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.     

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

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

San Jacinto-Tizate geothermal field,Nicaragua: Exploration and conceptual model

The results of the exploration of the San Jacinto-Tizate geothermal field during 1992–1995 included geological, hydrogeological and geophysical investigations (magnetotelluric, frequency soundings, subsurface temperature, and soil-gas surveys), and the drilling and testing of seven deep wells (728–2339 m). The geothermal field, located within a composite volcano-tectonic depression, can be divided into two main areas: San Jacinto and Tizate. The San Jacinto area shows evidence of a high-temperature (250–300°C) fossil geothermal system that at present has reservoir temperatures in the 180–190°C range. In the Tizate area there is an active geothermal system with temperatures of 250–285°C. An upflow zone with an excess pressure gradient exists in the central part of this area. Two hydraulically connected reservoirs exist: a shallow one at 550–1200 m depth, and a deeper one below 1600m. Two-phase conditions exist in the upper part of the shallow reservoir. Production tests demonstrate the commercial potential of both Tizate reservoirs.     

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.     

A geothermal field model based on geophysical and thermal prospectings in Nea Kessani (NE Greece)

The present study completes a study by Thanassoulas et al. (1986) Geophys. Prosp.34, 83–97 and deals with geophysical exploration for geothermal resources in Nea Kessani area, NE Greece. The results of some deep electrical soundings (AB = 6000 m) with the interpretation of a gravity profile crossing the investigated area are considered together with thermal investigations. All subsequent information, along with the conclusions of an earlier paper dealing with a reconnaissance geophysical survey of the same area, are used to highlight a subsurface geothermal field model.     

Resistivity monitoring at Cerro Prieto

In 1978 Lawrence Berkeley, Laboratory, in cooperation with Comisión Federal de Electricidad, began a program of dipole - dipole resistivity monitoring at the Cerro Prieto geothermal field. Dipole - Dipole measurements were first made in 1978, then repeated in 1979: (a) to determine whether the field boundaries could be defined by surface resistivity measurements; and (b) to determine if changes in reservoir conditions due to production may be monitored by surface measurements.In 1979 data accuracy was improved to where estimated measurement errors were less than 3 per cent. In addition, data coverage on a line over the field was expanded by 40 per cent for greater depth of investigation and more information on the newer, eastern part of the field. Resistivity modeling of the expanded 1979 profile indicates that the resistive body associated with the zone of production (Wilt et al., 1980) dips steeply eastward, and may underlie the eastern part of the field. The model also shows a thin steeply dipping conductor adjacent to the resistive body that may be associated with faulting and fluid movement. Model perturbation studies have shown that small changes associated with cold-water influx, fault zone migrations, and formation of a steam zone would all be detectable with precision dipole - dipole measurements. Telluric profile measurements taken along line E E′ were found to yield a significant amount of reconnaissance information but are unsuitable for monitoring purposes.     

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.     

Geothermal resources assessment in Hawaii

The Hawaii Geothermal Resources Assessment Program was initiated in 1978. The preliminary phase of this effort identified 20 Potential Geothermal Resource Areas (PGRAs) using available geological, geochemical and geophysical data. The second phase of the Assessment Program undertook a series of field studies, utilizing a variety of geothermal exploration techniques, in an effort to confirm the presence of thermal anomalies in the identified PGRAs and, if confirmed, to more completely characterize them. A total of 15 PGRAs on four of the five major islands in the Hawaiian chain were subject to at least a preliminary field analysis. The remaining five were not considered to have sufficient resource potential to warrant study under the personnel and budget constraints of the program.The results of these studies have allowed us to attempt an estimate of the probabilities of low- to moderate-temperature (50–125°C) and of moderate- to high-temperature (125 – 360°C) geothermal resources in 12 of the survey areas; inadequate data or interpretational difficulties did not allow a valid estimate to be made for the remaining three study sites. Table 11 presents estimated probabilities for these PGRAs that are based on all currently available data.The results of these studies have also demonstrated that no single surface geothermal exploration technique is capable of providing unequivocal proof of a subsurface thermal anomaly under all field conditions; it is more frequently the case that an estimate of the geothermal potential of a given PGRA must rely on a synthesis of all geological, geophysical and geochemical data available. Experience in the Kilauea East Rift Zone, a Known Geothermal Resource Area, has also demonstrated that none of the currently available surface exploration techniques are capable of yielding definitive information regarding the production capabilities of a specific parcel of a geothermal reservoir; the only technique that has proven capable of providing this information has been the drilling and flow testing of deep exploratory wells.The island of Kauai (Figs 1 and 2) was not studied during the current phase of investigation. Geothermal field studies were not considered to be warranted due to the absence of significant geochemical or geophysical indications of a geothermal resource. The great age of volcanism on this island would further suggest that, should a thermal resource be present, it would be of low temperature.The geothermal field studies conducted on Oahu focused on the caldera complexes of the two volcanic systems which form the island: Waianae volcano and Koolau volcano. The results of these studies and the interpreted probability for a resource are presented below.Lualualei Valley: (Figs 1 and 3). Geologic mapping located the focus of the late-stage eruptive activity near the back of Lualualei Valley and tentatively identified the Waianae caldera boundaries within the valley. Soil geochemistry studies defined anomalous zones of mercury concentrations and radon emanation that appeared to be coincident with the caldera boundary faults. Groundwater chemistry and temperature measurements identified a distinctly anomalous well near the back of the valley and several others with slightly anomalous conditions on the caldera boundary faults. Geophysical soundings indicated low subsurface resistivities within the valley that were interpreted to correspond to warm fresh to saline water-saturated basalt. On the basis of the available data, the probability for a low- to moderate-temperature resource (50–125°C) within 3 km of the surface is assessed at 10–20%. The probability for a higher temperature resource is less than 5%.Mokapu Peninsula and Koolau Caldera: (Fig. 3). Geologic mapping identified three post-erosional volcanic vents on Mokapu Peninsula; the inferred ages were on the order of 300,000 years. Geochemical studies on Mokapu were unable to identify a self-consistent pattern of soil geochemical anomalies or significant groundwater chemical anomalies that would suggest a geothermal resource. Resistivity soundings determined subsurface resistivities that were consistent with cold seawater-saturated sediment. The probability for even a low-temperature geothermal source at depths of 3 km or less beneath Mokapu is considered to be less than 5%.Results of preliminary soil geochemical studies and interpretation of available groundwater data to the south of Mokapu, within the Koolau caldera, suggest that some thermally induced alterations may be present. Interpretation of geophysical data indicates that the temperatures within the ancient Koolau magma chamber are less than 540°C and that the shallow subsurface resistivities show no evidence of thermal effects. On the basis of the rather sparse data currently available, the probability for a low- to moderate-temperature resource associated with the Koolau magma chamber is considered to be 10% or less.Due to the anticipated small demand for geothermal power on the island of Molokai in the foreseeable future, only preliminary efforts were made to assess the potential for a resource on this island. An abandoned well reported to have produced warm water when it was first drilled during the 1930s was located, but temperature measurements were unable to detect anomalies within the open portion of the hole; collapse of the lower third of the bore did not permit access to the water table, however. Soil geochemical analysis did not indicate significant mercury concentrations or unusual alteration minerals in the vicinity of the well. In the absence of detectable anomalies from the preliminary investigation, further studies were not considered to be warranted. The probability for a resource on West Molokai is not considered to be high; however, sufficient data are not available to offer an estimated probability for a resource.Geothermal assessment activities on Maui included an evaluation of the major rift zones and post-erosional volcanic vents on both West Maui volcano and Haleakala volcano. Field surveys conducted on West Maui yielded the following results (Fig. 19).Olowalu and Ukumehame Canyons: extensive geologic mapping characterized the southwest and southeast rift zones of West Maui volcano and interpreted these structures to suggest a migration of the rift zone activity late in the formation of West Maui. Numerous late-stage alkalic and trachitic dikes and plugs were also identified in the survey area. Ground-water geochemical and temperature measurements identified distinctly anomalous water chemistry and temperatures. Resistivity sounding data for the area was interpreted to indicate a thick layer of warm, fresh to saline water beneath the Olowalu and Ukumehame Canyons. The probability of a thermal resource having a temperature greater than or equal to 50°C is estimated to be 50–60%, whereas a temperature greater than or equal to 125°C has an estimated probability of 10% or less.Lahaina-Kaanapali: soil geochemical surveys were unable to identify a self-consistent pattern of soil mercury concentrations or radon emanation rates that would suggest a thermal resource. Groundwater temperature measurements and chemical analyses were similarly unable to detect significant thermal alterations. Geophysical soundings detected subsurface resistivities consistent with cold water-saturated alluvium and basalt. The probability of a thermal resource existing in this area is less than 5%.Honokowai: groundwater chemistry and temperature data for this area were unable to confirm the existence of any thermal impacts and geophysical soundings indicated normal subsurface resistivities. Hence the probability for a resource in this location is believed to be less than 5%.Field surveys on Haleakala were confined to the lower portions of the three major rift zones and yielded the following analyses:

• Haleakala Northwest Rift: soil geochemical and groundwater chemical studies in this area both indicate potential anomalies. The interpretation of the anomalies with regard to thermal alterations was not, however, unequivocal. Geophysical soundings were unable to identify significantly anomalous subsurface resistivities or self-potential variations. The probability of a low- to moderate-temperature resource is placed at 10–20%, whereas that for a high-temperature resource is less than 5%.
• Haleakala Southwest Rift: geologic mapping has determined that several flows on this rift are less than 10,000 years of age and that a few are less than 1000 years old. Preliminary geochemical studies were unable to identify unequivocal evidence of thermal effects on the lower rift zone area, whereas geophysical soundings indicated that thermal groundwaters may be present at depths of less than 3 km. The probability for a low- to moderate-temperature resource is estimated to be 30–40%, whereas that for a high-temperature resource is placed at 15–25%.
• Haleakala East Rift Zone: preliminary geochemical and geophysical surveys were performed in this area. The results of these efforts did not identify significant anomalies; however, difficulties in interpretation and the small amount of data available do not allow an assessment of geothermal potential to be made.

The island of Hawaii, being the youngest and most volcanically active island in the Hawaiian chain, was found to have the largest number of PGRAs (Fig. 34). The current assessment program performed field surveys in six of the most promising PGRAs on Hawaii, which yielded the following results:

• Kawaihae: geophysical surveys performed over this area indicate a set of magnetic and resistivity anomalies that suggest that an intrusive body, associated with the Puu Loa cinder cone, may be heating local groundwaters. Groundwater chemistry and temperature anomalies confirm the existence of a heat source in the vicinity; however, the temperatures are not indicated to be very high. The probability of a low- to moderate-temperature resource in the survey area is indicated to be 35 to 45% and a moderate- to high-temperature resource to be 15% or less.
• Hualalai: geologic mapping on the western flank of Hualalai suggests that frequent eruptive activity has occurred during the last 5000 years. Geophysical surveys have identified distinct magnetic, resistivity and self-potential anomalies near the summit of Hualalai, whereas the lower western flank has not shown significant thermal effects. Geochemical data on the lower flanks were similarly unable to identify any obvious thermally induced anomalies. These data suggest that there is a 35–45% probability of a low- to moderate-temperature thermal resource near the summit of Hualalai and a 20–30% probability of a high-temperature resource in this area. Probabilities for comparable resources existing on the lower flanks are estimated at 15–25 and 5% or less, respectively.
• Mauna Loa Southwest Rift: limited geophysical surveys performed on the lower southwest rift were unable to detect significant resistivity anomalies to depths equivalent to the local water table, and a self-potential traverse detected only one anomalous gradient that was interpreted to be the result of a downgoing streaming potential. No strong geothermal anomalies were identified; however, the limitations of the available data set do not allow a probability estimate to be made of the resource potential in this area.
• Mauna Loa Northeast Rift: geophysical and geochemical field studies performed in this PGRA were unable to detect any evidence of a geothermal anomaly in this location. The probability for even a low temperature resource is estimated to be less than 5%.
• Kilauea Southwest Rift: geologic mapping has indicated several areas of steaming ground and warm coastal springs adjacent to the rift systems. A re-analysis of available geophysical data for this area concluded that warm groundwater was present within the rift zone. Magnetic anomalies observed over the rift indicate that subsurface temperatures may exceed the Curie temperature. The probability for a low- to moderate-temperature resource on this rift is considered to be 100%, whereas that for a high-temperature resource on the upper rift is estimated at 70–80%.
• Kilauea East Rift Zone: an extensive body of geological, geophysical and geochemical data concerning the East Rift Zone is available and virtually all of this data indicates that a high-temperature thermal system is associated with the entire rift. Deep exploratory geothermal wells drilled into the rift zone have identified temperatures in excess of 350°C and continuous production from one of these wells for a period of more than two years indicates that sufficient recharge is available for production of geothermal electrical power. The probability for both a low- and high-temperature resource on this rift zone is 100%.

     

Tests of electric and electromagnetic methods in the travale geothermal field

Within the framework of an EEC project involving ten European laboratories, research on the application of several electric and electromagnetic methods to a geothermal field (Travale, Tuscany), has been undertaken by the B.R.G.M. The objective was to refine the conceptual model of the Travale field and, therefore, to describe the morphology of the basement (down to 2 km depth) and of the Rhaetian reservoir covered by impermeable series.The method best suited to this type of prospection appeared to be electrical dipole — dipole profiling, combined with a 2-D interpretation. Contrasts at more than 2 km depth (500 m long dipoles) could be seen and compared to the depths measured in the boreholes. The EM Melos method (Syscal equipment) should be improved (lower frequencies) to increase the depth penetration. It is, however, a good complementary method for the surface layers. An attempt at a computational interpretation of the self potential profiles gave promising results. To make full advantage of the bipole — dipole measurements efforts should be devoted to their computational interpretation.     

Does temperature affect electric resistivity of rocks in the Larderello region?

The geoelectric resistivity method of investigation (Schlumberger quadripôle array) has been applied in the Larderello region since 1950 for reconstructing the attitude of deep-seated geological features. From then to 1969, 6792 vertical electrical soundings (VES) were carried out by C.G.G. with an AB electrode separation from 2 to 6 km, covering an area of approximately 450 km² which includes the whole geothermal field of Larderello and its peripheral areas.The deep wells (more than 300) drilled in the central part of this area give an objective basis for VES calibration and construction of reliable cap-rock resistivity and resistive substratum maps. Furthermore, such wells provide thousands of thermometric measurements which enable the construction of thermal maps at different depths.     

Acoustic emission measurement of geothermal reservoir cracks in Takinoue (Kakkonda) field, Japan

Crack extension caused by a shut-down operation in production wells (build-up test) in Takinoue (Kakkonda) geothermal power plant, Iwate Prefecture, Japan, has been successfully measured by an acceleration-sensitive long-distance acoustic emission measurement technique. The distribution and activity of subsurface cracks in a geothermal reservoir can be determined by the combined procedure of the pressure build-up test and AE measurements. The feasibility and importance of the measurements are demonstrated by AE data and their interpretation.     

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.     

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.     

Characteristics and management of the Sumikawa geothermal reservoir, northeastern Japan

The subsurface temperature gradually increases southward in the Sumikawa geothermal field and decreases sharply toward the north. The geothermal reservoir contains a two-phase zone between the cap rock and hot water zone. The target for production was designated in the deep zone, in the high temperature southern area. The production and reinjection areas have been separated to recover thermal energy efficiently during the recycling of reinjection fluid; the wells have been spaced as far apart as possible to reduce well interference. To improve productivity and injectivity, cold-water well stimulation was applied, and this experiment reduced the number of wells required for 50 MW_e power generation.     

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

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

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.     

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.