistivity variations in the area. The models obtained from 2D ing the graben-horst structures, has been obtained. The results inversion of the data using different starting models mainly revealed that the depth of the high-resistivity basement is lo- showed that the inversion algorithm is stable. After that, com- cated approximately at depths between 800 and 900 m. The parison of the theoretical data resulted in reasonably good basement rises towards the surface in the area of the Dedetepe agreement with the observed data. In addition, the sensitivity Mountain and the Zeytindag˘ high zones, creating the faults analysis of the models calculating the model resolution matrix and fissured and fractured systems. In a region abundant in revealed that important model parameters are well resolved. volcanic activity, such elevations make heat transfer possible As a result, we demonstrated the stability of the 2D inversion and convey the heat towards the surface. This is also proved algorithm that can give reliable models. by the existence of the hot water that surfaces in the region of The 2D resistivity models for both profiles contrast the very Kaynarca, with its low-resistivity and graben characteristics low resistivities corresponding to the geothermal activity with in the 2D resistivity-depth section, and its density variations the high resistivity of the basement. However, it is difficult in the gravity model. Finally, it is important to note that re- to distinguish the structures of a geothermal system in detail interpretation of earlier data with new techniques is able to from the resistivity data without including borehole informa- provide valuable additional information. tion, because the stratigraphic structure of the study area is very complex and it is a product of volcanic and magmatic ACKNOWLEDGEMENTS activities of various ages. The very low resistivity zone be- low Dikili and Kaynarca, extending to depths of 500 m and Associate Editor Prof Pous, Dr Brasse and an anonymous 600 m (see Fig. 4a) may reflect either the hot-water reser- reviewer are gratefully acknowledged for their constructive voir or a hydrothermally altered zone. The hot water from reviews. This project was supported by the Scientific and Kaynarca and Dikili reported by JICA (1987) derived from the Technical Research Council of Turkey (TUBITAK, Grant no: same deep reservoir in the Yuntdag˘ volcanics I. These rocks YDABC¸ AG 431-G). We thank The General Directorate of

C 2006 European Association of Geoscientists & Engineers, Geophysical Prospecting, 54, 187–197 Deep resistivity structure of Dikili-Bergama region, Turkey 197

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