Electrical conductivity studies in the Travale geothermal field, Italy

Magnetotelluric and geomagnetic depth soundings have been carried out in the area of the Travale high enthalpy geothermal field (central Tuscany, Italy) in 1980 and 1981 to study the distribution of electrical conductivity in the geothermal anomaly and the crust beneath. Within this project the possible contributions of electromagnetic investigations to geothermal research were to be tested and a geothermal model of the Travale area was to be developed. The time-varying electric and magnetic fields have been recorded in a broad period range from 6–10,000 s, mainly on two profiles, the one parallel, the other perpendicular to the Travale graben. Strong lateral variations of apparent resistivities have been observed. Up to periods of 50–100 s the Travale graben is the dominating 2-D structure, but for longer periods of investigation the three-dimensionality of electrical conductivity structures has to be considered. The apparent resistivities inside the geothermal anomaly are extremely low, reaching not more than 50 ohm · m, even in the lower crust, but they increase to 100–300 ohm · m north of the geothermal field. Total conductance also indicates the geothermal field as a local conductivity anomaly, whereas further to the north the poorly conducting “barrier” has been confirmed. The cause of the high conductivity structures inside the geothermal area is to be seen in a highly fractured basement within this zone, allowing upward movement of hydrothermal fluids.     

First results of the application of the dipole electrical sounding method in the geothermal area of travale - radicondoli (Tuscany)

A new geoelectric prospecting method has been tested in the Travale - Radicondoli geothermal area. This method is based on the dipolar technique that permits investigation at very great depths with much fewer problems than encountered when using the classical electric prospecting techniques.The following steps were taken in order to operate with relatively low power from a 2 kW generator:

1. (i) the ground was energized with a series of current square waves at a frequency of less than 0.05 Hz in order to avoid the effects of electromagnetic coupling and induced polarization;

2. (ii) the voltage was recorded digitally at the measuring dipole;

3. (iii) the voltage recordings were processed by the spectral analysis method of “maximum likelihood”.

The resulting apparent resistivity diagrams were transformed into Schlumberger diagrams and then interpreted quantitatively.The six soundings are too limited in number to represent a real prospecting but refer to different geological and structural situations typical of a geothermal area. Two electrosoundings were sited for this purpose so as to be directly calibrated by the wells in the local geothermal field. The quantitative analysis of the resistivity diagrams in particular revealed the low resistivity values of the carbonate formation forming the geothermal reservoir, where the hot fluid circulation is particularly strong (15 Ω.m).The dipolar method has proved capable of distinguishing, in the geological situation of Travale area, the various structural features of the geothermal field such as “cover”, “reservoir”. substratum, uplifted structures and tectonic depressions.     

Well-testing in travale-radicondoli field. Part V. Concluding statement on pressure transient studies made from well tests in the Travele-Radicondoli reservoir

The theory and applications of pressure transient (well test) analysis have been studied intensively for more than 40 yr by petroleum reservoir engineers and groundwater hydrologists. Only in the past decade, however, have geothermal-fluid wells been tested for the purpose of making pressure transient studies. Results of these studies disclose various well conditions, for example, restrictions to fluid flow into the wellbore. They also disclose reservoir heterogeneities, boundaries and permeability-thickness products of reservoir rocks. Probably most important, they can be used in estimations of energy reserves. This powerful analytical tool is discussed with special reference to the Travale reservoir.This reservoir is complicated geologically and hydrologically. It lies on the margin of a graben near a widespread outcrop of the reservoir rocks, which also form an absorption area for the meteoric waters. The area explored can be divided into three zones: in one of these (the nearest to the absorption area) some noncommercial wells produce two-phase water-steam mixtures; in the second zone the wells produce superheated steam, while a well drilled in the graben itself produces a fluid with an uncondensable gas content of about 80%. The reservoir is described in relation to defining areas for further exploration. The nature of the reservoir has affected the design of programs for collecting pressure-production data and other well performance data. The performance history prior to the advent of pressure transient studies pertains mainly to what is known as the ‘old’ Travale reservoir to the southwest of the ‘new’ Travale-Radicondoli reservoir in which the more recent wells are drilled and in which modern well test analysis methods have been applied. Data on the “old” reservoir are discussed first.Because of its initial performance and relationship to nearby wells the most important well in the “new” reservoir is Travale well 22. It has been subjected to extensive well testing. Nearly all the wells in the “new” reservoir have been involved, however, through well-interference tests. In these tests the wells surrounding Travale well 22 are shut in and their pressure responses to different Travale well 22 production rates are measured. Well interference tests indicate the characteristics of fluid flow in the reservoir between test wells and in a qualitative way the heterogeneous nature of the reservoir itself.Pressure transient theory is developed from ideal system behavior: one vertical, fully-penetrating well producing at a constant rate from a horizontal reservoir of uniform thickness and of infinite extent in any direction from the wellbore. A great deal of research has been done to aid well-test analysts in their interpretation of pressure buildup and pressure drawdown curves constructed from data taken on wells in actual reservoirs. This research generally is accomplished with model studies. Some of the models developed in the present research fit reasonably well with the build-up behavior of Travale well 22.The research done on the Travale reservoir is summarized here with the objective of showing what has been learned, how it can be applied, and what should be done next. Confidence in applications of pressure transient analyses in the Travale reservoir has been gained. New concepts of the reservoir system have emerged as a result of the research. Additional testing and more precise measurements in the field should lead to good engineering estimates of energy reserves.     

Differential magnetic soundings in the travale geothermal area

In 1980 and 1982 the Laboratory of Applied Geophysics of the C.N.R.S. attempted to establish, by magnetic differential sounding, a possible conductivity anomaly linked with the geothermal field of Travale, Tuscany. Some 25 sites were occupied along two profiles, one between Siena and Populonia, near Piombino, the other between Siena and Cecina. An important anomaly of the transient magnetic field (some 15% of the normal field) was discovered in 1980 between Gerfalco (in the SW) and Frosini (in the NE). It exactly covers the geothermal area of Travale. The direction of the telluric currents causing the anomaly is parallel to the magnetic meridian and their maximum depth is of some 2000 m. The 1982 campaign showed that in the north of Travale, anomalous currents move in a NW - SE direction or even completely EW (SW of Volterra) and meet in the sea near Livorno. One possible interpretation of these phenomena as a whole is to assume the presence of very conductive layers between Larderello and Travale. The currents which circulate parallel to the coast are channelled locally by this structure, which could be closely linked with the geothermal field.     

Investigation of the geothermal anomaly of Travale (Tuscany) by telluric, magnetotelluric and geomagnetic deep sounding measurements

As part of the European Community research programme telluric, magnetotelluric and geomagnetic deep sounding measurements were undertaken at 40 sites within the geothermal area of Travale. In the period range of 6–10,000 s the telluric field inside the Travale graben is strongly polarized and directed, independent of the period, about parallel to the graben strike. The lateral variation of the telluric field amplitude is determined mainly by the distribution of the rocks (e.g. the central part of the geothermal anomaly inside the graben is correlated with a horst structure of resistive rocks) and an influence of the geothermal anomaly on the telluric field distribution cannot be observed. The apparent resistivity, as well as the phase curves, are rather similar at all sites within the graben, exhibiting 4–40 ohm · m for periods of 10 s and 50–500 ohm · m for periods of 10,000 s in E-polarization. In the period range of 10–100 s the E- and B-polarization of magnetotelluric measurements can be interpreted by the 2-D effect of the Travale graben, while with increasing period the induced current system becomes more and more 3-D below all sites. This limits the determination of the sedimentary cover thickness (max. 2500 m) by 1-D and 2-D model calculations to periods of less than 100 s.     

Gravity and elevation changes in the Travale geothermal field (Tuscany) Italy

The results of precise levelling measurements on a specially constructed network of benchmarks in the Travale geothermal area (Tuscany, Italy) revealed the subsidence of the central part of this area, at an average rate of 20 mm/year in the period 1978 – 1980. Two sets of gravity measurements over the same time-interval, using two Lacoste — Romberg gravimeters, have an average standard error of 2–4 μGal for the main network, and 4–8 μGal for the auxiliary network. The observed g variations fall within the error range in most of the stations. The variations noted in the stations in the south-western area of the field clearly fall outside the confidence interval, and cannot entirely be attributed to changes in elevation.An absolute gravity station was set up at Palazzo at Piano (Siena), where measurements were made by the IMGC absolute gravimeter, to detect any long-term gravity variations induced by geodynamic events.     

Hydrothermal metamorphism in the Larderello geothermal field

The various tectonic units underlying the Larderello — Travale geothermal region have undergone hydrothermal metamorphism. The hydrothermal mineral assemblages are generally consistent with the temperatures now measured in the wells, leading to the hypothesis that solid phases deposited from a liquid medium during a hot-water stage that preceded the vapour-dominated one.     

Differential attenuation of seismic waves at a dense array: A marker of the travale field from sources at regional distance

Dense arrays of three-component seismographs have been maintained in the Travale geothermal field during two periods of several weeks. The particular array configuration (20 stations on a 3 × 3 km area) was intended to allow a high horizontal resolution investigation of the field itself, by analysis of steep-incidence waves. Natural and artificial sources at 100 km distances provided phases reflected by deep crustal interfaces, from which differential attenuation factor estimates are obtained. Consequent signatures of the area around and within the productive field are discussed, with respect also to laboratory measurements on porous, fluid-filled rock samples.     

Magnetotelluric studies at Cerro Prieto

Ten magnetotelluric soundings were made near the production zone at Cerro Prieto, bringing to 17 the number of stations occupied in and around the geothermal field during 1978 and 1979. Results from the first seven soundings were reported elsewhere (Gamble et al., 1980). Data were analyzed in the field using a DEC LSI-11 microcomputer installed in a recording truck. The new soundings provide new information on the geometry of the geothermal system. We find evidence for a narrow resistive zone plunging southeastward, at a shallow angle, from a concealed apex a few hundred meters north of the power plant. This zone comes within about 500 m of the surface and can be traced roughly 5 km to the south. This zone correlates very well with a region of hydrothermal metamorphism, which has been identified by means of detailed studies of cores and cuttings from wells. The three dimensionality of this feature, combined with the influence of the large resistivity contrast between valley sediments and Cucapa Range granites 10 km to the west, makes a rigorous quantitative interpretation impractical. Although such an interpretation awaits further advancements in the techniques of calculating electromagnetic scattering by complex geological structures, the general picture seems clear. A comparison of the subsurface resistivity model with the position of the production zone and with subsurface geology suggests that the heat source lies at depth, roughly 4 km SSE of the present power plant.North of the power plant a two-dimensional interpretation of the magnetotelluric data is possible. A good fit between observed and calculated parameters is obtained for a subsurface model that is consistent with the model derived from dipole - dipole de resistivity measurements.     

Interpretation of magnetic anomalies over the Waimangu geothermal area, Taupo volcanic zone, New Zealand

The study area is located on the eastern side of the Taupo volcanic zone in central North Island of New Zealand. It lies a few kilometres to the southwest of Mt Tarawera, the site of the biggest New Zealand volcanic eruption in historical times (the June 1886 Tarawera eruption). The study area includes the Waimangu geothermal field and a small part of Waiotapu and Waikite fields. The extensive surface thermal expressions (boiling springs, hot lakes, craters, and sinter terraces) occurring at the Waimangu field were all formed following the 1886 Tarawera eruption. Another large area of less intense thermal manifestations (thermal ground and hydrothermally altered rocks) exists about 5 km southwest of Waimangu, extending towards the Waiotapu field in the south. In 1993 an aeromagnetic survey was conducted over the study area at an average altitude of about 350 m above the ground. The results show a subdued negative residual anomaly (about −100 nT) over the Waimangu field, which can be interpreted by near-surface hydrothermal demagnetisation of rhyolitic host rocks. The lateral distribution of the demagnetised rocks is much greater than the thermal area of Waimangu, and is consistent with the extent of low resistivity rocks across the study area. The magnetic interpretation also shows that two-high standing dacite domes situated about 5 and 7 km to the southwest of Waimangu have been affected by hydrothermal demagnetisation. There are negative residual anomalies outside the low resistivity zone that could be associated with reversely magnetised rocks (age >0.78 Ma). A strong positive residual anomaly (up to 450 nT) occurs to the east of the Waimangu field. Results from 3-D magnetic interpretation indicate some alternative models for this positive anomaly: (1) southwest–northeast trending, vertical basalt dykes (magnetisation 10 A/m), tops between −0.1 and −0.65 km RL (reduced LEVEL=relative to sea level), (2) a thick ( 1 km) sequence of rhyolites (magnetisation 2.5 A/m) extending from the surface down to about −0.8 km RL, and (3) a rather thin (0.35 km) sequence of rhyolites (from surface to sea level) underlain by basalt bodies similar to those of model (1).     

A broadband tensorial magnetotelluric study in the Travale geothermal field

As a contribution to the EEC study of the potential contribution of electric and electromagnetic techniques to geothermal exploration, magnetotelluric studies have been undertaken with a sounding bandwidth ranging from 2 to 7 decades of period at more than 30 sites within the chosen test area of Travale. This area must be one of the most unfavourable for the application of electrical techniques on account both of the thickness (up to 2 km) of conducting (< 1 ohm · m in some locations) cover formations and of the intensity of the artificial disturbances from local power stations and distribution lines. Nevertheless it has been possible to obtain good quality data over part of the sounding band employing an automatic in-field analysis system and rigorous data analysis and to penetrate to reservoir depths at the centre of the graben by undertaking broadband soundings (up to 10' s) at some sites. For interpretation of the data for periods up to about 100 s, 2-D modelling is both satisfactory and essential (1-D modelling provides correct layer resistivities but underestimates interface depths) and good agreement has been obtained for an electrical structure model and the relevant geological section. The 2-D models, which best fit the long period data, are characterised both by zones of highly conducting flysch cover formations and by an anomalously conducting basement. Restriction of the study to a test area within the Travate graben inhibits the unequivocal association of these conducting zones with the thermal anomaly.     

Thermal techniques for characterizing magma body geometries

The surface heat flux distribution resulting from emplaced magma bodies can be used to help characterize the magma source. Closed-form analytical solutions for the conduction heat transfer from various idealized magma geometries (dikes, sills, and spheres) are obtained using either the Schwarz-Christoffel transformation theorem (dikes and sills) or the ‘method of images’ with superposition (spheres). Comparison of these analytically determined heat flux distributions with field data from active geothermal areas at Yellowstone, Avachinsky volcano, Kilauea Iki, and the Coso geothermal area indicates that these fields may be conduction dominant, at least over certain depths. The comparison for Yellowstone implies that a sharp thermal boundary exists at a depth of approximately 1 km; this supports the suggestion by Morgan et al., (1977) that a strong hydrothermal zone exists at about that depth. The comparison for Avachinsky indicates that a spherical magma chamber exists at approximately 4·8 km depth; this is in close agreement with estimates by Fedotov et al., (1976) for a spherical magma chamber at 5 km depth. The comparison for Kilauea Iki indicates that the edge of the buried molten lava lense was 210 – 216 m from the center of the lake in 1975; this result is in good agreement with several independent geophysical measurements.     

Application of active audiomagnetotellurics (AAMT) in the geothermal field of Travale, Tuscany

In October 1981 the AAMT method was tested in the geothermal field of Travale. This method is based on the MT method, but uses artificial EM fields excited by a transmitter some kilometres from the receiving station. The transmitter consists of a switch mode amplifier for the lower frequency band (< 300 Hz) and six stacked linear amplifiers for the high frequency band. Maximum output is about 5 kW. For measurement of the very small EM field at the receiver the correlation technique is used to obtain best noise rejection. Interpretation of measured data is done by model calculations with the help of a near field theory. Cagniard's theory is found to be applicable only for high frequencies (i.e. short periods). The final result is a three-layer model in good agreement with known geology and resistivity models from other authors.     

P and S arrival time anomalies at a dense array: Marker of the travale field

Arrival times of seismic waves propagated to a dense array (20 three-component seismometers on a 3 × 3 km area) from natural sources beneath the Travale field, have been analysed for lateral variations which can be related to the extent of the productive reservoir. Significant teleseismic delays between sites only a few hundred meters apart correlate with productive well R9. Spatial variation of both P and S travel arrival times from local earthquakes at the edge of the array and 4 km deep beneath the reservoir consistently delineate its lateral extent to the south and east as documented by drillholes.     

An updated numerical model of the Larderello–Travale geothermal system, Italy

Larderello–Travale is one of the few geothermal systems in the world that is characterized by a reservoir pressure much lower than hydrostatic. This is a consequence of its natural evolution from an initial liquid-dominated to the current steam-dominated system. Beneath a nearly impermeable cover, the geothermal reservoir consists of carbonate-anhydrite formations and, at greater depth, by metamorphic rocks. The shallow reservoir has temperatures in the range of 220–250 °C, and pressures of about 20 bar at a depth of 1000 m, while the deep metamorphic reservoir has temperatures of 300–350 °C, and pressures of about 70 bar at a depth of 3000 m. The 3D numerical code “TOUGH2” has been used to conduct a regional modeling study to investigate the production mechanism of superheated steam, the interactions between the geothermal field and the surrounding deep aquifers, and the field sustainability. All the available geoscientific data collected in about one century of exploration and exploitation have been used to provide the necessary input parameters for the model, which covers an area (4900 km²) about 10 times wider than the Larderello–Travale geothermal field (400 km²). The numerical model explains the origin of the steam extracted in about one century of exploitation and shows that, at the current level, the production is sustainable at least for the next 100 years.     

The geophysical exploration of geothermal reservoirs: the travale test site (Italy)

Geothermal reservoirs are usually tied to geological structures that are not easily explored by one geophysical method only. The combined application of several different techniques seems to be the most promising strategy. The development of the best combination of methods, comparison of the available instruments, and a number of processing and interpretation techniques have to be tested on a well-studied geothermal reservoir. The Travale geothermal field (Tuscany, Italy) is a high enthalpy hydrothermal system with single phase (vapour or liquid) or two-phase conditions, dominating in different parts of the reservoir. This field was chosen as a test site for geophysicists of the EEC member states. Since 1980 electrical, electromagnetic, magnetic and seismic exploration techniques have been tested in cooperation with ENEL, the Italian Electricity Agency, under the sponsorship of the Community.     

Geothermal exploration of kos island, Greece: Magnetotelluric and microseismicity studies

This paper reports the results of magnetotelluric (MT) and microseismicity studies, conducted as part of a multi-disciplinary project to explore the geothermal potential of the island of Kos, Greece. The MT survey, comprising 18 soundings, was carried out in the bandwidth 128 Hz–40 s, in order to determine the deep conductivity structure in the geothermally prospective western part of the island. Rigorous dimensionality analysis has indicated that the geoelectric structure could adequately be approximated with 1-D interpretation tools. Two significant and seemingly communicating conductive zones of potential geothermal interest were found within the first 2 km. The first is extensive and shallow, detected at depths of 400–600 m; the second is deeper (1000–1300 m), but of considerably smaller lateral dimensions. A very deep relative conductor (<25 Ωm) was also detected at depths of 7–10 km, which is thought to comprise part of an old magma chamber with brine-saturated rocks. The microseismicity studies revealed the partial or total attenuation of shear waves in many microearthquake records. The analysis of these observations determined the vertical and lateral extent of that attenuation zone, the greatest part of which is located underneath the marine area between western Kos and Nissyros island to the south, extending approximately from near the surface to about 1.5 km depth. The nature of this zone is discussed in terms of fluid concentration due to the geothermal system of the area.     

Genesis of the plutonic-hydrothermal system around quaternary granite in the kakkonda geothermal system, Japan

The Kakkonda plutonic-hydrothermal system has as its heat source the Quaternary Kakkonda granite. The Kakkonda granite has a thick (1.3 km) contact-metamorphic zone, known mainly from the geothermal survey well WD-1a (total depth: 3729 m) drilled by the New Energy and Industrial Technology Development Organization (NEDO). The Kakkonda granite is a stock several tens of square kilometers in area with an upper contact about 1.5–3 km deep. It is a composite pluton varying from tonalite to granite. The early-stage granitic rocks are slightly metamorphosed to biotite grade by late-stage granitic rocks. K-Ar ages of separated minerals from the granitic rocks in both stages show the same cooling ages of 0.24–0.11 Ma for hornblende, 0.21–0.02 Ma for biotite, and 0.14–0.01 Ma for potassium feldspar. These are the youngest ages for granite in the world. The K-Ar ages become almost zero at 580°C for biotite and potassium feldspar, and at 350°C for illite. The Kakkonda granite intruded into a regional stress field in which the minimum principal stress was ENE–WSW and nearly horizontal. The regional stress field coincides with that of a previously recognized F2 fracture system before 0.4–0.3 Ma. Both stages of the Kakkonda granite and the contact aureole are fractured by recent tectonism, resulting in a zone of hydrothermal convection from about 2.5–3.1 km depth up to the surface. The boundary between the zone of hydrothermal convection and the underlying zone of heat conduction occurs 250–550 m below the upper contact of the Kakkonda granite, and has a temperature of 380–400°C.     

A model for the sulphur springs geothermal field St Lucia

A model to explain the behaviour of the Sulphur Springs geothermal field has been derived from downhole temperature records in the exploration boreholes. The model incorporates a main reservoir at 1 – 1.5 km depth, intersected by steeply inclined fissures which carry steam and gas to the well bores, and to the natural fumaroles. A substantial decline in the gas content of the steam could have serious consequences where the fissures are utilised as conduits between the boreholes and the deep reservoir. Further development of the field should concentrate on the fissures around 300 m or on the reservoir itself around 1000 – 1500 m.