Geothermal Province Research Articles

Application of lumped parameter model to estimate mean transit time (MTT) of the thermal water using environmental tracer (3H): Insight from uttarakhand geothermal area (India)

Uttarakhand geothermal area, a part of Himalayan geothermal province, is one of the most important geothermal fields in India. Thermal springs at Tapoban and Badrinanth area are the most well studied ones among several thermal manifestations present in this area. Lumped parameter models are used in this study to calculate the mean residence time of the thermal fluids using tritium concentrations of both precipitation and thermal water. Piston flow model (PFM), exponential mixing model or well mixed model (EMM), exponential piston flow (EPM) and dispersion model (DM) are used to estimate the mean transit time (MTT) of the thermal fluids. TracerLPM (version 1), an interactive Excel® (2007 or later) workbook program, is employed to carry out this modelling procedure. Historical records of weighted mean tritium concentration of precipitation at New Delhi GNIP station (representative of the study area) acts as input concentration in modelling procedure. Missing values of tritium concentrations at New Delhi GNIP station are derived by comparing the available tritium concentration data with the tritium records at Ottawa GNIP station, Canada. Ratio between tritium values of the two GNIP stations (New Delhi and Ottawa) indicates that tritium concentration in Ottawa can be scaled by the factor of 0.41 to match with the tritium concentration at New Delhi GNIP station. Among different lumped parameter models, the simulated tritium output concentration derived from EPM and DM matches closely with the measured tritium concentration of the thermal springs. The estimated mean transit time of the Tapoban thermal water (THS) is found to be between 40 and 44 years whereas for Badrinath thermal water (BTHS) it ranges from 102 to 112 years. Both the models (EPM and DM) suggest that the aquifer feeding the Tapoban thermal water (THS) contains mixture of water flow paths i.e. combination of both exponential and piston flow, whereas aquifer feeding the Badrinanth thermal water (BTHS) mostly contains exponential mixing portion. Hydrodynamic dispersion is found to be higher in the Badrinath system compared to the Tapoban thermal water system.

The measurement of soil gases and shallow temperature for determination of active faults in a geothermal area: a case study from Ömer–Gecek, Afyonkarahisar (West Anatolia)

Afyonkarahisar is a very important geothermal province of western Anatolia and has low and medium enthalpy geothermal areas. This study has been carried out for the preparation of distribution maps of soil gases (radon and carbon dioxide) and shallow soil temperature and the exploration of permeable tectonic regions associated with geothermal systems and reveal the origins of radon and carbon dioxide gases. The western district of the study area is characterized by the high radon concentration (168.30 kBq/m3), carbon dioxide ratio (0.30%), and soil temperature (21.0 °C) values. Fethibey and Demircevre faults, which allow the circulation of geothermal fluids, have been detected in the distribution maps of radon, carbon dioxide, and shallow depth temperature and the directions of the curves in these maps correspond to the strikes of Demircevre faults. The effect of the fault plays an important role in the change of carbon dioxide concentration along the W-E directional geological section prepared to determine the change of soil gas and shallow depth temperature values depending on lithological differences, fault existence, and geothermal reservoir depth. On the other hand, it was determined that Rn222 concentration and soil temperature changed as a function of geothermal reservoir depth or lithological difference. Tuffs in Koprulu volcano-sedimentary units are the main source of radon due to their higher uranium contents. Besides, the carbon dioxide in Omer–Gecek soils has geothermal origin because of the highest carbon dioxide content (99.3%) in non-condense gas. The similarities in patterns of soil temperature, radon, and carbon dioxide indicate that the variation in soil temperatures is related to radon and carbon dioxide emissions. It is concluded that soil gas and temperature measurements can be used to determine the active faults in the initial stage of geothermal exploration successfully.

Perspectives of geothermal water use in the Podhale Basin according to geothermal step distribution

At present, hydro-geothermal resources in which waters in bores are the carrier of heat energy have commercial importance in Poland. Geothermal waters should feature the highest possible temperature in the outflow, low mineral content, high capacity and low deposit depth. Even though geothermal energy is included in renewable sources of energy, complete depletion of the deposit is the largest risk in extracting geothermal waters. This is why these waters should also feature deposit renewability. There are currently several geothermal provinces with beneficial geothermal conditions in Poland, with the area of the Podhale Basin in the Inner Carpathians region being regarded as the most promising. The Podhale deposit also meets all of the above listed conditions. The possibility of using hydrothermal energy in Poland is practically available throughout the country, but operational use of hot groundwater must be profitable for investors. From another point of view, the environmental impact of such investments should be minimized. This paper is concerned with the special variety of temperature of geothermal waters extracted in the area of the Podhale Basin, which is the basic aspect in using deposits of this type.

The effect of offset on fracture permeability of rocks from the Southern Andes Volcanic Zone, Chile

The Southern Andes Volcanic Zone (SVZ) represents one of the largest undeveloped geothermal provinces in the world. Development of the geothermal potential requires a detailed understanding of fluid transport properties of its main lithologies. The permeability of SVZ rocks is altered by the presence of fracture damage zones produced by the Liquiñe-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF). We have therefore measured the permeability of four representative lithologies from the volcanic basement in this area: crystalline tuff, andesitic dike, altered andesite and granodiorite. For comparative purposes, we have also measured the permeability of samples of Seljadalur basalt, an Icelandic rock with widely studied and reported hydraulic properties. Specifically, we present the results of a systematic study of the effect of fractures and fracture offsets on permeability as a function of increasing effective pressure. Baseline measurements on intact samples of SVZ rocks show that the granodiorite has a permeability (10−18 m2), two orders of magnitude higher than that of the volcanic rocks (10−20 m2). The presence of throughgoing mated macro-fractures increases permeability by between four and six orders of magnitude, with the highest permeability recorded for the crystalline tuff. Increasing fracture offset to produce unmated fractures results in large increases in permeability up to some characteristic value of offset, beyond which permeability changes only marginally. The increase in permeability with offset appears to depend on fracture roughness and aperture, and these are different for each lithology. Overall, fractured SVZ rocks with finite offsets record permeability values consistent with those commonly found in geothermal reservoirs (>10−16 m2), which potentially allow convective/advective flow to develop. Hence, our results demonstrate that the fracture damage zones developed within the SVZ produce permeable regions, especially within the transtensional NE-striking fault zones, that have major importance for geothermal energy resource potential.

Isotope–geochemical characterization and geothermometrical modeling of Uttarakhand geothermal field, India

Uttarakhand geothermal area, located in the central belt of the Himalayan geothermal province, is one of the important high temperature geothermal fields in India. In this study, the chemical characteristics of the thermal waters are investigated to identify the main geochemical processes affecting the composition of thermal waters during its ascent toward the surface as well as to determine the subsurface temperature of the feeding reservoir. The thermal waters are mainly Ca–Mg–HCO3 type with moderate silica and TDS concentrations. Mineral saturation states calculated from PHREEQC geochemical code indicate that thermal waters are supersaturated with respect to calcite, dolomite, aragonite, chalcedony, quartz (SI > 0), and undersaturated with respect to gypsum, anhydrite, and amorphous silica (SI < 0). XRD study of the spring deposit samples fairly corroborates the predicted mineral saturation state of the thermal waters. Stable isotopes (δ18O, δ2H) data confirm the meteoric origin of the thermal waters with no oxygen-18 shift. The mixing phenomenon between thermal water with shallow ground water is substantiated using tritium (3H) and chemical data. The extent of dilution is quantified using tritium content of thermal springs and non-thermal waters. Classical geothermometers, mixing model, and multicomponent fluid geothermometry modeling (GeoT) have been applied to estimate the subsurface reservoir temperature. Among different classical geothermometers, only quartz geothermometer provide somewhat reliable estimation (96–140 °C) of the reservoir temperature. GeoT modeling results suggest that thermal waters have attained simultaneous equilibrium with respect to minerals like calcite, quartz, chalcedony, brucite, tridymite, cristobalite, talc, at the temperature 130 ± 5 °C which is in good agreement with the result obtained from the mixing model.

Geochemical evaluation of thermal springs in Odisha, India

The Indian state of Odisha has a number of thermal springs. These thermal springs are located at eight places (Attri, Tarabalo, Deulajhari, Magarmuhan, Bankhol, Badaberena, Taptapani and Boden) and belong to Mahanadi Geothermal Province, which is an Archean/Pre-Cambrian Geothermal Province. The thermal water discharging from these springs shows moderately acidic to moderately alkaline character (pH: 5.05–8.93) and the temperature ranges from 28 (Boden) to 58 °C (Tarabalo). Total dissolved solids (TDS) also shows a wide variation between 16.9 (Bankhol) and 595 mg/L (Deulajhari). A wide variation in the chemical characteristics of the thermal waters has been observed as they are located in different geological settings. Based on water chemistry, all the thermal springs can broadly be grouped into three water types: Na–Cl, Ca–HCO3 and Na–HCO3. The thermal spring water from Attri, Tarabalo and Deulajhari belongs to Na–Cl water type which is due to the circulation through granitic rocks. Higher concentrations of Cl and F in these thermal waters further suggest limited mixing and longer residence time as compared to the other areas where Ca–HCO3 and Na–HCO3 water types were found. Anion variation diagram clearly indicates that the thermal waters from Attri, Tarabalo and Deulajhari are fast ascending and fall in the mature water field; thus, their chemical signatures can be used to determine the reservoir temperature. However, in other areas, water chemistry is shaped by near-surface groundwater mixing processes and thus the chemical geothermometers may not be applicable to determine the reservoir temperature. No appreciable temporal variations have been observed in the water chemistry of the thermal waters.

The evolution of the Cappadocia Geothermal Province, Anatolia (Turkey): geochemical and geochronological evidence

Cappadocia Geothermal Province (CGP), central Turkey, consists of nine individual geothermal regions controlled by active regional fault systems. This paper examines the age dating of alteration minerals and the geochemistry (trace elements and isotopes) of the alteration minerals and geothermal waters, to assess the evolution of CGP in relation to regional tectonics. Ar–Ar age data of jarosite and alunite show that the host rocks were exposed to oxidizing conditions near the Earth’s surface at about 5.30 Ma. Based on the δ18O–δD relationhip, water samples had a high altitude meteoric origin. The δ34S values of jarosite and alunite indicate that water samples from the southern part of the study area reached the surface after circulation through volcanic rocks, while northern samples had traveled to the surface after interacting with evaporates at greater depths. REY (rare earth elements and yttrium) diagrams of alteration minerals (especially illite, jarosite and alunite) from rock samples, taken from the same locations as the water samples, display a similar REY pattern to water samples. This suggests that thermal fluids, which reached the surface along a fault zone and caused the mineral alteration in the past, had similar chemical composition to the current geothermal water. The geothermal conceptual model, which defines a volcanically heated reservoir and cap rocks, suggests there are no structural drawbacks to the use of the CGP geothermal system as a resource. However, fluid is insufficient to drive the geothermal system as a result of scanty supply of meteoric water due to evaporation significantly exceeding rainfall.

Comparative Analysis of 16S rRNA Gene Illumina Sequence for Microbial Community Structure in Diverse Unexplored Hot Springs of Odisha, India

ABSTRACTOdisha (East India) is home to several hot springs, of which Atri and Taptapani are the two with variation in temperature and located in the Mahanadi geothermal province having altitude 120 and 1800 ft., respectively, above sea level. Average temperature of Atri hot spring is as higher as 58 ± 5°C as compared to 48 ± 5°C of Taptapani. In-depth analysis of the microbial diversity of these hot springs through 16S rRNA deep sequencing analysis, targeting V3 region was performed using Illumina bar-coded sequencing platform. Existence of higher microbial diversity in Atri hot spring (1662 OTUs; 2708 species) as compared to Taptapani (1561 Operational Taxonomic Units [OTUs]; 2045 species) is supported by higher value of diversity indices for Atri (6.24, Shannon; 0.95, Simpson) than Taptapani (4.03, Shannon; 0.79, Simpson), probably due to favorable influence of environmental parameters around it. Irrespective of the four databases (GREENGENE, M5RNA, Ribosomal Database Project [RDP], and Small Subunit [SSU] databases) used for understanding community structure, the dominant phyla in the Atri hot spring were different from the predominant populations in the Taptapani in terms of percentage representation in different databases. From Principal Coordinates Analysis [PCoA] analysis, Atri and Taptapani metagenome, on comparison with other three metagenomes, were found to be matching with the community structure of hot springs of Gujarat, India, but differed from that of saline desert. Furthermore, predicted functional analysis in both the hot springs were found to be affiliated with carbohydrate, amino acids, energy, vitamins and cofactor, nucleotide, membrane transport metabolic pathways, and the genes involved in them, although their intensity of occurrence was varying as analyzed through PICRUSt and Tax4Fun probably due to physicochemical parameters prevailing around each hot spring. The present study for the first time has revealed the differential microbial community structure and predicted functional diversity of Atri and Taptapani hot springs of Odisha in such a great detail.

Fluid geochemical constraints on the heat source and reservoir temperature of the Banglazhang hydrothermal system, Yunnan-Tibet Geothermal Province, China

Longling, located in the Yunnan-Tibet Geothermal Province of China, is characterized by the wide distribution of hydrothermal areas, among which the Banglazhang system is the most geothermally active and marked by intensive hydrothermal activities. There is a lack of neutral Cl waters at Banglazhang, and the geothermal waters are mainly Na-HCO3 type with low but comparable SO4 and Cl concentrations. The SiO2 concentrations of the Banglazhang geothermal waters indicate equilibration in a shallow reservoir with a temperature around 150°C. While dilution by Mg-rich cold groundwaters complicates geothermometry, the Na-K geothermometer suggests that this shallow reservoir is fed by geothermal fluid from a deeper reservoir above 200°C. The geophysical investigations made in the neighboring regions of Banglazhang indicate that the Banglazhang system is likely to be located above the margin of a comparatively shallow-seated magma chamber. However, the hydrochemical and O, H and S isotopic evidence doesn't imply a mixing of magmatic fluid with Banglazhang geothermal waters. The geochemical scenario below Banglazhang is quite different than some typical magmatic-hydrothermal systems in China (such as Yangbajain and Rehai) where the geochemistry of geothermal waters are strongly affected by the input of magmatic fluid. The efforts made in this study might be suggestive for identifying the geothermal and geochemical origin of a large number of unexplored high-temperature hydrothermal systems in remote Yunnan and Tibet of China whose heat sources and geological conditions are unclear.

Magnetotelluric investigations to identify geothermal source zone near Chabsar hotwater spring site, Ahmedabad, Gujarat, Northwest India

The Cambay graben is one of the known geothermal provinces in the Gujarat state in Western India. Temperatures between 100 and 150°C have been recorded up to 3.5km depth in this zone. To understand the geoelectric structure and possible source zone of hot water spring at Chabsar, 50km southwest of Ahmedabad, on the left bank of Cambay basin, magnetotelluric (MT) investigations have been carried out at 24 sites with inter site spacing of ∼1.5km along four east-west profiles surrounding the hot water spring. The good quality high frequency MT data has been acquired to better resolve the image of shallow subsurface of the region. The recorded MT time series were processed and earth response functions were estimated in broad frequency range. The two-dimensional (2-D) inversion of the transverse electric (TE) and transverse magnetic (TM) mode data including vertical magnetic field component (Hz) of all four profiles have been performed after distortion and decomposition analysis. Combining 2-D structures of all the four profiles we infer a ∼900m thick Quaternary and Tertiary sediments overlain on Deccan traps of thickness around 1500m in the study region. The combined geoelectric structure after integrating the 2-D models of all four profiles shows the presence of a conductive zone (<10Ωm) at a depth of ∼1.0km with in traps possibly connected with Western Cambay Fault and extended down to 2–2.5km in central part of the study area. In view of deepening of basement below the high conductive zone and reported large heat flow values of the Cambay basin, we infer that the zone is presumably filled with fluids which may have a heat source at deeper levels.

The Cornubian geothermal province: heat production and flow in SW England: estimates from boreholes and airborne gamma-ray measurements

The Cornubian granite batholith provides one of the main high heat production and flow provinces within the UK. An extensive programme of borehole measurements was undertaken in the 1980s to characterise the geothermal resource. Here we revisit the published data on heat flow and heat production from 34 boreholes and revise the published heat flow values in accord with modern palaeoclimate knowledge. This leads to a more rigorous (and increased) set of estimated temperatures at depth across the granite outcrops. Predicted temperatures at a depth of 5 km largely exceed 200 °C and are 6–11 % higher than previously estimated values. We also reconsider the borehole heat production values in conjunction with new heat generation information from a recent regional-scale airborne geophysical survey. The radiometric (gamma-ray) data provide detailed (~70 m along-line) ground concentration estimates of the heat-producing radioelements. These are then combined to estimate heat production in the near surface. The airborne estimates are subject to attenuation by the soil profile. Here we demonstrate and then adopt an assumption that the observations of the soil–bedrock medium undergo a flux attenuation by a factor of about two compared to the response of the underlying material. The revised estimates are then correlated with their equivalent deeper borehole estimates. Linear regression is then used to correct the shallow airborne estimates to values that are consistent with the deeper borehole determinations. The procedure provides a detailed and extensive mapping of heat production at both on- and off-granite locations across SW England. The Dartmoor and Land’s End granite offer the greatest spatial geothermal potential in terms of their intrinsic radionuclide concentrations and associated heat production. District-scale heat production is studied using the airborne data acquired uniformly across conurbations. The analysis identifies the towns of Camborne, Penzance, St. Austell, Redruth and St Ives as having relatively high values (>4 μW m−3) within their urban perimeters.

Major and trace element concentrations in the geothermal springs along the west coast of Maharashtra, India

The west coast geothermal province in Maharashtra hosts a number of thermal springs along the 350 km linear stretch. Major and trace element analyses of thermal springs from 15 locations along with groundwater and surface water samples have been studied. The thermal waters had temperatures ranging from 40 to 71 °C, pH from 7.1 to 8.8 and total dissolved solids between 337 and 8698 mg/l. Most of the thermal waters are Na-Ca-Cl or Ca-Na-Cl type except one which is of Na-HCO3 type. The major ion data is comparable over three decades suggesting a mature geothermal system in the west coast. Trace elements concentrations were low in case of some elements such as<0.1 ppb of Ag and Cd, 1–3 ppb of Cr, Zn, Co, Cu, Pb and Ni, whereas elements like Li (19–386 ppb), B (104–1362 ppb), Sr (16–13,560 ppb), Rb (13–220 ppb), Cs (0.75–44 ppb) and Ba (3–2077 ppb) were much higher in concentration. The variations in the trace element concentration in the thermal waters indicate involvement of different rock types although all the thermal emergences are within the Deccan volcanic flows.

Geothermal energy resources of Jizan, SW Saudi Arabia

Jizan geothermal province has evolved over an oceanic ridge slice which is under plated a thin continental crust. The area is characterized by high heat flow and high geothermal gradient and hosts several thermal and warm springs whose chemistry indicates a high temperature reservoir within the granites. The geophysical data indicated that the thermal fluids are fault controlled. The residence time of the fluids, estimated based on 14C, is about 33 kilo years. Oxygen isotope shift suggests that the reservoir temperature is around 220°C. It is estimated that the province can generate electricity of the order of 134×106kWh which can be effectively utilized to meet future fresh water demand and save CO2 emissions by decreasing the domestic oil and gas consumption.

Reflection seismic sections across the Geothermal Province of Tuscany from reprocessing CROP profiles

We processed the seismic lines CROP-18 and the Tuscany segment of CROP-03, acquired in Southern Tuscany. The analyzed transects were originally designed to highlight deep crustal structures between the major reflector characterizing the geothermal fields (the K-horizon) and the Moho discontinuity. Our main target was to better depict the upper crustal structures, reprocessing the first 4–6s reflection times with the application of advanced techniques (wave equation datuming), improving signal/noise ratio and extracting information previously hidden by approximate static corrections and surface noise. We choose the wave equation datuming technique, a process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces. The wave equation datuming was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. The datuming procedure largely contributed to attenuate ground roll, enhance higher frequencies, increase resolution and improve the signal/noise ratio.The time migrated output sections show new evidence of regional continuity of high amplitude horizons, a better fitting with the surface geological setting and with well data resembling the local industry investigations. The interpretation confirms the fluids role, the contribution to the crustal reflectivity mostly by thermo-metamorphic processes and defines the tectonic setting.

Evolution of geothermal systems around the Red Sea

The current thermal regime around the red Sea was initiated by a mantle plume beneath Ethiopia, Eretria, Yemen and SW of Saudi Arabia at about 31 Ma before the present. The evolution of the geothermal systems around the Red Sea is coeval to the initial onset of volcanism over the separated land masses (Eritrea, Ethiopia, Djibouti, Yemen, Saudi Arabia and Egypt at 14 Ma. The entire tectono-magmatic activities around the Red Sea gave rise to several geothermal provinces over the continents surrounding the Red Sea, represented by thermal springs and fumaroles at several locations in Eretria, Djibouti, Ethiopia, Yemen and Saudi Arabia. These countries have the potential to develop these resources, like Ethiopia, to mitigate CO2 emissions in countries like Saudi Arabia and to enhance the GDP (gross domestic product) of the economically backward countries. The geothermal resources have the potential to make these countries energy independent in future.

Contribution of multi-method geophysics to the understanding of a high-temperature geothermal province: The Bouillante area (Guadeloupe, Lesser Antilles)

The Bouillante high-temperature geothermal field (Guadeloupe, French Lesser Antilles) is located on the west coast of Basse-Terre Island, about 15km from the currently active Soufrière volcano. We have studied the shallow structure of the geothermal zone using a compilation of all available gravity, magnetic and resistivity data, both subaerial and marine, acquired during the last 30years. Geophysical models have been developed based on geological constraints and previous geophysical interpretations. The resistivity distribution from electromagnetic surveys is interpreted in terms of water saturation of rocks, hydrothermal alteration and presence of hydrated minerals. Gravity is used to distinguish and characterize the denser formations from the low density areas. Finally, we are able to differentiate structures which pre- and post-date various recent magnetic inversions using the polarity of the magnetic anomalies. By integrating these results the previous conceptual model for the geothermal province has been improved, with the layering of the geological formations as defined by geophysical parameters now being taken into account, in agreement with the classical scheme of geothermal reservoirs. The shallow resistive layer is explained in terms of recent massive formations. The conductive intermediate layer marks the low density, demagnetized clay cover of the altered geothermal system. The deep resistive layer is associated with dense formations increasing in age with depth, and corresponds to the deep geothermal reservoir. Based on our geophysical data, the geothermal activity could be more developed towards the east and north compared to the previously proposed area. A volcano-structural control seems to affect the internal structure of the province, and consequently the development of geothermal activity. This current work could be used as a basis for future surveys of the Bouillante province in order to improve constraints, and to increase our knowledge of the island's overall evolution.

Advective heat transport associated with regional Earth degassing in central Apennine (Italy)

In this work we show that the main springs of the central Apennine transport a total amount of heat of ∼2.2×109Js−1. Most of this heat (57%) is the result of geothermal warming while the remaining 43% is due to gravitational potential energy dissipation. This result indicates that a large area of the central Apennines is very hot with heat flux values >300mWm−2. These values are higher than those measured in the magmatic and famously geothermal provinces of Tuscany and Latium and about 1/3 of the total heat discharged at Yellowstone. This finding is surprising because the central Apennines have been thought to be a relatively cold area. Translated by CO2 rich fluids, this heat anomaly suggests the existence of a thermal source such as a large magmatic intrusion at depth. Recent tomographic images of the area support the presence of such an intrusion visible as a broad negative velocity anomaly in seismic waves. Our results indicate that the thermal regime of tectonically active areas of the Earth, where meteoric waters infiltrate and deeply circulate, should be revised on the basis of mass and energy balances of the groundwater systems.

Possible evidence of remotely triggered and delayed seismicity due to the 2001 Bhuj earthquake (Mw = 7.6) in western India

Remote triggering by large earthquakes at regional distances is a globally observed phenomenon. However, there are no reports of observations of dynamic triggering at regional distances of several source lengths associated with the large Mw = 7.6 Bhuj earthquake of January 26, 2001, in western India. In the present study, a swarm of over 140 microearthquakes that occurred about 500 km southeast of Bhuj, in the geothermal province of the Western Ghats in the Deccan volcanic province (DVP) of India, immediately after the occurrence of the Bhuj earthquake in 2001 is investigated. The post-Bhuj seismicity (M < 2.0) occurred in three bursts spread over 2 months with each burst of intense activity lasting for 2–3 days. All the three bursts of seismicity occurred in the same volume along a 5-km-long NW–SE trending fault. The temporal coincidence and the sudden rise in seismicity that interrupts the characteristically low background seismicity strongly suggest that the Bhuj earthquake may have remotely triggered this activity. The triggered seismicity began approximately 2.5 h after the onset of the Bhuj mainshock and continued well after the passage of the surface waves, suggesting that the dynamic stresses possibly gave rise to secondary time-dependent mechanisms leading to the triggering. It is proposed that the triggered and delayed seismicity is possibly a consequence of the redistribution in pore fluid pressure due to the Bhuj earthquake. This is the first documented observation of remotely triggered seismicity at regional distances due to the Bhuj earthquake.

Joint evaluation of gravity, electrical and magnetotelluric methods for geothermal potential of fractured Aptian reefal carbonates in the Hmaima–El Gara area (Oued Serrat basin, Central–Western Tunisia)

Hmaima–El Gara area is located in Central–Western Tunisia and is known as an important geothermal province. In this study, we attempt to delineate the subsurface structures of the area using integrated interpretation of gravity, electrical and magnetotelluric data. The Hmaima thermal aquifer, associated with fractured Aptian reefal limestones, is characterized by high gravity and high resistivity. Horizontal gradient and Euler deconvolution method has been applied to the gravity data and provided fast information about both the depth and trends of the shallower subsurface structures in the area. As several of the mapped lineaments correlate with published geological fault trends, the other lineaments may be indicators of new insights for hydrothermal exploitation in the Hmaima–El Gara area (economical potential favorite zones).

A new 3D fault model of the Bouillante geothermal province combining onshore and offshore structural knowledge (French West Indies)

The Bouillante area hosts geothermal resources located in a complex structural area (Guadeloupe Island, French West Indies). On one hand, faults observed on the field mainly elongate along the E–W direction. On the other hand, offshore structures interpreted from marine seismic lines shows a larger range of directions. A coherent 3D interpretation is proposed through a fault model combining onshore and offshore structural knowledge in a zone crossing the island coastline. The fault network constructed reveals a hierarchy in the family of structures and highlights the prevalence of the NNW–SSE direction, associated with secondary NE–SW-trending structures, and the E–W direction. On a geographical point of view, the modelled faults are gathered in 3 clusters. Data available to build the 3D fault model are sometimes sparse, especially inland because of intense vegetation cover. Consequently, not only the results and impacts of the 3D fault model are discussed but also its limitations as well as its possible evolution.