Shallow Geothermal Reservoir Research Articles

Normal fault architecture, evolution, and deformation mechanisms in basalts, Húsavik, Iceland: Impact on fluid flow in geothermal reservoirs and seismicity

Faults within layered basaltic sequences significantly influence hydrothermal fluid flow in shallow geothermal reservoirs and potentially during CO2 sequestration and storage. Nevertheless, their characterization regarding fault zone architecture, fluid flow, deformation mechanisms, and seismic potential remains underdeveloped. This study addresses this gap by integrating structural and microstructural observations with X-ray diffraction analyses of exposed normal-transtensional faults associated with the seismically active Húsavík-Flatey Fault in the Tjörnes Fracture Zone, Northern Iceland. Our findings demonstrate that the evolution of basalt-hosted normal-transtensional faults progresses through distinct stages: (1) low-displacement fault propagation from pre-existing cooling joints; (2) fault linkage via dilational jogs; (3) damage zone/fault core growth through brecciation and cataclastic processes; (4) shear localization along sharp slip surfaces; and (5) smearing of volcaniclastic interbeds along the principal fault plane. Evidence of shear localization, truncated clasts, and hydrothermal breccias/veins suggests repeated seismic slip events facilitated by overpressured fluids. Conversely, the presence of clay-rich foliated cataclasite indicates aseismic slips during interseismic periods. Slip along fault jogs, bends, geometric irregularities, and orientation changes causes the dilatant opening of the fault planes and extensional horsetail fractures at fault tips. These structures create main tabular zones for lateral movement of hydrothermal fluids parallel to the fault strike in shallow geothermal reservoirs situated in active extensional-transtensional tectonic settings. In addition, the dilational jogs and the intersection of horsetail veins with the hosting faults may define linear zones of high structural permeability and intense localized fluid flow parallel to the σ2 paleostress orientation and finally mineral precipitation. The results of this study can be utilized to improve models of geothermal fluid flow for enhanced recovery in basaltic reservoirs and assess seismic risk in basaltic faults.

Theoretical analysis of using multiple borehole heat exchangers for production of heating and cooling energy in shallow geothermal reservoirs with underground water flow

In this work the feasibility of using the double-U borehole heat exchangers (BHE) in multiple arrangements for production of heating and cooling energy has been investigated.Theoretical case studies are given for a few hypothetical lithological cases corresponding to the lithology that can be found in City of Zagreb. The results show interference between the borehole heat exchangers and limits of use of shallow reservoir with respect to slow convection and loss of accumulated energy in the reservoir. Special emphasis is placed on the description of the groundwater flow determined by the parameters of the shallow geothermal reservoir, such as permeability, water saturation, thermal conductivity, specific heat capacity and soil density, topography, precipitation, etc. Simulations have been performed in FeFlow coupled with Python user-scripts for individual control of the BHE’s, and the surface equipment is modelled using RES2GEO. To assess the accuracy of numerical predictions in simulating thermal responses, FeFlow software was utilized to conduct Thermal Response Tests (TRT) for individual Borehole Heat Exchangers (BHEs). These numerical results were then juxtaposed with empirical TRT data collected from a site near Velika Gorica, a town within the City of Zagreb. For each case simulation is done for one year with one hour resolution. The temperature distribution analysis highlights notable differences between scenarios, where the temperature field with lower permeability experiences significant variations, particularly near the BHE. Here, temperatures can soar to 20 °C during cooling periods and plummet to 4 °C in heating seasons. These fluctuations cause overheating and subcooling of the reservoir, impairing the heat pump’s efficiency and necessitating increased electricity consumption. By using Multivariate linear regression and Multi-layer perceptron regressor method, the main parameter for determination of COP of ground source heat pump. The analysis reveals that the majority of errors fall within the range of ± 5 %, although the MLA prediction method exhibits slightly more frequent errors. Additionally, the R2 scores demonstrate strong performance: the MLA procedure achieves 0.95 for heating and 0.99 for cooling, while the MLP procedure attains 0.99 for both heating and cooling phases.

Distinct response of deep and shallow geothermal reservoir temperature following earthquakes in the Kangding Fault zone hydrothermal system

Study regionAs part of the active Xianshuihe Fault system, the Kangding geothermal field is a representative high-temperature hydrothermal system in the southeastern Tibetan Plateau, China. The strong tectonic and hydrothermal activity serves as a unique location to study the interaction between earthquakes and geothermal fluids. Study focusWe have collected water samples taken from thermal springs and boreholes in the geothermal field to identify fluid composition and reservoir temperature of the deep granite and shallow metamorphic rock using geochemical analysis and the Integrated Multicomponent Geothermometry (IMG) method. Two typical thermal springs that offer long-term hydrochemical data were selected to investigate the effect of earthquakes on the shallow and deep reservoir temperatures. New hydrological insights for the regionThe deep reservoir temperature in the Kangding geothermal field is in the range 222 ℃ to 256 ℃, while the shallow reservoir temperature ranges from 128 ℃ to 171 ℃. Both thermal springs show changes in reservoir temperature following the 2008 Wenchuan, the 2013 Lushan, and the 2014 Kangding earthquakes. The temperature changes are similar in the deep reservoir of the two springs but different in the case of the shallow reservoir. We propose that the changes in reservoir temperature, steam fraction, and dilution factor indicate that the deep reservoir is more sensitive to earthquake-induced permeability changes than the shallow reservoir.

Reservoirs and Hydrogeochemical Characterizations of the Yanggao Geothermal Field in Shanxi Province, China

Geothermal water is the product of deep circulation within the crust, and the understanding of its hydrogeochemical process can provide effective information for integrated research on its circulation pattern and formation mechanism. Based on the geothermal geological conditions of the Yanggao geothermal field, this study analyzed water samples from thermal springs and geothermal wells in the geothermal field, ascertaining their hydrochemical components, along with their hydrogen and oxygen isotopes. Using methods like piper diagrams, ionic component ratio characterization, Na–K–Mg equilibrium diagrams, and reverse path simulations, this study elucidated the recharge source of geothermal water in the study area, revealed the water–rock interactions the geothermal water experienced, and evaluated the geothermal reservoir temperatures. The results show that the geothermal water has hydrochemical types of Na–Cl–HCO3 and Na–HCO3–Cl, and is primarily recharged by the atmospheric precipitation in the northern mountainous area. The geothermal water has experienced extended water runoff and deep thermal circulation, and its hydrochemical composition primarily results from the weathering and dissolution of silicate rocks and evaporites. The major hydrogeochemical processes of the geothermal water involve the dissolution of calcite, dolomite, gypsum, and kaolinite. In addition, the canon-exchange also changes the chemical component of the geothermal water. The SiO2 Geothermometer, a multimineral equilibrium diagram, and the silica–enthalpy model reveal the presence of deep and shallow geothermal reservoirs in the study area, which exhibit temperatures of 73 °C and ranging from 125 to 150 °C, respectively. The open geothermal reservoir environment results in the mixing of geothermal water and cold water, with shallow and deep geothermal water mixing with cold water at ratios of 57% and 76%, respectively.

NUMERICAL ANALYSIS OF BOREHOLE HEAT EXCHANGER PERFORMANCE IN SHALLOW GRAVEL AQUIFERS AND CLAY-DOMINATED SOIL

The performance of geothermal heat extraction in shallow aquifers depends on both Borehole Heat Exchanger (BHE) and soil or aquifer properties. In this work, an analysis of the thermal yield of a shallow geothermal reservoir was made numerically with the finite element method used to simulate heat and mass transfer in the three-dimensional reservoir. The main parameters for analysis which have been considered are the geometry and physical parameters of the BHE and grout, as well as aquifer matrix and groundwater fluid. Physical parameters are thermal conductivity, flow conductivity, expansion coefficient, porosity, volumetric heat capacity, anisotropy and dispersivity. The numerical tests have been performed in single BHE line source configuration representing numerically modelled thermal response test for the estimation of sustainable heat extraction. The domain size was a 100x100 meter rectangle with a depth of 200 meters. Three main lithological configurations have been modelled: gravel aquifer with low and high convection of groundwater fluid, as well as a shallow geothermal reservoir dominated by clay material without convection. For selected cases, the analysis for temporal and spatial discretization was also made. Three-dimensional transient modelling was made in FEFLOW® software with pre- and post-processing done in user-defined Python scripts. The results show the most influential parameters to be considered when setting up the real case simulation of geothermal heating and cooling, as well as optimal temporal and spatial discretization set-up with respect to expected thermal gradients in the reservoir.

Modelling and optimization of shallow underground thermal energy storage

Shallow geothermal reservoirs are excellent candidates for low-enthalpy energy storage, and can serve as heat batteries providing constant discharge of base heat, as well as rapid discharge of heat in periods of high demand. Recharging can be done by pumping down hot water, heated using excess heat from e.g. waste incinerators. In addition to having a very low carbon footprint, such systems also require limited surface infrastructure, and can easily be placed near or under the end-user, such as below residential buildings. The geological setting is typically complex, with horizons, faults, and intertwined patterns of natural fractures, and the nearwell region is often hydraulically fractured to enhance inter-well communication. Therefore, in order to fully utilize the potential of shallow geothermal heat storage, numerical simulations are imperative. In this work, we show how to practically model such systems, including generation of computational grids with a large number of wells and fractures, numerical discretizations with discrete fractures, and complex storage strategies with multiple wells working together under common group targets. We also discuss how adjoint-based methods can be used to tune model parameters (e.g. well injectivities, rock properties, and hydraulic fracture conductivities) so that the model fits observed data, and to find well controls (e.g. rates and temperatures) that optimize storage operations. The methodology is demonstrated using one artificial and two real underground thermal energy storage projects currently under development, and we highlight important challenges and our suggested solutions related to each of them. Thematic collection: This article is part of the Earth as a thermal battery: future directions in subsurface thermal energy storage systems collection available at: https://www.lyellcollection.org/topic/collections/thermal-energy

A Thermal Conductivity Model for Deformable and Unsaturated Soils to Assess the Thermal Behaviour of Energy Piles

This paper presents an empirical model that predicts the thermal conductivity of soils by accounting for the effect of both the degree of saturation and void ratio on the heat exchange capacity of shallow geothermal reservoirs. The model is generated by the product of two terms: the former accounts for the effect of void ratio on the dry thermal conductivity, whereas the latter describes the influence of the degree of saturation on the moisture-dependent thermal conductivity. The model is a function of three parameters, which are easy to calibrate based on their physical meaning. Model predictions are validated against five different sets of experimental data from literature by means of two alternative approaches: blind prediction of thermal conductivity measurements not employed during calibration and numerical simulations of thermal tests performed on energy piles. Results show that the proposed model is capable of accurately predicting both the thermal conductivity of deformable unsaturated soils as well as reproducing the thermal behaviour of energy piles.

A decade of successful magnetotelluric surveys for delineating shallow geothermal reservoirs beneath nonvolcanic hot springs in Thailand

Magnetotelluric (MT) applications to probe the shallow reservoirs of nonvolcanic hot springs in Thailand have become ever more convincing during the past few decades. In 2013, the first modern MT survey was deployed over the Mae Chan hot spring. A shallow hot water reservoir within a network of fractured granite was imaged as a low-resistivity zone beneath the hot spring surrounded by a resistive granite heat source. With the success of the first MT deployment, in 2014 and 2015, MT measurements were expanded to five hot springs with the objective of 1 km deep drilling where the Muang Rae hot spring was chosen out of these five hot springs to assess the volume of hot fluid for a future geothermal power plant. The conductive zone was again detected at a depth of 200–600 m beneath the Muang Rae hot spring, which our team interpreted as a shallow hot fluid reservoir. However, others interpreted it as just the clay cap with no hot fluid inside, which occurs in most volcanic systems. Therefore, a 1 km well was drilled next to the conductor, instead of through it as we suggested. The well did not find any hot water flowing. In 2018, an MT survey with densely spaced stations was used around the Mae Chan hot spring to validate the shallow conductive zone beneath the hot spring through which five wells (<200 m) were drilled. The depth positions where the hot fluid was found match well with the conductor location at most wells. This is a good indicator that in the Thailand nonvolcanic system, the conductive zone beneath the hot spring is genuinely the shallow reservoir, and not the clay cap zone as in the volcanic systems.

Analysis of the Formation Mechanism of Medium and Low-Temperature Geothermal Water in Wuhan Based on Hydrochemical Characteristics

Wuhan and its surrounding areas have obvious geothermal spring outcrops, which are unexplored potential geothermal resources. The degree of geothermal resource development in Wuhan is low, and there is a lack of systematic research on their hydrochemical characteristics and formation mechanism. The Wuhan area is bounded by the Xiang-Guang fault, the South Qinling-Dabie orogenic belt in the north, and the Yangtze landmass in the south, with Silurian and Quaternary outcrops and little bedrock outcrops. The Silurian is the main water barrier in the region, which separates the upper Triassic and Paleogene as shallow aquifers and the lower Cambrian and Ordovician as deep aquifers. Different strata are connected by a series of fault structures, which constitute Wuhan’s unique groundwater water-bearing system. Eleven geothermal water (23~52 °C) and six surface water samples (around 22 °C) were collected from the study area. The geothermal water in the study area is weakly alkaline, with a pH of 7.04~8.24. The chemical type of geothermal water is mainly deep SO42− with a higher TDS and shallow HCO3− type water with a lower TDS. Isotopic analysis indicates that atmospheric precipitation and water-rock interaction are the main ionic sources of geothermal water. The chemical composition of geothermal water is dominated by ion-exchange interactions and the dissolution of carbonates and silicates. The characteristic coefficients, correlation analysis, water chemistry type, recharge elevation, geothermal water age, reservoir temperature, and cycle depth were also analyzed. The performance was similar in the same geothermal reservoir, which could be judged as an obviously deep and shallow geothermal fluid reservoir, and the genetic conceptual model of Wuhan geothermal was preliminarily deduced. DXR-8 and DXR-9 had the best reservoir conditions, hydrodynamic conditions, rapid alternation of water bodies, and large circulation depth, which is a favorable location for geothermal resource development and will bring considerable economic and social benefits.

Hydrochemical Characteristics and Genetic Mechanism of Geothermal Springs in the Aba Area, Western Sichuan Province, China

Geothermal resources have been a source of significant clean energy in the world. The Sichuan Province is famous for its abundant geothermal resources in China, especially in western Sichuan. The Aba area is a significant minority region in northwestern Sichuan with abundant geothermal resources. In this study, hydrochemical and D-O analyses were conducted on the eight collected geothermal springs to investigate the genetic mechanism of the geothermal resource in the Aba area. The exposed temperatures and pH values of the geothermal springs ranged from 23 °C to 48 °C and from 6.6 to 9.5, respectively. Based on the hydrochemical characteristics, the eight geothermal springs were classified into two types: class A and class B. The class A geothermal springs belonged to the hydrochemical type of Ca-Mg-HCO3-SO4 and Ca-Mg-HCO3 and were affected by the weathering and dissolution of carbonate and silicate. The class B hydrochemical type of geothermal spring was Na-HCO3, which was determined by the weathering and dissolution of evaporite and silicate. A Na-K-Mg triangle diagram revealed that the geothermal springs belonged to immature water. A chalcedony geothermometer indicated that the temperature of the class A shallow geothermal reservoir in the Aba area was 59.70–73.00 °C and 70.65–120.91 °C for class B. Silicon enthalpy approaches showed that the initial reservoir temperature for class A was 181.36–203.07 °C (mixed by 85.76–89.44% cold water) and 271.74–295.58 °C (mixed by 87.39–87.54% cold water) for class B. The recharge elevation of the geothermal spring was 3415–3495 m as calculated by the D-O isotopes. We have proposed these genetic models of the two typical geothermal springs. The achievements provide a vital reference for the further development of geothermal water and the sustainable utilization of geothermal resources in the Aba area.

Sequential coupled numerical simulations of an air/ground-source heat pump: Validation of the model and results of yearly simulations

Numerical simulations are important tools for the assessment of exploiting geothermal energy in heat pump applications. They can be used to evaluate the performance of the system, the long-term production scenarios and the sustainability of the geothermal reservoir. The present work introduces and describes a numerical model, in which a dedicated Matlab® script has been realized to allow sequentially coupled simulations of a shallow geothermal reservoir and of a heat pump system. A mathematical model of a dual-source heat pump, working alternatively with the ground or the air as heat source/sink, has been developed in Matlab® environment. The heat exchangers of the heat pump have been modelled considering the equations that govern the physical phenomena. The dynamic numerical simulator FEFLOW®, based on the finite element method, has been used to simulate the behaviour of the geothermal reservoir, subjected to heat extraction/reinjection by a closed loop vertical heat exchangers field. This methodological approach is useful to evaluate the performance of the coupled system in the long term, and it is important for understanding the advantages and limits of the dual-source heat pump in assuring sustainability over time avoiding the depletion of geothermal resources. The models and their coupling have been calibrated and validated with experimental data from a shallow geothermal plant located in Tribano (Padova, IT). It consists of eight coaxial borehole heat exchangers 30 m deep, connected to a 16 kW dual-source heat pump prototype. The heat pump system provides heating and cooling to an office area. The coupled model has been used to compare the performance of the heat pump when working in air-mode only or in ground-mode only. This allowed the development of a switching control strategy between the two thermal sources. Yearly simulations with the switching strategy have shown that the seasonal performance factor of the dual-source heat pump during the heating mode can be 13.8 % higher compared to the one obtained with a conventional air source heat pump and 3.8 % higher with respect to a ground source heat pump.

Effects of Seawater Recharge on the Formation of Geothermal Resources in Coastal Areas and Their Mechanisms: A Case Study of Xiamen City, Fujian Province, China

The southeast coastal areas of China have abundant geothermal resources. Most especially, seawater-recharged geothermal systems in the coastal areas have large quantiles of recharge but suffer water salinization and low water temperature. Moreover, the geothermal water development in these areas may induce seawater intrusion. Understanding the genetic patterns of geothermal resources is significant for rational exploration and protection. This study analyzed the hydrochemical and environmental isotopic characteristics of geothermal water, groundwater, and surface water samples collected in the area with geothermal resources in Xiamen Province in the southeast coastal areas of China. Based on this, the recharge of geothermal water circulation and the genetic patterns of geothermal resources were revealed. The results of this study indicate that the geothermal water in mountainous areas and piedmonts in Xiamen is mainly recharged by rainfall infiltration. In contrast, the geothermal water in coastal areas in Xiamen is recharged by seawater mixing to different extents, as indicated by hydrochemical types, isotopic characteristics, and the C1-/Br- ratio of geothermal water. As revealed by the calculation results using the Cl−mixing model, 10 of 13 geothermal fields in Xiamen are recharged by seawater mixing, with a mixing ratio of up to 73.20% in the Pubian geothermal field. After being recharged by rainfall in the low mountainous areas, geothermal water migrates toward deep parts along NW-trending faults. Then, it converges with regional NE-trending deep faults to absorb heat conducted from deep parts to form deep geothermal reservoirs. The deep geothermal reservoirs were estimated to be 185–225°C using the silica-enthalpy mixing model. The geothermal water is mixed with cold water or seawater while rising along faults. The temperature of shallow geothermal reservoirs was estimated to be 71–145°C using SiO2geothermometers.

Performance of Multi-Well Exploitation and Reinjection in a Small-Scale Shallow Geothermal Reservoir in Huailai County

The sustainable development of a shallow aquifer geothermal reservoir is strongly affected by the reinjection–production strategy. However, the reinjection–production strategy optimization of a small-scale exploitation unit with tens of meters of well spacing is site specific and has not yet been fulfilled. This study numerically investigates sustainable heat extraction based on various reinjection–production strategies which were conducted in a single-phase aquitard–aquifer geothermal system in Huailai County, Hebei Province, China. The response of the water level and production temperature is mainly discussed. The numerical results show that production without reinjection induces the highest production temperature and also the water level drawdown. Although reinjection in a single doublet well system is conducive to the control of water level drawdown, the introduction of the thermal breakthrough problem causes a decrease in the production temperature. The thermal breakthrough and sustainability of geothermal reservoirs highly depend on the well spacing between the production and reinjection wells, especially for the small-scale field. Therefore, a large well spacing is suggested. A multi-well system facilitates the control of water level drawdown while bringing intensive well interference and thermal breakthrough. Large spacing between the production and reinjection wells is also the basic principle for the design of the multi-well system. A decrease in openhole length leads to an increase in the production temperature and output thermal power. An increase in the production rate affects the thermal breakthrough highly and shortens the lifetime of the geothermal system. Furthermore, the extracted thermal energy is highly affected by the reduction in the reinjection temperature. The results in this study can provide references to achieve sustainable geothermal exploitation in small-scale geothermal reservoirs.

Application of ORC in a Distributed Integrated Energy System Driven by Deep and Shallow Geothermal Energy

This study presents a distributed integrated energy system driven by deep and shallow geothermal energy based on forward and reverse cycle for flexible generation of cold, heat and electricity in different scenarios. By adjusting the strategy, the system can meet the demand of heat-electricity in winter, cool-electricity in summer and electricity in transition seasons. The thermodynamic analysis shows that the thermal efficiency of the integrated energy system in the heating and power generation mode is 16% higher than that in the cooling and power generation mode or the single power generation mode. Meanwhile, the annual heat-obtaining quantity of the system is reduced by 11% compared with that of the independent power generation system, which effectively alleviates the imbalance of the temperature field of the shallow geothermal reservoir. In terms of net power generation, the integrated energy system can generate approximately 31% more electricity than the conventional independent cooling and heating system under the same cooling and heating capacity. An integrated system not only realizes the comprehensive supply of cold and thermal ower by using clean geothermal efficiency, but also solves the temperature imbalance caused by the attenuation of a shallow geothermal temperature field. It provides a feasible way for carbon emission reduction to realize sustainable and efficient utilization of geothermal energy.

An assessment of a shallow geothermal reservoir of Mae Chan hot spring, northern Thailand via magnetotelluric surveys

In a non-volcanic geothermal system, like Mae Chan hot spring and many other hot springs in Thailand, hot water is heated deep underground and seeps to the surface through fractures and faults. Some of the hot water may aggregate in a hydrothermal alteration zone along the fracture zones of granite rocks to form shallow “hot water reservoirs”. These networks lower the bulk resistivity of the granitic rock to form a low resistivity zone associated with the hot water reservoir, which can then be imaged via a magnetotelluric (MT) survey. A series of magnetotelluric surveys from 2013 to 2018 was conducted in order to assess the location, size and depth of the shallow geothermal reservoir of the Mae Chan hot spring. All data were combined to produce the final 3-D resistivity structure. The final MT survey had a high density of MT sites across the zone of interest which allowed us to precisely image the shallow reservoir for drilling purposes. Using the final MT results, five new boreholes with a maximum depth of 200 m were drilled. Hot water was found at various depths from each borehole with perfect agreement with the final resistivity structure derived from the MT data. This 3-D resistivity outline will be useful in developing the field with future production and re-injection wells.

Integration of Photovoltaic Electricity with Shallow Geothermal Systems for Residential Microgrids: Proof of Concept and Techno-Economic Analysis with RES2GEO Model

The European Union aims to reduce Greenhouse Gas (GHG) emissions by 55% before 2030 compared to 1990 as a reference year. One of the main contributions to GHG emissions comes from the household sector. This paper shows that the household sector, when organised into a form of prosumer microgrids, including renewable sources for electric, heating and cooling energy supply, can be efficiently decarbonised. This paper investigates one hypothetical prosumer microgrid with the model RES2GEO (Renewable Energy Sources to Geothermal). The aim is to integrate a carbon-free photovoltaic electricity source and a shallow geothermal reservoir as a heat source and heat sink during the heating and cooling season. A total of four cases have been evaluated for the Zagreb City location. The results represent a balance of both thermal and electric energy flows within the microgrid, as well as thermal recuperation of the reservoir. The levelised cost of energy for all cases, based on a 20-year modelling horizon, varies between 41 and 63 EUR/MWh. On the other hand, all cases show a decrease in CO2 emissions by more than 75%, with the best case featuring a reduction of more than 85% compared to the base case, where electricity and gas for heating are supplied from the Distribution System Operator at retail prices. With the use of close integration of electricity, heating and cooling demand and supply of energy, cost-effective decarbonisation can be achieved for the household sector.

Seismoelectric surface-wave analysis for characterization of formation properties, using dispersive relative spectral amplitudes

We have studied seismoelectric (SE) surface-wave signals and found that they can be used to infer changes in the SE coupling properties at depth. SE surface-wave signals have much higher amplitudes than SE body-wave signals. We measured the seismic and the electrical potential or electromagnetic (EM) field along the surface of the earth. We used dispersive relative spectral amplitudes (DRSAs) that measure the frequency-dependent relative strength of electrical signals versus seismic signals associated with SE surface-wave signals. We find that DRSAs have sensitivity to contrasts in the electrokinetic coupling coefficient and other relevant petrophysical properties at depth. Our discovery can mitigate the major limitation that plagues body-wave-based SE methods: the relative weakness of the converted EM signals from seismic body waves. We envision applications to characterize subsurface rock, fluid, and fluid-flow properties (e.g., porosity, permeability, and dynamic fluid viscosity, salinity) in the near surface, for aquifers, and shallow geothermal reservoirs.

Three-dimensional resistivity structure in the hydrothermal system beneath Ganzi Basin, eastern margin of Tibetan Plateau

The Ganzi geothermal system located in the eastern margin of Tibetan Plateau, is found to distribute the strongly active surface manifestations, such as boiling springs, and fumaroles. The resistivity structure of high-temperature hydrothermal system beneath Ganzi Basin has been obtained from the three-dimensional (3-D) magnetotelluric inversion of 103 broadband MT stations in period range from 320 Hz to 1000s. The MT stations have an average spacing of 500 m and completely cover the Ganzi Basin. The detailed 3-D resistivity structure which the authors found has been described characteristics of the shallow and deep geothermal reservoirs as well as to correlate with the structural features present in the Ganzi Basin. The shallow geothermal reservoir is mainly composed of the sedimentary layer, with a depth of 300−800 m, while the deep geothermal reservoir mainly consists of the fault fracture zone with a depth of 1−5 km. The porosity of the geothermal reservoirs is in the range 5–10 %, estimated based on the resistivity model. The Ganzi fault provides the pathway for transport heat and the circulation of geothermal fluids. The possible heat source for the Ganzi geothermal system has been attributed to tectonic deformation heating in the crust, radiogenic heat production derived from the decay of U, Th and K, and heat from the mantle.

A Simple Thermal Conductivity Model for Unsaturated Geomaterials Accounting for Degree of Saturation and Dry Density

This paper proposes a simple thermal conductivity model for geomaterials accounting for the combined effect of both degrees of saturation and dry density. The model only requires the determination of the thermal conductivity under dry conditions (i.e., at a degree of saturation equal to zero) and as little as two additional measurements of thermal conductivity performed at different levels of degree of saturation and dry density. The model is a function of only two fitting parameters, namely the moisture factor {m}_{f} and the density factor {m}_{d}. Despite its simplicity, the model can correctly predict the thermal conductivity of geomaterials and this has been validated against five sets of experimental data obtained on a very broad range of materials ranging from fine (e.g., bentonite) to coarser soils (e.g., a mix of gravel, coarse sand and silt) tested at different levels of degree of saturation and dry density. The paper also shows that the model can be applied to different engineering contexts such as (a) the thermal behaviour of earth materials used for building construction, (b) the thermal performance of bentonites employed for the storage of nuclear waste and (c) the estimation of the heat exchange in shallow geothermal reservoirs. Finally, the proposed model can be easily implemented in a finite element code to perform numerical simulations to study the heat transfer in unsaturated geomaterials.

Shallow structure of Los Humeros (LH) caldera and geothermal reservoir from magnetotellurics and potential field data

SUMMARYThis study focuses in the analysis of the internal structure of the upper 3 km of Los Humeros (LH) caldera and the relation of electrical and hydrothermal anomalies. For this purpose, we measured, processed and interpreted 78 broad-band magnetotelluric (MT) soundings. We performed a 3-D inversion of the data set (ModEM) using all MT soundings, although only half of the available frequencies per sounding due to limited computed power. We also carried out the 2-D inversions (NLCG) of the invariant determinant along two orthogonal profiles (EW and NS) crossing the caldera structure; their comparison yields similar resistivity and structural models results. The resistivity modelling is complemented with the results of a joint 3-D inversion of an accurate gravity database of 720 stations, and total field aeromagnetic data (SGM) from the caldera crater. The combined results provide novel details about the structure of the shallow geothermal reservoir of the resurgence caldera complex hosting the active hydrothermal system. Density and resistivity models show the existence of a composed crater basin structure separated by an EW high-density structure; the northern basin is associated to the LH crater, whereas the southern basin associates to the emergent Los Potreros (LP) caldera basin. The magnetization model indicates that there is a common source for the magnetic volcanic products observed at the caldera surface, and that the LP fault is the more magnetized fault of the geothermal system. The propylic zoning under the geothermal field, which according to the MT model results has resistivities above ∼100 Ω-m, was extrapolated using this and additional criteria to obtain the distribution of other hypothetical propylitic zones of hydrothermal potential.