THE RESISTIVITY STRUCTURE OF HIGH-TEMPERATURE GEOTHERMAL SYSTEMS IN ICELAND

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1 Presented at Short Course III on Exporation for Geotherma Resources, organized by UNU-GTP and KenGen, at Lake Naivasha, Kenya, October 24 - November 17, GEOTHERMAL TRAINING PROGRAMME Kenya Eectricity Generating Co., Ltd. THE RESISTIVITY STRUCTURE OF HIGH-TEMPERATURE GEOTHERMAL SYSTEMS IN ICELAND Knutur Arnason, Ragna Karsdottir, Hjamar Eysteinsson, Oafur G. Fovenz and Steinar Thor Gudaugsson Orkustofnun Grensasvegi 9, 108 Reykjavik ICELAND ka@isor.is ABSTRACT Eectrica and eectro-magnetic methods have been used extensivey to identify and deineate high-temperature geotherma reservoirs in Iceand. A hightemperature systems, within the basatic crust in Iceand, have a simiar resistivity structure, characterized by a ow resistivity cap at the outer margins of the reservoir, underain by a more resistive core towards the inner part. This is found in fresh-water systems as we as brine systems, with the same character but ower resistivities in the brine systems. Comparison of this resistivity structure with data from wes shows a good correation with ateration mineraogy. The ow resistivity in the ow-resistivity cap is dominated by conductive mineras in the smectite-zeoite zone in the temperature range of ºC. At temperatures ºC zeoites disappear and the smectite is graduay repaced by the resistive chorite. At temperatures exceeding 250ºC chorite and epidote are the dominant mineras and the resistivity is probaby dominated by the pore fuid conduction in the high-resistivity core. The important consequence of this is that the observed resistivity structure can be interpreted in terms of temperature distribution. A simiar resistivity structure is to be expected in acidic rocks. Due to different ateration mineraogy, however, the transition from the conductive cap to the more resistive core presumaby occurs at temperatures ower than 200 C. 1. INTRODUCTION Resistivity methods have been used in geotherma surveying for decades in Iceand. From the mid sixties, DC-methods, mosty Schumberger soundings, were used to identify and deineate hightemperature systems. In the mid eighties the DC methods were succeeded by centra-oop TEMsoundings (Transient Eectro-Magnetic). The TEM-soundings have proven to be more downward focused and have better resoution at depth than the DC-methods As geotherma exporation progressed and resistivity data were obtained from different geotherma fieds, a characteristic resistivity structure of the high-temperature geotherma system started to emerge. The fieds have a distinctive ow resistivity zone at their outer margins which is underain by higher resistivity towards the interior of the reservoir. This resistivity structure was found to 1

2 Arnason et a. 2 Resistivity of HT systems in Iceand contradict the conceptua mode that the resistivity shoud generay decrease with increasing temperature. As data from geotherma wes became more abundant, a comparison of the resistivity structure with geoogica and geophysica we data, was made possibe. This comparison showed that the resistivity structure coud be correated to the ateration mineraogy, which on the other hand, basicay refects the therma conditions in the geotherma system. The idea, that the resistivity was affected by the ateration came up within the geophysica group at Orkustofnun in the eary eighties and was first presented in a workshop within the Geoogoca Society of Iceand in Mars Increasing data have supported and carified these observations (Arnason et a., 1987; Arnason and Fovenz, 1992). Recenty, simiar observations have been made in Japan (Uchida, 1995). In this paper we review and present severa exampes of this resistivity srtucture and discuss its causes and impications. 2. RESISTIVITY OF ROCKS The resistivity of water-saturated rocks is in genera dependent on many physica parameters such as porosity, the sainity of the saturating fuid, temperature, conductivity of the rock matrix, and therma ateration. The interpay of these parameters is quite compex and in some respects not fuy comprehended. Empirica equations have been proposed as to the infuence of the different parameters, but they are usuay based on measurements of resistivity in rock sampes under different and often simpified conditions. A genera formua describing the resistivity of saturated rocks is bound to have many free parameters in order to account for the different factors affecting the resistivity. The compiation of such a genera formua is further hampered by the difficuty in controing individua factors in a reproducibe manner. Many usefu simpified formuas do exist, that can be good approximations under certain conditions. The most simpe, and probaby the most widey used, is Archie s aw (Archie, 1942): m w a 1) where, w and are the buk resistivity, the resistivity of the saturating fuid, and porosity, respectivey, and a and m are empirica coefficients. This formua seems to be a fairy good approximation when the conductivity is dominated by the saturating fuid. The empirica coefficients a and m are usuay reported around 1 and 2, respectivey. Another usefu, but simpified, formua was put forward by Rink and Schopper 1976), where they, in addition to pore fuid conduction, as described by Archie s aw, incude interface conduction. Fovenz et. a. 1985) expored the reationship between the buk resistivity, fuid resistivity, porosity, and temperature, for rocks in the uppermost kiometre of the Iceandic crust, outside the vocanic zones. From fied data and core-sampe measurements, they compied a semi-empirica reation, based on the so-caed doube porosity mode (Stefansson et a., 1982). They found that for rocks, saturated with fuids with resistivity higher than about 2 m, at room temperature, the buk resistivity is practicay independent of the resistivity of the fuid, but dependent on porosity and temperature. Fovenz et a. concuded that, except for rocks saturated with highy saine waters (sea-water), eectrica conduction in the Iceandic crust, outside the vocanic zones, is mainy controed by ateration mineras (cay mineras and zeoites).

3 Resistivity of HT systems in Iceand 3 Arnason et a. 3. THE RESISTIVITY STRUCTURE REVEALED The first appication of the resistivity method on high-temperature fieds in Iceand was carried out in the eary seventies on Reykjanes geotherma fied (Bjornsson et a., 1972), and Krisuvik geotherma fied (Arnorsson et a., 1975). The first arge-scae resistivity survey for a high-temperature geotherma exporation was performed in the eary seventies in the Krafa geotherma fied, NE Iceand (Karsdottir et. a. 1978). DCmethods, mainy Schumberger soundings, were appied. A we defined ow resistivity anomay was detected in reativey resistive host-rocks. A soundings within the ow-resistivity anomay showed, however, increasing apparent resistivity in the datapoints for the argest eectrode spacing. This was found to be in contradiction with the conception that the resistivity shoud generay decrease with increasing temperature at depth. Attempts were made to expain the increasing apparent resistivity by atera resistivity variations or eectromagnetic effects. These possibiities were rued out by theoretica cacuations and fied tests, showing, that the resistivity did indeed increase again beow a reativey thin ow-resistivity anomay. The next extensive resistivity survey was a reconnaissance survey in the Svartsengi-Edvorp- Reykjanes geotherma fieds on the outer part of the Reykjanes peninsua, SW Iceand, using Schumberger soundings. The outer peninsua is penetrated by sea-water and the geotherma systems were manifested by ow resistivity anomaies (0.5 2 m) in a reativey conductive host rock (5 10 m) (Georgsson and Tuinius; 1983). No cear evidence was found indicating increasing resistivity towards the centre of the geotherma systems as was observed in the fresh-water saturated geotherma system in Krafa. In the years of 1985 to 1987 a detaied resistivity survey was carried out in Nesjaveir geotherma fied, in a NNE-SSW trending fissure swarm, north of the Hengi centra vocano in SW Iceand (Arnason et.a; 1986, 1987). DC-resistivity methods, both Schumberger soundings and haf- Schumberger head-on resistivity profiing, where used to coect arge data sets on severa profies, designed for a joint 2D-modeing of the Schumberger and head-on data. The 2D-modeing resuted in highy constrained and detaied resistivity sections through the uppermost one kiometer of the reservoir. The modes showed a we defined ow resistivity ayer of 3 5 m on the outer margins of the reservoir, and underain by, about an order of magnitude higher resistivity deeper in the geotherma system.. At that time, numerous wes had aready been dried into the Nesjaveir geotherma system, and abundant geoogica and geophysica data were avaiabe. The resistivity mode for each section was compared to geoogica and geophysica data from nearby wes (within 100m from the profie). No obvious correation was observed between ithoogy and resistivity. A good and cear correation was, on the other hand, found between the ateration mineraogy and resistivity. Comparison with porosity ogs did not revea any obvious correation. The porosity is, however, strongy correated to ithoogy, as is to be expected. The resistivity modes correated we with the resistivity ogs in the shaower parts of the wes, but at depth, the resistivity ogs had quaitativey simiar pattern as the 2D modes but showed consideraby ower vaues. This was at first considered to mean that the increasing resistivity towards the inner part oft the reservoir was due to partia boiing. This was ater rued out, based on pressureand temperature ogs. A coser inspection of the resistivity ogs showed some inconsistencies in the data and that the resistivity ogs ony gave reative information at depth. The probem turned out be an instrumenta probem of the resistivity-ogging too. This was ater soved, and the discrepancies were no onger present.

4 KG-8 KG-10 KJ-15 KJ-19 KJ-20 KJ-16 KJ-18 NJ-11 NG-7 NG-10 Arnason et a. 4 Resistivity of HT systems in Iceand Figure 1 shows a smoothed 2D mode for one of the profies from Nesjaveir, perpendicuar to the fissure swarm (so 2D assumption is we justified). A cear resistivity anomay is seen, with a cap of resistivities of the order of 5 m at the margins and higher resistivity deeper in the reservoir. The reservoir is confined by dykes and fauts in the fissure swarm and has very sharp near-vertica boundaries and some atera fow near the surface. Three wes are cose to the profie. On Figure 1, the wes are projected onto the section, showing the zones of dominant ateration mineras. Formation temperature isotherms, based on temperature ogs from the wes are aso shown. The figure shows very good m a.s m 200 Temperature C Resistivity Ateration > 25 m Unatered rocks m Smectite - zeoite zone 2-10 m ow resistivity cap Mixed ayered cay zone Chorite zone High resistivity core Chorite-epidote zone OS RK FIGURE 1: Resistivity cross-section from Nesjaveir geotherma fied, ateration zoning in wes and temperature correation between the resistivity and temperature. The resistivity is high in the cod, unatered rocks outside the reservoir and decreases strongy at the onset of geotherma ateration, in the smectitezeoite zone, when the temperature has reached about 100 C. It is ow, generay ower than 5 m, down to the mixed-ayered cay zone, where it increases consideraby again and stays reativey high in the chorite and chorite-epidote zones at temperatures exceeding 250 C. Shorty after the survey in the Nesjaveir fied, the DC-methods were succeeded by centra-oop TEM-soundings. Since that time severa resistivity surveys have been carried out at various freshwater saturated high-temperature geotherma fieds. During the same period, severa wes have been dried and more we data have become avaiabe for comparison with the resistivity structure. A these surveys reveaed basicay the same genera resistivity structure. In a cases where we data are avaiabe for comparison, the resistivity structure correates with the ateration mineraogy, but no obvious correation is found with ithoogy. A good correation is generay found between resistivity and temperature, but with some important exceptions. These exceptions are found where parts of the reservoir have recenty been cooed down and the ateration mineraogy is no onger in equiibrium with the temperature Figure 2 shows a simpified resistivity cross-section through the eastern part of the Krafa geotherma system, NE Iceand, based on detaied centra-oop TEM survey (Árnason and Karsdottir, 1996). The zoning of ateration mineraogy in nearby wes and estimated reservoir isotherms has been projected onto the cross-section. The figure shows a very consistent correation between the ateration minera zones and resistivity. The ow-resistivity cap (resistivity ower 500 _ m a.s. _ 0 _ _ Cadera rim 300 Unatered rocks Smectite - zeoite zone Mixed ayered cay zone Chorite zone _ Chorite-epidote zone m OS RK Resistivity > 100 m m 1-6 m ow resistivity cap High resistivity core Ateration FIGURE 2: Resistivity cross-section from Krafa geotherma fied, ateration zoning and temperature 250 TEM sounding Temperature C

5 Depth (m) Resistivity of HT systems in Iceand 5 Arnason et a. than 10 m) coincides with the smectite-zeoite zone, which extends to the surface in the we fied, and the increase in resistivity with depth very consistenty foows the top of the mixed ayered cay zone. The correation with temperature is, however, not as good. It is evident from Figure 2 that the ateration mineraogy is not in equiibrium with present temperature in the system (the reation between reservoir temperature and ateration mineraogy of basatic rocks wi be summarised in a ater section). Two distinct anomaies in the temperature-ateration reation are found, i.e. in we KG- 10 in the western part of the section and in we KJ-18 in the eastern. The Krafa geotherma system has undergone severa phases with heating and recooing in different parts of the reservoir. The hypothesis has been put forward (Saemundsson, 1991; Árnason and Karsdottir, 1996) that 2000 years ago, a dyke was injected and bocked the fow of geotherma fuid from west, resuting in considerabe cooing around we KJ-18. A simiar phenomenon was observed in the Nesjaveir geotherma system. In 1988, Orkustofnun conducted a resistivity survey in the Asa Rift, Djibouti, East Africa, using centra-oop TEM-soundings (Árnason and Fovenz, 1995). The Asa Rift is an active spreading zone with basatic vocanism and hosts a geotherma system with highy saine fuids. The TEM-soundings show high resistivity from the surface and down to about 100m above the water-tabe, where the resistivity drops beow 2-12 m, which was expained by partia saturation. At the water tabe the resistivity decreased further, but in most of the soundings the resistivity increased again at depth. We data was sparse from the survey area, but comparison coud be made with data from two wes. In both cases a distinctive owering the resistivity coincided with the water tabe, as was to be expected, but the increasing resistivity with depth coincided with the top of the chorite zone in both of the wes. Figure 3 shows the comparison for the we Asa-4 and the nearby TEM-sounding DJ- ASAL FIGURE 3: Ateration and temperature in we Asa-4 in Djibouti and resistivity from a nearby TEM-sounding 11. Due to the high sainity brine, the eectrica conduction was thought to be dominant in the saturating fuid, and that resistivity variations were mainy due to differences in saturation and porosity as described by Archie s aw. The increased resistivity at the top of the chorite zone was therefore thought to refect decreasing porosity due to minera precipitation. In 1996 and 1997 a second resistivity survey was carried out on the outer part of the Reykjanes peninsua. This time the centra-oop TEM method was appied. The resut of this survey ceary demonstrated that the centra-oop TEM method has much better resoution and more penetration depth than the DC-method. The TEM data reveaed a cear resistivity image of the brine hightemperature geotherma systems in the peninsua (Svartsengi-Edvorp and Reykjanes). The surrounding rocks have resistivity of the order of 5-15 m and the geotherma systems appear as a ow-resistivity cap, with resistivities ranging from m, with an underying high-resistivity core with resistivities in the range of 7-15 m (Karsdottir, 1997, 1998). This is ceary seen in Figure 4, which shows a resistivity section from the Reykjanes in the west and to Edvorp and Svartsengi in the east. Severa wes have been dried into the geotherma systems and the zones of dominant ateration mineras have been projected onto the section. Here again the resistivity ayering shows an obvious correation with the ateration mineraogy, but no obvious correation was found with ithoogy. Figure 5 shows the ateration zones, ithoogica and resistivity ogs from we SJ-18 in the Svartsengi geotherma fied as we as a resistivity mode from a nearby TEM-resistivity section C 150 C 200 C 250 C TEM-sounding DJ-11 > 260 m 2.3 m WATER TABLE 1.5 m 2.6 m Resistivity Ateration > 260 m m 2.6 m Smectite - zeoite zone Mixed ayered cay zone Chorite zone Chorite-epidote zone 100 C Temperature OS RK

6 Arnason et a. 6 Resistivity of HT systems in Iceand Figure 4 shows roughy the same correation between the resistivity and ateration mineras as in the fresh-water systems, but there are some minor differences. In the fresh-water systems, the boundary between the ow resistivity cap and the resistive core correates with the boundary between the smectite-zeoite zone and the mixed-ayered cay zone. According to Figures 4 and 5, this boundary seems to be within the mixed-ayered cay zone. We EG-2 stands out, indicating that the ow resistivity cap is we within the chorite zone. This is not significant because the we is to the side of the section, where the resistivity is steepy dipping, perpendicuar to the section. The survey on the Reykjanes peninsua therefore indicates a sighty different correation between ateration and resistivity. The correation found in the highy saine system in the Asa Rift (Figure 3) supports the hypothesis that the transition from the ow-resistivity cap to the resistive core is moved towards the chorite zone. This can possiby be expained by sighty different ateration zoning in the saine systems. REYKJANES ELDVÖRP SVARTSENGI H6 H2 H9 H8 ST01 EG2 H9 H12 0 m 0 m We km Mixed ayered cay zone Chorite/epidote zone ALTERATION 5-9 m Seawater mixed with m groundwater RESISTIVITY 5-15 m m ow-resistivity cap High-resistivity core TEM-sounding FIGURE 4: Resistivity cross-section from Reykjanes peninsua and ateration zoning in wes

7 Resistivity of HT systems in Iceand 7 Arnason et a. 0 Depth (m) m Geoogica section Ateration zones Resistivity og ( m) > Resistivity from nearby TEM soundings Lithoogy Fine-medium grained basat Medium-coarse grained basat Basatic breccia Basatic tuff No data Ateration Unatered zone Smectite-zeoite zone Mixed-ayered zone Chorite zone Chorite-epidote zone Epidote-amphiboite zone FIGURE 5: Geoogica section, ateration zoning, resistivity ogs in we SJ-18 in Svartsengi and resistivity from nearby TEM-soundings 4. ALTERATION OF BASALTIC ROCKS Due to water-rock interaction and chemica transport by the geotherma fuids, the primary mineras in the host rock matrix are transformed, or atered, into different mineras. The ateration process and the resuting type of ateration mineras are dependent on the type of primary mineras, chemica composition of the geotherma fuid and temperature. The intensity of the ateration is furthermore dependent on the temperature, but aso on time and the texture of the host rocks. The ateration process and the resuting ateration mineraogy of the basatic rocks in high-temperature geotherma systems in Iceand have been studied quite extensivey (Kristmannsdóttir, 1979). The primary mineraogy of the basatic host rocks in the vocanic zones of Iceand is reativey homogeneous. The geotherma fuids can be divided into two reativey homogeneous types, i.e. ow sainity or fresh-

8 Arnason et a. 8 Resistivity of HT systems in Iceand water and saine fuids. Due to this homogeneity, the stabiity and formation of ateration mineras is mainy dependent on temperature. There is no room here for a engthy discussion so we wi ony discuss the main features and the dominant mineras or minera casses. At temperatures ower than 220 C, ow-temperature zeoites and the cay minera smectite are formed. The ateration intensity is normay ow for temperatures beow C. The range where ow temperature zeoites and smectite are abundant is caed the smectite-zeoite zone. In the temperature range from 220 C to about C, the ow temperature zeoites disappear and the smectite is transformed into chorite in a transition zone, the so-caed mixed ayered cay zone, where smectite and chorite coexist in a mixture. At about 250 C the smectite has disappeared and chorite is the dominant minera, marking the beginning of the chorite zone. At sti higher temperatures, about C, epidote becomes abundant in the so-caed chorite-epidote zone. This zoning appies for fresh water systems. In brine systems, the zoning is simiar but the mixed ayered cay zone extends over a wider temperature range or up to temperatures near 300ºC. A simiar ateration zoning, dependent on temperature, is observed in geotherma systems in acidic rocks, but with somewhat different ateration mineras. (Kristmannsdottir, 1985). This correspondence of different stabe and dominant ateration mineras with different temperatures is used extensivey in geotherma exporation and driing. Anayses of dri-cuttings during driing are used to estimate the unperturbed formation temperature. Comparison of estimated formation temperature from temperature ogs, and the ateration mineraogy, can be used to te whether present temperatures are in equiibrium with the ateration, or if cooing or heating has recenty occurred. Such a comparison ceary shows that the rocks around wes KJ-18 and KG-10 on Figure 2 have recenty been cooed. 5. CONDUCTION MECHANISMS Athough the reevant conduction mechanisms behind the observed resistivity structure of the hightemperature geotherma systems are not known in detais, they can be quaitativey understood in terms of the structura and physica properties of the different dominant ateration mineras. If pore fuid is the dominant conductor, measured resistivities of the geotherma fuids of the freshwater systems m at room temperature) and reasonabe vaues for porosity 10-15%) and for the coefficients, a and m, in Archie s aw (eq. 1), give the resistivity in the range of 15 to 50 m for temperatures in the range of 200 to 250 C. This is higher than the observed resistivity in the owresistivity cap, by a factor of 3 to 10, showing that a different conduction mechanism is dominant. The smectite cay minera is an obvious candidate. The smectite and chorite mineras are cosey reated. Both are so-caed sheet siicates (Deer et a., 1962). Smectite has hydrated and oosey bound cations between the siica pates, making the minera conductive and with a high cation exchange capacity. In the chorite minera the cations are on the other hand fixed in a crysta attice, making the minera resistive. In the case of fresh-water geotherma systems, the conduction in the surrounding rocks is probaby due to ow intensity and ow temperature ateration mineras and/or pore fuid conduction in very fresh rocks. In the ow resistivity cap the conduction is dominated by the highy conductive ateration mineras. The above estimate for the contribution of pore fuid conduction at high temperatures roughy agrees with the vaues found in the resistive core, indicating that the pore fuid conduction is dominant.

9 Resistivity of HT systems in Iceand 9 Arnason et a. In the saine systems the resistive inner core has resistivity of the order of 10 m. If it is assumed that this is, ike in the case of fresh-water systems, mainy due to porefuid conduction, it must foow, that the ateration mineras contribute significanty to the conductivity in the ow resistivity cap. 6. RESISTIVITY AS A THERMOMETER The correation between the resistivity structure of high-temperature geotherma systems in basatic rocks and ateration mineraogy can be summarised as foows: The resistivity is reativey high in cod unatered rocks outside the reservoir. The smectite-zeoite zone forms a ow resistivity cap on the outer margins of the reservoir. The resistivity increases again towards the interior of the reservoir at the top of, or within, the mixed ayer cay zone. This observation is of great importance, because the temperature dependence of the ateration mineraogy makes it possibe to interpret the resistivity ayering in terms of temperature, provided that the temperature is in equiibrium with the dominant ateration. The upper boundary of the owresistivity cap corresponds to temperatures in the range of C, depending on the intensity of the ateration. The transition from the ow resistivity cap to the resistive core corresponds to temperature in the range of C. Thus, if ateration is in equiibrium with temperature, the mapping of the resistivity structure is in fact mapping of isotherms. It is evident from Figure 2, that the resistivity refects the ateration, but not the present temperature, if cooing has recenty taken pace. In this case the resistivity is to be considered as a maximum thermometer. The dominant high temperature mineras, ike chorite and epidote, are stabe at ower temperatures and do not degenerate to ower temperature mineras. Under proonged cooing, higher temperature ateration can, however, probaby be so heaviy overprinted by ow-temperature mineras that the resistivity refects the new therma conditions, but no data exist, at present, which concusivey confirm this. If the reservoir, or parts of it, is heated up, ower temperature mineras ike smectite can transform to chorite and mixed ayered cays, and it is beieved that the dominant ateration and the resistivity can adjust reativey quicky to increased temperatures. In acidic rocks, a structura transition of dominant ateration mineras, simiar to the smectite-chorite transition in basatic rocks, occurs with temperature, but at temperatures ower than 200 C (Kristmannsdottir, 1985). A transition from the ow resistivity cap to a more resistive core is therefore expected in acidic rocks, at ower temperatures than in basatic rocks. 7. CONCLUSIONS Surface resistivity surveys of high-temperature geotherma systems in the basatic rocks of the vocanic zones of Iceand aways seem to revea basicay the same resistivity structure. A ow resistivity cap is observed on the outer margins of the reservoirs and underain by a more resistive core. Extensive comparison of this resistivity structure to we data has reveaed a consistent correation to the zones of dominant ateration mineras, where the ow-resistivity cap coincides with the smectite-zeoite zone and the transition to the more resistive core occurs at the boundary, or within the mixed ayer cay zone. The ateration mineraogy is, on the other hand, mosty predicted by temperature. This has the important consequence that, the resistivity structure can be interpreted directy in terms of temperature, if the ateration is in equiibrium with temperature. The upper boundary of the ow-resistivity cap is found where the temperature is in the range of C and the transition to the resistive core occurs at temperatures in the range of C.

10 Arnason et a. 10 Resistivity of HT systems in Iceand REFERENCES Archie, G.E., 1942: The eectrica resistivity og as an aid in determining some reservoir characteristics. Tran. AIME, 146, Arnason, K., Haradsson, G.I., Johnsen, G. V, Thorbergsson, G. Hersir, G.P., Saemundsson, Georgsson, L.S. and Snorrason, S.P., 1986: Nesjaveir; A geoogica and geophysica survey Orkustofnun report OS-86017/JHD-02 96p. (in Iceandic). Arnason K., Haradsson, G.I., Johnsen, G.V., Thorbergsson, G., Hersir, G.P., Saemundsson, K., Georgsson, L.S., Rognvadsson, S.Th. and Snorrason, S.P., 1987: Nesjavei Okeduhasr; A geoogica and geophysica survey Orkustofnun report OS-86018/JHD p. (in Iceandic). Arnason, K. and Fovenz, O.G. 1992). Evauation of physica methods in geotherma exporation of rifted vocanic crust. GRC transactions, vo.16 October Arnason, K., Fovenz, O.G., Georgsson, L.S. and Hersir, G.P., 1987: Resistivity structure of hightemperature geotherma systems in Iceand. From: Abstracts V.2 Internationa Union of Geodesy and Geophysics XIX Genera Assemby in Vancuver Canada p447. Arnason, K. and Fovenz, O.G., 1995: Geotherma exporation by TEM-soundings in the Centra Asa Rift in Djibouti, East Africa. From: Proceedings of the WGC Congress in Forence, Itay Arnason, K and Karsdottir, R., 1996: A TEM resistivity survey of the Krafa high-temperature fied. Orkustofnun report OS-96005/JHD-03 96p. (in Iceandic). Arnorsson S., Gudmundsson, G., Sigurmundsson, S., Bjornsson, A., Bjornsson, S., Einarsson, P., Gisason, G., Gunnaugsson, E. and Jonsson, J., 1975: Krisuvik-fied. A report on the geotherma survey. Orkustofnun report. OSJ-HD p. (in Iceandic). Bjornson S., Arnorsson, S. and Tomasson, J., 1972: Economic evauation of Reykjanes therma brine area, Iceand. The American Association of Petroeum Geoogist Buetin. Vo 6 No. 12, December Deer W.A.; Howie R.A. and Zussman J., 1962: Rock-Forming Mineras, Vo 3 Sheet Siicates. Longmans, Green and Co Ltd, London. 270p. Fovenz O.G., Georgsson, L.S. and Arnason, K., 1985: Resistivity structure of the upper crust in Iceand. J. Geophys. Res., 90, Georgsson, L.S. and Tuinius, H., 1983: A resistivity survey of the outer part of the Reykjanes Peninsua in Orkustofnun report OS-83049/JHD-09 70p. (in Iceandic). Karsdottir R., Johnsen, G.V., Björnsson, A., Sigurdsson O. and Hauksson, E., 1978: Krafa hightemperature fied. A report on a geophysica survey in Orkustofnun report OS-JHD p. (in Iceandic). Karsdottir, R., 1997: A TEM-survey of the outer part of the Reykjanes Peninsua. Orkustofnun report OS p. (in Iceandic). Karsdottir, R., 1998: A TEM-resistivity survey of Svartsengi high-temperature fied in Orkustofnun report OS p. (in Iceandic).

11 Resistivity of HT systems in Iceand 11 Arnason et a. Kristmannsdottir, H., 1979: Ateration of basatic rocks by hydrotherma activity at C. Internationa cay conference Esevier Sci. Pub. Company, Amsterdam 1979, Kristmannsdottir, H., 1985: The roe of mineras in geotherma energy research. From: Uppsaa Symposium Cay Mineras-Modern Society p Rink, M. and Shopper, J.R., 1976: Pore structure and physica properties of porous sedimentary rocks. Pure App. Geophys. 114; Saemundsson, K., 1991: The geoogy of the Krafa geotherma system. Nattura Myvatns pub. by the Iceandic Society of Natura Sciences (in Iceandic). Stefansson, V., Axesson, G. and Sigurdsson, O., 1982: Resistivity ogging of fractured basat. From: Proceedings of Eight Workshop Geotherma Reservoir Engineering; p Stanford University Caifornia. Uchida, T., 1995: Resistivity structure of Sumikawa geotherma fied, north eastern Japan, obtained from magneoteuric data. From WGC proceedings. Vo