Advective Heat Transport Research Articles

The origin of deep geothermal anomalies in the German Molasse Basin: results from 3D numerical models of coupled fluid flow and heat transport

The European Molasse Basin is a Tertiary foreland basin at the northern front of the Alps, which is filled with mostly clastic sediments. These Molasse sediments are underlain by Mesozoic sedimentary successions, including the Upper Jurassic aquifer (Malm) which has been used for geothermal energy production since decades. The thermal field of the Molasse Basin area is characterized by prominent thermal anomalies. Since the origin of these anomalies is still an object of debates, especially the negative ones represent a high risk for geothermal energy exploration. With our study, we want to contribute to the understanding of the thermal configuration of the basin area and with that help to reduce the exploration risk for future geothermal projects in the Molasse Basin. For this, we conducted 3D basin-scale coupled fluid and heat transport simulations to reproduce the present-day thermal field of the Molasse Basin by considering conduction, advection, and convection as heat-driving mechanisms. Within this paper, we show how the temperature distribution of the Molasse Basin, including the pronounced thermal anomalies, can be reproduced by coupled fluid flow and heat transport simulations following a multi-scale 3D-modelling approach. We find that the shallow thermal field is strongly affected by basin-wide fluid flow. Furthermore, we show that the temperature distribution at the depth of the Malm aquifer is strongly influenced by the hydraulic conductivity of the Foreland and Folded Molasse Sediments and that hydraulically conductive faults have only a minor influence on the regional temperature distribution. Moreover, we show that the positive and negative thermal anomalies are caused by the superposed effects of conductive and advective heat transport and correlated with the geological structure.

Thermal history of the lower Carboniferous Culm Basin in the Nízký Jeseník Mts. (NE Bohemian Massif, Czech Republic and Poland)

Raman microspectroscopy of organic matter, vitrinite reflectance and fluid inclusion data were used to reconstruct the thermal history in the lower Carboniferous accretionary prism of the Culm Basin in the Nizký Jesenik Mts. (NE Bohemian Massif). The model involves the Variscan (mid–late Carboniferous) burial diagenesis, which was overprinted by a post-Variscan, probably Permian and/or early Mesozoic, thermal pulse(s) in its central and western parts. The latter may have been related to advective heat transport and the circulation of hot fluids. In the siliciclastic rocks of the Culm Basin, the maximum palaeotemperatures varied from ~200 ± 30 °C in the E (in the Hradec-Kyjovice Formation) to ~350 ± 30 °C in the NW (in the Andělska Hora Formation).

Controls on permafrost thaw in a coupled groundwater-flow and heat-transport system: Iqaluit Airport, Nunavut, Canada

Numerical simulations of groundwater flow and heat transport are used to provide insight into the interaction between shallow groundwater flow and thermal dynamics related to permafrost thaw and thaw settlement at the Iqaluit Airport taxiway, Nunavut, Canada. A conceptual model is first developed for the site and a corresponding two-dimensional numerical model is calibrated to the observed ground temperatures. Future climate-warming impacts on the thermal regime and flow system are then simulated based on climate scenarios proposed by the Intergovernmental Panel on Climate Change (IPCC). Under climate warming, surface snow cover is identified as the leading factor affecting permafrost degradation, including its role in increasing the sensitivity of permafrost degradation to changes in various hydrogeological factors. In this case, advective heat transport plays a relatively minor, but non-negligible, role compared to conductive heat transport, due to the significant extent of low-permeability soil close to surface. Conductive heat transport, which is strongly affected by the surface snow layer, controls the release of unfrozen water and the depth of the active layer as well as the magnitude of thaw settlement and frost heave. Under the warmest climate-warming scenario with an average annual temperature increase of 3.23 °C for the period of 2011–2100, the simulations suggest that the maximum depth of the active layer will increase from 2 m in 2012 to 8.8 m in 2100 and, over the same time period, thaw settlement along the airport taxiway will increase from 0.11 m to at least 0.17 m.

Thermal modeling and heat flow density interpretation of the onshore Northwest Java Basin, Indonesia

Studies pertaining to heat flow density distribution in Indonesian tertiary sedimentary basins have been conducted intermittently throughout more than two decades. Even so, all of these studies have always concentrated on the basic compilations of thermal gradient, thermal conductivity, and terrestrial heat flow datasets or the study of basin thermal history rather than on the investigations into their present thermal regime. In addition, syntheses on the possible causes to the observed heat flow density distribution in these basins have been largely determined only on the basis of its correlation with geological and geophysical evidences. One basin experiencing this particular problem is the onshore Northwest Java Basin, where knowledge of the present-day subsurface temperature structure is virtually absent and interpretation of heat flow distribution has been mainly qualitative. For these reasons, in this study we have attempted to quantitatively characterize the present subsurface temperature distribution and surface heat flow density of the onshore Northwest Java Basin using a simple numerical thermal modeling approach, based on the most recent heat flow datasets. Taking into account a conductive heat transfer process, the modeling exercise is aimed at deriving a 3-D subsurface temperature distribution. The temperature distribution is translated into the predicted surface heat flow distribution. The modeled temperature field reveals its dependence on the geometrical aspects of the basin’s basement. Comparison between the computed and the observed heat flow densities, taking into account the uncertainties of both, suggests that advective heat transport by groundwater flow may be present, in addition to conduction. The modeling results are thus capable of demonstrating the importance of a quantitative approach in studying the present thermal state of a sedimentary basin.

A review of thermal history and timescales of tectonometamorphic processes in Sikkim Himalaya (NEIndia) and implications for rates of metamorphic processes

AbstractThe lesser and greater Himalayan sequences (LHandGH) in Sikkim have been studied in detail recently. TheLHconsists mainly of pelitic lithologies with detrital zircon ages >1800 Ma andεNd(0) of −27.7 to −23.4 (with intercalated calcsilicates, metabasites and a sheared granite gneiss – the Lingtse gneiss). The metamorphic sequence is characterized by (i) a remarkably continuous and systematic metamorphic field gradient (~60–70 °C kbar−1), (ii) peak pressures as well as temperatures of metamorphism increasing at higher metamorphic grades (~5 kbar, 500 °C at the garnet zone to ~8 kbar, 700 °C at the sillimanite–muscovite‐zone), (iii) clockwiseP–Tpaths of a tight ‘hairpin’ shape with a prograde slope of 20–30°C kbar−1, (iv) initiation of garnet growth (Lu–Hf chronometry) at systematically older ages at higher grades (11–10.5 Ma at garnet zone to 16.8 Ma at sillimanite–K‐feldspar zone), (v) peak of metamorphism simultaneously at all grades (13–11 Ma), (vi) deformed and folded metapelitic rocks but with little evidence of pervasive shearing and (vii) duration of melt formation and segregation on the order of a few 100,000 years. The degree of melting was low, as indicated by the presence of muscovite all through the sequence. This evidence indicates that theLHbehaved as a coherent block, and thermomechanical models quantitatively reproduce many of these features as well as explain the formation of inverted metamorphic sequence in such a coherent block. The greater Himalayan sequence (GH), also largely metapelitic, has different protoliths (with components as young as 800 Ma) and a different metamorphic history (extensive migmatization at higher grades ~8–12 kbar, 750–850 °C at an older age of largely >20 Ma along a different clockwiseP–Tpath characterized by sharp, isothermal decompression followed by near isobaric cooling). Geochronological data (monazite, zircon, garnet) as well as cooling rate calculations (geospeedometry) reveal that theGHis made up of at least two blocks with distinct, complex and multistageP–T–thistories. Different stages of the high‐Tcooling were rapid (100's °C Ma−1to 10's °C Ma−1at different stages) and at least some of it was unrelated to exhumation; synthesis of these results with low‐Tthermochronometry allows the detailed multistage cooling and exhumation history of this block to be reconstructed. The reconstructed cooling and exhumation histories show that a number of interlinked processes operating on a hierarchy of timescales leads to the overall evolution. TheP–Thistory with rapid cooling can be reproduced by thermomechanical models that consider conductive as well as advective heat transport and there is no need to invoke additional processes (such as the role of fluids). Metabasic inclusions within both theLHand theGHappear to record a different (? older in part) history. The role of these in the overall evolution, as well as the difference in metamorphic ages of theLHandGHremain open questions that need to be addressed in future research.

Using a Thermal Proxy to Examine Sediment–Water Exchange in Mid-Continental Shelf Sandy Sediments

Fluid exchange across the sediment–water interface in a sandy open continental shelf setting was studied using heat as a tracer. Summertime tidal oscillation of cross-shelf thermal fronts on the South Atlantic Bight provided a sufficient signal at the sediment–water interface to trace the advective and conductive transport of heat into and out of the seabed, indicating rapid flushing of ocean water through the upper 10–40 cm of the sandy seafloor. A newly developed transport model was applied to the in situ temperature data set to estimate the extent to which heat was transported by advection rather than conduction. Heat transported by shallow 3-D porewater flow processes was accounted for in the model by using a dispersion term, the depth and intensity of which reflected the depth and intensity of shallow flushing. Similar to the results of past studies in shallower and more energetic nearshore settings, transport of heat was greater when higher near-bed velocities and shear stresses occurred over a rippled bed. However, boundary layer processes by themselves were insufficient to promote non-conductive heat transport. Advective heat transport only occurred when both larger boundary layer stresses and thermal instabilities within the porespace were present. The latter process is dependent on shelf-scale heating and cooling of bottom water associated with upwelling events that are not coupled to local-scale boundary layer processes.

Application limits of the interpretation of near-surface temperature time series to assess groundwater recharge

Summary The main objective of this study was to test the application limits of a groundwater recharge assessment technique based on the inversion of a vertical one-dimensional numerical model of advective–conductive heat transport, using temperature time series at three different depths (1, 3, 5 m) in the unsaturated zone. For this purpose, several synthetic hourly datasets of subsurface temperatures, representing various weather, ground cover, and soil texture conditions, thus covering a wide range of groundwater recharge values, were produced with the vertical one-dimensional coupled heat and moisture transport simulator SHAW (Simultaneous Heat and Water model). Estimates of the vertical flux of water in the soil were then retrieved from these realistic temperature profiles using a simple one-dimensional numerical simulator of advective and conductive heat transport in the unsaturated zone that was developed as part of this study. The water flux was assumed constant on a weekly, monthly, semiannual, and annual basis. From these vertical water flux estimates, annual (potential) groundwater recharge rates were then computed and results were compared to those calculated previously with SHAW to assess the accuracy of the method. Results showed that, under ideal conditions, it would be possible to estimate annual recharge rates that are above 200 mm/y, with an acceptable error of less than 20%. These “ideal” conditions include the resolution of the water flux on a weekly basis, error-free temperature measurements below the soil freezing zone, and model parameter values (thermal conductivity and heat capacity of the soil) known a priori with no uncertainty. However, this work demonstrates that the accuracy of the method is highly sensitive to the uncertainty of the input model parameters of the numerical model used to carry out the inversion and to measurement errors of temperature time series. For the conditions represented in this study, these findings suggest that, despite the best modeling and field instrumentation practices, heat-based techniques for the assessment of diffuse groundwater recharge rates are likely not well suited for real field conditions, but could still represent a viable approach for applications carried out in engineered materials and under controlled conditions.

Stationary and transient thermal states of barometric pumping in the access pit of an underground quarry

The transition zone between free and underground atmospheres hosts spectacular phenomena, as demonstrated by temperature measurements performed in the 4.6m diameter and 20m deep vertical access pit of an abandoned underground quarry located in Vincennes, near Paris. In summer, a stable stratification of the atmosphere is maintained, with coherent temperature variations associated with atmospheric pressure changes, with a barometric tide S2 larger than 0.1°C peak to peak. When the winter regime of turbulent cold air avalanches is initiated, stratification with pressure induced signals can be restored transiently in the upper part of the pit, while the lower part remains fully mixed and insensitive to pressure variations. The amplitude of the pressure to temperature transfer function increases with frequency below 5×10−4Hz, with values at 3×10−5Hz varying from 0.1°C·hPa−1 at the bottom up to 2°C·hPa−1 towards the top of the pit. These temperature variations are accounted for by cave breathing, which is pressure induced motion of air amplified by the large volume of the quarry. This understanding is supported by a numerical model including advective heat transport, heat diffusion, and heat exchange with the pit walls. Mean lifetime in the pit is of the order of 9 to 13h, and barometric pumping results in an effective ventilation rate of the quarry of the order of 10−7s−1. This study illustrates the important role of barometric pumping in heat and matter transport between atmosphere and lithosphere. The resulting stationary and transient states, revealed in this pit, are probably a general feature of functioning interface systems, and therefore are an important aspect to consider in problems of contaminant transport, or the preservation of precious heritage such as rare ecosystems or painted caves.

Advection and dispersion heat transport mechanisms in the quantification of shallow geothermal resources and associated environmental impacts

Borehole Heat Exchangers (BHEs) are increasingly being used to exploit shallow geothermal energy. This paper presents a new methodology to provide a response to the need for a regional quantification of the geothermal potential that can be extracted by BHEs and the associated environmental impacts. A set of analytical solutions facilitates accurate calculation of the heat exchange of BHEs with the ground and its environmental impacts. For the first time, advection and dispersion heat transport mechanisms and the temporal evolution from the start of operation of the BHE are taken into account in the regional estimation of shallow geothermal resources.This methodology is integrated in a GIS environment, which facilitates the management of input and output data at a regional scale. An example of the methodology's application is presented for Barcelona, in Spain. As a result of the application, it is possible to show the strengths and improvements of this methodology in the development of potential maps of low temperature geothermal energy as well as maps of environmental impacts. The minimum and maximum energy potential values for the study site are 50 and 1800W/m2 for a drilled depth of 100m, proportionally to Darcy velocity. Regarding to thermal impacts, the higher the groundwater velocity and the energy potential, the higher the size of the thermal plume after 6months of exploitation, whose length ranges from 10 to 27m long. A sensitivity analysis was carried out in the calculation of heat exchange rate and its impacts for different scenarios and for a wide range of Darcy velocities. The results of this analysis lead to the conclusion that the consideration of dispersion effects and temporal evolution of the exploitation prevent significant differences up to a factor 2.5 in the heat exchange rate accuracy and up to several orders of magnitude in the impacts generated.

The hydrothermal power of oceanic lithosphere

Abstract. We have estimated the power of ventilated hydrothermal heat transport, and its spatial distribution, using a set of recently developed plate models which highlight the effects of axial hydrothermal circulation and thermal insulation by oceanic crust. Testing lithospheric cooling models with these two effects, we estimate that global advective heat transport is about 6.6 TW, significantly lower than most previous estimates, and that the fraction of that extracted by vigorous circulation on the ridge axes (< 1 My old) is about 50 % of the total, significantly higher than previous estimates. These new estimates originate from the thermally insulating properties of oceanic crust in relation to the mantle. Since the crust is relatively insulating, the effective properties of the lithosphere are "crust dominated" near ridge axes (a thermal blanketing effect yielding lower heat flow) and gradually approach mantle values over time. Thus, cooling models with crustal insulation predict low heat flow over young seafloor, implying that the difference of modeled and measured heat flow is due to the heat transport properties of the lithosphere, in addition to ventilated hydrothermal circulation as generally accepted. These estimates may bear on important problems in the physics and chemistry of the Earth because the magnitude of ventilated hydrothermal power affects chemical exchanges between the oceans and the lithosphere, thereby affecting both thermal and chemical budgets in the oceanic crust and lithosphere, the subduction factory, and the convective mantle.

Geothermal modelling of faulted metamorphic crystalline crust: a new model of the Continental Deep Drilling Site KTB (Germany)

The area of the 9.1-km-deep Continental Deep Drillhole (KTB) in Germany is used as a case study for a geothermal reservoir situated in folded and faulted metamorphic crystalline crust. The presented approach is based on the analysis of 3-D seismic reflection data combined with borehole data and hydrothermal numerical modelling. The KTB location exemplarily contains all elements that make seismic prospecting in crystalline environment often more difficult than in sedimentary units, basically complicated tectonics and fracturing and low-coherent strata. In a first step major rock units including two known nearly parallel fault zones are identified down to a depth of 12 km. These units form the basis of a gridded 3-D numerical model for investigating temperature and fluid flow. Conductive and advective heat transport takes place mainly in a metamorphic block composed of gneisses and metabasites that show considerable differences in thermal conductivity and heat production. Therefore, in a second step, the structure of this unit is investigated by seismic waveform modelling. The third step of interpretation consists of applying wavenumber filtering and log-Gabor-filtering for locating fractures. Since fracture networks are the major fluid pathways in the crystalline, we associate the fracture density distribution with distributions of relative porosity and permeability that can be calibrated by logging data and forward modelling of the temperature field. The resulting permeability distribution shows values between 10-16 and 10-19 m(-2) and does not correlate with particular rock units. Once thermohydraulic rock properties are attributed to the numerical model, the differential equations for heat and fluid transport in porous media are solved numerically based on a finite difference approach. The hydraulic potential caused by topography and a heat flux of 54 mW m(-2) were applied as boundary conditions at the top and bottom of the model. Fluid flow is generally slow and mainly occurring within the two fault zones. Thus, our model confirms the previous finding that diffusive heat transport is the dominant process at the KTB site. Fitting the observed temperature-depth profile requires a correction for palaeoclimate of about 4 K at 1 km depth. Modelled and observed temperature data fit well within 0.2 degrees C bounds. Whereas thermal conditions are suitable for geothermal energy production, hydraulic conditions are unfavourable without engineered stimulation.

Shape Optimization Methods Locating Layer Interfaces in Geothermal Reservoirs

Subsurface structures have a strong influence on fluid flow and heat transport in geothermal systems. We examine whether the position and shape of an interface between two lithological bodies can be detected based on temperature-depth measurements. We use a level set function to describe the interface, and a shape optimization method in combination with the adjoint variable based on the heat transport equation to invert for position and shape. Specifically, we investigate how advective heat transport affects the identification of the interface and show that the method successfully retrieves interface positions in synthetic 2D cases of two-layer models.

PALEOMAGNETISM. A Hadean to Paleoarchean geodynamo recorded by single zircon crystals.

Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport. The existence of a terrestrial magnetic field before the Late Heavy Bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior.

Direct current (DC) resistivity and induced polarization (IP) monitoring of active layer dynamics at high temporal resolution

With permafrost thawing and changes in active layer dynamics induced by climate change, interactions between biogeochemical and thermal processes in the ground are of great importance. Here, active layer dynamics have been monitored using direct current (DC) resistivity and induced polarization (IP) measurements at high temporal resolution and at a relatively large scale at a heath tundra site on Disko Island on the west coast of Greenland (69°N). At the field site, the active layer is disconnected from the deeper permafrost, due to isothermal springs in the region. Borehole sediment characteristics and subsurface temperatures supplemented the DC-IP measurements. A time-lapse DC-IP monitoring system has been acquiring at least six datasets per day on a 42-electrode profile with 0.5m electrode spacing since July 2013. Remote control of the data acquisition system enables interactive adaptation of the measurement schedule, which is critically important to acquire data in the winter months, where extremely high contact resistances increase the demands on the resistivity meter. Data acquired during the freezing period of October 2013 to February 2014 clearly image the soil freezing as a strong increase in resistivity. While the freezing horizon generally moves deeper with time, some variations in the freezing depth are observed along the profile. Comparison with depth-specific soil temperature indicates an exponential relationship between resistivity and below-freezing temperature. Time-lapse inversions of the full-decay IP data indicate a decrease of normalized chargeability with freezing of the ground, which is the result of a decrease in the total unfrozen water and of the higher ion concentration in the pore-water. We conclude that DC-IP time-lapse measurements can non-intrusively and reliably image freezing patterns and their lateral variation on a 10–100m scale that is difficult to sample by point measurements. In combination with laboratory experiments, the different patterns in resistivity and chargeability changes will enable the disentanglement of processes (e.g., fluid migration and freezing, advective and diffusive heat transport) occurring during freezing of the ground. The technology can be expanded to three dimensions and also to larger scale.

Basin-scale conceptual groundwater flow model for an unconfined and confined thick carbonate region

Application of the gravity-driven regional groundwater flow (GDRGF) concept to the hydrogeologically complex thick carbonate system of the Transdanubian Range (TR), Hungary, is justified based on the principle of hydraulic continuity. The GDRGF concept informs about basin hydraulics and groundwater as a geologic agent. It became obvious that the effect of heterogeneity and anisotropy on the flow pattern could be derived from hydraulic reactions of the aquifer system. The topography and heat as driving forces were examined by numerical simulations of flow and heat transport. Evaluation of groups of springs, in terms of related discharge phenomena and regional chloride distribution, reveals the dominance of topography-driven flow when considering flow and related chemical and temperature patterns. Moreover, heat accumulation beneath the confined part of the system also influences these patterns. The presence of cold, lukewarm and thermal springs and related wetlands, creeks, mineral precipitates, and epigenic and hypogenic caves validates the existence of GDRGF in the system. Vice versa, groups of springs reflect rock–water interaction and advective heat transport and inform about basin hydraulics. Based on these findings, a generalized conceptual GDRGF model is proposed for an unconfined and confined carbonate region. An interface was revealed close to the margin of the unconfined and confined carbonates, determined by the GDRGF and freshwater and basinal fluids involved. The application of this model provides a background to interpret manifestations of flowing groundwater in thick carbonates generally, including porosity enlargement and hydrocarbon and heat accumulation.

Determining the Origins of Advective Heat Transport Convergence Variability in the North Atlantic

AbstractA recent state estimate covering the period 1992–2010 from the Estimating the Circulation and Climate of the Ocean (ECCO) project is utilized to quantify the roles of air–sea heat fluxes and advective heat transport convergences in setting upper-ocean heat content anomalies H in the North Atlantic Ocean on monthly to interannual time scales. Anomalies in (linear) advective heat transport convergences, as well as Ekman and geostrophic contributions, are decomposed into parts that are due to velocity variability, temperature variability, and their covariability. Ekman convergences are generally dominated by variability in Ekman mass transports, which reflect the instantaneous response to local wind forcing, except in the tropics, where variability in the temperature field plays a significant role. In contrast, both budget analyses and simple dynamical arguments demonstrate that geostrophic heat transport convergences that are due to temperature and velocity variability are anticorrelated, and thus their separate treatment is not insightful. In the interior of the subtropical gyre, the sum of air–sea heat fluxes and Ekman heat transport convergences is a reasonable measure of local atmospheric forcing, and such forcing explains the majority of H variability on all time scales resolved by ECCO. In contrast, in the Gulf Stream region and subpolar gyre, ocean dynamics are found to be important in setting H on interannual time scales. Air–sea heat fluxes damp anomalies created by the ocean and thus are not set by local atmospheric variability.

Linear and nonlinear modelling of a theoretical travelling-wave thermoacoustic heat engine

AbstractWe have carried out three-dimensional Navier–Stokes simulations, from quiescent conditions to the limit cycle, of a theoretical travelling-wave thermoacoustic heat engine (TAE) composed of a long variable-area resonator shrouding a smaller annular tube, which encloses the hot (HHX) and ambient (AHX) heat exchangers, and the regenerator (REG). Simulations are wall-resolved, with no-slip and adiabatic conditions enforced at all boundaries, while the heat transfer and drag due to the REG and HXs are modelled. HHX temperatures have been investigated in the range 440–500 K with the AHX temperature fixed at 300 K. The initial exponential growth of acoustic energy is due to a network of travelling waves thermoacoustically amplified by looping around the REG/HX unit in the direction of the imposed temperature gradient. A simple analytical model demonstrates that such instability is a localized Lagrangian thermodynamic process resembling a Stirling cycle. An inviscid system-wide linear stability model based on Rott’s theory is able to accurately predict the operating frequency and the growth rate, exhibiting properties consistent with a supercritical Hopf bifurcation. The limit cycle is governed by acoustic streaming – a rectified steady flow resulting from high-amplitude nonlinear acoustics. Its key features are explained with an axially symmetric incompressible model driven by the wave-induced stresses extracted from the compressible calculations. These features include Gedeon streaming, Rayleigh streaming in the resonator, and mean recirculations due to flow separation. The first drives the mean advection of hot fluid from the HHX to a secondary heat exchanger (AHX2), in the thermal buffer tube (TBT), necessary to achieve saturation of the acoustic energy growth. The direct evaluation of the nonlinear energy fluxes reveals that the efficiency of the device deteriorates with the drive ratio and that the acoustic power in the TBT is balanced primarily by the mean advection and thermoacoustic heat transport.

Investigation of a recent extreme high‐temperature event in the Tokyo metropolitan area using numerical simulations: the potential role of a ‘hybrid’ foehn wind

A record‐breaking high temperature of 39.8 °C for June in Japan was observed at 1420 Japan Standard Time on 24 June 2011 60 km northwest of central Tokyo. In this extreme high‐temperature (EHT) event, surface air temperatures above 37.0 °C were recorded in and around Kumagaya, an area just north of the convergence line between westerly winds from the Chubu Mountains and a southwesterly wind from the Pacific Ocean. To determine the mechanism of this EHT event, we applied various analyses using the Weather Research and Forecasting (WRF) model and observational data. According to the heat budget analysis obtained from the WRF model, during the morning most of the sensible heat supply to the mixed layer came from the net heat input, due to surface sensible heat transported by subgrid‐scale turbulent diffusion. However, most of the net heat input came from advective heat transport after noon, when the westerly wind penetrated the EHT area. This westerly wind, according to backward trajectory, Lagrangian energy budget and Eulerian forward tracer analyses, arose from a combination of two kinds of foehn flow (‘hybrid’‐type foehn wind). Specifically, the westerly wind became a foehn wind that was caused by dry‐adiabatic heating and wet‐diabatic heating with water vapour condensation. This ‘hybrid’ foehn wind was an important factor in causing the present EHT event.

Crustal permeability: Introduction to the special issue

The topic of crustal permeability is of broad interest in light of the controlling effect of permeability on diverse geologic processes and also timely in light of the practical challenges associated with emerging technologies such as hydraulic fracturing for oil and gas production (‘fracking’), enhanced geothermal systems, and geologic carbon sequestration. This special issue of Geofluids is also motivated by the historical dichotomy between the hydrogeologic concept of permeability as a static material property that exerts control on fluid flow and the perspective of economic geologists, geophysicists, and crustal petrologists who have long recognized permeability as a dynamic parameter that changes in response to tectonism, fluid production, and geochemical reactions. Issues associated with fracking, enhanced geothermal systems, and geologic carbon sequestration have already begun to promote a constructive dialog between the static and dynamic views of permeability, and here we have made a conscious effort to include both viewpoints. This special issue also focuses on the quantification of permeability, encompassing both direct measurement of permeability in the uppermost crust and inferential permeability estimates, mainly for the deeper crust. The directly measured permeability (k) of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 m in intact crystalline rock, intact shales, and fault gouge, to values as high as 10 m in well-sorted gravels. The permeability of Earth’s upper crust can be regarded as a process-limiting parameter, in that it largely determines the feasibility of advective solute transport (k ~ >10 20 m), advective heat transport (k ~ ≥10 16 m), and the generation of elevated fluid pressures (k ~ ≤10 17 m) – processes which in turn are essential to ore deposition, hydrocarbon migration, metamorphism, tectonism, and many other fundamental geologic phenomena. The hydrodynamics of fluids in the brittle upper crust, where topography and magmatic heat sources dominate patterns of flow and externally derived (meteoric) fluids are common (e.g. Howald et al. 2015) are distinct from the ductile lower crust, dominated by devolatilization reactions and internally derived fluids (e.g. Connolly & Podladchikov, 2015). The brittle–ductile transition between these regimes occurs at 10–15 km depth in typical continental crust. Permeability below the brittle–ductile transition is non-negligible, at least in active orogenic belts (equivalent to mean bulk k of order 10 19 to 10 18 m) so that the underlying ductile regime can be an important fluid source to the brittle regime (e.g. Ingebritsen & Manning 2002). The overall objective of this special issue is to synthesize current understanding of static and dynamic permeability through representative publications from multiple disciplines. The objective of this introduction to the special issue is to define crucial nomenclature and the ‘static’ and ‘dynamic’ permeability perspectives and to briefly summarize the contents of this special issue, which is divided into the following sections: the physics of permeability, static permeability, and dynamic permeability.

Nature and Evolution of the Lithospheric Mantle beneath the Hoggar Swell (Algeria): a Record from Mantle Xenoliths

Peridotite xenoliths exhumed by Quaternary alkaline magmatism in the Tahalgha district, southern Hoggar, represent fragments of the subcontinental lithospheric mantle beneath the boundary between the two major structural domains of the Tuareg Shield: the ‘Polycyclic Central Hoggar’ to the east and the ‘Western Hoggar’, or ‘Pharusian Belt’, to the west. Samples were collected from volcanic centres located on both sides of a major lithospheric shear zone at 4°35' separating these two domains. Although showing substantial variations in their deformation microstructures, equilibrium temperatures and modal and chemical compositions, the studied samples do not display any systematic changes of these features across the 4°35' fault. The observed variations rather record small-scale heterogeneities distributed throughout the study area and reflecting the widespread occurrence of vein conduits and metasomatized wall-rocks related to trans-lithospheric melt circulation during the Cenozoic. These features include partial annealing of pre-existing deformation microstructures, post-deformation metasomatic reactions, and trace-element enrichment, coupled with heating from 750–900°C (low-temperature lherzolites) to 900–1150°C (intermediate-T lherzolites and high-T harzburgites and wehrlites). Trace-element modelling confirms that the range of rare earth element (REE) variations observed in the Tahalgha clinopyroxenes may be accounted for by reactive porous flow involving a single stage of basaltic melt infiltration into a light REE (LREE)-depleted protolith. Whole-rock compositions record the final entrapment of disequilibrium metasomatic melts upon thermal relaxation of the veins–wall-rock system. The striking correlations between equilibrium temperatures and trace-element enrichment favor a scenario in which the high-temperature peridotites record advective heat transport along melt conduits, whereas the intermediate- and low-temperature lherzolites reflect conductive heating of the host Mechanical Boundary Layer. This indicates that the lithosphere did not reach thermal equilibrium, suggesting that the inferred heating event was transient and was rapidly erased by thermal relaxation down to the relatively low-temperature present-day geotherm. The low-T (<900°C) deformed lherzolites (porphyroclastic to equigranular) are characterized by only incipient annealing and LREE-depleted clinopyroxene compositions. They were only weakly affected by the Cenozoic events and could represent relatively well-preserved samples from rejuvenated Pan-African lithosphere. Extensive lithospheric rejuvenation occurred either regionally during the Pan-African orogeny, as a result of lithospheric delamination or thermomechanical erosion after thickening, or more locally along the meridional shear zones. The low-T Tahalgha lherzolites are comparable with lherzolites from Etang de Lherz, southern France, interpreted as lithospheric mantle rejuvenated by melt-induced refertilization during a late stage of the Variscan orogeny.