Subsurface Thermal Regime Research Articles

Investigating the thermal state of permafrost with Bayesian inverse modeling of heat transfer

Abstract. Long-term measurements of permafrost temperatures do not provide a complete picture of the Arctic subsurface thermal regime. Regions with warmer permafrost often show little to no long-term change in ground temperature due to the uptake and release of latent heat during freezing and thawing. Thus, regions where the least warming is observed may also be the most vulnerable to permafrost degradation. Since direct measurements of ice and liquid water contents in the permafrost layer are not widely available, thermal modeling of the subsurface plays a crucial role in understanding how permafrost responds to changes in the local energy balance. In this work, we first analyze trends in observed air and permafrost temperatures at four sites within the continuous permafrost zone, where we find substantial variation in the apparent relationship between long-term changes in permafrost temperatures (0.02–0.16 K yr−1) and air temperature (0.09–0.11 K yr−1). We then apply recently developed Bayesian inversion methods to link observed changes in borehole temperatures to unobserved changes in latent heat and active layer thickness using a transient model of heat conduction with phase change. Our results suggest that the degree to which recent warming trends correlate with permafrost thaw depends strongly on both soil freezing characteristics and historical climatology. At the warmest site, a 9 m borehole near Ny-Ålesund, Svalbard, modeled active layer thickness increases by an average of 13 ± 1 cm K−1 rise in mean annual ground temperature. In stark contrast, modeled rates of thaw at one of the colder sites, a borehole on Samoylov Island in the Lena River delta, appear far less sensitive to temperature change, with a negligible effect of 1 ± 1 cm K−1. Although our study is limited to just four sites, the results urge caution in the interpretation and comparison of warming trends in Arctic boreholes, indicating significant uncertainty in their implications for the current and future thermal state of permafrost.

Long‐Term, High‐Resolution Permafrost Monitoring Reveals Coupled Energy Balance and Hydrogeologic Controls on Talik Dynamics Near Umiujaq (Nunavik, Québec, Canada)

AbstractRising temperatures in the Arctic and subarctic are driving the rapid thaw of permafrost by reducing permafrost cooling, increasing active layer thickness, and promoting talik formation. In this study, the cyrohydrogeology of a permafrost mound located within the discontinuous permafrost zone near Umiujaq (Nunavik, Québec, Canada) is characterized through the analysis of a dataset covering more than two decades of monitoring. This dataset captures a high degree of interannual variability in air temperature and ground thermal conditions, as well as the formation and closure of a supra‐permafrost talik. Data indicate that variable saturation and advective heat transport directly contribute to the expansion and contraction of the talik. Data further indicate the presence of two distinct thermo‐hydrologic settings resulting from differences in surface conditions, as well as subsurface thermal and flow regimes. The first, found at the top of the mound feature, is characterized by very low moisture contents (<0.05 m3/m3), while the second, found at the side of the mound feature, shows higher annual moisture contents that strongly influence the dynamics of heat and groundwater flow. The data were synthesized into a detailed conceptual model of the cyrohydrogeological dynamics that highlights the important role of hydrogeological characterization and long‐term data sets in understanding the effects of groundwater flow on seasonal frost and permafrost dynamics. Specifically, the results presented here show that in the absence of long‐term data sets, longer‐period transient phenomena such as talik opening and closure may be misrepresented as uni‐directional feedback loops, as opposed to highly dynamic temporary phenomena.

City-wide monitoring and contributing factors to shallow subsurface temperature variability in Nanjing, China

Shallow temperatures down to a depth of 100 m were measured over one year in 19 closed boreholes located in Nanjing, China, to reveal the conditions and factors influencing the subsurface thermal regime. A monitoring concept with distributed temperature sensing, fiber Bragg grating-based sensor, and a type of Resistance Temperature Detectors, is implemented, providing spatial distribution characteristics of subsurface temperatures. The results show that temperatures near the surface are most dynamic, influenced by the air temperature. The temperature remains stable at the depth of 10–20 m. The mean transition temperature is 18.1 °C. Borehole measurements are interpolated by using satellite images and surface temperature records to obtain large-scale surface temperature distributions and temporal variations of subsurface temperature. Geological and hydrological conditions are primary factors by affecting subsurface upward heat flux and heat loss. Urban land cover change and enhanced heat release from urbanization contribute to a subsurface urban heat island with intensities of 1.0–4.4 °C. The altered subsurface thermal regime is of primary concern for the management of shallow geothermal energy use. The monitoring concept in this study can provide spatially-temporally continuous profiles of subsurface temperature and become a reference for city-wide geothermal monitoring in other urban areas.

Increasing the Depth of a Land Surface Model. Part I: Impacts on the Subsurface Thermal Regime and Energy Storage

Abstract The representation of the thermal and hydrological states in land surface models is important for a realistic simulation of land–atmosphere coupling processes. The available evidence indicates that the simulation of subsurface thermodynamics in Earth system models is inaccurate due to a zero-heat-flux bottom boundary condition being imposed too close to the surface. To assess the influence of soil model depth on the simulated terrestrial energy and subsurface thermal state, sensitivity experiments have been carried out in piControl, historical, and RCP scenarios. A deeper bottom boundary condition placement has been introduced into the JSBACH land surface model by enlarging the vertical stratification from 5 to 12 layers, thereby expanding its depth from 9.83 to 1416.84 m. The model takes several hundred years to reach an equilibrium state in stand-alone piControl simulations. A depth of 100 m is necessary, and 300 m recommendable, to handle the warming trends in historical and scenario simulations. Using a deep bottom boundary, warming of the soil column is reduced by 0.5 to 1.5 K in scenario simulations over most land areas, with the largest changes occurring in northern high latitudes, consistent with polar amplification. Energy storage is 3–5 times larger in the deep than in the shallow model and increases progressively with additional soil layers until the model depth reaches about 200 m. While the contents of Part I focus on the sensitivity of subsurface thermodynamics to enlarging the space for energy, Part II addresses the sensitivity to changing the space for water and improving hydrological and phase-change interactions.

Modeling shallow ground temperatures around hot buried pipelines in cold regions

Transmitting fluids at high operating temperatures through buried pipelines is common in the oil and gas industry. Pipelines are insulated to maintain the fluid temperature within an operating threshold and reduce heat loss to the subsurface. In cold regions, damaged insulation around pipelines can raise ground temperatures by tens of degrees and keep ground frost free during the winter. This can lead to environmental and geotechnical issues, operational inefficiencies and, in extreme cases, pipeline failure. When supported by field data, modeling of coupled heat and water transfer in the subsurface can be used to understand baseline and disturbed ground temperatures and to design thermal remediation plans. However, simulating ground temperatures under the influence of snow cover, freeze-thaw conditions as well as a shallow heat source is challenging. This case study presents a simple and effective approach using a thermal boundary layer and a time-varying Dirichlet boundary condition to simulate these conditions with coupled vadose zone and groundwater models. The approach was validated by comparing the model results to a temperature survey along a boiler feed water pipeline corridor in northern Alberta, Canada. Model results were used to better understand the influence of high-temperature pipelines on subsurface thermal regimes, to help design a ground temperature monitoring program, and successfully identify and remediate pipeline insulation damage.

Shallow subsurface thermal structure onshore Denmark: temperature, thermal conductivity and heat flow

Available measured temperatures and thermal conductivities covering Danish onshore areas to a depth of about 300 m have been compiled and analysed. Temperature data from 236 borehole sites, including 56 boreholes with detailed temperature profiles, are applied together with thermal conductivities mea- sured on samples collected at 34 well-characterised outcrops and on core material from 20 boreholes. Significant thermal variations in the shallow subsurface are observed. At a depth of 50 m, a mean temperature of 8.9 ± 0.8°C is found, close to the mean annual surface temperature. Higher mean values of 9.7 ± 1.1°C found at 100 m and 11.6 ± 2.2°C at 200 m reflect a general increase of temperatures with depth. In contrast to the assumption commonly held, we observe significant lateral variations both lo- cally and regionally. At a depth of 100 m, temperatures vary between 7.3 and 13.0°C across Denmark, and at 250 m between 9.6 and 17.9°C. Mean values of the thermal conductivities lie within a range of 0.6–6 W/(m·K) measured water- saturated at laboratory conditions. The majority of values are within the interval of 1–3 W/(m·K) and show a strong correlation with lithology. The content of quartz and the rock porosity (the content of water) are found to be two main factors controlling the observed variations. Characteristic temperature gradients are in the range 1–4°C/100 m. Following Fourier’s law of heat conduction, a clear correlation is observed between temperature gradients and thermal conductivities of different lithologies. Intervals of quartz-rich sand deposits with high thermal conductivity show low temperature gradients, chalk and limestone intervals with intermediate conductivity display intermediate gradients, while sections with fine grained clay deposits of low thermal conductivity show high gradient values. A correlation analysis provides an estimate of regional shallow heat flow of 37 ± 5 mW/m2, consistent with local, classically determined heat-flow values from shallow borehole data. However, it is significantly below deep background heat flow, and this is believed to be caused by long-term paleoclimatic effects. The shallow subsurface thermal regime across the Danish area is largely controlled by thermal conduction. Only locally, and in rare cases, do we observe temperature perturbations due to ground- water migration. In addition to general geoscientific purposes, our results are important for several applications including exploitation of shallow geothermal energy and the use of the subsurface for heat storage and cooling purposes.

Single-year thermal regime and inferred permafrost occurrence in the upper Ganglass catchment of the cold-arid Himalaya, Ladakh, India

Cold-arid regions of the trans-Himalaya in the Indian Himalayan Region (IHR) is suspected to have a significant area of permafrost. However, information on the ground thermal regime of these permafrost areas is so far not available. This study bridge this knowledge gap by analysing the sub-surface thermal regime of selected sites in the Ganglass catchment, Ladakh range. Near surface ground temperature data recorded during September 2016 to August 2017 using 24-miniature temperature data loggers distributed across 12 plots and covering an elevation range of 4700–5612 m a.s.l. are used in this study. Permafrost characteristics including plausible ranges of thermal offset, active-layer thickness and mean annual ground temperature at 10 m depth were estimated by driving a one-dimensional heat conduction model. Two statistical models were used to map first order estimates of permafrost area in this 15.4 km2 catchment. Study suggest permafrost occurrence at all sites above 4900 m a.s.l. with active-layer thickness ranging from 0.1 to 4.2 m and the mean annual ground surface temperature ranging from between −10.0 and −0.85 °C for these sites. MAAT at these sites range from −4.1 to −8.9 °C and the surface offsets vary from −1.1 to 3.9 °C. Estimated thermal offset range from −0.9 to 0 °C. Both statistical models show comparable results and suggest 95% mean permafrost cover in the catchment above 4727 m a.s.l. These results strongly indicate existence of significant permafrost areas across the high elevations of the cold-arid regions of IHR. So far, permafrost processes are not considered for assessing present and future estimates of water and regional climate and as a causative factor for disasters like debris flows and landslides in the region. This study highlight the need for greater research efforts on Himalayan permafrost to have a comprehensive understanding of Himalayan cryosphere.

A New Method for Simulation of Lunar Microwave Brightness Temperatures and Evaluation of Chang'E‐2 MRM Data Using Thermal Constraints From Diviner

AbstractWe used the bolometric brightness temperatures (TBol) derived from the Lunar Reconnaissance Orbiters Diviner Lunar Radiometer (Diviner) as an upper boundary condition in our thermal model. We then calculated temperature profiles at any local time based on our improved thermal model at low to middle latitudes (70° N/S). Based on the temperature profiles, we modeled the midnight brightness temperature at 19.35 (TB19) and 37 GHz (TB37). Comparing to the Chang'E‐1 and Chang'E‐2 (CE‐1/2) observations, we found that CE‐1 showed a better data quality than that of CE‐2, especially for the TB37 data. Assuming that the issue with the CE‐2 data is caused by heat contamination of the cold‐reference antennas, we performed an empirical normalization of the CE‐2 microwave radiometer data near midnight following the approach of Hu et al. (2017). The results show that TB difference (modeled values minus modified TB) for 19.35 GHz is less than 3.40 K for ∼80% of the pixels. At 37 GHz, ∼67% of the pixels have TB difference less than 2.88 K. Additionally, we identified some areas of low microwave temperature in our modified TB maps. These low‐TB features can be characterized by two types: (1) low TB spots at fresh craters with high rock abundance and bright rays and (2) high‐Ti lunar mare surfaces with a low content of rock fragments. Investigating these low‐TB regions with the modified TB data can reveal more information about subsurface thermal regime and properties and help us better understand the evolution of regolith on the Moon.

Disturbances of Temperature-Depth Profiles by Surface Warming and Groundwater Flow Convection in Kumamoto Plain, Japan

Subsurface temperatures depend on climate and groundwater flow. A lack of observations of subsurface temperature collected over decades limits interpretation of the combined influences of surface warming and groundwater flow on subsurface thermal regimes. Subsurface temperature-depth profile data acquired for Kumamoto Plain, Japan, between 1987 and 2012 were collected and analyzed to elucidate regional groundwater and heat flows. The observed and simulated temperature-depth profiles showed the following: subsurface water flows from northeast to southwest in the study area; the combined influence of surface warming and water flow perturbation produces different temporal changes in thermal profiles in recharge, intermediate, and discharge areas; and aquifer thermal properties contribute more than hydraulic parameters to the perturbation of temperature-depth profiles. Spatial and temporal evolution features of subsurface thermal regimes may be utilized to investigate the influence of surface warming events on subsurface water and heat flows at the basin scale.

Heat as a groundwater tracer in shallow and deep heterogeneous media: Analytical solution, spreadsheet tool, and field applications.

Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expected conduction-dominated regime can be analysed to estimate vertical water fluxes. A number of analytical approaches have been proposed for using heat as a groundwater tracer, and these have typically assumed a homogeneous medium. However, heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata and at finer scales in streambeds. Herein, we apply the analytical solution of Shan and Bodvarsson (2004), developed for estimating vertical water fluxes in layered systems, in 2 new environments distinct from previous vadose zone applications. The utility of the solution for studying groundwater-surface water exchange is demonstrated using temperature data collected from an upwelling streambed with sediment layers, and a simple sensitivity analysis using these data indicates the solution is relatively robust. Also, a deeper temperature profile recorded in a borehole in South Australia is analysed to estimate deeper water fluxes. The analytical solution is able to match observed thermal gradients, including the change in slope at sediment interfaces. Results indicate that not accounting for layering can yield errors in the magnitude and even direction of the inferred Darcy fluxes. A simple automated spreadsheet tool (Flux-LM) is presented to allow users to input temperature and layer data and solve the inverse problem to estimate groundwater flux rates from shallow (e.g., <1 m) or deep (e.g., up to 100 m) profiles. The solution is not transient, and thus, it should be cautiously applied where diel signals propagate or in deeper zones where multi-decadal surface signals have disturbed subsurface thermal regimes.

The thermal impact of subsurface building structures on urban groundwater resources – A paradigmatic example

Shallow subsurface thermal regimes in urban areas are increasingly impacted by anthropogenic activities, which include infrastructure development like underground traffic lines as well as industrial and residential subsurface buildings. In combination with the progressive use of shallow geothermal energy systems, this results in the so-called subsurface urban heat island effect.This article emphasizes the importance of considering the thermal impact of subsurface structures, which commonly is underestimated due to missing information and of reliable subsurface temperature data. Based on synthetic heat-transport models different settings of the urban environment were investigated, including: (1) hydraulic gradients and conductivities, which result in different groundwater flow velocities; (2) aquifer properties like groundwater thickness to aquitard and depth to water table; and (3) constructional features, such as building depths and thermal properties of building structures.Our results demonstrate that with rising groundwater flow velocities, the heat-load from building structures increase, whereas down-gradient groundwater temperatures decrease. Thermal impacts on subsurface resources therefore have to be related to the permeability of aquifers and hydraulic boundary conditions. In regard to the urban settings of Basel, Switzerland, flow velocities of around 1 md−1 delineate a marker where either down-gradient temperature deviations or heat-loads into the subsurface are more relevant. Furthermore, no direct thermal influence on groundwater resources should be expected for aquifers with groundwater thicknesses larger 10m and when the distance of the building structure to the groundwater table is higher than around 10m.We demonstrate that measuring temperature changes down-gradient of subsurface structures is insufficient overall to assess thermal impacts, particularly in urban areas. Moreover, in areas which are densely urbanized, and where groundwater flow velocities are low, appropriate measures for assessing thermal impacts should specifically include a quantification of heat-loads into the subsurface which result in a more diffuse thermal contamination of urban groundwater resources.

Subsurface thermal regime to delineate the paleo-groundwater flow system in an arid area, Al Kufra, Libya

The purpose of this study was to understand the groundwater flow system in Al Kufra basin, Libya, as a case study of arid areas using subsurface temperature. The temperature-depth profiles and water levels were measured in eight boreholes in the area. Well 6 is considered a recharge type profile with low geothermal gradient (0.0068°C/m) and an estimated paleo-temperature around 19.5°C. The other profiles are of discharge type with higher geothermal gradient (0.0133 to 0.0166°C/m). The constructed horizontal 2D distribution maps of the hydraulic heads and the subsurface temperature measurements reveal that the main recharge area is located to the south with low temperature while the main discharge area is located to the north with higher temperature. Vertical 2D distribution maps show that location of well 4 has low hydraulic heads and higher temperature indicating that the fault defined in the area may have affected the groundwater flow system. The estimated groundwater flux ranges from 0.001 to 0.1mm/day for the recharge area and from −0.3 to −0.7mm/day in average in the discharge area.

Modeling the spatiotemporal variability in subsurface thermal regimes across a low-relief polygonal tundra landscape

Abstract. Vast carbon stocks stored in permafrost soils of Arctic tundra are under risk of release to the atmosphere under warming climate scenarios. Ice-wedge polygons in the low-gradient polygonal tundra create a complex mosaic of microtopographic features. This microtopography plays a critical role in regulating the fine-scale variability in thermal and hydrological regimes in the polygonal tundra landscape underlain by continuous permafrost. Modeling of thermal regimes of this sensitive ecosystem is essential for understanding the landscape behavior under the current as well as changing climate. We present here an end-to-end effort for high-resolution numerical modeling of thermal hydrology at real-world field sites, utilizing the best available data to characterize and parameterize the models. We develop approaches to model the thermal hydrology of polygonal tundra and apply them at four study sites near Barrow, Alaska, spanning across low to transitional to high-centered polygons, representing a broad polygonal tundra landscape. A multiphase subsurface thermal hydrology model (PFLOTRAN) was developed and applied to study the thermal regimes at four sites. Using a high-resolution lidar digital elevation model (DEM), microtopographic features of the landscape were characterized and represented in the high-resolution model mesh. The best available soil data from field observations and literature were utilized to represent the complex heterogeneous subsurface in the numerical model. Simulation results demonstrate the ability of the developed modeling approach to capture – without recourse to model calibration – several aspects of the complex thermal regimes across the sites, and provide insights into the critical role of polygonal tundra microtopography in regulating the thermal dynamics of the carbon-rich permafrost soils. Areas of significant disagreement between model results and observations highlight the importance of field-based observations of soil thermal and hydraulic properties for modeling-based studies of permafrost thermal dynamics, and provide motivation and guidance for future observations that will help address model and data gaps affecting our current understanding of the system.

Shallow subsurface temperature in the environs of El-Nubaria canal, northwestern Nile Delta of Egypt: implications for monitoring groundwater flow system

Borehole temperatures from 47 wells were utilized to comprehend the spatial subsurface thermal regime in the study area. The recharge regions are situated in reclaimed desert lands (southern and southwestern parts) and have high subsurface temperatures, whereas the discharge areas are situated in the old agricultural lands (northern and northeastern parts) and have lower subsurface temperatures. This thermal system is attributed to differences in the surface air temperature where higher values are recorded in the developed desert (Wadi El-Natrun station average annual 23.15 °C) and the lower values in the old agricultural land (Damanhour station average annual 20.37 °C). The geothermal gradient is low in the recharge area (average 0.0198 °C/m) and high in the discharge area (average 0.0343 °C/m). The effect of the irrigation canals as gaining or losing streams on the thermal system decreases gradually and disappears at 90 m below sea level. Two-dimensional groundwater flow and heat transport models were simulated to estimate the vertical groundwater flow velocity and predict the factors controlling the groundwater flow system. Permeability was the most sensitive parameter during the simulation process. The assessed vertical linear groundwater velocities are, up to 3.2 mm/day in the main recharge region toward the southwestern course, up to 2.4 and 2 mm/day under El-Bustan and El-Hager canals, respectively, and up to 1.4 mm/day in the contact between the reclaimed desert and the old agricultural lands. The average vertical linear groundwater discharge velocities are, more than 2.4 mm/day in the southwestern corner, 0.8 and 0.6 mm/day in the northeastern and northern directions, respectively and 1.2 and 2.4 mm/day under El-Nubaria and Ferhash canals, respectively.

Eleven years of ground–air temperature tracking over different land cover types

ABSTRACTWe have analyzed series of air, near‐surface and shallow ground temperatures under four land cover types, namely bare clayey soil, sand, short‐cut grass and asphalt; the samples were collected between 2002 and 2013 and monitored at the Geothermal Climate Change Observatory Sporilov, Prague (50°02.43′N, 14°28.54′E, 226 m a.s.l.). A comparison of all of the obtained temperature series revealed a strong dependence of the subsurface thermal regime on the respective surface cover material. The ground ‘skin’ temperature was generally warmer than the surface air temperature over all monitored surfaces; however, the temperatures over different land cover types differed significantly. Asphalt exhibited the highest temperatures, and temperatures below the grassy surface were the lowest. Special attention was paid to assessing the value of the ‘temperature offset’, the instant value of which sometimes varied dramatically, on both daily and annual scales, by up to 30+ K; however, on a long‐time scale, the temperature offset was generally constant and reflected the surface material. The characteristic 2003–2013 mean values for the individual covers are as follows: asphalt 4.1 K, sand 1.6 K, clay 1.4 K and grass 0.2 K. All four surface covers revealed typical daily and inter‐annual cycles, which were monitored and are discussed in detail. Incident solar radiation was the primary variable for determining the amount and temporal changes of the temperature offset values. A linear relationship between air–ground temperature differences and incident solar radiation was detected. The mean slope of the linear regression between both variables is clearly surface cover dependent. The greatest value, 3.3 K per 100 W m−2, was found for asphalt cover; rates of 1.0–1.2 apply to bare soil and sand cover, and a negative slope of −0.44 K per 100 W m−2 represents grass cover.

The contribution of gravity method in geothermal exploration of southern part of the Gulf of Suez–Sinai region, Egypt

The Gulf of Suez region represents the most promising area in Egypt for geothermal exploration which is characterized by superficial thermal manifestations represented by a cluster of hot springs with varying temperatures from 35 to 72°C. The main purpose of the present study was to shed the light on the integration between gravity work and geothermal data in detecting the main subsurface structures in addition to expecting the geothermal sources in the area under consideration.Correction was applied on the bottom hole temperature data to obtain the true formation equilibrium temperatures that can provide useful information about the subsurface thermal regime. Based on these logging data, temperature gradient and heat flow values were computed at each well, and it is found that the mean geothermal gradient of the study area is 32°C/km; nevertheless, some local geothermal potential fields were located with more than 40°C/km. Also, heat flow values are ranging from 45 to 115mW/m2.The Bouguer anomaly map of the study area was used for delineating the subsurface structures and tectonic trends that have resulted in a potential heat source. The gravity inversion revealed a good correlation between areas of high temperature gradients, high heat flow and positive gravity anomalies. The high temperature gradient and heat flow values suggested being associated with a noticeable hydrothermal source of heat anomaly located at relatively shallow depths which is expected to be due to the uplift of the basement in the area.

Use of the subsurface thermal regime as a groundwater-flow tracer in the semi-arid western Nile Delta, Egypt

Temperature profiles from 25 boreholes were used to understand the spatial and vertical groundwater flow systems in the Western Nile Delta region of Egypt, as a case study of a semi-arid region. The study area is located between the Nile River and Wadi El Natrun. The recharge areas, which are located in the northeastern and the northwestern parts of the study area, have low subsurface temperatures. The discharge areas, which are located in the western (Wadi El Natrun) and southern (Moghra aquifer) parts of the study area, have higher subsurface temperatures. In the deeper zones, the effects of faults and the recharge area in the northeastern direction disappear at 80 m below sea level. For that depth, one main recharge and one main discharge area are recognized. The recharge area is located to the north in the Quaternary aquifer, and the discharge area is located to the south in the Miocene aquifer. Two-dimensional groundwater-flow and heat-transport models reveal that the sealing faults are the major factor disturbing the regional subsurface thermal regime in the study area. Besides the main recharge and discharge areas, the low permeability of the faults creates local discharge areas in its up-throw side and local recharge areas in its down-throw side. The estimated average linear groundwater velocity in the recharge area is 0.9 mm/day to the eastern direction and 14 mm/day to the northwest. The average linear groundwater discharge velocities range from 0.4 to 0.9 mm/day in the southern part.

Indication of deep groundwater flow through the crystalline rocks of southern Norway

We assess the major aspects of the subsurface thermal regime within crystalline rocks of southern Norway. Results of two-dimensional modeling of coupled groundwater flow and heat transfer demonstrate that advective cooling due to groundwater flow is an important factor for the reduction of subsurface temperatures in southwestern Norway (the Bergen and Stavanger areas) where the normal annual precipitation is high on the western (windward) side of the Scandes mountains. On the other hand, the influence of groundwater flow on subsurface temperatures is most likely relatively low in southeastern Norway (the Moss area), which represents the rain-shadow area with light precipitation and is characterized by smoothed landforms. Therefore, where the reduced subsurface temperatures are associated with high atmospheric precipitation and complex surrounding topography, deep groundwater flow is most likely present within crystalline rocks. In this case, the observed low values of the thermal gradient within crystalline rocks can be used as groundwater flow tracers.

Thermal characteristics of permafrost in the steep alpine rock walls of the Aiguille du Midi (Mont Blanc Massif, 3842 m a.s.l)

Abstract. Permafrost and related thermo-hydro-mechanical processes are thought to influence high alpine rock wall stability, but a lack of field measurements means that the characteristics and processes of rock wall permafrost are poorly understood. To help remedy this situation, in 2005 work began to install a monitoring system at the Aiguille du Midi (3842 m a.s.l). This paper presents temperature records from nine surface sensors (eight years of records) and three 10 m deep boreholes (4 years of records), installed at locations with different surface and bedrock characteristics. In line with previous studies, our temperature data analyses showed that: micro-meteorology controls the surface temperature, active layer thicknesses are directly related to aspect and ranged from &lt;2 m to nearly 6 m, and that thin accumulations of snow and open fractures are cooling factors. Thermal profiles empirically demonstrated the coexistence within a single rock peak of warm and cold permafrost (about −1.5 to −4.5 °C at 10 m depth) and the resulting lateral heat fluxes. Our results also extended current knowledge of the effect of snow, in that we found similar thermo-insulation effects as reported for gentle mountain areas. Thick snow warms shaded areas, and may reduce active layer refreezing in winter and delay its thawing in summer. However, thick snow thermo-insulation has little effect compared to the high albedo of snow which leads to cooler conditions at the rock surface in areas exposed to the sun. A consistent inflection in the thermal profiles reflected the cooling effect of an open fracture in the bedrock, which appeared to act as a thermal cutoff in the sub-surface thermal regime. Our field data are the first to be obtained from an Alpine permafrost site where borehole temperatures are below −4 °C, and represent a first step towards the development of strategies to investigate poorly known aspects in steep bedrock permafrost such as the effects of snow cover and fractures.

Correlation of Aerogravity and BHT Data to Develop a Geothermal Gradient Map of the Northern Western Desert of Egypt using an Artificial Neural Network

The northern part of the Western Desert of Egypt represents the second most promising area of hydrocarbon potential after the Gulf of Suez province. An artificial neural network (ANN) approach was used to develop a new predictive model for calculation of the geothermal gradients in this region based on gravity and corrected bottom-hole temperature (BHT) data. The best training data set was obtained with an ANN architecture composed of seven neurons in the hidden layer, which made it possible to predict the geothermal gradient with satisfactory efficiency. The BHT records of 116 deep oil wells (2,000–4,500 m) were used to evaluate the geothermal resources in the northern Western Desert. Corrections were applied to the BHT data to obtain the true formation equilibrium temperatures, which can provide useful constraints on the subsurface thermal regime. On the basis of these corrected data, the thermal gradient was computed for the linear sections of the temperature-versus-depth data at each well. The calculated geothermal gradient using temperature log data was generally 30 °C/km, with a few local high geothermal gradients in the northwestern parts of the study area explained by potential local geothermal fields. The Bouguer gravity values from the study area ranged from −60 mGal in the southern parts to 120 mGal in the northern areas, and exhibited NE–SW and E–W trends associated with geological structures. Although the northern Western Desert of Egypt has low regional temperature gradients (30 °C/km), several potential local geothermal fields were found (>40 °C/km). The heat flow at each well was also computed by combining sets of temperature gradients and thermal conductivity data. Aerogravity data were used to delineate the subsurface structures and tectonic framework of the region. The result of this study is a new geothermal gradient map of the northern Western Desert developed from gravity and BHT log data.