Magnetotelluric observations over the Chaves geothermal field (NE Portugal)—Preliminary results

PHYSICS OFTHE EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 91 (1995) 203-211

Letter section

Magnetotelluric observations over the Chaves geothermal field (NE Portugal)--preliminary results F.A. Monteiro Santos"'*, A. Dupis b, A.R. Andrade mfonso a, L.A. Mendes-Victor a a Departamento de Fisica da Unioersidade de Lisboa, K Escola Politdcnica, 58, Lisboa, Portugal b Centre de Recherches Gdophysiques, 58150 Garchy, France

Received 8 December 1994; revision accepted 28 April 1995

Abstract

On the Portuguese mainland the most important low enthalpy field occurs in the graben of the Chaves region, where magnetotelluric measurements have been carried out to determine the crustal electrical conductivity distribution. From the impedance tensor decomposition two distinct regional directions (N-S and N65 ° + 10°E) have been found. These directions are consistent with the geology and related to the main faults. The apparent resistivity and phase curves derived from the determinant of the impedance tensor were used to obtain 1-D and 2-D interpretations. The regional models indicate the existence of a conductive layer at a depth between 7 and 12 km (resistivity in the range of 100-300 II m, from 1-D modelling and 60 II m from 2-D model). The 2-D interpretation provides a first estimate of the regional geothermal gradient (28°C km - I ) in the Hercynian granite. The 1-D models from the soundings located within the graben suggest that local tectonics could play a significant role in the CO 2 extraction and in migration of the deep CO2, as indicated from isotopic data.

I. Introduction

Magnetotelluric (MT) surveys were carried out by the Centre de Recherches G6ophysiques de Garchy (CNRS) and the D e p a r t m e n t of Physics of the Faculdade de Ci~ncias de Lisboa (University of Lisbon), in the northeastern part of Portugal (Chaves region), to determine crustal geoelectrical structures. The studied zone is located in the sub-zone of Galicia-Trfis-os-Montes, where several mineral and thermal springs can be found.

* Corresponding author.

On the Portuguese mainland the low enthalphy occurrences that are represented by hot springs can be associated with two main geologic groups: the first group occurs along the tectonic lineaments of the Hesperic massif in the northern part of the country (e.g. the Chaves hot springs); the second group occurs in the Meso-Cenozoic borders related to faults owing to the alpine orogeny (Aires-Barros, 1989). In the Chaves area, and close to Chaves city, a hot spring reaching a t e m p e r a t u r e of 78°C has been known at least since R o m a n times. The waters belong to the b i c a r b o n a t e - s o d i u m - C O 2rich type, and the relatively high CI content (about 45 mg 1-1) indicates that the geothermal reservoir

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204

F.A. Monteiro Santos et al. / Physics of the Earth and Planetary Interiors 91 (1995) 203-211

The area under research is dominated by the Chaves basin (elevation about 350 m), a graben with a NNE-SSW axis, bounded by the Hercynian granite (syn-tectonic and post-tectonic) and Silurian schistose formations (Fig. 1). Two main faults play an important role: the NNE-SSW Chaves-Verin fault, which corresponds to a lateHercynian episode and the N70°-80°E fault system crossing the graben in the neighbourhood of Fai6es and S. Est~v~o. The area was intersected by intense neotectonic activity that has reactivated the old fractures and caused, in the sedimentary basin, a complex pattern of faults. The present regional tectonic stress field, with the direction of maximum horizontal compressive

is liquid rather than vapour dominant (Aires-Barros et al., 1994). Although several geochemical results have been obtained, the source of the free CO2 (about 350 ppm) is not well defined at the present time. Isotopic analysis on ~80 and D, performed by Aires-Barros et al. (1994), seems to confirm that Chaves thermal Waters are of meteoric origin, with the most probable recharge area northeastwards of the graben, at higher elevations (8001000 m). Geothermometric interpretations of geochemical composition, based on silica geothermometers, show that the thermal waters have experienced a temperature of about 120°C (Alres-Bairros et al., 1994).

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F,,t. Monteiro Santos et al. / Physics of the Earth and Planetary Interiors 91 (1995) 203-211

stress trending NW-SE to WNW-ESE, favours the ascent of geothermal fluid at the crossing of the NNE-SSW, NNW-SSE and N70°-80°E faults (UTAD Report, 1992). In this paper we present the preliminary results obtained from the interpretation of the MT data, obtained in the Chaves graben and in its neighbourhood, in an attempt to obtain a first model of the electrical structure in that region and to establish a preliminary model of geothermal fluid circulation.

2. Acquisition and data processing The MT measurements were performed during two periods--May 1992 and July 1993. The survey comprised 41 MT soundings (Fig. 1) in the frequency range of 222-0.011 Hz, from four selected bands (180-12 Hz; 20-1 Hz; 1.6-0.1 Hz and 1 / 8 - 1 / 1 2 5 Hz), with sampling periods of 1.37 ms, 16 ms, 120 ms and 2 s, respectively. Processing and preliminary interpretation of the MT data included the following:

MT-32

(1) Regional and local strikes were determined, using the algorithm described by Zhang et al. (1987). (2) A 1-D interpretation of the apparent resistivity and phase curves derived from the determinant of the impedance tensor (Zdet) was carried out. The resistivity-depth models were iteratively modified until one optimal adjustment between the observed and model response curves was obtained (trial-and-error method). (3) A 2-D interpretation of those curves, including the distortion tensors in the modelling procedures, was carried out. To take into account lateral changes in the Earth's conductivity, a 2-D finite elements program was used.

3. MT results and interpretation Tensor decomposition has been performed at some stations, assuming that the local and regional structures are 2-D with different strikes (Zhang et al., 1987). The local structure is charac-

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206

F.A. Monteiro Santos et al. / Physics of the Earth and Planetary Interiors 91 (1995) 203-211

position two main features are observed (Fig. 1): the direction of the graben (which has a roughly N - S axis) and the directions defined northeastern of Fai6es and in Bolideira zone (N65 ° + 10*E). The former direction is correlated with deep sectors of the Chaves-Verin fault and the latter shows a good correlation with the fracturing related to the D 3 phase before the late-Hercynian episode. At Sites 27, 28 and 29, located southeast of Chaves, an alignment related to the schistose complex is clear. At those sites there is an exposed sequence of graphitic slates, which could be responsible for such observations. A more uniform distribution is observed in the local strikes (Monteiro Santos, 1994), and it was possible to define an average direction (N40°E). In the Mairos-Bolideira region the local strike is near N-S, reflecting the influence of the strongly fractured granite outcrop.

terized by a thickness which is small compared with the skin depth. Fig. 2 presents the results for two typical soundings: one outside the graben (Site 45) and another inside the basin (Site 32). At Site 45 the regional strike is near N78°E and the local strike is N37°E. For the station within the depression (32) the regional strike is N - S and the local strike is near N50°E. The distortion parameters show some dependence on the frequency and some dispersion for periods greater than 3 s related to the loss of data quality owing to man-made noise. However, the relative stability of the strike values confirms the validity of the 2-D hypothesis. It should be noted that t a n - l a ~< -45 ° (in Sounding 32, and in the period ranging from 0.01 to 1 s), satisfying the condition for the validity of a local strike, for measurements on a conductive terrain (Zhang et al., 1987). From the synthesis of all the regional strikes that have been calculated from the tensor decomgranite

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F~4. Monteiro Santos et al. / Physics of the Earth and Planetary Interiors 91 (1995) 203-211

zones, correlated to the graben fill and hydrothermal ascending fluid phases as indicated by audio-MT surveys (Monteiro Santos et al., 1995). An examination of the phase curves of the invariant, corresponding to stations on granite, reveals similarities for several soundings. We have calculated the average of those curves, and from them, and from the 1-D interpretation, we have built two regional models. The principal characteristics of such models are shown in Fig. 4: the first layer, with a thickness of 1 km, corresponds to the sediments and altered granite (resistivity of 150 ~ m) and less altered granite (600 f~ m); the second one, with a thickness of 8 km and resistivity in the range of 2000-2500 ~ m, is related to

Berdichevsky and Dimitriev (1976) defined the invariant of the impedance tensor Zde t = v F Z x x Z y y - Z x y Z y ~ . According to those workers, 3-D effects caused by shallow geoelectrical structures are attenuated by the use of that invariant. In addition, the phase is not affected and the determinant phases can be useful in determining the main conductivity features. The 1-D interpretation of the Zdet apparent resistivity and phase curves using a trial-and-error procedure and the code based on Patra and Mallick (1980) is shown in Fig. 3. In general, the models for the sites outside the depression have five layers. The models for the sites in the graben show more diversity, revealing good conductive

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F.A. Monteiro Santos et al. /Physics of the Earth and Planetary Interiors 91 (1995) 203-211

208

The 1-D regional models have been the basis for a more detailed model of the middle and upper crust. Following Zhang et al. (1987), we have calculated the 2-D regional model for the N - S profile in the Mairos-Bolideira region. The model has been obtained by a trial-and-error method, using a finite element forward program (Rijo, 1977), including the distortion tensors in the characterization of the shallow structures. The regional impedance tensor is represented by a two-dimensional model with a N70°E strike. At depths greater than 12 kin, a layer model has been assumed. The period interval 0,01-1 s was used to estimate the distortion tensor. The best model and the distortion diagrams are shown in Fig. 5. The upper crust has a resis-

less altered granite, with some water content; the third layer, with a thickness between 3 and 4 km and resistivity ranging from 100 to 300 11 m, probably has a great water content and may represent the granite-granulite transition zone; the layer below, which is more resistive (3500 fl m) and more impermeable, corresponds to the granulite. Nevertheless, the location of the top (62-68 km) of the last layer and its resistivity (100 fl m) are not well resolved. In the models there is no evidence of the Moho interface, which, according to the seismic results, should be close to 30 km (T611ez et al., 1993). The final interpretation, which is now in progress, will have to take that interface into consideration. N

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F.4. Monteiro Santos et al. / Physics of the Earth and Planetary Interiors 91 (1995) 203-211

tivity of about 1500 ~ m; this, if compared with the normal resistivity of the granite, seems to indicate some water content. The middle crust (10-20 km) rocks with resistivity of 3500 l'l m represent the less fractured substratum (granulite facies). All stations show distortion vectors and the distortion diagrams present a complex distortion pattern caused by localized multi-conducting structures. As pointed out by Olhoeft (1981), the electrical properties of the granite are principally conditioned by the amount of free water content and by the temperature. However, the interpretation of several seismic surveys carried out in the schistose domain of Galicia-Trfis-os-Montes (T611ez et al., 1993), did not show evidence of low-velocity layers that could be related to partial melting; thus we interpret the more conductive zone (60 1~ m, at a depth in the range of 7-12 km), as being correlated with the effects of infiltrated water.

4. Thermal gradient in the middle crust

Shankland and Ander (1983) have compiled world-wide field data and plotted field conductivities vs. temperature. They observed thermal dependences and they associated different conductive regimes with different tectonic regions. Shankland and Ander (1983) suggest that their curves can be used empirically to estimate regional geotherms from electrical resistivity profiles. The comparison of our 2-D resistivity model with the curves of Shankland and Ander allows us to forecast a temperature in the range of 300-400°C at a depth between 7 and 12 kin. As Hermance and Grillot (1974) pointed out, the change of temperature with changing resistivity can be estimated more readily than the absolute value of the temperature from the conductivity values. Adopting such a point of view and assuming that the upper crust is more or less homogeneous outside the graben, and furthermore that the temperature dependence on resistivity is a function of the activation energy, the

209

expression for the average geothermal gradient is (Hermance and Grillot, 1974) kT 2

pl

GT = ---~-zIn p---~, where Pl and ,02 are the resistivities at depths z~ and z2, respectively, T 1 is the absolute temperature at depth z 1, E is the activation energy, k is the Boltzman's constant and z is the difference Z 2 -- ZI .

According to our regional 2-D model, the resistivity changes by a factor of 25 from a depth of 1 to 10 km; considering that at 1 km the temperature is 30°C and assuming an activation energy of 0.1 eV, we obtain an average geothermal gradient of 28°C kin-1. The value 0.1 eV corresponds to an activation energy for the 'wet' granite and is constant with increasing temperature (Olhoeft, 1981). That gradient, which may reflect the influence of the upper to middle crust, is below the mean geothermal gradient (30°C km-1) evaluated from recent temperature measurements in boreholes located on the schistose zone south of the Chaves basin (R. Duque, personal communication, 1994). Vigneresse et al. (1988), have reported gradient values of the same order (23-25°C km -1) for the granitic Hercynian formations of the Armorican Massif in Brittany (Western France).

5. Origin of CO 2 Carbon dioxide may be of surface origin, but it may also come from the deep crust or the upper mantle, and remain trapped at various levels in overlying sedimentary layers (Arthaud et al., 1994). From examination of 13C contents of free and dissolved CO2, it is possible to determine the origin of the carbon. According to Arthaud et al. (1994), various workers have proposed limit ratios which indicate that origin. For free CO 2 and values of 8~3C%o vs. PDB (the Peedee Belemnite standard) of - 4 to - 8, a deep magmatic origin is proposed. In Chaves, analysis of ~3C contents (with values close to -5.72; Moitinho de Almeida, 1982),

210

EA. Monteiro Santos et al. / Physics of the Earth and Planetary Interiors 91 (1995) 203-211

indicate a mantle origin of the carbon. However, the isotopic analysis cannot provide the answers to the questions related to spatial gas storage and its possible migration towards the surface during the geodynamic evolution of the basin, although the 1-D resistivity models from the soundings on the graben (Fig. 3) show low resistivity zones at great depth, suggesting deep water circulation and that local tectonic setting could play an important role in CO 2 extraction and migration from the mantle to the surface.

6. C o n c l u s i o n s An analysis of the regional geoelectrical structure of thee Chaves region has been made using the magnetotelluric method. The main conclusions of that analysis are as follows: (1) two distinct directions exist, related to the deep faults. The C h a v e s - V e r i n fault has a N35 ° + 5°E direction in the u p p e r part and it seems almost N - S at deep sectors. The deep fault systems play an important role in the geothermal system control, mainly in water circulation and constraint on ascent. (2) T h e r e is a predominance of 2-D structures in the overburden, in the graben zone and its neighbourhood. (3) A conductive layer exists at a depth ranging from 7 to 12 km. The results, in spite of their preliminary character, support the hypothesis of a deep origin for CO2, namely in some sectors of the C h a v e s - V e r i n fault. The estimated geothermal gradient is comparable with those obtained for Hercynian areas of Brittany. F r o m the combination of those studies with the geological and geochemical data, a global circulation model was proposed. Its main characteristics are as follows: (1) the recharge zone seems to be located in the northeast (Bolideira-Mairos zone). The E N E - W S W faults will allow the precipitation water to infiltrate and flow at depth towards the graben. (2) The zones of ascent are linked to the N70°-80°E, N20°-30°W and N N E - S S W fault

systems that cross the graben at several sites, favouring hot fluid ascent and C O 2 extraction. (3) The shallow circulation of hot water in the graben will be mainly conditioned by the neotectonic faults.

Acknowledgements

This work was financed by the EC as part of the P r o g r a m m e Joule I (Contract JOUG-0009-C) and by JNICT. We would like to thank A. Choquier for his assistance with field work. We also thank anonymous reviewers for their careful critiques and comments.

References

Aires-Barros, L., 1989. Geothermal resources in Portugal. An. UTAD, 2: 11-22. Aires-Barros, L., Gra~a, R.C. and Marques, J.M., 1994. The low temperature geothermal system of Chaves (Northern Portugal): a geochemical approach. In: Proc. Geothermics 94 in Europe. Doc. BRGM, 203: 67-73. Arthaud, F., Dazy, J. and Grillot, J-C., 1994. Distribution of deep carbon dioxide in relation to the structure and tectonic evolution of south-east France. Geodin. Acta, 7(2): 86-102. Berdichevsky, M.N. and Dimitriev, V.I., 1976. Basic principles of interpretation of magnetotelluric curves. In: A. Ad~im (Editor), Geoelectric and Geothermal Studies. Akaddmiai Kiad6, Budapest, pp. 165-221. Hermance, J.F. and Grillot, L.R., 1974. Constraints of temperature beneath Iceland from magnetotelluric data. Phys. Earth Planet. Inter., 8: 1-12. Moitinho de Almeida, F., 1982. Novos dados geotermom6tricos sobre as figuas de Chaves e S. Pedro do Sul. Comun. Serv. Geol. Port., 68(2): 179-190. Monteiro Santos, F.A., 1994. Interpreta~o integrada de dados de resistividade e magnet~teifrica: aplica~o ao estudo de reservat6rios geot6rmicos de baixa entalpia. Ph.D. Thesis, University of Lisbon, 280 pp. Monteiro Santos, F.A., Dupis, A., Andrade Afonso, A.R. and Mendes-Victor, L.A., 1995. An AMT survey over the Chaves geothemral field (NE Portugal). Geothermics, submitted. Olhoeft, G.R., 1981. Electrical properties of granite with implications for the lower crust. J. Geophys. Res., 68: 931-936. Patra, H.P. and Mallick, K., 1980. Geosounding Principles, 2: Time-varying Oeoelectric Soundings. Elsevier, Amsterdam, 410 pp.

F~4. Monteiro Santos et al. /Physics of the Earth and Planetary Interiors 91 (1995) 203-211 Rijo, L., 1977. Modelling of electric and electromagnetic data. Ph.D. Thesis, University of Utah, Salt Lake City. Shankland, T.J. and Ander, M.E., 1983. Electrical conductivity, temperature and fluid in the lower crust. J. Geophys. Res., 88: 9475-9484. T611ez, J., Matias, L.M., C6rdoba, D. and Mendes-Victor, L.A., 1993. Structure of the crust in the schistose domain of Galicia-Trfis-os-Montes (NW Iberian Peninsula). Tectonophysics, 221: 81-93.

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Universidade de Trgs-os-Montes e Alto Douro (UTAD), 1992. Evaluation of geothermal resources between Lamego and Vila Verde da Raia. Geological final report of the Programme Joule/ECE-JOUG-0009-C. Vigneresse, J.L., Jolivet, J., Cuney, M. and Bienfait, G., 1988. Etude G6othermique du Massif Armoricain. Hercynica, IV(I): 45-55. Zhang, P., Roberts, R.G. and Pedersen, L.B., 1987. Magnetotelluric strike rules. Geophysics, 51: 267-278.