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Layout of magnetic and telluric sensors for synchronous recording. Two complete MT sites (2 telluric and 3 magnetic components) were recorded together with 8 other telluric dipoles, 200 m long. Southern Tuscany, Capraia Island and major railways are also shown.
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A magnetotelluric (MT) survey was performed in an area south-west of the Mt. Amiata geothermal field to assess its usefulness for detecting deep, water-dominated geothermal systems. The survey comprised 28 sites spaced approximately 1 km apart along a SW-NE profile. Two unique aspects of the data collection included (1) a remote reference site loca...
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... to record only electric field data, which, when combined with the magnetic data at the central main sites, provided a continuous profile of MT data. On these telluric-magnetotelluric (TMT) sites only high frequency data were recorded (0.1-125 Hz). All dipoles were 200 m long and were aligned parallel and perpendicular to the profile direction (Fig. 2). Two MT systems 1 km apart recorded high frequency data simultaneously on two main (MT) sites and on the 8 adjacent TMT sites. A total of about 3 hours of data was acquired for local remote-referencing of the TMT data in late afternoon and early morning, aiming to take advantage of relatively short but quiet periods between noisy data ...
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To date, magnetotelluric monitoring of processes during reservoir engineering of geothermal systems have been carried out only at three sites world-wide. Here, we add a new survey at the Reykjanes peninsular (Iceland). The MT data acquisition at RN-15/IDDP-2 covered the last third of the drilling period and thermo-hydraulic stimulation. Drilling wa...
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... Another fault zone (Sanyi Fault -FAZ1710) was observed at depths of 1500 -1710 m. Since the electrical resistivity of rocks is sensitive to the presence of fluids, geophysical methods that remotely sense sub-surface resistivity, such as Magnetotellurics (MT), can be a powerful tool in investigating the fluid distribution in the shallow crust (Chen and Chen 1998; Fiordelisi et al. 2000; Chiang et al. 2005; Santos et al. 2006). The effectiveness of MT in imaging fault zones has been demonstrated by studies of the San Andreas Fault zone in California, the US and elsewhere (Unsworth et al. 1999; Chen et al. 2007). ...
The Chi-Chi earthquake occurred on September 21st, 1999 in the Western Foothills of central Taiwan. This Mw=7.6 earthquake produced a 90 km long surface rupture and caused severe damage across Taiwan. The coseismic displacement on the Chelungpu fault was one of the largest ever observed. The Taiwan Chelungpu drilling project (TCDP) began in 2003 and resulted in a 2,000 m well that recovered cores from the fault zone at A-hole and finished in 2005 with two boreholes (A-hole and B-hole) being completed. The Chelungpu fault that caused the Chi-Chi earthquake was observed in the core at a depth of 1,111 m (FAZ1111). Another fault zone (Sanyi Fault - FAZ1710) was observed at depths of 1,500~1,710 m. Since the electrical resistivity of rocks is sensitive to the presence of fluids, geophysical methods that remotely sense sub-surface resistivity, such as Magnetotellurics (MT), can be a powerful tool in investigating the fluid distribution in the shallow crust. The effectiveness of MT in imaging fault zones has been demonstrated by studies of the San Andreas Fault zone in California, the U.S. and elsewhere. In magnetotellurics, the depth of exploration increases as the signal frequency decreases. Thus for imaging shallow fault zone structure at the TCDP site, the higher frequency audio-magnetotelluric (AMT) method is the most suitable. In this paper, AMT data collected at the TCDP site from 2004 to 2006 are presented. Spatial and temporal variations are described and interpreted in terms of the tectonic setting. Audio-magnetotelluric (AMT) measurements were used to investigate electrical resistivity structure at the TCDP site from 2004~2006. These data show a geoelectric strike direction of N15°E to N30°E. Inversion and forward modeling of the AMT data were used to generate a 1-D resistivity model that has a prominent low resistivity zone (< 10 ohm-m) between depths of 1,100 and 1,500 m. When combined with porosity measurements, the AMT measurements imply that the ground water has a resistivity of 0.55 ohm-m at the depth of the fault zone. Time variations in the measured AMT data were observed from 2004~2005 with maximum changes of 43% in apparent resistivity and 18° in phase. The change in apparent resistivity is greatest in the 1,000~100 Hz frequency band. These frequencies are sensitive to the resistivity structure of the upper 500 m of the hanging wall of the Chelungpu Fault. The decrease in resistivity over time appears to be robust and could be caused by an increase in porosity and a re-distribution of the groundwater.
... Another fault zone (Sanyi Fault-FAZ1710) was observed at depths of 1500-1710 m. Since the electrical resistivity of rocks is sensitive to the presence of fluids, geophysical methods that remotely sense sub-surface resistivity, such as Magnetotellurics (MT), can be a powerful tool in investigating the fluid distribution in the shallow crust (Chen and Chen 1998;Fiordelisi et al. 2000;Chiang et al. 2005;Santos et al. 2006). The effectiveness of MT in imaging fault zones has been demonstrated by studies of the San Andreas Fault zone in California, the US and elsewhere (Unsworth et al. 1999;Chen et al. 2007). ...
The Taiwan Chelungpu-fault drilling project (TCDP) has undertaken scientific drilling and directly sampled the sub-surface rupture of the 1999 Chi-Chi earthquake. Audio-magnetotelluric (AMT) measurements were used to investigate electrical resistivity structure at the TCDP site from 2004 - 2006. These data show a geoelectric strike direction of N15 Et o N30E. Inversion and forward modeling of the AMT data were used to generate a 1-D resistivity model that has a prominent low resistivity zone (< 10 ohm-m) between depths of 1100 and 1500 m. When combined with porosity measurements, the AMT measurements imply that the ground water has a resistivity of 0.55 ohm-m at the depth of the fault zone. Time variations in the measured AMT data were observed from 2004 - 2005 with maximum changes of 43% in apparent resistivity and 18 in phase. The change in apparent resistivity is greatest in the 1000 - 100 Hz frequency band. These frequencies are sensitive to the resistivity structure of the upper 500 m of the hanging wall of the Chelungpu Fault. The decrease in resistivity over time appears to be robust and could be caused by an increase in porosity and a re-distribution of the groundwater.
... This paper deals with new geological data from the Mount Amiata volcano-geothermal region and the Roccalbegna area (Fig. 2), providing a kinematic evolutionary model for the formation of the extensional Tuscan Nappe boudins. Indeed, the Roccalbegna area has been recently investigated for geothermal purposes by ENEL GreenPower [27]. Integration of fieldwork data, seismic interpretations of two new seismic reflection profiles (lines RA01 and RA02 inFig. ...
Fieldwork data, borehole stratigraphy and seismic interpretations revealed that stretched upper crust typifies the southern part of the Mt. Amiata geothermal area. Crustal stretching took place during Miocene extensional tectonics which gave rise to widespread tectonic delamination within the tectonic pile of the inner Northern Apennines, stacked from the Cretaceous to the Early Miocene. Extensional horses (boudins) composed of the Tuscan Nappe, the deepest non-metamorphic tectonic unit of the Northern Apennines, were generated from subhorizontal detachments linked to east-dipping ramps. Minor detachments and related ramps produced minor Tuscan Nappe boudins composed of only part of the Tuscan Nappe succession. West and south of the Mt. Amiata geothermal area, three main Tuscan Nappe boudins have been discovered and geometrically investigated by boreholes, fieldwork and seismic data. These geological bodies partially crop out in the Sasso d'Ombrone, Roccalbegna-Torrente Rigo and Castell'Azzara areas. Seismic images and boreholes have allowed better control of the geo-metry of the Roccalbegna-Torrente Rigo boudin and reconstruction of its possible developmental mechanism, characterised by superposed normal faulting. The Tuscan Nappe boudinage produced structural depressions which were mainly infilled by continental sediments. The Cinigiano-Baccinello basin lies between the Roccalbegna and Sasso d'Ombrone Tuscan Nappe boudins and was mainly infilled by Late Miocene continental deposits. Seismic profiles imaging the geometrical relationships between the Tuscan Nappe boudins and the Late Miocene deposits infilling the Cinigiano-Baccinello basin in its eastern margin, indicate that this basin was a boudinage-related basin.
... In this region, the last extensional phase following the Apennine orogeny gave rise to NNW-SSE normal faults and formed tectonic depressions where volcanic products as well as Neogene sediments were deposited. Later the region underwent a general uplift, so that older units outcrop at some places (Fiordelisi et al., 2000). Recent volcanism of several Ma to 0.1 Ma age serves as the present-day heat source (Gianelli and Ruggieri, 2000). ...
Surface heat flow measurements over active geothermal systems indicate strongly positive thermal anomalies. Whereas in “normal“ geothermal settings, the surface heat flow is usually below 100–120 mW m−2, in active geothermal areas heat flow values as high as several watts per meter squared can be found. Systematic interpretation of heat flow patterns sheds light on heat transfer mechanisms at depth on different lateral, depth and time scales. Borehole temperature profiles in active geothermal areas show various signs of subsurface fluid movement, depending on position in the active system. The heat transfer regime is dominated by heat advection (mainly free convection). The onset of free convection depends on various factors, such as permeability, temperature gradient and fluid properties. The features of heat transfer are different for single or two-phase flow. Characteristic heat flow and heat transfer features in active geothermal systems are demonstrated by examples from Iceland, Italy, New Zealand and the USA.Two main factors affect the rheology of the lithosphere in active geothermal areas: steep temperature gradients and high pore fluid pressures. Combined with lithology and structure, these factors result in a rheological zonation with important consequences both for geodynamic processes and for the exploitation of geothermal energy. As a consequence of anomalously high temperature, the mechanical lithosphere is thin and its total strength can be reduced by almost one order of magnitude with respect to the average strength of continental lithosphere of comparable age and thickness. The top of the brittle/ductile transition is located within the upper crust at depths less than 10 km, acts as the root zone of listric normal faults in extensional environments and, at least in some cases, is visible on seismic reflection lines. These structural and rheological features are well illustrated in the Larderello geothermal field in Tuscany.
... An MT survey carried out in 1994 on the western margin of the Mt. Amiata area highlighted its complex structure and the relations between conductive anomalies and the geothermal reservoir (Manzella et al., 1999;Fiordelisi et al., 2000). ...
... An MT survey carried out in 1994 on the western margin of the Mt. Amiata field (Manzella et al., 1999;Fiordelisi et al., 2000;B-B 0 in Fig. 1a) and another MT survey in the Larderello geothermal area (Fiordelisi et al., 1995) provided interesting results for defining the characteristics of the EM noise affecting the geothermal areas of southern Tuscany. ...
... The telluric dipoles of the main sites and the satellite sites were 200 m long and were aligned parallel and perpendicular to the profile direction, wherever topography and vegetation permitted. The satellite sites data, combined with the synchronous magnetic data at the central main sites, provided a continuous MT profile that has proved effective in defining the details of the shallower structures (Manzella et al., 1999;Fiordelisi et al., 2000). Data were acquired during the night, when test soundings indicated that noise tended to be at a minimum. ...
During 1999 a magnetotelluric (MT) survey was carried out on the southern margin of the Mt. Amiata geothermal region (Tuscany, Italy), with the aim of defining the shallow and deep electric structures related to the local geothermal reservoirs and system heat recharge. Local and remote data were collected along a SW–NE profile and processed with two different robust algorithms. After a detailed study of the EM strike, the data were inverted and two-dimensional (2D) models of electrical resistivity and impedance phase were computed. The interpretation revealed a good correlation between the features of the geothermal field and resistivity distribution at depth. In particular, a shallow conductor (0.5–4 km) detected by the MT survey shows a good correlation with the areal extension of the geothermal reservoirs.
... In this region, the last extensional phase following the Apennine orogeny gave rise to NNW-SSE normal faults and formed tectonic depressions where volcanic products as well as Neogene sediments were deposited. Later the region underwent a general uplift, so that older units outcrop at some places (Fiordelisi et al., 2000). Recent volcanism of several Ma to 0.1 Ma age serves as the present-day heat source (Gianelli and Ruggieri, 2000). ...
... The presence of a deep anomalous body is also suggested by geophysical data. Gravimetric data in this area show very low values despite the uplift of denser units, and imply the presence of a low density body at depth (Bernabini et al. 1995;Fiordelisi et al., 2000). Moreover, the seismic reflection data have revealed the presence of a zone in which there are no seismic reflection signals. ...
... Moreover, the seismic reflection data have revealed the presence of a zone in which there are no seismic reflection signals. The location and shape of the seismic-transparency zone show a similarity with the deep low-density body of the gravity anomaly, so that the zone has been interpreted as a batholith (Fiordelisi et al., 2000). Geophysical investigations have revealed some peculiar features of the shallow structure in the Amiata area. ...
Two geothermal fields, Bagnore and Piancastagnaio, are in the southern sector of the Quaternary Amiata volcano (southern Tuscany, Italy). Fluid inclusions trapped in quartz, adularia and calcite found in core-sample in deep drilled wells (between 1690 and 3220 m below the ground level) of the geothermal fields provided information on the physical-chemical characteristics of circulating fluids in past. Three types of fluid inclusions related to the geothermal activity were recognized: type I (liquid-rich) inclusions trapped an aqueous fluid with CO 2 concentration (1.27-2.68 mol/kg) that is significantly higher than that of present-day fluids (0.11-0.05 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with lesser CO 2 (<0.85 mol/kg). Type III (vapor-rich) inclusions record an early fluid circulation under boiling conditions and in some samples coexist with type I inclusions. The decrease of CO 2 concentration from type I inclusions to type II inclusions, and then to present-day concentrations can be related to boiling with gas loss and/or mixing. In general type I and II inclusions show variable salinities (0.11-3.90 mol/kg of Na + and Cl -) and Th (150-335°C), which can also be associated with boiling and mixing processes. In particular, the data suggest that mixing between a hot low-salinity fluid and a cooler saline fluid may has occurred in samples where present-day temperatures are in the range 195-280°C. On the other hand, the present-day hottest areas (295-320°C) appear to be nearly in thermal stability from the time of fluid inclusion trapping and mixing processes are not evident.
Simplified geothermal occurrence models using attributes identified at Coso and elsewhere were developed and applied in preparing the recently completed Department of Defense-funded evaluation of geothermal potential on U.S. military bases. An interpretation of the spatial associations between selected characteristics was used to direct field investigations. Several potential targets were identified using this method, and field investigations at two bases provided evidence supporting the likelihood of geothermal occurrences. Enough data and supporting evidence now exist to suggest that the development and application of more sophisticated geothermal occurrence models may be useful for focusing geothermal development in existing fields and grass-roots geothermal exploration.
Electrical conductivity of the subsurface is known to be a crucial parameter for the characterization of geothermal settings. Geothermal systems, composed by a system of faults and/or fractures filled with conducting geothermal fluids and altered rocks, are ideal targets for electromagnetic (EM) methods, which have become the industry standard for exploration of geothermal systems. This review paper presents an update of the state-of-the-art geothermal exploration using EM methods. Several examples of high-enthalpy geothermal systems as well as non-volcanic systems are presented showing the successful application of EM for geothermal exploration but at the same time highlighting the importance of the development of conceptual models in order to avoid falling into interpretation pitfalls. The integration of independent data is key in order to obtain a better understanding of the geothermal system as a whole, which is the ultimate goal of exploration.
