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.     

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.     

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.     

Geoelectric study of some indian geothermal areas

The electrical resistivity technique has been used extensively in the Indian sub-continent for the exploration of geothermal areas. The first systematic application of the resistivity method for locating the geothermal reservoir was made in the Puga area, which is situated very close to the collision junction of the Indian and the Asian plates and has numerous hot springs with temperatures varying from 30 to 84°C (boiling point at that altitude). The resistivity depth probes indicated the presence of a conductive zone, with a value of 10–25 ohm·m and a thickness varying from 50 to 300 m over an area of 3 km², which was inferred to correspond to a shallow thermal reservoir. Thermal surveys also revealed a significant anomaly corresponding to this zone, which, when drilled, encountered a reservoir of wet steam with a temperature of up to 135°C, thus confirming the results of the resistivity surveys. Somewhat similar results have been obtained in the adjoining area, where much thicker zones with moderate electrical conductivity have been mapped.Another significant application of the electrical resistivity method has been made in the NNW-SSE extending West Coast geothermal belt of India, which is covered by Traps (Basalts) of the Cretaceous-Eocene. The area is characterized by the existence of a number of hot springs, with temperature up to 70°C, along a 400 km long alignment, associated with steep gravity gradients and an isolated occurrence of native mercury in the zone of a gravity “high”. The enigmatic geology of this area has been mapped, giving quantitative estimates of the thickness of the Traps and inferring the structural features. In addition, the electrical resistivity depth probes have also been used to identify the pre-Trappean geology, thereby locating the probable areas which could act as geothermal reservoirs.This paper presents the results of the electrical resistivity surveys in the form of geoelectric sections for some of the geothemal fields in the Indian sub-continent.     

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.     

Magnetotelluric soundings in the travale area, Tuscany

Long period MT soundings have been completed on ten sites already recorded on the Travale geothermal field in order to check the reproducibility of past data and provide new information on the geothermal structure and, eventually, on reservoir properties. A versatile equipment allowing for recording signals in the 100 – 0.01 Hz range was used with recording periods up to 6 hours. Although noise was a serious problem, especially in the 1 – 3 s window, it was possible to achieve acceptable signal to noise ratios and in most cases to produce reliable signal processing. Records also display a strong anisotropy depending on structural strikes. Interpretation did not call for sophisticated 2D or 3D modelling but instead focussed on 1D inversion along the less disturbed principal direction. It is concluded that because of a geoelectrical setting with a conductive cover overlying a resistive reservoir and basement, electrical conductance is a parameter representative neither of reservoir structure nor of soaking fluid properties. Essentially it reflects conductivity and/or thickness variations within cover formations. The best fit was found for a three layer structure associating a superficial resistive horizon, an intermediate, generally 1000 m thick, conductive complex and a resistive basement. It is not possible to discriminate within this resistive complex any conductive anomaly or other reservoir feature. In conclusion, the information yielded by the MT method is essentially of structural nature. The top of the carbonate reservoir could be mapped with fair confidence in good agreement with available drilling data.     

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.     

The shallow structure of Los Humeros and Las Derrumbadas geothermal fields, Mexico

Los Humeros caldera and Las Derrumbadas rhyolitic domes located in the State of Puebla (Mexico) are being extensively studied by Comisión Federal de Electricidad (CFE) to assess the geothermal potential of that area. Both geothermal sites have been locally studied with geological, geochemical and geophysical methods. In order to get a geological picture of the sub-surface of the area comprising both sites, a regional structural survey was conducted together with gravity and magnetics studies. These studies showed a regional weakness zone (the Libres-Oriental depression), where the volcanic activity related to the geothermal manifestations took place.The tectonic events in the area are represented by two systems of structures. The first one (N140°–N170°) gave rise to the folds and overthrusts affecting the sequence of calcareous rocks resting upon the basement. This system is affected by a second system (N40°–70°E). Regionally these tectonic events remodelled the sedimentary sequence giving rise to a series of topographic highs that constitute the core of several ranges, and to depressions that were filled with volcano-sediments during the ensuing volcanic activity.The rhyolitic domes of Las Derrumbadas are emplaced in a regional NW-SE lineament, which reflects clearly on the Bouguer anomaly, and is interpreted as a front of overthrust affecting the marine sedimentary sequence. Estimations of depths to the basement, based on magnetic data, showed that the basement under Los Humeros caldera is shallower than in Las Derrumbadas. The gravity data were interpreted in terms of tabular bodies associated to calcareous rocks, of igneous intrusions, and of depressions filled with volcano-sediments. A joint interpretation of the geological and geophysical data led to a model of the structure of the shallow crust in the area.     

Appraisal of the Tokaanu–Waihi geothermal field and its relationship with the Tongariro geothermal field, New Zealand

Tokaanu–Waihi geothermal field is situated near the southern end of the Taupo Volcanic Zone, New Zealand. Neutral chloride thermal waters discharge at Tokaanu and Waihi in the north of the field on flat land between the andesite volcanoes Tihia and Kakaramea and the shore of Lake Taupo, while steam-heated thermal features occur at Hipaua on the northern flanks of Kakaramea. Electrical resistivity surveys have been made over the field using several different measurement techniques. In the north of the field where roads and tracks allow vehicle access, resistivity profiling using Schlumberger arrays with electrode spacings (AB/2) of 500 m and 1000 m show that Tokaanu, Waihi and Hipaua all lie within a continuous region of low apparent resistivity (5–20 Ωm) and are thus part of the same geothermal system. Along the eastern edge of the system there is a sharp transition to apparent resistivities greater than 100 Ωm in the cold surrounding region. Surveys on Lake Taupo using an equatorial bipole-bipole electrode array towed behind boats (spacing equivalent to AB/2=500 m) found that the low resistivity zone extends offshore by about 1 km. The steep, bush-clad, southern part of the field was surveyed with magnetotelluric (MT) resistivity measurements using both naturally occurring signals and the 50 Hz radiation from the power wires as sources. These measurements found low resistivities over the north-eastern slopes and around the summits of Tihia and Kakaramea, indicating thermal activity. However, the measurements were too widely spaced to allow the field boundary to be clearly delineated. Interpretation of the resistivity and other data suggests that the Tokaanu–Waihi thermal waters rise nearly vertically from a source deep beneath the elevated southwestern part of the field to the water table. These waters then flow north to discharge at the surface near Lake Taupo. Neighbouring geothermal systems, which occur at Tongariro about 18 km south of Tokaanu–Waihi, and at Motuoapa about 10 km to the northeast, are separated from the Tokaanu–Waihi field by high resistivity ground. This suggests that the thermal fluids discharging at the three fields do not have a common source, as has been suggested previously.     

Chilean geothermal resources and their possible utilization

Most of the hot spring areas in Chile are located along the Andean Cordillera, associated with Quaternary volcanism. The volcanic—geothermal activity is mainly controlled by the subduction processes of the Nazca and Antarctic oceanic plates under the South America continental plate, and occurs at three well-defined zones of the Chilean Andes: the northern zone (17°30′–28°S), the central—south zone (33φ–46°S) and the southern-most or Austral zone (48°–56°S).Some tested high temperature geothermal fields, and geological and geochemical surveys of many other hot spring areas, evidence a great potential of geothermal resources in this country. Both electrical and non-electrical applications of this potential are considered in this paper.Taking into account the potentially available geothermal resources, the development of natural resources, the geographic and social—economic conditions existing in the different regions of Chile, it is concluded that power generation, desalination of geothermal waters, recovery of chemicals from evaporite deposits and brines and sulfur-refining are the main possible applications of geothermal energy in northern Chile; in central—south Chile geothermal energy is suitable for agribusiness such as greenhouses, aquaculture and animal husbandry.     

Gravity survey and interpretation of bouguer anomalies in the campidano geothermal area (Sardinia, Italy)

A gravity survey of the Campidano geothermal fields and surrounding region was conducted in 1981. It covered an area of 1900 km² and included 952 uniformly distributed stations. The Bouguer anomaly is generally negative within the Campidano graben, reaching −10 mgal in the central zone, whereas a positive Bouguer anomaly prevails outside the graben, exceeding 20 mgal in several areas. The gravity data were interpreted using spectral analysis and two-dimensional models, to determine the thickness of sediments and andesitic volcanics within the graben. The total thickness of these formations reaches 3000 m in the centre, but is reduced elsewhere, especially towards the sides of the graben. The thermal springs on both the eastern and western sides of the graben are associated with residual positive anomalies and are near very steep gradients in the Bouguer anomaly.     

The Iceland Deep Drilling Project: a search for deep unconventional geothermal resources

The Iceland Deep Drilling Project (IDDP) is a long-term program to improve the economics of geothermal energy by producing supercritical hydrous fluids from drillable depths. Producing supercritical fluids will require the drilling of wells and the sampling of fluids and rocks to depths of 3.5–5 km, and at temperatures of 450–600 °C. The IDDP plans to drill and test a series of such deep boreholes in the Krafla, Nesjavellir and Reykjanes geothermal fields in Iceland. Beneath these three developed high-temperature systems frequent seismic activity continues below 5 km, indicating that, even at supercritical temperatures, the rocks are brittle and therefore likely to be permeable, even where the temperature is assumed to exceed 550–650 °C. Temperature gradients are greater and fluid salinities smaller at Nesjavellir and Krafla than at Reykjanes. However, an active drilling program is underway at Reykjanes to expand the existing generating capacity and the field operator has offered to make available one of a number of 2.5 km deep wells to be the first to be deepened to 5 km by the IDDP. In addition to its potential economic significance, drilling deep at this location, on the landward extension of the Mid-Atlantic Ridge, is of great interest to the international science community. This paper examines the prospect of producing geothermal fluids from deep wells drilled into a reservoir at supercritical temperatures and pressures. Since fluids drawn from a depth of 4000–5000 m may prove to be chemically hostile, the wellbore and casing must be protected while the fluid properties are being evaluated. This will be achieved by extracting the fluids through a narrow retrievable liner called the “pipe”. Modelling indicates that if the wellhead enthalpy is to exceed that of conventionally produced geothermal steam, the reservoir temperature must be higher than 450 °C. A deep well producing 0.67 m³/s steam (2400 m³/h) from a reservoir with a temperature significantly above 450 °C could, under favourable conditions, yield enough high-enthalpy steam to generate 40–50 MW of electric power. This exceeds by an order of magnitude the power typically obtained from a conventional geothermal well in Iceland. The aim of the IDDP is to determine whether utilization of heat from such an unconventional geothermal resource at supercritical conditions will lead to increased productivity of wells at a competitive cost. If the IDDP is an economic success, this same approach could be applied in other high-temperature volcanic geothermal systems elsewhere, an important step in enhancing the geothermal industry worldwide.     

Conductive electrique de sels doubles du type iodure d'argent-diiodures de piperazinium et de N, N′-alklylpiperazinium

The electrical conductivity has been studied for four binary systems of the type silver iodide-piperazinium or N, N′-disubstituted piperazinium diiodides. At 25 °C, the conductivity shows a maximum of 0.1 (ohm cm)⁻¹ for the system silver iodide-N, N′ tetramethylpiperazinium diiodide at 93.25% mol silver iodide. The ionic nature of the conductivity has been proven.     

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.     

Structure and hydrology of the Ogiri field, West Kirishima geothermal area, Kyushu, Japan

The geothermal system in the West Kirishima area is controlled by a system of faults and fractures oriented along two main directions, northwest to southeast and east–northeast to west–southwest. The Ginyu fault extends through the Ogiri field in the Ginyu area, which is one of the east–northeast to west–southwest striking faults in this area. This fault is the reservoir target for developing the geothermal resources in the Ogiri field. The Ginyu fault is a near planar fracture with a uniform temperature of 232°C and has near-neutral pH, chloride fluids. Based on the results of a detailed analysis of the Ginyu fault, all production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, securing steam production for a 30 MW_e power plant. A typical fracture-type geothermal model for the Ogiri field was developed on the basis of the geology, electric and geophysical logs, fluid chemistry, and well test data.     

Geothermal systems in Iceland: Structure and conceptual models—II. Low-temperature areas

A review and assessment of data pertaining to the origin and nature of low-temperature geothermal activity in Iceland are presented. This activity is widely distributed in Quaternary and Tertiary formations on the American plate in western Iceland west of the active belts of volcanism and rifting but it is very sparse on the European plate east of these belts. Low-temperature systems occur in a few places within the active volcanic belts. Temperatures range from just above ambient to a little over 150°C. Generally speaking, resevoir temperatures decrease with increasing distance from the active volcanic belts. The distribution of the low-temperature areas can be correlated to a large extent with active tectonism. In Iceland the European plate is tectonically stable but in the American plate the shear stress field is complicated, leading to complex fracturing and faulting of the crust at present. No single generalized conceptual model describes the basic features of all low-temperature areas in Iceland. Low-temperature geothermal activity is considered to develop by one of the following four processes, or any combination of them: (1) deep flow of groundwater from highland to lowland areas through permeable structures driven by the hydraulic gradient; (2) convection in young fractures formed by tectonic movements in old and relatively impermeable bedrock; (3) drift of high-temperature geothermal systems out of the active volcanic belts in conjunction with their cooling and extinction of the magma heat source; and (4) magma intrusion into Quaternary or Tertiary formations adjacent to the active volcanic belts. Formation of permeable fractures by recent earth movements is probably the most common process responsible for the development of low-temperature activity through convection in these fractures. Convection in low-temperature systems with temperatures above some 60°C is probably mostly driven by pressure differences created by a relatively light hot water column within the system and a denser cold water column outside it. In systems of lower temperature the convection is driven by hydrostatic head in the recharge areas. The source of the low-temperature waters is largely meteoric. However, in some coastal areas a significant seawater-groundwater component is present, up to 10%. Waters not containing a seawater component are low in dissolved solids, or in the range 150–500 ppm. The reason is the low content of anions, particularly Cl, in the basaltic rock forming soluble salts with the major aqueous cations. Geothermal waters from the low-temperature areas in Iceland typically possess lower δD-values (more negative) than the local precipitation. This difference is variable; most often it lies in the range of 10–30% δD, but it may be as large as 70‰. This difference has been considered to indicate that the recharge areas to the low-temperature areas lie inland on higher ground, the distance being as much as 150 km. The interpretation favoured here is that at least some of the low-temperature waters contain a component of “ice-age water”, i.e. water that is older than about 10, 000 years. The “ice-age water” is depleted in deuterium relative to today's precipitation. When “ice-age water” is present in the geothermal water, deuterium cannot be used as a tracer to locate the recharge areas to the geothermal areas and in this way to deduce about regional groundwater flow.     

An estimate of the resource potential of New Zealand geothermal fields for power generation

The basic similarity between most of the New Zealand geothermal fields suggests that the exploited fields of Wairakei and Broadlands can be used as indicators of the potential of other fields as resources for steam for power production. Assuming adequate permeability will be obtained in fields yet to be tested, the two parameters controlling this potential are areal extent (as defined by resistivity survey) and temperature at depth. As most field temperatures are bracketed by Wairakei (270°C maximum) and Broadlands (310°C maximum), field potential per unit area should also be bracketed by the field potentials per unit area of these two fields, i.e. Wairakei at 10–11 MWe/km² and Broadlands at 13–14 MWe/km².Based upon our present knowledge of the fields in question we may thus assess their proven, inferred and speculative reserves. Our totals for all fields of 450 MWe proven, 750 MWe inferred and 1300 MWe speculative suggests that New Zealand has some 1300–2500 MWe available from its geothermal resources should it desire to exploit these for electrical power.These figures can only be confirmed and improved by drilling and ultimately by exploitation. The most promising tool for a full assessment of a field potential, the reservoir model, can only really be set up once the field has been exploited sufficiently to have been disturbed. In future cases this may only be the case once a power station has been established and has been operating for some time.     

Takigami geothermal system, northeastern Kyushu, Japan

The Takigami geothermal reservoir is bounded by a system of faults and fractures oriented along two main directions, north to south and east to west. The Noine fault has a large vertical displacement and trends north to south, dividing the subsurface characteristics of resistivity, permeability, temperature and reservoir depth. The Takigami geothermal fluid has a near neutral pH and is of the Na–Cl type, with a chloride content ranging from 400 to 600 ppm. The southwestern part of the area has the highest subsurface temperature, up to 250°C. The deep fluid originates from the southwest, and flow is mainly to the north and partly to the east along faults and fractures, decreasing in temperature with increasing lateral flow.