Geothermal Research Research Articles

Cyclic waterfrac stimulation to develop an Enhanced Geothermal System (EGS)—Conceptual design and experimental results

The design and results of a cyclic hydraulic fracturing experiment performed to enhance the productivity of the geothermal research well at Groß Schönebeck (Germany) are presented. The stimulation carried out in the low-permeability volcanics of the Lower Rotliegend (Lower Permian) included alternating stages with cyclic changes of low and high flow rates with up to 150 L/s over six days in conjunction with the addition of quartz sand to support fracture opening. There was rapid water level increase in an adjacent well due to the stimulation (i.e. water injection). The subsequent production test showed the success of the fracture treatment, with the overall productivity of the treated well being increased by a factor of four.

Effects of internal structure and local stresses on fracture propagation, deflection, and arrest in fault zones

The way that faults transport crustal fluids is important in many fields of earth sciences such as petroleum geology, geothermal research, volcanology, seismology, and hydrogeology. For understanding the permeability evolution and maintenance in a fault zone, its internal structure and associated local stresses and mechanical properties must be known. This follows because the permeability is primarily related to fracture propagation and their linking up into interconnected clusters in the fault zone. Here we show that a fault zone can be regarded as an elastic inclusion with mechanical properties that differ from those of the host rock. As a consequence, the fault zone modifies the associated regional stress field and develops its own local stress field which normally differs significantly, both as regard magnitude and orientation of the principal stresses, from the regional field. The local stress field, together with fault-rock heterogeneities and interfaces (discontinuities; fractures, contacts), determine fracture propagation, deflection (along discontinuities/interfaces), and arrest in the fault zone and, thereby, its permeability development. We provide new data on the internal structure of fault zones, in particular the fracture frequency in the damage zone as a function of distance from the fault core. New numerical models show that the local stress field inside a fault zone, modelled as an inclusion, differ significantly from those of the host rock, both as regards the magnitude and the directions of the principal stresses. Also, when the mechanical layering of the damage zone, due to variation in its fracture frequency, is considered, the numerical models show abrupt changes in local stresses not only between the core and the damage zone but also within the damage zone itself. Abrupt changes in local stresses within the fault zone generate barriers to fracture propagation and contribute to fracture deflection and/or arrest. Also, analytical solutions of the effects of material toughness (the critical energy release rate) of layers and their interfaces show that propagating fractures commonly become deflected into, and often arrested at, the interfaces. Generally, fractures propagating from a compliant (soft) layer towards a stiffer one tend to become deflected and arrested at the contact between the layers, whereas fractures propagating from a stiff layer towards a softer one tend to penetrate the contact. Thus, it is normally easier for fractures to propagate from the host rock into the damage zone than vice versa. Similarly, it is easier for fractures to propagate from the outer, stiffer parts of the damage zone to the inner, softer parts, and from the stiff host rock to the outer damage zone, than in the opposite directions. These conclusions contribute to increased understanding as to how fractures propagate and become arrested within fault zones, and how the fault zone thickness is confined at any particular time during its evolution.

A Review of Discussions in the Organized Session on Sustainable Development of Geothermal Energy at 2003 Annual Meeting of Geothermal Research Society of Japan

A Review of Discussions in the Organized Session on Sustainable Development of Geothermal Energy at 2003 Annual Meeting of Geothermal Research Society of Japan

Pressure-dependent Production Efficiency of an Enhanced Geothermal System (EGS): Stimulation Results and Implications for Hydraulic Fracture Treatments

A series of stimulation experiments were carried out at the geothermal research well in Gros Schonebeck (EGrSk 3/90) located in the northeastern part of Germany. The intended purpose of these experiments was to develop concepts for a productivity increase of the geothermal well to create an Enhanced Geothermal System (EGS). Two different kinds of stimulation types were performed. Hydraulic gel-proppant stimulations were conducted in sandstone sections with high initial permeability. Then a different fracturing concept was applied injecting high amounts of water. This waterfrac stimulation was realized in the entire open section including sandstones and volcanic rocks. Evidence of the creation and properties of a very long vertical fracture was retrieved from pressure response analyses demonstrating a bilinear flow regime. The production efficiency of the produced artificial fractures shows a strong dependence on reservoir pressure. At increased reservoir pressure the artificial fractures of all stimulated intervals are highly conductive and subsequently become less conductive during pressure decline. Hence the range of a suitable reservoir pressure is constrained by this fracture efficiency and limits the usage of this well as an injection well for geothermal power production.

Impact of Poroelastic Response of Sandstones on Geothermal Power Production

During geothermal power production using a borehole doublet consisting of a production and injection well, the reservoir conditions such as permeability k, porosity π and Skempton coefficient B at the geothermal research site Gross Schoenebeck/Germany will change. Besides a temperature decrease at the injection well and a change of the chemical equilibrium, also the pore pressure Pp will vary in a range of approximately 44 MPa ±10 MPa in our reservoir at −3850 to −4258 m depth. This leads to a poroelastic response of the reservoir rocks depending on effective pressure P eff (difference between mean stress and pore pressure), resulting in a change in permeability k, porosity π and the poroelastic parameter Skempton coefficient B. Hence, we investigated the effective pressure dependency of Flechtinger sandstone, an outcropping equivalent of the reservoir rock via laboratory experiments. The permeability decreased by 21% at an effective pressure range from 3 to 30 MPa, the porosity decreased by 11% (P eff=6 to 65 MPa) and the Skempton coefficient decreased by 24% (p eff=4 to 25 MPa). We will show which mechanisms lead to the change of the mentioned hydraulic and poroelastic parameters and the influence of these changes on the productivity of the reservoir. The most significant changes occur at low effective pressures until 15 to 20 MPa. For our in situ reservoir conditions p eff=43 MPa a change of 10 MPa effective pressure will result in a change in matrix permeability of less than 4% and in matrix porosity of less than 2%. Besides natural fracture systems, fault zones and induced hydraulic fractures, the rock matrix its only one part of geothermal systems. All components can be influenced by pressure, temperature and chemical reactions. Therefore, the determined small poroelastic response of rock matrix does not significantly influence the sustainability of the geothermal reservoir.

Interpretation and mathematical modeling of temporal changes of temperature observed in borehole Yaxcopoil-1 within the Chicxulub impact structure, Mexico

Summary Geothermal research of the Chicxulub impact structure on the Yucatan Peninsula, Mexico, included repeated temperature logs following 0.3–0.8, 15, 24, 34 and 50 months after shut-in of drilling operations at the 1.5 km deep Yaxcopoil-1 borehole. A gradual distortion of the linear temperature profile by a cold wave propagating downward from 145 m to 317 m was detected within the observational period of 49 months (March 2002–April 2006). The amplitude of the cold wave was increasing with depth and time in the range of 0.8–1.6 °C. As an explanation of this unusual phenomenon, the hypothesis of downward migration of a large volume of drilling mud, reported lost during drilling within the overlying and cooler highly porous and permeable karstic rocks, has been proposed. The thermal effects of the migrating fluid have been evaluated by solving numerically the heat conduction–convection equation in appropriate geothermal models. The best coincidence between the observed data and the simulations was yielded by the model of the drilling mud migrating as a large body. Parameters of this model are constrained by the measured temperature logs relatively tightly: (i) the vertical extent of the downward migrating fluid body is about 5–10 m, possibly increasing within the observational period of 49 months by a factor of 2; (ii) the horizontal extent of the body must be at least 15–20 m, i.e. by order(s) of magnitude larger than the diameter of the borehole; (iii) the average speed of the migration is about 5 metres per month and (iv) the fluid must migrate through a highly porous rock (80% porosity or more). This high porosity, which is necessary for the model to fit the observed data, and the observed relatively stable velocity of the migration indicate the existence of a well-developed system of interconnected cavities down to more than 300 m about 150 m more than the deepest cave system known in Yucatan yet.

Time-dependent chamber and vent conditions during explosive caldera-forming eruptions

We use a modified version of the CPIUC model [Macedonio, G., Neri, A., Martí, J., Folch, A., 2005. Temporal evolution of flow conditions in sustained explosive eruptions, Journal of Volcanology and Geothermal Research, 143, 153-172] to simulate chamber and vent conditions during the different phases of a piston-like caldera-forming eruption. Our idealized caldera-forming scenario assumes an initial central-vent conduit that, after critical chamber decompression, migrates to a fissure-vent peripheral conduit(s). Further decompression leads to final piston-like subsidence which stops only after the virtual destruction of the magmatic reservoir. The simulations find that the pressure at the conduit entrance drops during the decompression phases at a rate depending on the conduit geometry, chamber volatile zonation and fragmentation threshold. The higher the volume contrast between the initial central-vent and the final peripheral fissure-vent conduits, the higher the pressure drop and the jump in the mass eruption rate. Pressure increases back to lithostatic during piston subsidence while some compressible magma remains within the chamber. Finally, during the later phase, pressure experiments a gentle increase or decrease depending on the balance between deposition of intra-caldera material and decrease in the contents of volatiles as deeper chamber levels are tapped.

Petrology of 1977 to 1998 eruptions of Piton de la Fournaise, La Réunion Island

Geochemical and petrological variations in 35 historical eruptions from the April 1977 eruption to the March–September 1998 eruption at Piton de la Fournaise have been analyzed for their major elements abundances, mineralogical compositions and oxygen isotopes. During these 21 years, samplings were repeated several times during eruptions as well as temperature measurements. Since the end of 1977 to 1998, all erupted lavas, except 1998-Hudson basalt, belong to the group of the “steady state basalts”. However, evidences for small degrees of differentiation can be observed. During a single eruption, the earliest lava emitted contains less phenocrysts and has almost always a lower MgO⁎ (= MgO/[MgO + FeOt]) than the lavas emitted later, suggesting that it is more differentiated. From one eruption to the next, between 1977 and 1998, bulk compositions are more and more differentiated towards the first lava of 1998. A process of low-pressure fractional crystallisation, involving small proportions of olivine, clinopyroxene and plagioclase, allows to explain this evolution. A major result of our study is to show that this evolution does not correspond to a unique fractionation trend. Over the considered period, different types of magmas have been distinguished. The first magma type, erupted from October 1977 to November 1987, was richer in potassium (0.82 wt.% in average) than the second one, and apparently derived from the liquid of the 1977 oceanite. The second magma type, erupted from February 1988 to September 1998, was poorer in K 2O (0.69 wt.% in average). In March 1998, a third magma type (Hudson basalt) erupted simultaneously with the second one. Hudson basalt has undergone a wehrlitic fractionation and is supposed to come from a deeper magma reservoir than other erupted magmas for this period. From 1977 to 1998, the ∂ 18O of lavas progressively decreases with time, together with the increasing chemical differentiation of the magmas by low-pressure fractional crystallisation. This variation agrees with the hypothesis of magma contamination at shallow level in the altered volcanic edifice (Vlastélic, I., Deniel, C., Bosq, C., Télouk, P., Boivin, P., Bachèlery, P., Famin, V., Staudacher, T., 2009-this issue. Pb isotope geochemistry of Piton de la Fournaise historical lavas. Journal of Volcanology and Geothermal Research, 184, 63−78 (this issue)). Overall, petrological variations are in good agreement with a model of a shallow reservoir discontinuously supplied between 1977 and 1998. The speed of the variations, the coexistence of different liquids, the chemical evolution of the lavas during the same eruption, lead us to favour a model of chemically stratified reservoir, constituted by a complex of dykes, sills and laccoliths close to the surface and located above larger magma bodies.

Magmatic degassing at Erta 'Ale volcano, Ethiopia

Here we report measurements of the chemical composition and flux of gas emitted from the central lava lake at Erta 'Ale volcano (Ethiopia) made on 15 October 2005. We determined an average SO 2 flux of ∼ 0.69 ± 0.17 kg s − 1 using zenith sky ultraviolet spectroscopy of the plume, and molar proportions of magmatic H 2O, CO 2, SO 2, CO, HCl and HF gases to be 93.58, 3.66, 2.47, 0.06, 0.19 and 0.04%, respectively, by open-path Fourier transform infrared (FTIR) spectrometry. Together, these data imply fluxes of 7.3, 0.7, 0.008, 0.03 and 0.004 kg s − 1 for H 2O, CO 2, CO, HCl and HF, respectively. These are the first FTIR spectroscopic observations at Erta 'Ale, and are also some of the very few gas measurements made at the volcano since the early 1970s (Gerlach, T.M., 1980b. Investigation of volcanic gas analyses and magma outgassing from Erta 'Ale lava lake, Afar, Ethiopia. Journal of Volcanology and Geothermal Research, 7(3–4): 415–441). We identify significant increases in the proportion of H 2O in the plume with respect to both CO 2 and SO 2 across this 30-year interval, which we attribute to the depletion of volatiles in magma that sourced effusive eruptions during the early 1970s and/or to fractional magma degassing between the two active pit craters located in the summit caldera.

Vent conditions for expected eruptions at Vesuvius

Determining consistent sets of vent conditions for next expected eruptions at Vesuvius is crucial for the simulation of the sub-aerial processes originating the volcanic hazard and the eruption impact. Here we refer to the expected eruptive scales and conditions defined in the frame of the EC Exploris project, and simulate the dynamics of magma ascent along the volcanic conduit for sub-steady phases of next eruptions characterized by intensities of the Violent Strombolian (VS), Sub-Plinian 2 (SP2), and Sub-Plinian 1 (SP1) scale. Sets of conditions for the simulations are determined on the basis of the bulk of knowledge on the past history of Vesuvius [Cioni, R., Bertagnini, A., Santacroce, R., Andronico, D., Explosive activity and eruption scenarios at Somma–Vesuvius (Italy): towards a new classification scheme. Journal of Volcanology and Geothermal Research, this issue.]. Volatile contents (H 2O and CO 2) are parameterized in order to account for the uncertainty in their expected amounts for a next eruption. In all cases the flow in the conduit is found to be choked, with velocities at the conduit exit or vent corresponding to the sonic velocity in the two-phase non-equilibrium magmatic mixture. Conduit diameters and vent mixture densities are found to display minimum overlapping between the different eruptive scales, while exit gas and particle velocities, as well as vent pressures, largely overlap. Vent diameters vary from as low as about 5 m for VS eruptions, to 35–55 m for the most violent SP1 eruption scale. Vent pressures can be as low as less than 1 MPa for the lowest volatile content employed of 2 wt.% H 2O and no CO 2, to 7–8 MPa for highest volatile contents of 5 wt.% H 2O and 2 wt.% CO 2 and large eruptive scales. Gas and particle velocities at the vent range from 100–250 m/s, with a tendency to decrease, and to increase the mechanical decoupling between the phases, with increasing eruptive scale. Except for velocities, all relevant vent quantities are more sensitive to the volatile content of the discharged magma for the highest eruptive scales considered.

Anhydrite in the 1989–1990 lavas and xenoliths from Redoubt Volcano, Alaska

The eruption of Redoubt Volcano in Alaska produced a moderate sulfur emission (estimated at 1 × 10 tons SO 2), but relatively small volume of lava (0.11 km ) with pre-eruption estimates of 840–950 °C and fO 21.5 to 2.0 log units above NNO (Swanson, S.E., Nye, C.J., Miller, T.P., Avery, V.F., 1994. Magma mixing in the 1989–1990 eruption of Redoubt Volcano: Part II. Evidence from mineral and glass chemistry. Journal of Volcanology and Geothermal Research 62, 453–468). Petrologic estimates of sulfur production (Sigurdsson, H., Devine, J.D.,Davis, A.N., 1985. The petrologic estimation of volcanic degassing. Jokull 35, 1–8) from this eruption (Gerlach, T., Westrich, H.R., Casadevall, T.J., Finnegan, D.L., 1994. Vapor Saturation and accumulation in magmas of the 1989–1990 eruption of Redoubt Volcano, Alaska. Journal of Volcanology and Geothermal Research 62, 317–337) are considerably less than the measured sulfur emission, leading workers to propose the existence of a pre-eruption vapor phase to explain the “excess” sulfur. Initial examination of the 1989–1990 Redoubt eruptive products reported anhydrite (Nye, C.J., Swanson, S.E., Avery, V.F., Miller, T.P., 1994. Geochemistry of the 1989–1990 eruption of Redoubt Volcano: Part I, whole-rock, major- and trace-element chemistry. Journal of Volcanology and Geothermal Research 62, 429–452.) in interstitial glass from some cognate gabbroic xenoliths, but anhydrite was not noted in any of the andesites. A Boeing 747 encountered the ash plume from the initial eruptive phase on December 15, 1989 and provided ash samples that reportedly contained gypsum (Bayhurst, G.K., Wohletz, K.H., Mason, A.S., 1994. A method for characterizing volcanic ash from the December 15, 1989, eruption of Redoubt Volcano, Alaska. U.S. Geological Survey Bulletin 2047, 13–17). However, the identification was based on EDS analyses on a SEM and the mineral could have been anhydrite. Reexamination of the 1989–1990 Redoubt lavas and xenoliths revealed the presence of rare, fine-grained (10s of microns) anhydrite crystals in the lavas and larger (10s to 100s of microns) crystals in the xenoliths. The anhydrite forms elongate crystals with sharp contacts against the groundmass glass. Grains show rounded terminations suggesting breakdown of the anhydrite, perhaps during eruption. The anhydrite is a sparse phase in the lavas, but occurs in lavas and cognate xenoliths erupted throughout the eruption. The presence and breakdown of anhydrite may explain the “excess” sulfur associated with the Redoubt eruption. A careful search of other freshly erupted andesitic lavas solidified from hydrous, oxidizing magmas should reveal other examples of anhydrite and cause a rethinking of the source of sulfur in volcanic emissions.

Stability analysis of Hawaiian Island flanks using insight gained from strength testing of the HSDP core

Hawaiian Island flank failures are recognized as the largest landslide events on Earth, reaching volumes of several thousand cubic kilometers and lengths of over 200 km and occurring on an average of once every 100 000 years. The 3.1 km deep Hawaii Scientific Drilling Project (HSDP) enabled an investigation of the rock mass strength variations on the island of Hawaii [Schiffman, P., Watters, R.J., Thompson, N., Walton, A.W., 2006. Hyaloclastites and the slope stability of Hawaiian volcanoes: insights from the Hawaiian Scientific Drilling Project's 3-km drill core. Journal of Volcanology and Geothermal Research, 151 (1–3): 217–228]. This study builds on that of Schiffman et al. [Schiffman, P., Watters, R.J., Thompson, N., Walton, A.W., 2006. Hyaloclastites and the slope stability of Hawaiian volcanoes: Insights from the Hawaiian Scientific Drilling Project's 3-km drill core. Journal of Volcanology and Geothermal Research, 151 (1–3): 217–228] by considering more in-depth rock mass classification and strength testing methods of the HSDP core. Geotechnical core logging techniques combined with laboratory strength testing methods show that rock strength differences exist within the edifice. Comparing the rock strength parameters obtained from the various volcano lithologies identified weak zones, suggesting the possible location of future slip surfaces for large flank failures. Relatively weak rock layers were recognized within poorly consolidated hyaloclastite zones, with increases in strength based on degree of alteration. Subaerial and submarine basalt flows are found to be significantly stronger. With the aid of digital elevation models, cross-sections have been developed of key flank areas on the island of Hawaii. Limit equilibrium slope stability analyses are performed on each cross-section using various failure criteria for the rock mass strength calculations. Based on the stability analyses the majority of the slopes analyzed are considered stable. In cases where instability (i.e. failure) is predicted, decreased rock mass quality (strength) of the altered and highly poorly consolidated lithologies is found to have a significant influence. These lithologies are present throughout the Hawaiian Islands, representing potential failure surfaces for large flank collapses. Failure criterion input parameters are considered in sensitivity analyses as are the influences of certain external stability factors such as sea level variation and seismic loading.

Comment on “Recent unrest at Canary Islands' Teide Volcano?”

Many seismic, geochemical, hydrological, and microgravity signatures support Tenerife's recent volcanic unrest [Péerez et al., 2004; Marrero et al., 2005; Almendros et al., 2007; Gottsmann et al., 2006; López et al., 2006; Tarraga et al., 2006; Pérez et al., 2007]. Volcanic unrest at Tenerife may or may not affect Teide's shallow magmatic system and therefore prompt its reawakening. García et al. [2006] claimed a reawakening of Teide, and a coauthor (R. Ortiz) predicted an explosive eruption with a 70% probability [Olaizola, 2005]. They reported a picture of a “plume” at the volcano's summit on October 2004 as evidence of Teide's reawakening.On the contrary, continuous monitoring of carbon dioxide emissions at Teide as well as a visual and close inspection of the summit cone did not support any outburst of gases at that time. Other data that do not support the reactivation of the shallow magma chamber beneath Teide are the low rate of sulfur dioxide emissions [Weber et al., 2006], small observed fluctuation of the helium isotope ratio, and other geochemical signatures from the fumaroles. Also, the first results of a permanent GPS network do not report deformation (T. Sagiya et al., Establishment of the Canary Islands GPS network for volcano monitoring and its initial results, submitted to Journal of Volcanology and Geothermal Research, 2007).

The Waldemar Lindgren Award for 2006 Citation of Thomas Monecke

President Goldfarb, SEG members, and guests, it is my honor and privilege this evening to present to you this year’s winner of the Waldemar Lindgren Award, Dr. Thomas Monecke. This award is presented annually to an early career scientist whose published research represents an outstanding contribution to economic geology. This award is critically important to the Society of Economic Geologists as it supports and encourages our future. It is important to appreciate that the Lindgren award winners this decade will be the leaders of our Society in the decades to come. I know you will agree with me that with a recipient like Thomas, the Society and the science of economic geology are in good and capable hands. Thomas is an exceptional geologist whose publications represent an outstanding contribution to economic geology, primarily in the fields of ancient and modern volcanic-hosted massive sulfide deposits, alteration mineralogy, mineral trace element geochemistry, and fractal frequency distribution. He is both an outstanding field geologist and a laboratory scientist, a rare combination for a young geoscientist. Dr. Monecke’s contribution to the science of economic geology is outstanding. Since 2000, Thomas has published over 20 papers, the majority as senior author, in leading international journals such as Economic Geology , Earth and Planetary Science Letters , Journal of Volcanology and Geothermal Research , Chemical Geology , Contributions to Mineralogy and Petrology , Clays and Clay Minerals , Geochimica et Cosmochimica Acta , and Mineralium Deposita. This is an exceptional output from an early career researcher and his publications show that he has developed an international reputation for both fundamental and applied research. Thomas received his bachelor’s with honors degree in 1995 from Freiberg University in Germany, completing a thesis on “X-ray Excited Optical Luminescence of …

White House seeks to cut geothermal research funds

White House seeks to cut geothermal research funds

Volcán Tancítaro, Michoacán, Mexico, 40Ar/ 39Ar constraints on its history of sector collapse

Volcán Tancítaro is a 97 ± 3 km 3 stratovolcano located in the Michoacán Guanajuato volcanic field (MGVF), part of the Trans Mexican Volcanic Belt. Prior to this study, there was only one K–Ar date (530 ± 60 ka; [Ban, M., Hasenaka, T., Delgado-Granados, H., Takaoka, N., 1992. K–Ar ages of lavas from shield volcanoes in the Michoacán–Guanajuato volcanic field, Mexico. Geofisica Internacional 31 (4), 467–473.] and one sector-collapse event reported for this volcano in the literature [Garduño-Monroy V.H., Corona-Chavéz, P., Israde-Alcantara, I., Mennella, L., Arreygue, E., Bigioggero, B., Chiesa, S., 1999. Carta Geológica de Michoacán, scale 1:250,000. Universidad Michoacana de San Nicolás de Hidalgo.; Capra, L., Macías, J.L., Scott, K.M., Abrams, M., Garduño-Monroy, V.H., 2002. Debris avalanches and debris flows transformed from collapses in the Trans-Mexican Volcanic Belt, Mexico — Behavior, and implications for hazard assessment. Journal of Volcanology and Geothermal Research 113, 81–110.]. Twenty-six new 40Ar/ 39Ar ages indicate that Volcán Tancítaro became active ≥ 793 ± 22 ka and that the most recent effusive activity occurred at 237 ± 34 ka. Two catastrophic sector-collapse events are identified and dated; the first one occurred on the west side between 694 and 571 ka, whereas the second one occurred on the east side between 261 and 238 ka. The older collapse produced a 2.3–3.4 km 3 debris-avalanche and laharic deposit spread over ∼ 567 km 2, whereas the more recent collapse left a 3-km wide, horseshoe-shaped scar on the eastern flank and produced a 3.6–7.0 km 3 debris-avalanche and laharic deposit that covers ∼ 654 km 2. Reconstruction of the main edifice of Volcán Tancítaro using ArcGIS software and digital elevation models indicates that the volume removed during the eastern sector collapse was ∼ 4.7 km 3.

Historical volcanoes of Armenia and adjacent areas: What is revisited?

The validity of some data in Karakhanian et al. [Karakhanian, A., Djrbashian, R., Trifonov V., Philip H., Arakelian S., Avagian, A., 2002. Holocene–historical volcanism and active faults as natural risk factor for Armenia and adjacent countries. Journal of Volcanology and Geothermal Research, 113, 1, 319–344; Karakhanian, A., Jrbashyan, R., Trifonov, V., Philip, H., Arakelian, S., Avagyan, A., Baghdassaryan, H., Davtian, V., Ghoukassyan, Yu., 2003. Volcanic hazards in the region of the Armenian nuclear power plant. Journal of Volcanology and Geothermal Research, 126/1–2, 31–62] that are revisited by R. Haroutiunian is considered. A conclusion is made that the revisions suggested by Haroutiunian concern unessential parts of the content of work by Karakhanian et al. [Karakhanian, A., Djrbashian, R., Trifonov V., Philip H., Arakelian S., Avagian, A., 2002. Holocene–historical volcanism and active faults as natural risk factor for Armenia and adjacent countries. Journal of Volcanology and Geothermal Research, 113, 1, 319–344; Karakhanian, A., Jrbashyan, R., Trifonov, V., Philip, H., Arakelian, S., Avagyan, A., Baghdassaryan, H., Davtian, V., Ghoukassyan, Yu., 2003. Volcanic hazards in the region of the Armenian nuclear power plant. Journal of Volcanology and Geothermal Research, 126/1–2, 31–62]. This article presents new evidence and re-proves the earlier conclusions that are disputed or revised by R. Haroutiunian.

Timescales of degassing and crystallization implied by 210Po– 210Pb– 226Ra disequilibria for andesitic lavas erupted from Arenal volcano

Disequilibrium between 210Po, 210Pb, and 226Ra was measured on whole rocks and plagioclase mineral separates erupted between 1968 and 2003 from Arenal volcano with a goal of monitoring the volatile fluxing and crystallization in the decades and years leading up to eruption. Degassing during the eruption was found to remove nearly all 210Po from Arenal lavas, which appears to be true of lava eruptions in general. Most of Arenal's lavas have ( 210Pb)/( 226Ra) ratios within 20% of equilibrium, indicating that most of the magmas involved in this eruption did not have strong, persistent fluxes of 222Rn in or out of the system during the decades leading to eruption. This is consistent with a time-frame of differentiation from basalt to basaltic andesite exceeding a century. Lava erupted in 1971 had ( 210Pb) in excess of ( 226Ra) by as much as a factor of 2. These lavas were the first to mark the change in geochemical trends that were likely caused by the arrival of a new magma at the surface at Arenal [Ryder, C.H., Gill, J.B., Tepley III, F., Ramos, F., Reagan, M., this issue. Closed to open system differentiation at Arenal Volcano (1968–2003). Journal of Volcanology and Geothermal Research.], suggesting that the 210Pb excess was related to the first appearance of this magma. The high ( 210Pb)/( 226Ra) ratio in this lava apparently reflects Rn-degassing from large volumes of underlying magma and/or extraction of Rn from conduit-area rocks or fluids due to deformation and heating. Plagioclase mineral separates had 210Po– 210Pb– 226Ra disequilibrium patterns suggesting a growth period stretching over a period of more than 50 years up to the time of eruption.

Geoquímica de fuentes minerales y termales del Complejo Volcánico Cerro Bravo – Cerro Machín, Colombia

El Complejo Volcánico Cerro Bravo-Cerro Machín, localizado entre los departamentos de Caldas, Risaralda, Quindío y Tolima, tiene un área aproximada de 2.000 km2 y, de norte a sur, está conformado por el volcán Cerro Bravo, los volcanes del Parque Nacional Natural Los Nevados (PNNN) y el volcán Cerro Machín.
 En el área de este complejo volcánico, se localiza un gran número de manantiales, muchos de ellos identificados y caracterizados químicamente en el marco la Investigación Geotérmica del Macizo Volcánico del Ruiz (CHEC et al., 1983). Tomando como base dicha investigación y la información generada por Ingeominas, a través de la vigilancia geoquímica de la actividad de los volcanes de la zona y de un inventario de manantiales termales del municipio de Santa Rosa de Cabal, se llevó a cabo la reinterpretación de la geoquímica de manantiales para estimar las temperaturas de los reservorios geotérmicos del área e identificar procesos de ebullición, dilución y mezcla de los fluidos geotérmicos. Adicionalmente, y con base en la composición química y la temperatura, se plantearon recomendaciones sobre usos potenciales de las fuentes minerales y termales.
 De acuerdo con su distribución espacial y su posible asociación a los volcanes, los cien ( 100) manantiales registrados, fueron divididos en nueve (9) grupos: 1) volcán Cerro Bravo, 2) Nevado del Ruiz, sector oriental , 3) Batolito de El Bosque, 4) Nevado del Ruiz, sector occidental, 5) Santo Domingo, 6) Santa Rosa de Cabal, 7) El Bosque (Santa Isabel -Cerro España), 8) Nevado del Tolima y 9) volcán Cerro Machín.
 De los 74 manantiales termales registrados (con temperaturas de descarga superiores a 4°C por encima de la temperatura ambiente promedio), 27 son de agua caliente (temperatura mayor a 50ºC). Las temperaturas de descarga más elevadas, cercanas al punto de ebullición del agua, se encuentran en manantiales de los grupos 4, 6 y 9. Adicionalmente se registran 26 manantiales de agua mineral de baja temperatura.
 La geoquímica de los manantiales señala la probabilidad de existencia de sistemas de temperatura intermedia (entre 125 y 225°C), en el Batolito de El Bosque (grupo 3), en el área de Santo Domingo (grupo 5) y en el Nevado del Tolima (grupo 8) y de temperatura alta (>250ºC), en el área del volcán Cerro Bravo, en la zona occidental del Nevado del Ruiz (grupo 4), en Santa Rosa de Cabal (grupo 6)y en el volcán Cerro Machín (grupo 9).
 En Santa Rosa de Cabal, se identifica un área de máximo interés como recurso energético y desde el punto de vista geoquímico, dado que los geotermómetros indican la mayor temperatura en el reservorio de toda el área de estudio y la composición de los manantiales permite identificar con mucha claridad procesos de ebullición y posterior dilución (manantiales de San Vicente) y dilución directa a partir del agua del reservorio (manantiales de Santa Rosa). De acuerdo con la Investigación Geotérmica del Macizo Volcánico del Ruiz, es difícil asociar los manantiales de Santa Rosa de Cabal con una fuente de calor cercana, dado que el volcán más cercano, Paramillo de Santa Rosa, es relativamente antiguo comparado con los otros volcanes de la zona. No obstante, la temperatura del reservorio inferida a partir de los manantiales de Santa Rosa, mayor a la inferida para la zona occidental del Nevado del Ruiz (Botero Londoño), permite plantear la posibilidad de que la fuente de calor de Santa Rosa sea independiente y quizás asociada al Paramillo de Santa Rosa (Alfara & Jaramillo, 2002), aun cuando la cámara magmática en enfriamiento no haya promovido una actividad eruptiva reciente.
 El potencial de aplicación del recurso geotérmico identificado en la zona de estudio es muy amplio y suficiente aún para los usos de máximo requerimiento energético (generación eléctrica). Sin embargo teniendo en cuenta la facilidad de implementación de los diferentes desarrollos geotérmicos posibles, los usos mineromedicinales son quizás los más viables. Estos elevarían la calidad de vida de las comunidades locales e impulsaría actividades paralelas como el turismo, con el consecuente impacto positivo en la economía local.

New and classical applications of heat flow studies

This special issue of Journal of Geophysics and Engineering is dedicated to a collection of papers which resulted from an international workshop held in Aachen, Germany, on 4-7 October 2004, called 'New and Classical Applications of Heat Flow Studies'. This was the third in a series of topical geothermal workshops arranged by the Geothermal Working Group of the German Geophysical Society (DGG) and was organized by the Institute of Applied Geophysics at RWTH Aachen University under the auspices of the International Heat Flow Commission of the International Association of Seismology and Physics (IASPEI). The meeting was attended by some 60 scientists from 14 countries and three continents. Financial assistance, granted by DGG and IASPEI, allowed us to provide partial support for a total of eight students, young scientists and eminent researchers from eastern Europe and overseas. The convenors of the meeting were Christoph Clauser (Aachen), Thomas Kohl (Zurich) and Makoto Taniguchi (Kyoto). The main local organizers were Volker Rath (scientific programme) and Ute Kreutz (accommodation and financial affairs). The topics addressed in more than 50 oral and poster presentations indicated that today intriguing new applications of heat flow studies have emerged, complementing the classical topics of heat flow mapping and the tectonic implications of heat flow. In classical applications, for instance, thermal signatures of water flow or downward diffusion of variations in the Earth's mean temperature are considered as noise which needs to be corrected prior to further use of the data. In contrast, in several new applications it is exactly the information contained in these signatures which has been extracted and interpreted. For instance, over the past two decades, work on the most prominent of these new applications has been devoted to inverting the variation of the Earth's past mean ground surface temperature (GST). As of today, GST provided by the geothermal method has become widely recognized as a valuable supplement to established climate proxies. In contrast to these, GST is directly linked to climate-related temperature variations. It is based in a straightforward manner on heat transport and thermal physics and, in contrast to climate proxies, does not require transfer functions for relating temperature-sensitive phenomena to temperature.The second new application makes use of the signature of heat advection to identify and quantify flow in the deep subsurface. Based on high-quality data of sufficient number and distribution, the method's detection limit is far below that of any other technique. These are but two examples where, under a new perspective, noise has turned into signal. Last but not least, the potential of geothermal energy to supply low-pollution, low-carbon-dioxide electric power and heat has been recognized and exploited for more than 100 years, primarily in tectonically active regions with natural steam reservoirs. However, the ever-increasing price of fossil fuels and their exhaust of greenhouse gases have turned geothermal energy into an attractive option, even in geothermally less favourable regions. In this more demanding situation, understanding different heat transport processes requires more refined techniques for data correction, parameter identification and numerical simulation. This special issue comprises a selection of 11 papers which emerged from this meeting. Corresponding to the scope of the meeting, four papers address paleoclimatic topics, four papers deal with topics related to subsurface flow, and five papers discuss various aspects related directly or indirectly to geothermal energy. Thus, this special issue reflects exciting trends in current geothermal research. It illustrates that today geothermal research comprises new and challenging fields, with applications in environmental sciences, reservoir engineering, and climate and energy research.