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faults using integrated geophysicals methods over lesvos island geothermal field, Greece
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Geothermics, Vol. 20, No. 5/6, pp. 355-368, 1991. Printed in Great Britain.
LOCATION OF POSSIBLY PRODUCTIVE GEOTHERMAL FRACTURE ZONES/FAULTS USING INTEGRATED GEOPHYSICAL METHODS OVER LESVOS ISLAND GEOTHERMAL FIELD, GREECE C. T H A N A S S O U L A S and N. X A N T H O P O U L O S Institute of Geology and Mineral Exploration (IGME), Departmentof Geophysical Research, 70 Messoghion Avenue, Athens, 115-26, Greece (Received November 1989;acceptedfor publication September1991) Abstract--In 1988, Geotest Hellas carried out a geophysicalsurveyon Lesvosisland using gravity, vertical electrical soundings and self-potential methods in order to study the geothermal potential of the island. This paper is based on the primary results presented by Geotest Hellas, which were re-examined and presented in a different form to facilitate the selection of the areas of strong ~eothermal interest. In particular, application of the gravity, vertical electrical sounding (VES) and self-potential (SP) methods has providedinformationon the geophysicalstructure of the geothermaltarget areas of Lesvosisland. The gravitysurveyprovidedvaluable informationon the morphologyand tectonicsof the crystalline basement (and on the peridotitic body and pyroclastic sediments), for the study of the geothermal potential of the area. The electrical surveyprovided information on the highlyconductivezones (usually associated with geothermal fields), the electrical stratigraphy and the tectonics of the main geologicalunits of the target area. The SP survey mapped areas where electrical polarization is observed in the faulting system. An integrated interpretation of the results of all these surveys led to the selection of five target areas considered to be of high geothermal potential. The location of these target areas is shown in Fig. 14, ranked from 1 to 5 in decreasing order of priority.
INTRODUCTION As part of its geothermal research programme in Greece, the Public Power Corporation (PPC) launched a few years ago a long-term exploration project in the most promising geothermal areas of Greece. Milos, Soussaki, Poros-Methana, Nisyros, Lesvos, Mygdonian basin, Strymon Valley, Platystomo, and Aidipsos were selected as target areas on the basis of already known geothermal manifestations. Field operations over these areas and the results have mainly been presented in I G M E (Institute of Geology and Mineral Exploration) reports, I G M E being the main contractor on behalf of the PPC; the Italian Electricity Board presented an integrated interpretation of the data collected by I G M E ( E N E L , 1978). Interest in the geothermal potential of Lesvos island was first stimulated many years ago by the presence of surface manifestations, which were studied by many scientists and research groups (Papastamataki and Katsikatsos, 1969; Hecht, 1972; Di Paola, 1973; Dominco, 1973; Katsikatsos and Sfetsos, 1978; E N E L , 1978; Papastamataki and Leonis, 1982; Geotermica Italiana, 1983; Sfetsos, 1985). The promising results of these studies and the definition of three main areas of geothermal interest (Petra-Argenos, Kalloni-Stipsi, Polyhnitos) by Geotermica Italiana (1983), were to justify in part the start of the next phase of geothermal research, borehole drilling. The combined geophysical survey of the Lesvos project was supported financially by the E E C ( D G X I I , Contact No. EN3G-0087-GR). The field operations and initial evaluation were 355
C. Thanassoulas and N. Xanthopoulos
356
LES
N I
.his
c? ,5
Fig. 1. Location of Lesvos island, Greece. Geologicalsketch map and location of main geothermal fields, mb = metamorphic basement, p = peridotites, nv = Neogene volcanism, nls = Neogenic lacustrine sediments, qs = Quaternary sediments, Iv = late volcanics, mf = main faults. 1 = Polyhnitos, 2 = Kalloni-Stipsi, 3 = Petra-Argenos geothermal fields. performed by Geotest Hellas (1988), while the final assessment was made by I G M E (Department of Geophysical Research). The location of Lesvos island is shown in Fig. 1. GEOLOGICAL
A N D T E C T O N I C S E T T I N G OF L E S V O S I S L A N D
The main geological and tectonic features of Lesvos island are as follows. Lesvos island is characterized by two main stratigraphic-structural units: 1. An epimetamorphic series (crystalline sequence) forming the stratigraphic " b a s e m e n t " of the island, locally and tectonically associated with peridotitic bodies. The basement outcrops mainly in the southeastern part of the island, along with peridotite massif outcrops in the central-eastern part. 2. An overlying sequence of volcanic rocks of calc-alkaline affinity that mainly occur in the west. The geological sketch map in Fig. 1 shows a main fracture trend ( N W - S E ) and a less developed E - W trend. The most important trend is the N W - S E , which corresponds to normal faulting during the Neocene episode that continued during the Pleocene (Simeakis and Someritis, 1982). A series of tectonic features of the basement such as lows, highs and normal faults trending N N E - S S W and N W - S E , some of which are still active at present, have facilitated the ascent of hot fluids from depth. GEOPHYSICAL SURVEYS Previous geophysical surveys on Lesvos island have focussed on local geothermal and hydrogeological problems. The first geophysical project (VES) was carried out by O i k o n o m o u
Integrated Geophysical Methods Over Lesvos Geothermal Field • "".
LESVOSI S L A N d )
357
t
Ni
G'B
Fig. 2. Temperature (°C) at 250 m depth belowground surface (after Taktikos, 1985), and locationof gravityprofile G8. (1971) to study Polyhnitos geothermal field. Nathanael (1979) applied the geoelectrical method (VES) in the Kalloni area. Thanassoulas (1982) carried out 10 deep VES (AB/2 = 4 km) in a reconnaissance study of the deep regional geological and tectonic features of Lesvos island. The gravity map of the island was compiled by Hamburg University (Professor Makris) in cooperation with the IGME (1982). A small-scale geophysical study was performed by Xanthopoulos et al. (1988), for the evaluation of the geothermal potential of Argenos area. Taktikos (1985) compiled a temperature map of Lesvos island (-250 m below ground surface) that is presented in Fig. 2. The areas of main geothermal interest are characterized by temperature highs of up to 90°C (Polyhnitos area), and up to 98°C (Stipsi area). Deep fracturing of the basement and the tectonic regime in the Lesvos geothermal fields meant that a detailed geophysical study had to be carried out in the most promising geothermal areas (central part of the island). The topography and tectonics of the crystalline basement were investigated by interpreting the results of the gravity survey, which revealed the density contrast between the overlying pyroclastics and sediments and the basement. The main geoelectrical formations, the high conductivity zones, which are frequently associated with geothermal fields, and the fractured zones and faults of the crystalline basement, were investigated by the VES, using the Schlumberger method. Finally, the geothermally active fracture zones and faults were identified by the self potential method. PRESENTATION OF GEOPHYSICAL RESULTS
Gravity survey As a first approach to the study of the deep structure of the crystalline basement of Lesvos island, the existing Bouguer anomaly map was analyzed. Its interpretation was carried out by
358
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.
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Fig. 3. Gravity modelling and its interpretation along profile G8. Densities used: 2.1 g/cm3 for pyroclastics, 3.1 g/cm3 for peridotitic body and 2.67 g/cm 3 for the crystalline basement (after Geotest Hellas, 1988).
Geotest Hellas (1988) along gravity profiles, using semi three-dimensional inversion algorithms. Gravity modelling covered the area of main geothermal interest (central part of the island). The gravity models were interpreted in terms of: (a) surface pyroclastic formation (density = 2.1 g/cm3), (b) peridotitic body (density = 3.1 g/cm 3) and (c) crystalline basement (density = 2.67 g/cm3). A sample of gravity modelling and its interpretation along profile No. G8 (Fig. 2) is shown in Fig. 3. From the gravity models along profiles, the depth to the top of the crystalline basement was resampled and the corresponding map was compiled (Fig. 4). The outlook of the map and the shape of the depth contours reveal the highly tectonized character of the crystallin~ basement. Local highs are indicated by crosses at the appropriate uplift of the basement, while lows are marked with a minus sign. Generally, the crystalline basement is very deep in the western part of the island (up to -2750 m), whereas in the eastern part it is uplifted to -500 m. A graben striking in a WSWENE direction and crossing the entire island can also be observed. Large variations in depth within short horizontal distances indicate large step faults and intense tectonics. These features are favorable for the development of geothermal fields through deep fluid circulation. A typical example is the case of Polyhnitos geothermal field (Fig. 2), where a regional crystalline basement is observed at 750 m depth, with a 500 m fault step (graben depth of 1250 m) along the SW-NE coastline. The location of the crystalline basement faults was determined from the gravity models as they were presented by Geotest Hellas (1988), although in some cases (Gulf of Kalloni) the location of the faults as indicated in the map depends only on interpolated data over the Gulf. A closer inspection of these interpretational models shows the presence of abrupt changes in depth
Integrated Geophysical Methods Over Lesvos Geothermal Field
359
Fig. 4. Depth (in km) to the top of the crystallinebasement, calculatedfrom gravitydata. of the crystalline basement. This is attributed to fracturing and faulting systems that exist in these areas. The location of the determined fracture zones and faults is shown in Fig. 5. The SWNE Kalloni Gulf graben direction is revealed very well, along with a NW-SE direction. Mapping of the crystalline basement faults is considered very important, because these are the areas where possible geothermal targets will be sought.
Electrical survey A total of 50 geoelectrical soundings utilizing the Schlumberger method (AB/2 = 3 km), were used for the geoelectrical study of the target areas. The location of the VES and a sample of an interpretational model along profile T9 are shown in Figs 6 and 7 (Geotest Hellas, 1988). In Fig. 7, apart from the surface pyroclastic formations and the morphology (faulting) of the crystalline basement, some conductive (p < 10 ohm m) zones are observed. These zones are associated with intense faulting of the crystalline basement. The resistivity map for depth -1000 m was compiled, and is presented in Fig. 6. At this depth low resistivity zones (<10 ohm m) are observed over the areas of main geothermal interest. Lesvos island is mainly covered by pyroclastics and sediments, which prevent us from accurately mapping the existing faults and fracture zones, because of the weathering of the surface soft formations. Apart from that, it is rather difficult to estimate the magnitude of fault throws of the crystalline basement without any surface manifestations. The observed faults in each particular interpretational model of the geoelectrical cross-sections, presented by Geotest Hellas (1988), are shown in Fig. 8. Solid triangles indicate the dip direction of the faults as revealed by the geoelectrical models. A correlation made between the faults/fracture zones mapped by the geoelectrical method and those mapped by geological observations (Fig. 8) reveals the hidden intense tectonic character of the basement of Lesvos island which was so difficult to study from surface observations.
C. Thanassoulas and N. Xanthopoulos
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Integrated Geophysical Methods Over Lesvos Geothermal Field .5,
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In the northern part of the island, where there is a pyroclastic cover, only a few geological faults have been mapped. The electrically mapped faults not only coincide with them but provide a far clearer picture of the fracturing of the formations underlying the pyroclastics. All the zones characterized as low resistivity/high conductivity zones are presented in map form in Fig. 9. In this way we have achieved an integrated picture of the spatial distribution of
LESVOS ISLAND
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Fig. 8. Location of the fracture zones and faults, determined from the interpretational models presented by Geotest Hellas (1988).
362
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LESVOSISLAND , - J ' , ~ - , " h
0 6kin
Fig. 9. Location of the low resistivity/high conductivity zones. Shaded areas indicate shallow conductive zones (up to 300 m below ground surface), while "open" areas indicate deep seated zones. The numbers indicate the depth (in km) of the conductive zone, in places, below ground surface.
the conductive zones. The high conductivity zones can be divided into two groups. The first one represents the shallow conductive zones, and is represented in the map by the hatched areas. The second group represents the deep seated conductive zones and is not hatched. The depth of the conductive zones is marked by the corresponding values emplaced in the conductive regions. As was expected, some conductive zones extend from ground surface to large depths. Generally, only the deep seated conductive zones in the crystalline basement are possible geothermal targets.
Self potential survey Research by Thanassoulas (1989) has shown that the SP method is sensitive and responds quite well over fracture zones and faults where geothermal fluids circulate. Application of the SP method on Lesvos island was therefore aimed at detecting faults and fracture zones where convection of geothermal fluids occurs. A total of 208.4 km of SP lines was surveyed in all the areas of geothermal interest of Lesvos. The two main areas surveyed with the SP method are shown in Fig. 10a (Geotest Hellas, 1988). The compiled SP maps are presented in Figs 10b and 10c. The SP anomalies to be interpreted were each selected from the original SP profiles. These SP anomalies are generated by electrochemical mechanisms caused by the convection of thermal fluids in the crystalline basement fracture zones. Therefore, the "patch model" (Fitterman, 1984) was adopted for the interpretation procedure. The selected SP anomalies were modelled and mapped in Fig. 11 on top of the geological faults. Each model is assigned a letter from a to o.
Integrated Geophysical Methods Over Lesvos Geothermal Field
363
(a) N I LESVOS ISLAND
B
A
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:
I
O
L
Fig. 10. (a) Map showinglocationof areas A and B (SP measurements) (after Geotest Hellas, 1988). (b) SP map (mV) of area A (after Geotest Hellas, 1988). (Continuedoverleaf.) DISCUSSION O F G E O P H Y S I C A L R E S U L T S - - C O N C L U S I O N S Fracturing and faulting of the crystalline basement of Lesvos island plays an important role in the development of its geothermal potential. Consequently, the scope of this geothermal project was first to define the deep-seated fracture/fault zones of the crystalline basement by interpreting the gravity map of Lesvos island. In Fig. 12 a correlation is made between the gravity mapped
C. Thanassoulas and N. Xanthopoulos
364
(c)
Fig. 10
(continued). (c) SP map (mV) of area B (after Geotest Hellas, 1988)•
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Fig. 11. Map showing SP models and fracture zones and faults• SP models (labelled a to o) are indicated by the thick black line. The arrow indicates the dip direction of the fault.
Integrated Geophysical Methods Over Lesvos Geothermal Field
365
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Fig. 12. Comparison of the fracture zones and faults mapped by the gravity method with the location of the SP models. Fracture zones and faults are indicated by thin lines, the solid triangle indicating the dip direction. The thick black lines denoted by the letters a to o show the location of the SP models and the arrow indicates the dip direction of the model.
fracture zones and faults and the location of the electrically polarized fault planes, namely the SP models. In Polyhnitos area SP models a and b show the same strike attitude as the gravity faults in the same area. The same is observed for SP models o, j, k, l, m, g, h in Kalloni-Stipsi area, with the exception of model i and n for which no corresponding crystalline basement fault direction has been mapped. In comparing the exact locations of faults to SP models it must be taken into account that gravity faults are produced by crystalline basement fault steps, while the SP models are generally produced in the fracture zones located in the crystalline basement itself. A fracture zone in the crystalline basement without a concurrent step fault cannot be discriminated in gravity interpretation methods. From Fig. 12 it is evident that the fracturing/faulting system of the crystalline basement of the island is in part electrically polarized. This is very important, since electrical polarization occurs in places where hot fluids migrate in the fracturing/faulting systems. In Fig. 13 the SP models have been mapped on top of the crystalline basement contour map. Depth to the top of the electrically polarized planes (D), and depth extent (E) are given in Table 1, with reference to the associated SP models. The SP models are known to be generated by electrochemical mechanisms triggered by convection of thermal fluids in the crystalline basement fracture zones. Consequently, the top of the polarized plane will generally be located at a depth corresponding to the depth of the top of the crystalline basement in the same location. This must hold for the deep seated models. SP
366
C. Thanassoulas and N. Xanthopoulos
Fig. 13. Comparison of the location of the SP models with the contour of the top of the crystalline basement. The thick black lines denoted by the letters a to o show the location of the SP models and the arrow indicates the dip direction of the model. The thinner lines show the contours of the isodepths of the crystalline basement in km.
Table 1. SP models and depth to the top of electrically polarized planes (D), and depth extent (E) SP model
D (m)
E (m)
a b c d e f g h i j k 1 m n o
780 890 200 270 410 510 290 280 1620 650 350 740 660 370 1280
820 440 890 110 380 170 2500 130 1800 5150 190 950 580 3320 2910
m o d e l s d, e, f at P o l y h n i t o s a r e a a r e s h a l l o w t a r g e t s o f n o i n t e r e s t . S P m o d e l c b a r e l y c o m e s w i t h i n t h e c r y s t a l l i n e b a s e m e n t . SP m o d e l s a a n d b a r e e n t i r e l y e m p l a c e d in t h e c r y s t a l l i n e b a s e m e n t , w h i l e a r e m a r k a b l e c o i n c i d e n c e is o b s e r v e d f o r t h e d e p t h to t h e t o p o f t h e s e p o l a r i z e d planes and the depth of the crystalline basement. I n K a l l o n i - S t i p s i a r e a , SP m o d e l s h a n d k a r e s h a l l o w t a r g e t s a n d t h e r e f o r e o f n o g e o t h e r m a l
Integrated Geophysical Methods Over Lesvos Geothermal Field
367
Fig. 14. Areas considered to have a high geothermal potential and proposed for drilling. Numbers indicate decreasing order of priority. interest. O f particular interest are the SP models o, n, and q in Stipsi area, all of which penetrate the crystalline b a s e m e n t to great depth in a zone where other geological and geochemical m e t h o d s have detected strong g e o t h e r m a l activity (Papastamataki and Leonis, 1982). By c o m p a r i n g the deep-seated conductive zones (Fig. 9) and the m a p p e d fracture zones and faults of the crystalline b a s e m e n t (Fig. 5), as well as the location of the SP models (Fig. 11) and the t e m p e r a t u r e distribution (Fig. 2), it then b e c o m e s evident that fracturing controls the high conductivity zones. In the same area where conductive zones and faults/fracture zones co-exist, SP anomalies have b e e n detected. Finally, f r o m integration of the results of the applied m e t h o d s , five drilling sites (Fig. 14) have b e e n p r o p o s e d , considered to be of high geothermal potential, based on criteria such as the presence of electrically polarized bodies (SP models), deep seated high conductivity zones and fracturing/faulting of the crystalline basement. These areas are shown in Fig. 14, r a n k e d from 1 to 5 in decreasing o r d e r of priority. REFERENCES Di Paola, G. M. (1973) Rapport geologique et geothermique sur l'ile de Lesvos (Mer Egee). IGME. Dominco, E. (1973) Geochemical analysis of Lesvos thermal springs. IGME. ENEL (1978) Geothermal reconnaissance study of Lesvos island--general report. PPC. Fitterman, D. V. (1984) Thermoelectrical self potential anomalies and their relationship to solid angles subtended by the source region. Geophysics 49, 165-170. Geotermica Italiana (1983) Island of Lesvos. Geothermal Project---evaluation of existing data. IGME. Geotest Hellas (1988) Geothermal research of Lesvos island, Athens, Greece. Hecht, J. (1972) GeologicalMap of Greece 1:50,000, Lesvos Island. IGME. IGME-Hamburg University (1982) Bouguer anomaly map compilation of Lesvos island. IGME. Katsikatsos, G. and Sfetsos, K. (1978) Report on the hydrogeological reconnaissance research of Lesvos island SPA. IGME.
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Nathanael, I. (1979) Geoelectrical investigations of Kalloni, Lesvos island area. IGME 2941, 1-15. Oikonomou, P. (1971) Geoelectrical investigations of the Polyhnitos, Lesvos island area. IGME 1914, 1-12. Papastamataki, A. and Katsikatsos, G. (1969) The SPA of Polyhnitos area, Lesvos island. IGME E1645, 1-24. Papastamataki, A. and Leonis, K. (1982) Geochemical investigations for the geothermal exploration of Lesvos island. IGME E3700, 1-32. Sfetsos, K. (1985) Hydrogeological investigation in the frame of geothermal study of Lesvos island on behalf of PPC. IGME E4554, 1-8. Simeakis, C. and Someritis, Y. (1982) Etude n6otectonique-mikrotectonique, Etude de fracturation, Etude geologique et perspectives g6othermique de l'fle de Lesvos. IGME. Taktikos, S. (1985) Temperature distribution on Lesvos island at -250 m from ground surface. (Unpublished map of IGME.) Thanassoulas, C. (1982) Deep geoelectrical soundings (VES) on Lesvos island. IGME 3562, 1-10. Thanassoulas, C. (1989) Application of the SP potential technique over the Milos geothermal test site. Geothermics18, 497-507. Xanthopoulos, N., Karmis, P. and Thanassoulas, C. (1988) Geophysical study of the Argenos geothermal field, northern Lesvos island. IGME E5683, 1-15.
Geothermics, Vol. 20, No. 5/6, pp. 355-368, 1991. Printed in Great Britain.
LOCATION OF POSSIBLY PRODUCTIVE GEOTHERMAL FRACTURE ZONES/FAULTS USING INTEGRATED GEOPHYSICAL METHODS OVER LESVOS ISLAND GEOTHERMAL FIELD, GREECE C. T H A N A S S O U L A S and N. X A N T H O P O U L O S Institute of Geology and Mineral Exploration (IGME), Departmentof Geophysical Research, 70 Messoghion Avenue, Athens, 115-26, Greece (Received November 1989;acceptedfor publication September1991) Abstract--In 1988, Geotest Hellas carried out a geophysicalsurveyon Lesvosisland using gravity, vertical electrical soundings and self-potential methods in order to study the geothermal potential of the island. This paper is based on the primary results presented by Geotest Hellas, which were re-examined and presented in a different form to facilitate the selection of the areas of strong ~eothermal interest. In particular, application of the gravity, vertical electrical sounding (VES) and self-potential (SP) methods has providedinformationon the geophysicalstructure of the geothermaltarget areas of Lesvosisland. The gravitysurveyprovidedvaluable informationon the morphologyand tectonicsof the crystalline basement (and on the peridotitic body and pyroclastic sediments), for the study of the geothermal potential of the area. The electrical surveyprovided information on the highlyconductivezones (usually associated with geothermal fields), the electrical stratigraphy and the tectonics of the main geologicalunits of the target area. The SP survey mapped areas where electrical polarization is observed in the faulting system. An integrated interpretation of the results of all these surveys led to the selection of five target areas considered to be of high geothermal potential. The location of these target areas is shown in Fig. 14, ranked from 1 to 5 in decreasing order of priority.
INTRODUCTION As part of its geothermal research programme in Greece, the Public Power Corporation (PPC) launched a few years ago a long-term exploration project in the most promising geothermal areas of Greece. Milos, Soussaki, Poros-Methana, Nisyros, Lesvos, Mygdonian basin, Strymon Valley, Platystomo, and Aidipsos were selected as target areas on the basis of already known geothermal manifestations. Field operations over these areas and the results have mainly been presented in I G M E (Institute of Geology and Mineral Exploration) reports, I G M E being the main contractor on behalf of the PPC; the Italian Electricity Board presented an integrated interpretation of the data collected by I G M E ( E N E L , 1978). Interest in the geothermal potential of Lesvos island was first stimulated many years ago by the presence of surface manifestations, which were studied by many scientists and research groups (Papastamataki and Katsikatsos, 1969; Hecht, 1972; Di Paola, 1973; Dominco, 1973; Katsikatsos and Sfetsos, 1978; E N E L , 1978; Papastamataki and Leonis, 1982; Geotermica Italiana, 1983; Sfetsos, 1985). The promising results of these studies and the definition of three main areas of geothermal interest (Petra-Argenos, Kalloni-Stipsi, Polyhnitos) by Geotermica Italiana (1983), were to justify in part the start of the next phase of geothermal research, borehole drilling. The combined geophysical survey of the Lesvos project was supported financially by the E E C ( D G X I I , Contact No. EN3G-0087-GR). The field operations and initial evaluation were 355
C. Thanassoulas and N. Xanthopoulos
356
LES
N I
.his
c? ,5
Fig. 1. Location of Lesvos island, Greece. Geologicalsketch map and location of main geothermal fields, mb = metamorphic basement, p = peridotites, nv = Neogene volcanism, nls = Neogenic lacustrine sediments, qs = Quaternary sediments, Iv = late volcanics, mf = main faults. 1 = Polyhnitos, 2 = Kalloni-Stipsi, 3 = Petra-Argenos geothermal fields. performed by Geotest Hellas (1988), while the final assessment was made by I G M E (Department of Geophysical Research). The location of Lesvos island is shown in Fig. 1. GEOLOGICAL
A N D T E C T O N I C S E T T I N G OF L E S V O S I S L A N D
The main geological and tectonic features of Lesvos island are as follows. Lesvos island is characterized by two main stratigraphic-structural units: 1. An epimetamorphic series (crystalline sequence) forming the stratigraphic " b a s e m e n t " of the island, locally and tectonically associated with peridotitic bodies. The basement outcrops mainly in the southeastern part of the island, along with peridotite massif outcrops in the central-eastern part. 2. An overlying sequence of volcanic rocks of calc-alkaline affinity that mainly occur in the west. The geological sketch map in Fig. 1 shows a main fracture trend ( N W - S E ) and a less developed E - W trend. The most important trend is the N W - S E , which corresponds to normal faulting during the Neocene episode that continued during the Pleocene (Simeakis and Someritis, 1982). A series of tectonic features of the basement such as lows, highs and normal faults trending N N E - S S W and N W - S E , some of which are still active at present, have facilitated the ascent of hot fluids from depth. GEOPHYSICAL SURVEYS Previous geophysical surveys on Lesvos island have focussed on local geothermal and hydrogeological problems. The first geophysical project (VES) was carried out by O i k o n o m o u
Integrated Geophysical Methods Over Lesvos Geothermal Field • "".
LESVOSI S L A N d )
357
t
Ni
G'B
Fig. 2. Temperature (°C) at 250 m depth belowground surface (after Taktikos, 1985), and locationof gravityprofile G8. (1971) to study Polyhnitos geothermal field. Nathanael (1979) applied the geoelectrical method (VES) in the Kalloni area. Thanassoulas (1982) carried out 10 deep VES (AB/2 = 4 km) in a reconnaissance study of the deep regional geological and tectonic features of Lesvos island. The gravity map of the island was compiled by Hamburg University (Professor Makris) in cooperation with the IGME (1982). A small-scale geophysical study was performed by Xanthopoulos et al. (1988), for the evaluation of the geothermal potential of Argenos area. Taktikos (1985) compiled a temperature map of Lesvos island (-250 m below ground surface) that is presented in Fig. 2. The areas of main geothermal interest are characterized by temperature highs of up to 90°C (Polyhnitos area), and up to 98°C (Stipsi area). Deep fracturing of the basement and the tectonic regime in the Lesvos geothermal fields meant that a detailed geophysical study had to be carried out in the most promising geothermal areas (central part of the island). The topography and tectonics of the crystalline basement were investigated by interpreting the results of the gravity survey, which revealed the density contrast between the overlying pyroclastics and sediments and the basement. The main geoelectrical formations, the high conductivity zones, which are frequently associated with geothermal fields, and the fractured zones and faults of the crystalline basement, were investigated by the VES, using the Schlumberger method. Finally, the geothermally active fracture zones and faults were identified by the self potential method. PRESENTATION OF GEOPHYSICAL RESULTS
Gravity survey As a first approach to the study of the deep structure of the crystalline basement of Lesvos island, the existing Bouguer anomaly map was analyzed. Its interpretation was carried out by
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C. Thanassoulas and N. Xanthopoulos
14] 9 t
o o
-'tt/ ' -6/
d.
....
4
~
/"
o o + + o
.... .
o
Profile G8
calculated .
.
8
~
~/' .j.
.
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\
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- km 28
0
-3 ~ ~
32
sl
-
crystalline basement
Fig. 3. Gravity modelling and its interpretation along profile G8. Densities used: 2.1 g/cm3 for pyroclastics, 3.1 g/cm3 for peridotitic body and 2.67 g/cm 3 for the crystalline basement (after Geotest Hellas, 1988).
Geotest Hellas (1988) along gravity profiles, using semi three-dimensional inversion algorithms. Gravity modelling covered the area of main geothermal interest (central part of the island). The gravity models were interpreted in terms of: (a) surface pyroclastic formation (density = 2.1 g/cm3), (b) peridotitic body (density = 3.1 g/cm 3) and (c) crystalline basement (density = 2.67 g/cm3). A sample of gravity modelling and its interpretation along profile No. G8 (Fig. 2) is shown in Fig. 3. From the gravity models along profiles, the depth to the top of the crystalline basement was resampled and the corresponding map was compiled (Fig. 4). The outlook of the map and the shape of the depth contours reveal the highly tectonized character of the crystallin~ basement. Local highs are indicated by crosses at the appropriate uplift of the basement, while lows are marked with a minus sign. Generally, the crystalline basement is very deep in the western part of the island (up to -2750 m), whereas in the eastern part it is uplifted to -500 m. A graben striking in a WSWENE direction and crossing the entire island can also be observed. Large variations in depth within short horizontal distances indicate large step faults and intense tectonics. These features are favorable for the development of geothermal fields through deep fluid circulation. A typical example is the case of Polyhnitos geothermal field (Fig. 2), where a regional crystalline basement is observed at 750 m depth, with a 500 m fault step (graben depth of 1250 m) along the SW-NE coastline. The location of the crystalline basement faults was determined from the gravity models as they were presented by Geotest Hellas (1988), although in some cases (Gulf of Kalloni) the location of the faults as indicated in the map depends only on interpolated data over the Gulf. A closer inspection of these interpretational models shows the presence of abrupt changes in depth
Integrated Geophysical Methods Over Lesvos Geothermal Field
359
Fig. 4. Depth (in km) to the top of the crystallinebasement, calculatedfrom gravitydata. of the crystalline basement. This is attributed to fracturing and faulting systems that exist in these areas. The location of the determined fracture zones and faults is shown in Fig. 5. The SWNE Kalloni Gulf graben direction is revealed very well, along with a NW-SE direction. Mapping of the crystalline basement faults is considered very important, because these are the areas where possible geothermal targets will be sought.
Electrical survey A total of 50 geoelectrical soundings utilizing the Schlumberger method (AB/2 = 3 km), were used for the geoelectrical study of the target areas. The location of the VES and a sample of an interpretational model along profile T9 are shown in Figs 6 and 7 (Geotest Hellas, 1988). In Fig. 7, apart from the surface pyroclastic formations and the morphology (faulting) of the crystalline basement, some conductive (p < 10 ohm m) zones are observed. These zones are associated with intense faulting of the crystalline basement. The resistivity map for depth -1000 m was compiled, and is presented in Fig. 6. At this depth low resistivity zones (<10 ohm m) are observed over the areas of main geothermal interest. Lesvos island is mainly covered by pyroclastics and sediments, which prevent us from accurately mapping the existing faults and fracture zones, because of the weathering of the surface soft formations. Apart from that, it is rather difficult to estimate the magnitude of fault throws of the crystalline basement without any surface manifestations. The observed faults in each particular interpretational model of the geoelectrical cross-sections, presented by Geotest Hellas (1988), are shown in Fig. 8. Solid triangles indicate the dip direction of the faults as revealed by the geoelectrical models. A correlation made between the faults/fracture zones mapped by the geoelectrical method and those mapped by geological observations (Fig. 8) reveals the hidden intense tectonic character of the basement of Lesvos island which was so difficult to study from surface observations.
C. Thanassoulas and N. Xanthopoulos
360
//~, / "~ \" \/
(~
9kin
/
~
390(3"
~ I
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I
'
'"
Fig. 5. Fracture zones and faults of the crystalline basement as determined by the interpretational gravity models presented by Geotest Hellas (1988).
N
I
L E S V O S ISLAND
,..,
0
0
o
o
o o
\
i
6kin
J
Fig. 6. Spatialdistributionofelectricalresistivity(ohmm)at 1000mbelowsealevelandlocationofVES(°pencircles)
Integrated Geophysical Methods Over Lesvos Geothermal Field .5,
361
T9
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In the northern part of the island, where there is a pyroclastic cover, only a few geological faults have been mapped. The electrically mapped faults not only coincide with them but provide a far clearer picture of the fracturing of the formations underlying the pyroclastics. All the zones characterized as low resistivity/high conductivity zones are presented in map form in Fig. 9. In this way we have achieved an integrated picture of the spatial distribution of
LESVOS ISLAND
\
6kin i
Fig. 8. Location of the fracture zones and faults, determined from the interpretational models presented by Geotest Hellas (1988).
362
C. Thanassoulas and N. Xanthopoulos
LESVOSISLAND , - J ' , ~ - , " h
0 6kin
Fig. 9. Location of the low resistivity/high conductivity zones. Shaded areas indicate shallow conductive zones (up to 300 m below ground surface), while "open" areas indicate deep seated zones. The numbers indicate the depth (in km) of the conductive zone, in places, below ground surface.
the conductive zones. The high conductivity zones can be divided into two groups. The first one represents the shallow conductive zones, and is represented in the map by the hatched areas. The second group represents the deep seated conductive zones and is not hatched. The depth of the conductive zones is marked by the corresponding values emplaced in the conductive regions. As was expected, some conductive zones extend from ground surface to large depths. Generally, only the deep seated conductive zones in the crystalline basement are possible geothermal targets.
Self potential survey Research by Thanassoulas (1989) has shown that the SP method is sensitive and responds quite well over fracture zones and faults where geothermal fluids circulate. Application of the SP method on Lesvos island was therefore aimed at detecting faults and fracture zones where convection of geothermal fluids occurs. A total of 208.4 km of SP lines was surveyed in all the areas of geothermal interest of Lesvos. The two main areas surveyed with the SP method are shown in Fig. 10a (Geotest Hellas, 1988). The compiled SP maps are presented in Figs 10b and 10c. The SP anomalies to be interpreted were each selected from the original SP profiles. These SP anomalies are generated by electrochemical mechanisms caused by the convection of thermal fluids in the crystalline basement fracture zones. Therefore, the "patch model" (Fitterman, 1984) was adopted for the interpretation procedure. The selected SP anomalies were modelled and mapped in Fig. 11 on top of the geological faults. Each model is assigned a letter from a to o.
Integrated Geophysical Methods Over Lesvos Geothermal Field
363
(a) N I LESVOS ISLAND
B
A
L6km 0
:
I
O
L
Fig. 10. (a) Map showinglocationof areas A and B (SP measurements) (after Geotest Hellas, 1988). (b) SP map (mV) of area A (after Geotest Hellas, 1988). (Continuedoverleaf.) DISCUSSION O F G E O P H Y S I C A L R E S U L T S - - C O N C L U S I O N S Fracturing and faulting of the crystalline basement of Lesvos island plays an important role in the development of its geothermal potential. Consequently, the scope of this geothermal project was first to define the deep-seated fracture/fault zones of the crystalline basement by interpreting the gravity map of Lesvos island. In Fig. 12 a correlation is made between the gravity mapped
C. Thanassoulas and N. Xanthopoulos
364
(c)
Fig. 10
(continued). (c) SP map (mV) of area B (after Geotest Hellas, 1988)•
\, \'
~r
j
,---\,
, 6kin
Fig. 11. Map showing SP models and fracture zones and faults• SP models (labelled a to o) are indicated by the thick black line. The arrow indicates the dip direction of the fault.
Integrated Geophysical Methods Over Lesvos Geothermal Field
365
N I LESVOS I S L A N O
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J/ ' i / ~ /
\
/
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/
/
/,
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Fig. 12. Comparison of the fracture zones and faults mapped by the gravity method with the location of the SP models. Fracture zones and faults are indicated by thin lines, the solid triangle indicating the dip direction. The thick black lines denoted by the letters a to o show the location of the SP models and the arrow indicates the dip direction of the model.
fracture zones and faults and the location of the electrically polarized fault planes, namely the SP models. In Polyhnitos area SP models a and b show the same strike attitude as the gravity faults in the same area. The same is observed for SP models o, j, k, l, m, g, h in Kalloni-Stipsi area, with the exception of model i and n for which no corresponding crystalline basement fault direction has been mapped. In comparing the exact locations of faults to SP models it must be taken into account that gravity faults are produced by crystalline basement fault steps, while the SP models are generally produced in the fracture zones located in the crystalline basement itself. A fracture zone in the crystalline basement without a concurrent step fault cannot be discriminated in gravity interpretation methods. From Fig. 12 it is evident that the fracturing/faulting system of the crystalline basement of the island is in part electrically polarized. This is very important, since electrical polarization occurs in places where hot fluids migrate in the fracturing/faulting systems. In Fig. 13 the SP models have been mapped on top of the crystalline basement contour map. Depth to the top of the electrically polarized planes (D), and depth extent (E) are given in Table 1, with reference to the associated SP models. The SP models are known to be generated by electrochemical mechanisms triggered by convection of thermal fluids in the crystalline basement fracture zones. Consequently, the top of the polarized plane will generally be located at a depth corresponding to the depth of the top of the crystalline basement in the same location. This must hold for the deep seated models. SP
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C. Thanassoulas and N. Xanthopoulos
Fig. 13. Comparison of the location of the SP models with the contour of the top of the crystalline basement. The thick black lines denoted by the letters a to o show the location of the SP models and the arrow indicates the dip direction of the model. The thinner lines show the contours of the isodepths of the crystalline basement in km.
Table 1. SP models and depth to the top of electrically polarized planes (D), and depth extent (E) SP model
D (m)
E (m)
a b c d e f g h i j k 1 m n o
780 890 200 270 410 510 290 280 1620 650 350 740 660 370 1280
820 440 890 110 380 170 2500 130 1800 5150 190 950 580 3320 2910
m o d e l s d, e, f at P o l y h n i t o s a r e a a r e s h a l l o w t a r g e t s o f n o i n t e r e s t . S P m o d e l c b a r e l y c o m e s w i t h i n t h e c r y s t a l l i n e b a s e m e n t . SP m o d e l s a a n d b a r e e n t i r e l y e m p l a c e d in t h e c r y s t a l l i n e b a s e m e n t , w h i l e a r e m a r k a b l e c o i n c i d e n c e is o b s e r v e d f o r t h e d e p t h to t h e t o p o f t h e s e p o l a r i z e d planes and the depth of the crystalline basement. I n K a l l o n i - S t i p s i a r e a , SP m o d e l s h a n d k a r e s h a l l o w t a r g e t s a n d t h e r e f o r e o f n o g e o t h e r m a l
Integrated Geophysical Methods Over Lesvos Geothermal Field
367
Fig. 14. Areas considered to have a high geothermal potential and proposed for drilling. Numbers indicate decreasing order of priority. interest. O f particular interest are the SP models o, n, and q in Stipsi area, all of which penetrate the crystalline b a s e m e n t to great depth in a zone where other geological and geochemical m e t h o d s have detected strong g e o t h e r m a l activity (Papastamataki and Leonis, 1982). By c o m p a r i n g the deep-seated conductive zones (Fig. 9) and the m a p p e d fracture zones and faults of the crystalline b a s e m e n t (Fig. 5), as well as the location of the SP models (Fig. 11) and the t e m p e r a t u r e distribution (Fig. 2), it then b e c o m e s evident that fracturing controls the high conductivity zones. In the same area where conductive zones and faults/fracture zones co-exist, SP anomalies have b e e n detected. Finally, f r o m integration of the results of the applied m e t h o d s , five drilling sites (Fig. 14) have b e e n p r o p o s e d , considered to be of high geothermal potential, based on criteria such as the presence of electrically polarized bodies (SP models), deep seated high conductivity zones and fracturing/faulting of the crystalline basement. These areas are shown in Fig. 14, r a n k e d from 1 to 5 in decreasing o r d e r of priority. REFERENCES Di Paola, G. M. (1973) Rapport geologique et geothermique sur l'ile de Lesvos (Mer Egee). IGME. Dominco, E. (1973) Geochemical analysis of Lesvos thermal springs. IGME. ENEL (1978) Geothermal reconnaissance study of Lesvos island--general report. PPC. Fitterman, D. V. (1984) Thermoelectrical self potential anomalies and their relationship to solid angles subtended by the source region. Geophysics 49, 165-170. Geotermica Italiana (1983) Island of Lesvos. Geothermal Project---evaluation of existing data. IGME. Geotest Hellas (1988) Geothermal research of Lesvos island, Athens, Greece. Hecht, J. (1972) GeologicalMap of Greece 1:50,000, Lesvos Island. IGME. IGME-Hamburg University (1982) Bouguer anomaly map compilation of Lesvos island. IGME. Katsikatsos, G. and Sfetsos, K. (1978) Report on the hydrogeological reconnaissance research of Lesvos island SPA. IGME.
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Nathanael, I. (1979) Geoelectrical investigations of Kalloni, Lesvos island area. IGME 2941, 1-15. Oikonomou, P. (1971) Geoelectrical investigations of the Polyhnitos, Lesvos island area. IGME 1914, 1-12. Papastamataki, A. and Katsikatsos, G. (1969) The SPA of Polyhnitos area, Lesvos island. IGME E1645, 1-24. Papastamataki, A. and Leonis, K. (1982) Geochemical investigations for the geothermal exploration of Lesvos island. IGME E3700, 1-32. Sfetsos, K. (1985) Hydrogeological investigation in the frame of geothermal study of Lesvos island on behalf of PPC. IGME E4554, 1-8. Simeakis, C. and Someritis, Y. (1982) Etude n6otectonique-mikrotectonique, Etude de fracturation, Etude geologique et perspectives g6othermique de l'fle de Lesvos. IGME. Taktikos, S. (1985) Temperature distribution on Lesvos island at -250 m from ground surface. (Unpublished map of IGME.) Thanassoulas, C. (1982) Deep geoelectrical soundings (VES) on Lesvos island. IGME 3562, 1-10. Thanassoulas, C. (1989) Application of the SP potential technique over the Milos geothermal test site. Geothermics18, 497-507. Xanthopoulos, N., Karmis, P. and Thanassoulas, C. (1988) Geophysical study of the Argenos geothermal field, northern Lesvos island. IGME E5683, 1-15.