Geophysical exploration for geothermal low enthalpy resources in Lipari Island, Italy

Integrated geophysical surveys were performed in two sites, Fossa di Fuardo and Terme di San Calogero in Lipari Island, Southern Italy with the intent of the exploration of low-enthalpy geothermal fluids. Both sites show strong geochemical and geologic evidences of hydrothermal activity. The geophysical methods consist of two microgravimetric surveys, two 2D geoelectric profiles, a seismic reflection profile and a five seismic refraction profiles. The seismic methods allowed us to locate the main subsurface seismic discontinuities and to evaluate their geometrical relationships. The gravity field was used to constraint the seismic discontinuities, while the electric prospecting let discriminate more conductive areas, which could correspond to an increase in thermal fluid circulation in the investigated sites.The results obtained by the different geophysical methods are in good agreement and permit the definition of a reliable geo-structural model of the subsurface setting of the two investigated areas. A low-enthalpy geothermal reservoir constituted by a permeable pyroclastic and lava sequence underlying two shallow impermeable formations was found at Fossa del Fuardo. The reservoir is intersected by some sub-vertical faults/fractures that probably play an important role in convoying the thermal water up to the surface. At the other site, Terme di S. Calogero, the geophysical surveys showed that an intense circulation of fluids affects the subsurface of the area. This circulation concentrates along a ENE-trending fault located at a little distance from the thermal resort. The hot fluids may upraise along the fault if the width of the ascent area is smaller than 20 m.     

The self-sealing geothermal field

A geothermal field producing dry steam or high temperature water is a trap for convection currents generated in an aquifer of high permeability and of sufficient thickness by a deep heat source. A basic implication of this concept is, that a geothermal field requires a cap-rock of more or less impermeable rocks above the producing aquifer. In Larderello, Monte Amiata, and Salton Sea geothermal fields, a clearly reconnaissable tight formation overlies the producing zone and limits the upward movements of the convection currents. In other fields,i.e. The Geysers (California), Wairakei and Waiotapu (New Zealand) we do not know a geologically well defined cap-rock formation, presenting a large difference in permeability in comparison with the reservoir formation. The hot water circulating in a hydrothermal system without a cap-rock can produce deposits and rock alteration in proper places along the flow paths. The tracture and pore filling and any other permeability reducing factors increase resistance to the water circulation: those processes can originate an effective cap-rock. By such processes a hydrothermal system can become a self-sealed geothermal field. The silica deposition is probably the main self-sealing process. In fact, 1) silica is very common. 2) it is available with almost no limitation, 3) its deposition is strictly related to temperature changes, and 4) it is likely to produce very effective patterns of deposition. Where an unlimited CO₂ supply is available at depth, the calcium carbonate deposition appears to be a noticeable sealing process, which is controlled by pressure, at relatively shallow depth. In other cases CaCO₃ precipitation should not be an important factor in the self-sealing of geothermal fields. Argillization appears to be an important shallow process. It is especially effective in the acid environment of many thermal shows, thus determining their migration and/or extinction. According to our analysis and to present evidence those three self-sealing processes are the most important ones. In The Geysers Field, the wells penetrated the same formation, the Franciscan graywackes, from top to bottom. The Franciscan Formation has a very low primary permeability; secondary or fissure permeability is at the contrary very high. It is evident that there is no recognizable cap-rock in the accepted sense of petroleum geology. The wells produce superheated steam; the producing zone begins at 300 m depth or so; the quantity of steam increases with the thickness of the producing zone penetrated by the holes. Beginning in 1964, the wells have been drilled with air as circulating medium. No steam or water has been observed in the top few hundreds meters drilled: we can safely conclude that the graywackes are impervious in the upper section of the holes. Cores and cuttings show frequent fissures filled with silica in different mineral forms and hydrothermally altered rocks are common. In the Geysers area, hot springs, steam vents, carbon dioxide and hydrogen sulphide fumaroles are numerous, and wide zones of rocks, altered by past hydrothermal activity, are prominent features. As usual in many hyperthermal areas, also in The Geysers the manifestations of surface heat change frequently in place, in size, and in fluids discharge. The filling of rock fissures by mineral deposition seems the simplest and most natural explanation of the place changes of the individual springs. The active faults continually generate new fissures, limit the sealing action, and account for the persistent surface thermal activity of the area. The composition of the waters from the hot springs at The Geysers has been re-considered, in comparison with both surface waters and natural steam. The hot springs mainly originate by natural steam condensation, as Allen and Day stated in 1927. This conclusion is now strenghtened and extended: the perched water table producing hot springs at The Geysers is purely condensed steam. Practically all its characteristics can be explained by this condition alone. Separation from other shallow water bodies is extremely sharp. Let us sumarize: the impermeability of the upper section of the holes is demonstrated by the lack of fluids in the Sulphur Bank area, whereas the geochemistry of the hot springs compared with shallow waters indicates that similar conditions occur in the Geysers and Little Geysers areas. Furthermore, silica and other fissure-filling processes occur all over the region, as well as argillization of graywackes. We conclude that:

1. a)
   a cap-rock exists in The Geysers Field; this fact readily explains the production of dry steam;     
   
   

Groundwater flow analysis using different geothermal constraints: The case study of Acqui Terme area,northwestern Italy

We review some analytical techniques that use underground thermal data as tracers of groundwater flow. These techniques allow the evaluation of the Darcy velocity in shallow aquifers of mid-low permeability and the evaluation of heat gain/loss by conduction in deeper aquifers. Examples of application are then given for the Acqui Terme hydrothermal system, located in the Tertiary Piedmont Basin (northwestern Italy). The analysis of borehole temperatures allowed the inference of the hydraulic features of the sedimentary cover of the hydrothermal system. The results show the presence of a relatively weak flow, with upward and horizontal components, only in conglomerates occurring at the base of the marly impermeable cover. The analysis of the heat transported in the deep parts of the hydrothermal system was approached by splitting the water path into different sections, each with given shape, slope and hydraulic properties. The recharge area is situated in the upland, south of the discharge area. Meteoric water initially descends and then flows horizontally within the fractured metamorphic basement of the basin, heating by conduction. Finally, from a reservoir positioned at intermediate depths, hot water reaches rapidly the surface through a sub-vertical fault. This scheme of deep water flow is constrained by the regional surface heat flow and the local geothermal gradient, and it is consistent with data of rock–water equilibrium temperature.     

Trace-element distribution in an active hydrothermal system, Roosevelt hot springs thermal area, Utah

Chemical interaction of thermal fluids with reservoir rock in the Roosevelt Hot Springs thermal area, Utah, has resulted in the development of characteristic trace-element dispersion patterns. Multielement analyses of surface rock samples, soil samples and drill cuttings from deep exploration wells provide a three-dimensional perspective of chemical redistribution within this structurally-controlled hot-water geothermal system.Five distinctive elemental suites of chemical enrichment are recognized, each characteristic of a particular combination of physical and chemical conditions within the geothermal system. These are: (1) concentrations of As, Sb, Be, and Hg associated with siliceous material at locations of liquid discharge, fluid mixing or boiling; (2) concentrations of Mn, Ba, W, Be, Cu, Co, As, Sb and Hg in manganese and iron oxide deposits; (3) high concentrations of Hg in argillized rock near fumaroles and lower concentrations in a broad diffuse halo surrounding the thermal center; (4) concentrations of As in sulfides and Li in silicate alteration minerals immediately surrounding high-temperature fluid flow-controlling fractures; (5) deposits of CaCO₃ at depth where flashing of brine to steam has occurred due to pressure release. The geochemical enrichments are not, in general, widespread, pervasively developed zones of regular form and dimension as are typical in many ore-forming hydrothermal systems.As the geothermal system develops, changes and eventually declines through time, the chemical deposits are developed, remobilized or superimposed upon each other, thus preserving within the rocks a record of the history of the geothermal system. Recognition of trace-element distribution patterns during the exploration of a geothermal system may aid definition of the present geometry and interpretation of the history of the system.     

Geochemistry of the thermal springs and fumaroles of Basse-Terre Island,Guadeloupe, Lesser Antilles

 The purpose of this work was to study jointly the volcanic-hydrothermal system of the high-risk volcano La Soufrière, in the southern part of Basse-Terre, and the geothermal area of Bouillante, on its western coast, to derive an all-embracing and coherent conceptual geochemical model that provides the necessary basis for adequate volcanic surveillance and further geothermal exploration. The active andesitic dome of La Soufrière has erupted eight times since 1660, most recently in 1976–1977. All these historic eruptions have been phreatic. High-salinity, Na–Cl geothermal liquids circulate in the Bouillante geothermal reservoir, at temperatures close to 250  °C. These Na–Cl solutions rise toward the surface, undergo boiling and mixing with groundwater and/or seawater, and feed most Na–Cl thermal springs in the central Bouillante area. The Na–Cl thermal springs are surrounded by Na–HCO₃ thermal springs and by the Na–Cl thermal spring of Anse à la Barque (a groundwater slightly mixed with seawater), which are all heated through conductive transfer. The two main fumarolic fields of La Soufrière area discharge vapors formed through boiling of hydrothermal aqueous solutions at temperatures of 190–215  °C below the "Ty" fault area and close to 260  °C below the dome summit. The boiling liquid producing the vapors of the Ty fault area has δD and δ¹⁸O values relatively similar to those of the Na–Cl liquids of the Bouillante geothermal reservoir, whereas the liquid originating the vapors of the summit fumaroles is strongly enriched in ¹⁸O, due to input of magmatic fluids from below. This process is also responsible for the paucity of CH₄ in the fumaroles. The thermal features around La Soufrière dome include: (a) Ca–SO₄ springs, produced through absorption of hydrothermal vapors in shallow groundwaters; (b) conductively heated, Ca–Na–HCO₃ springs; and (c) two Ca–Na–Cl springs produced through mixing of shallow Ca–SO₄ waters and deep Na–Cl hydrothermal liquids. The geographical distribution of the different thermal features of La Soufrière area indicates the presence of: (a) a central zone dominated by the ascent of steam, which either discharges at the surface in the fumarolic fields or is absorbed in shallow groundwaters; and (b) an outer zone, where the shallow groundwaters are heated through conduction or addition of Na–Cl liquids coming from hydrothermal aquifer(s). Received: 9 November 1998 / Accepted: 15 July 1999     

Natural tracers for identifying the origin of the thermal fluids emerging along the Aegean Volcanic arc (Greece): Evidence of Arc-Type Magmatic Water (ATMW) participation

The Aegean volcanic arc is the result of a lithosphere subduction process during the Quaternary time. Starting from the Soussaki area, from west to east, the arc proceeds through the islands of Egina, Methana, Milos, Santorini, the Columbus Bank, Kos and Nisyros. Volcano-tectonic activities are still pronounced at Santorini and Nisyros in form of seismic activity, craters of hydrothermal explosions, hot fumaroles and thermal springs. A significant number of cold water springs emerge in the vicinity of hot waters on these islands.Chemical and isotopic analyses were applied on water and fumaroles samples collected in different areas of the volcanic arc in order to attempt the assessment of these fluids. Stable isotopes of water and carbon have been used to evaluate the origin of cold and thermal water and CO₂.Chemical solute concentrations and isotopic contents of waters show that the fluids emerging in Egina, Soussaki, Methana and Kos areas represent geothermal systems in their waning stage, while the fluids from Milos, Santorini and Nisyros proceed from active geothermal systems.The δ²H–δ¹⁸O–Cl^? relationships suggest that the parent hydrothermal liquids of Nisyros and Milos are produced through mixing of seawater and Arc-Type Magmatic Water (ATMW), with negligible to nil contribution of local ground waters and with very high participation of the magmatic component, which is close to 70% in both sites. A very high magmatic contribution to the deep geothermal system could occur at Santorini as well, perhaps with a percentage similar to Nisyros and Milos, but it cannot be calculated because of steam condensation heavily affecting the fumarolic fluids of Nea Kameni before the surface discharge.The parent hydrothermal liquid at Methana originates through mixing of local groundwaters, seawater and ATMW, with a magmatic participation close to 19%. All in all, the contribution of ATMW is higher in the central–eastern part of the Aegean volcanic arc than in the western sector. This difference, which is spotted in the variable isotopic composition of the sampled fluids from west to east along the arc, is probably due to several causes, including the tectonic regime, the depth of the deep reservoir below sea level, the age of volcanic activity and in general the geomorphologic state of each island.     

REGIONAL GEOTHERMAL EXPLORATION IN EGYPT*

Although Egypt is not characterized by abundant Cenozoic igneous activity, its location in the northeastern corner of the African plate suggests that it may possess geothermal resources, especially along its eastern margin. Regional geothermal exploration has been carried out in Egypt using the thermal gradient/heat flow technique and groundwater temperature/chemistry technique. In the thermal gradient/heat flow study, existing oil-well bottom-hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled. Groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements. On a very modest budget, a regional thermal high has been discovered along the eastern margin of Egypt, and a local thermal anomaly has been discovered in this zone. Published geological information suggests that the sandstones of the Nubian Formation may be a suitable reservoir for geothermal fluids. The new data indicate that temperatures of 150°C or higher may be found in this reservoir in the Gulf of Suez and Red Sea coastal zone where it lies at a depth of 4 km and deeper.     

天水地震区深部地温场的有限元计算

用有限元方法计算了天水地震区上地壳地温场。结果表明，影响该区浅部（层）地温场的主要因素是基底构造和断裂构造。祁连褶皱系为高热异常区，秦岭褶皱系为低热异常区，西秦岭北缘断裂带为地温梯度带。认为该带历史地震的发生与热应力差有关     

Geoelectromagnetic and geothermic investigations in the Ihlara Valley geothermal field

The Ihlara Valley is situated within a volcanic arc that is formed by the collision of the eastern Mediterranean plate system with the Anatolian plate. In this study we will present data from a reservoir monitoring project over the Ihlara-Ziga geothermal field, located 22 km east of Aksaray, in central Anatolia.Although identified geothermal resources in the Ihlara Valley are modest, substantial undiscovered fields have been inferred primarily from the volcanic and tectonic setting but also from the high regional heat flow (150–200 mWm⁻²) on the Kir ehir Massif.In 1988 and 1990, geoelectromagnetic surveys were undertaken by MTA-Ankara to confirm the presence of a relatively shallow (≈ 0.5–1 km), hydrothermally caused conductive layer or zone. CSAMT and Schlumberger resistivity data show good correspondence with each other, and 2-D geoelectric models are also in harmony with geologic data and gravity anomalies.The depth of the resistive basement, which is interpreted as Paleozoic limestone, is 200–250 m in the western part and increases eastward (≈ 600–750 m). This may imply N-S-oriented normal faulting within the survey area. The parameters of the top layer are a resistivity of 25 to 95 ohm m and a thickness of between 100 and 250 m. The thickness of the conductive tuffs between the top layer and the basement, whose resistivity is about 4–5 o hmm, also increases eastward (from 100 to 450 m). The apparent resistivity maps for the frequencies between 32 and 2 Hz reveal a localized low resistivity anomaly to the east of Belisirma.     

New evidence relating to the structure of the zone of hydrothermal discharges in the East Pauzhetka thermal field,Kamchatka

This paper is concerned with a discussion of a fundamental problem in geothermal research, that of reaching an understanding of the structure and physical nature of zones of thermal discharges, taking the Pauzhetka hydrothermal system as an example along with the eponymous geothermal field situated at the southern tip of Kamchatka. We combined geological and geophysical results from magnetic surveying, electrical surveying, gravity surveying, soil thermometry, pitting and well drilling in hydrothermal clay, to carry out a detailed study of a large zone of hydrothermal discharges in the structure of this hydrothermal system, that is, the East Pauzhetka thermal field. We identified a system of blocks that control aquifers, mineralization zones at the base of the sequence of hydrothermal clay, steam-charged and water-saturated volumes, and volumes where ascending hydrothermal brines circulate via fissures and pores. We hypothesize the existence of a shallow (a few tens of meters) top of a subintrusive body of an intermediate or basic composition. The intrusion of that body may have given rise to the formation of a tectono-magmatic uplift in the East Pauzhetka thermal field. The resulting geological and geophysical data stimulate multidisciplinary surveys of other areas in the Pauzhetka geothermal field and make their contribution to the solution of a major scientific and applied problem, which is to determine the source of heat for the field.     

Geochemistry of geopressured hydrothermal waters in the Niigata Sedimentary Basin, Japan

Abstract   Geothermal waters in the Niigata Sedimentary Basin, central Japan, are divided into four groups based on their chemical composition (i.e. Na-SO₄-type, Na-SO₄-Cl-type, Na-Cl-type and Na-Cl-HCO₃-type). The Na-SO₄-type geothermal water forms as a consequence of water–rock interaction and generally occurs in the outer part of the basin. The Na-Cl-type geothermal water is further subdivided into the original Na-Cl-type geopressured thermal water and the mixed Na-Cl-type geothermal water, in terms of its geochemical and isotopic composition. The original Na-Cl-type geopressured thermal water originates from a geopressured hydrothermal system containing the altered fossil formation waters that are sealed at depth. It moves up to the upper part of the depositional succession or the ground, and generally does not mix with groundwater that is of meteoric origin. This type of water is cooled by heat conduction. The concentration of Cl^– in this type of thermal water is very similar to that in seawater. The δD and δ¹⁸O values are approximately constant and independent of temperature. The original Na-Cl-type geopressured thermal water is distributed mainly along anticlinal axes in folded Neogene formations. The mixed Na-Cl-type geothermal water is related to the expulsion activity of the geopressured hydrothermal system and occurs mostly along active faults. It is formed by shallow groundwater of meteoric origin being mixed with geopressured hydrothermal water when the geopressured hydrothermal system was expulsed along active faults by paroxysmal tectonic events.     

Heat flow at standard depth

Secular and long-term periodic changes in surface temperature cause perturbations to the geothermal gradient which may be significant to depths of at least 1000 m, and major corrections are required to determine absolute values of heat flow from the Earth's interior. However, detailed climatic models remain contentious and estimates of error in geothermal gradients differ widely. Consequently, regions of anomalous heat flow which could contain geothermal resources may be more easily resolved by measuring relative values at a standard depth (e.g. 100 m) so that all data are subject to similar corrections.Regional heat flow data obtained in existing deep holes show reasonable correlation with values determined at shallow depth. Hence geothermal resources of low enthalpy can be characterised by extrapolating temperatures from relative heat flow data readily obtained from shallow boreholes. Regional control can be provided by casing deep boreholes drilled for other purposes.For routine geothermal exploration, borehole temperatures can be measured using gradient probes with fixed sensor separation (e.g. 5 m), allowing very accurate determinations of the geothermal gradient at a single depth. Values of relative heat flow can then be obtained after determining the thermal resistivity of the corresponding core interval. Sampling errors can be minimised by multiple determinations of thermal conductivity over the complete interval.     

北京南口—孙河断裂带水热系统特征与成因分析

中低温对流型地热资源在华北地区广泛分布,是一种清洁的替代能源.与活动断裂带相关的水热型地热资源是中低温地热系统的重要组成部分.本文基于高精度重力测量、微动测深及钻孔温度测量等数据,从热源、通道、储层和盖层四个方面探讨了南口—孙河断裂带水热系统特征.低重力异常揭示的燕山期花岗二长岩、闪长岩岩体范围为23.8 km~2和14.3 km~2,放射性测井数据计算得到其生热率均值为3.14μW·m~(-3),侏罗系火山岩生热率均值为1.65μW·m~(-3),隐伏岩体和火山岩均难以构成地热系统的附加热源.重力异常显示南口—孙河断裂带宽度约500~800 m,断裂带切割蓟县系雾迷山组白云岩热储层.钻井温度曲线显示断裂带内水热活动强烈,说明该断裂带是导水、导热的重要通道.断裂带南西侧马池口一带第四系松散层与侏罗系火山岩形成了热储盖层,微动测深显示火山岩最大厚度约1500 m.综上源、通、储、盖四个要素分析,该地热系统为热传导一对流复合型,来自京西北山区的大气降水经远距离径流深循环吸收地层热量后沿南口—孙河断裂上移到达裂隙发育的白云岩地层中形成热水.总之,沿南口—孙河断裂带具备了良好的地热地质条件,可达到规模开采的条件.     

Hydrothermal clays and pyrite in geothermal fields: Their significance for the geochemistry of present-day endogenous processes in southern Kamchatka

The studies reported in this paper were carried out in the Pauzhetka and Nizhne-Koshelevskii geothermal fields situated in the southern Kamchatka Peninsula within the Pauzhetka-Kambalnyi-Koshelevskii geothermal area. Layer-by-layer sampling of clays was carried out by stripping, pitting, and hand-operated drilling of core holes in the Verkhne-Pauzhetka thermal field and the Nizhne-Koshelevskii thermal anomaly, which were studied previously using several geological, geophysical, and hydrogeothermal techniques. Hydrothermal clays were found to compose a nearly continuous sheet on the surface of the thermal field and of the thermal anomaly. The sheet has an average thickness of 1.3 to 1.5 m. The chemical and mineralogic composition of the clays have been characterized. The concentrations of Au, Hg, Pb, and Ag (a total of 41 elements) were determined in clay layers selected every 15–20 cm in vertical sections. The elements show inhomogeneous distributions, both along the strike and in vertical sections of the hydrothermal clay sheet, which can be accounted for by the physicochemical, hydrogeochemical, and temperature conditions prevailing during the generation of these clays in specific areas of the thermal fields. It was found that the hydrothermal clay sheet lying on the ground surface of the geothermal fields has a significance of its own as an independent geological body, not only is it an aquifer and a heat-isolating horizon; it also serves as a dynamically active geochemical barrier in the structure of the present-day hydrothermal system. Pyrite is a concentrator of ore elements in hydrothermal clays, in addition to sulfates of Ca, Fe, Mg, Ba, and Al, and (possibly) alumosilicates.     

Application of subsurface temperature measurements in geothermal prospecting in Iceland

In geothermal areas in Iceland aquifers are in most cases found to occur in highly permeable near-vertical fractures in the low permeability basaltic crust. Therefore heat transfer in the rocks surrounding the aquifers is mainly conductive.Temperature profiles in shallow non-flowing boreholes are used to construct a two dimensional model of the temperature distribution in the vicinity of near vertical aquifers. This is done by finite element solution of the equation of heat transfer which requires knowledge of the regional temperature gradient outside the area of geothermal activity and some constraints on the temperature within the aquifers. The model is helpful in estimating dip and location of near-vertical water bearing fractures and thus in siting production wells.An example of successful use to the method and of soil temperature measurements from a geothermal field in North-Iceland is demonstrated.     

Source mechanism of long-period events at Kusatsu–Shirane Volcano, Japan, inferred from waveform inversion of the effective excitation functions

We investigate the source mechanism of long-period (LP) events observed at Kusatsu–Shirane Volcano, Japan, based on waveform inversions of their effective excitation functions. The effective excitation function, which represents the apparent excitation observed at individual receivers, is estimated by applying an autoregressive filter to the LP waveform. Assuming a point source, we apply this method to seven LP events the waveforms of which are characterized by simple decaying and nearly monochromatic oscillations with frequency in the range 1–3 Hz. The results of the waveform inversions show dominant volumetric change components accompanied by single force components, common to all the events analyzed, and suggesting a repeated activation of a sub-horizontal crack located 300 m beneath the summit crater lakes. Based on these results, we propose a model of the source process of LP seismicity, in which a gradual buildup of steam pressure in a hydrothermal crack in response to magmatic heat causes repeated discharges of steam from the crack. The rapid discharge of fluid causes the collapse of the fluid-filled crack and excites acoustic oscillations of the crack, which produce the characteristic waveforms observed in the LP events. The presence of a single force synchronous with the collapse of the crack is interpreted as the release of gravitational energy that occurs as the slug of steam ejected from the crack ascends toward the surface and is replaced by cooler water flowing downward in a fluid-filled conduit linking the crack and the base of the crater lake.     

Fluid geochemistry of Montserrat Island,West Indies

 Two geochemical surveys carried out in March 1991 and September 1992 revealed the existence of a hydrothermal system in the southern portion of Montserrat Island, below Soufrière Hills Volcano. This conclusion is supported by the presence of: (a) the thermal springs of Plymouth which are fed by deep Na–Cl waters (Cl concentration ∼25 000 mg/kg, temperature ca. 250  °C) mixed with shallow steam-heated waters; (b) the four fumarolic fields of Galway's Soufrière, Gages Upper Soufrière, Gages Lower Soufrière, and Tar River Soufrière, where acid to neutral, steam-heated waters are present together with several fumarolic vents, discharging vapors formed through boiling of hydrothermal aqueous solutions. Involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by: (a) the high ³He/⁴He ratios of fumarolic fluids, i.e., 8.2 R_A at Galway's Soufrière and 5.9 R_A at Gages Lower Soufrière; (b) the δD and δ¹⁸O values of Na–Cl thermal springs and steam condensates, indicating the involvement of arc-type magmatic water in the formation of deep geothermal liquids; and (c) the CH₄/CO₂ ratios of fumarolic fluids, which are lower than expected for equilibrium with the FeO–FeO_1.5 hydrothermal rock buffer, but being shifted towards the SO₂–H₂S magmatic gas buffer. Received: 26 March 1996 / Accepted: 19 July 1996     

共和盆地恰卜恰地热区现今地热特征

恰卜恰地热区位于青海共和盆地的东北部,是我国重要的具有干热岩地热资源勘探开发潜力的地区之一.自2013年起,不同的研究者针对该区开展了大量地球物理探测工作,然而现今地热场的研究相对较少.本文基于4口干热岩钻孔的稳态测温资料和81块岩芯样品的热导率测试数据,计算了研究区4个大地热流值.研究结果表明：研究区基底花岗岩层现今地温梯度为39.0~45.2℃·km^-1,平均值为41.3℃·km^-1,大地热流值介于93.3~111.0 mW·m^-2之间,平均值为102.2 mW·m^-2,与我国主要的克拉通型盆地（如柴达木盆地、四川盆地和鄂尔多斯盆地）和新生代裂谷型盆地（如渤海湾盆地）相比,该区属于青藏高原高热流背景下的局部异常高地温梯度和高大地热流区.分析认为,研究区高地热异常可能暗示共和盆地浅部（20 km以浅）存在局部异常热源体（岩浆囊）.     

Modeling ground deformations of Panarea volcano hydrothermal/geothermal system (Aeolian Islands, Italy) from GPS data

Panarea volcano (Aeolian Islands, Italy) was considered extinct until November 3, 2002, when a submarine gas eruption began in the area of the islets of Lisca Bianca, Bottaro, Lisca Nera, Dattilo, and Panarelli, about 2.5 km east of Panarea Island. The gas eruption decreased to a state of low degassing by July 2003. Before 2002, the activity of Panarea volcano was characterized by mild degassing of hydrothermal fluid. The compositions of the 2002 gases and their isotopic signatures suggested that the emissions originated from a hydrothermal/geothermal reservoir fed by magmatic fluids. We investigate crustal deformation of Panarea volcano using the global positioning system (GPS) velocity field obtained by the combination of continuous and episodic site observations of the Panarea GPS network in the time span 1995–2007. We present a combined model of Okada sources, which explains the GPS results acquired in the area from December 2002. The kinematics of Panarea volcano show two distinct active crustal domains characterized by different styles of horizontal deformation, supported also by volcanological and structural evidence. Subsidence on order of several millimeters/year is affecting the entire Panarea volcano, and a shortening of 10⁻⁶ year⁻¹ has been estimated in the Islets area. Our model reveals that the degassing intensity and distribution are strongly influenced by geophysical-geochemical changes within the hydrothermal/geothermal system. These variations may be triggered by changes in the regional stress field as suggested by the geophysical and volcanological events which occurred in 2002 in the Southern Tyrrhenian area.     

A geothermal study of the Jharia Gondwana basin (India): Heat flow results from several holes and heat production of basement rocks

Temperatures have been measured in nine boreholes (ranging from 400 to 900 m in depth) in the Jharia Gondwana sedimentary basin of the Indian shield. About two hundred thermal conductivity determinations have been made on core samples from these holes. Temperature profiles, Bullard plots and heat flow profiles of these holes indicate different types of disturbances in the shallow geothermal regime, attributable principally to groundwater movement. Heat flow in the region of the “anticlinal high” is about 0.4 HFU higher than the heat flow in the main synclinal region of the basin. The possible sources for this variation are regional groundwater movement and upwelling of thermal waters through a deep-seated fault/fracture system. The heat flow of 1.9 HFU characterizing the main synclinal region, taken as the regional value for the basin as a whole, has been related to the heat generation of the Precambrian basement rocks. A plot of heat flow vs. heat generation falls in line with three plots for the Precambrian complexes of the Indian shield, indicating the absence of a thermal anomaly due to deeper crustal conditions underneath this basin.