Hengill geothermal volcanic complex (Iceland) characterized by integrated geophysical observations

Structural features of volcanic and hydrothermal systems can be used to infer the location of magma chambers or productive geothermal areas. The Hengill volcanic triple-junction complex has a well-developed geothermal system, which is being exploited to extract hot fluids that are used for electrical power and heat production. In the framework of the I-GET project, a 4-month temporary seismological network including seven high-dynamic broadband instruments was deployed and 1D transient electromagnetic soundings (TEM) and 3D magnetotelluric (MT) surveys were performed to improve the understanding of the relationships between structural features, seismic activity and fluid production at the Hengill geothermal system. The MT and TEM data set are analysed elsewhere. The analysis of the seismological data set allowed the detection and classification of more than 600 earthquakes, among which long-period (LP) earthquakes were observed for the first time in this area. This work focuses first on a joint inversion for the 3D velocity structure and determination of the locations of the hypocentres from about 250 local volcano-tectonic earthquakes with clear P- and S-wave arrival times. The results confirm those from earlier tomography studies in this area. Integrating the seismic velocity and resistivity models in a semi-quantitative approach by cross-plotting the resistivity model with the velocity ratio V_P/V_S delineates a structural body with a high seismic velocity ratio and low resistivity that is interpreted as the main heat source of the geothermal system.     

Integrated seismic and magnetotelluric exploration of the Skierniewice,Poland, geothermal test site

The purpose of the research activities at the Skierniewice geothermal test site is to develop and apply an exploration methodology for low-enthalpy systems in sedimentary formations. Work included seismic and magnetotelluric surveys carried out close to well Kompina-2 to create a detailed structural–geologic model and characterize the anisotropic fracture system around the borehole. The study included the reprocessing of archival data from selected boreholes and 2D seismic lines. The collected data were used to identify formations with high fracture permeability and the presumed flow path of geothermal (∼110 °C) brine in high productivity zones, and determining rock porosities and salinity distribution in the subsurface. The next stage of the investigations will focus on siting a second borehole and studying the possibility of installing a plant for electrical generation or direct geothermal heat applications.     

Revised hypocenter solutions for microearthquakes in the kakkonda geothermal field, japan

The hypocenters of microearthquakes in the Kakkonda geothermal field have been relocated along the Kakkonda River using a new velocity structure model. Compared to the solution used by the previous velocity model, the depth of the hypocenters is shallower in the relocation. The microearthquakes occurred in a highly fractured region, as suggested by geological and petrological studies based on well data, but did not occur along major tectonic folds and faults. An alternative hydraulic condition might be necessary to trigger the earthquakes. Seismicity in 1995 was lower than in 1988 in Kakkonda. The decrease in the number of events is possibly due to the decrease in the amount of reinjection fluid or the change in the characteristics of the geothermal reservoir.The number of microearthquakes decreases rapidly at 1–2 km below sea-level. Probability density of seismic energy distribution is utilized to indicate the active seismic regions. The model also shows that a contour map of the lower boundary of the high seismic energy region corresponds to the occurrence of cordierite, which was produced by heat from the neo-granitic pluton body, implying that the occurrence of microearthquakes in the Kakkonda geothermal field is controlled by the neo-granitic rocks at depth. The top of the granite can be imaged, using the probability density of seismic energy distribution.     

Three-dimensional imaging of a geothermal system using temperature and geological models derived from a well-log dataset

The accurate imaging of geothermal systems from the ground surface down to great depths is an interdisciplinary problem common to geothermal resource exploration and development. Rocks can be characterized mainly in terms of their lithology, mineralogy, fracture distribution, permeability, thermal conductivity and porosity, and similarly the geothermal fluid (and its circulation) by its geochemistry, flow pattern, velocity, temperature and pressure. Some of these data are obtained by well logging and from laboratory tests conducted on drillhole cores. In general, the distribution of geothermal wells is not random, and well data are limited in terms of quantity and depth range. Accordingly, a sophisticated spatial modeling technique is indispensable in the three-dimensional imaging of geothermal systems. We describe a versatile 3-D modeling method that can be used to determine the temperature, flow velocity, and distribution of geological units within a geothermal field based on well log data. The model results for the Hohi geothermal area, Japan, provide plausible estimates of temperature, flow velocity, and geology to a depth of 3000 m. Superimposition of the three spatial models we obtained shows that, at Hohi, two geothermal reservoirs are localized near highly fractured fault zones that provide paths for the ascent of thermal fluids from depth.     

Inverse modeling and forecasting for the exploitation of the Pauzhetsky geothermal field,Kamchatka, Russia

A three-dimensional numerical model of the Pauzhetsky geothermal field has been developed based on a conceptual hydrogeological model of the system. It extends over a 13.6-km² area and includes three layers: (1) a base layer with inflow; (2) a geothermal reservoir; and (3) an upper layer with discharge and recharge/infiltration areas. Using the computer program iTOUGH2 [Finsterle, S., 2004. Multiphase inverse modeling: review and iTOUGH2 applications. Vadose Zone J. 3, 747–762], the model is calibrated to a total of 13,675 calibration points, combining natural-state and 1960–2006 exploitation data. The principal model parameters identified and estimated by inverse modeling include the fracture permeability and fracture porosity of the geothermal reservoir, the initial natural upflow rate, the base-layer porosity, and the permeabilities of the infiltration zones. Heat and mass balances derived from the calibrated model helped identify the sources of the geothermal reserves in the field. With the addition of five make-up wells, simulation forecasts for the 2007–2032 period predict a sustainable average steam production of 29 kg/s, which is sufficient to maintain the generation of 6.8 MWe at the Pauzhetsky power plant.     

Tomographic study of the seismic velocity at the Soultz-sous-Forêts EGS/HDR site

The 2000 and 2003 hydraulic stimulations carried out at the Soultz-sous-Forêts Hot Dry Rock (HDR) site, Alsace, France, generated a continuous, albeit of small magnitude, seismic activity. For each experiment, the Ecole et Observatoire des Sciences de la Terre (EOST) installed a seismic network of 18 stations (in 2000) and 23 stations (in 2003). More than 3500 seismic events were detected and located in both cases.Changes in the physical properties of the reservoir due to fluid circulation have been estimated by means of a 4D tomography in which the 3D seismic velocity structure is calculated for different time windows. The entire data set has been apportioned to different temporal subsets with a fixed number of events. We used the tomographic algorithm of [Thurber, C.H., 1983. Earthquake locations and 3D crustal structure in the Coyote Lake area, central California. J. Geophys. Res. 88, 8226–8236] for the 2000 data and a new, more accurate method that is based on a double-difference tomography [Zhang, H., Thurber, C.H., 2003. Double-difference tomography: the method and its application to the Hayward fault, California. Bull. Seismol. Soc. Am. 93 (5), 1875–1889] for the 2003 set. The latter approach gives a good resolution within the reservoir volume, and one that is slightly less so in its vicinity. This tomographic study points out the changes in reservoir velocity structure occurring during the stimulation.We discuss the variation of the seismic velocity with time during each of the hydraulic stimulations and compare the velocity changes observed in 2000 and 2003. This analysis has allowed us to define the region of influence of hydraulic activity and determine the effects of water circulation on geothermal reservoir properties.     

Geothermal exploration using geoelectric methods in Kestanbol, Turkey

A self-potential survey was carried out in the Kestanbol area in order to investigate the fault zones that might be associated with geothermal activity. Two fault zones, which could be interpreted as planes electrically polarized by geothermal activity, have been detected by compilation of a self-potential map. A resistivity survey, using the Wenner configuration, was able to identify the geoelectrical structures that are caused by different rocks with variable degrees of fracturation and also affected by geothermal activity. The resistivity maps and two-dimensional geoelectric models indicated a good conductive region (<10 ohm-m) in the southwest part of Kestanbol. On the basis of the results of both methods, a new location is proposed for drilling in this conductive zone.     

Geophysical investigations of the Seferihisar geothermal area, Western Anatolia, Turkey

Self-potential (SP), magnetic and very low frequency electromagnetic (EM-VLF) surveys were carried out in the Seferihisar geothermal area to identify major and minor fault zones and characterize the geothermal system. The SP study provided useful information on the local faults and subsurface fluid flow. The main SP anomalies appear mostly along and near active fault zones in the area of the Cumalı, Tuzla and Doğanbey hot springs. Two of these anomalies near the Tuzla hot springs were further evaluated by SP modelling. Total magnetic field values increase from the Doğanbey to the Cumalı hot springs. Modelling performed on the magnetic data indicates that between these two spring areas are four different regions or units that can be distinguished on the basis of their magnetic susceptibility values. Fraser filtering of EM-VLF data also indicates that there are three significant conductive zones in the regions around the Cumalı, Tuzla and Doğanbey hot springs, and that they lie between important fault systems. The EM-VLF and total (stacked) SP data show that the conductive tilt anomalies obtained by Fraser filtering generally coincide with negative SP areas.According to our geophysical investigations, new exploratory wells should be drilled into the conductive zones located between the Cumalı and Tuzla hot springs. We further recommend that resistivity and magnetotelluric methods be carried out in the area to obtain additional information on the Seferihisar geothermal system.     

Results of a 3D seismic survey at the Travale (Italy) test site

In the last 15 years geothermal exploration in Tuscany, Italy, has addressed deep reservoirs (depth ≥ 3000 m), hosted within complex geological systems, such as metamorphic formations and/or intrusive bodies. Reservoir productivity is linked to fractured and permeable zones that are rather confined and not uniformly distributed. In this context, the seismic methods represent one of the most reliable geophysical techniques for locating potential drilling targets. A 3D seismic survey has been acquired at the Travale test site, and its results have been used to develop a geological and structural model of the site, and to identify and characterize fractured zones inside the deep geothermal reservoir. A correlation between a high-amplitude reflector (H marker) and fractured contact-metamorphic rocks has been highlighted. More than 70% of the total geothermal fluid production at the Travale area comes from this seismic marker.     

Seismic reflector imaging by prestack time migration in the Kakkonda geothermal field, Japan

We show that a prestack migration method improves the S/N ratio of seismic reflection profiling in the Kakkonda geothermal field where seismic reflection data are of poor quality. We use non-iterative prestack time migration (PSTM), which does not require multiple iterations to determine the velocity structure for prestack time migration. The optimum constant migration velocity can be determined at each image point from a migration velocity analysis based on primary diffraction patterns. Our results delineate a strong reflector beneath a zone of high seismicity. According to the correspondence between the fracture distribution, the distribution of microearthquakes, and geothermal structure, this reflector is interpreted to be a zone of low-angle fractures saturated with hydrothermal fluids, and to be strongly controlled by the geothermal structure.     

Te Kopia geothermal system (New Zealand) — the relationship between its structure and extent

The Te Kopia geothermal system is one of several high-temperature systems in the Taupo Volcanic Zone (TVZ) of New Zealand. It is located along the Paeroa Fault Zone, a major active fault system trending NE in the central TVZ. Three independent studies, i.e. resistivity survey, magnetic interpretation, and detailed topographic analysis of faults and fractures, indicate the existence of another fault system, trending NW, that also significantly influences the Te Kopia geothermal system. Results from these studies also show that, at Te Kopia, a resistivity low and hydrothermally demagnetised rocks (both are indicators of a geothermal reservoir in volcanic rocks) clearly coincide with a zone of high fault and fracture density. Hence, the Te Kopia field is a good example of the significant influence that geological structures (major fault systems) have on the extent of a geothermal reservoir, by creating zones of fractured rocks that provide permeable paths for thermal fluids.     

Characterization of geothermal reservoirs with electrical surveys: Beowawe geothermal field

The work reported here was undertaken to test the utility of electrical surveys for geothermal reservoir characterization using existing exploration and well data sets from the operating Beowawe geothermal field located in the Basin and Range Province of western USA. The STAR geothermal reservoir simulator was used to model the natural state of the system, and to compute the subsurface distributions of temperature and salinity, which were in turn utilized to calculate pore-fluid resistivity. Archie's law, which relates formation resistivity to porosity and pore-fluid resistivity, was adopted to infer the formation resistivity distribution. Subsequently, direct current (DC) resistivity, magnetotelluric (MT) and self-potential (SP) postprocessors were used to compute the expected response corresponding to available survey data. The measured apparent resistivity distribution from a dipole–dipole DC resistivity survey is in good agreement with the computed values. The calculated self-potential distribution agrees with the main features of an available SP survey. Although the computed MT apparent resistivity sounding curves reproduce the shapes of the measured MT sounding curves, an overall scale factor exists between the measured and calculated MT responses, and similarly with the computed dipole–dipole resistivity model. Possible reasons are static shifts in the coarsely sampled MT stations, and resistivity anisotropy due to the stratigraphy. Taken as a whole, the results of this study support the view that a suite of carefully designed electrical surveys (DC, MT, and SP) may be employed to infer favorable subsurface geothermal reservoir characteristics.     

Temperature dependence of seismic properties in geothermal rocks at reservoir conditions

Petrophysical experiments on two Icelandic geothermal rock samples at simulated in situ reservoir conditions are analysed to delineate the effect of temperature on seismic velocity and attenuation. A goal of the present work is to predict the effect of the saturating pore fluid on seismic velocity using the Gassman equation, which has been modified for this purpose. To include the temperature effect in the equation, two assumptions are made: (1) the grain/mineral and dry bulk moduli are independent of temperature; and (2) the temperature dependence follows solely from the thermophysical characteristics of the saturating fluid through the fluid bulk modulus and fluid density. Laboratory measurements show that P-wave velocities decrease with increasing temperature. This change is related to the thermophysical characteristics of the saturating fluid; at higher temperatures bubbles and thermal microfractures are formed affecting seismic velocities. The measurements also show that at low temperatures seismic attenuation decreases with temperature due to the rapid decrease in the fluid viscosity. On the other hand, at higher temperatures the attenuation increases because of the generation of bubbles and thermal microfractures. Although having data from only two samples and that no measurements on dry samples were done, thus limiting the generality of the claims that can be made, the study presents a plausible approach to relate changes in seismic properties to geothermal system temperatures.     

Heat flow map of South America

The results of geothermal investigations carried out in South America have been compiled with the purpose of preparing regional maps of terrestrial heat flow. The compilation revealed that 655 heat flow values had been determined, giving an overall data density of 37/10⁶ km² and a representative mean heat flow of 63+-36 mW/m². The quality of the data set is variable, depending on the nature of the primary geothermal data, and the geographic distribution of the data set is also non-uniform. In spite of such difficulties a careful analysis of the data set, following suitable priority schemes, has allowed not only the determination of reliable mean heat flow values for a large number of major geological structures in South America, but also the preparation of mosaics of regional heat flow variations. Heat flow is extremely variable in the Cordilleran regions, with the eastern and southern parts having relatively high values compared to the western and northern parts. The general trend of increasing heat flow from the western coastal regions towards inland areas is interrupted by a N-S trending low heat flow belt in the Pre-Cordilleran basins. In the eastern part of the continent heat flow is low to normal (<75 mW/m²) but there are indications that in the Patagonian Platform it is higher than in the Brazilian Platform. There are, however, several isolated localities of high heat flow in the northeastern and south-central parts of Brazil. The Mesozoic rift basins (Potiguar, Recôncavo and Taubaté) are also characterized by relatively high values.In order to examine the tectonic significance of variations in the regional geothermal regime, heat flow maps have been prepared using manual and automatic contouring methods. The comparative study of automatic contour maps generated by means of a variety of data interpolation and gridding schemes has led to the identification of some geothermal features that are believed to be related to tectonic processes affecting the South American continent. Prominent among these are E-W trending belts of low heat flow in northern Peru and in central Chile (extending into the Sierras Pampeans in Argentina), as well as high heat flow belts in northern Chile (extending into the Altiplano in Bolivia) and southern Chile (extending into western Argentina). The low heat flow belts coincide approximately with zones of sub-horizontal subduction, while the high heat flow belts are situated in regions of high-angle subduction. Some of these features correlate well with the results of studies on anelastic attenuation, electrical resistivity distribution and some patterns of global seismic tomography. On the other hand, many of these features are not evident in the recent spherical harmonic analysis of global heat flow, which suggests that the use of empirical predictors based on a heat flow-age relation in devising global heat flow maps should be restricted to tectonically stable areas.     

Investigations of deep resistivity structures at the Wairakei geothermal field

The Wairakei geothermal field was the proving ground for the use of electrical resistivity methods for geothermal exploration. At this site it was first demonstrated that a large contrast in resistivity existed between geothermal ground and the cold surroundings. Within the top 500 m of the geothermal field, low-resistivity (5–10 Ωm) reflects the effects of both the hot saline water in the pore spaces and the conductive rock-matrix. The first surveys at Wairakei used a Wenner array (a ∼550 m) to measure the resistivity values along tracks throughout the field; contour maps of the resistivities were used to estimate the lateral extent of the geothermal waters at a few hundred metres depth. In the late 1960s the Wenner array was superseded by the Schlumberger array (AB/2 = 500 m and 1000 m), which enabled deeper penetration and better definition of the extent of the geothermal waters. These early surveys showed that the bounds of the geothermal waters were often sharp, leading to the concept that a ‘resistivity boundary’ could be defined for New Zealand's liquid-dominated geothermal fields. As new methods of measuring electrical structure with greater precision became available, Wairakei was often chosen as the testing ground.     

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.     

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.     

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.     

Diffusion of fluids in porous media with memory

In some geothermal areas the fluids may precipitate minerals in the pores ofthemedium thus diminishing their size To study the flow of these fluids we modify the law ofDarcyby introducing a memory formalism represented by a derivative of fractional ordersimulating theeffect of a decrease of the permeability in time The diffusion in these media isstudied giving thedistribution of the pressure of the fluid in a half space when the fluid pressureat the planebounding the half space is constant or sinusoidal or in the form of a box in the timedomain Thevelocity of propagation of the phase allows us to model the flow in several cases ofpracticalinterest A method is suggested to retrieve the two parameters defining the diffusion anumericaltest of the method gives encouraging results © 1999 CNR Published by ElsevierScience Ltd