Northern Alsace Research Articles

Using highly accurate land gravity and 3D geologic modeling to discriminate potential geothermal areas: Application to the Upper Rhine Graben, France

New land gravity data results acquired in northern Alsace were presented. Compared to the available old Bouguer anomaly, we recovered an accurate Bouguer anomaly field showing data uncertainties [Formula: see text]. A qualitative data analysis using pseudotomographies reveals several negative anomalies suggesting a decrease of the bulk density at the depth of geothermal interest. We have performed a quantitative study on the basis of the existing 3D geologic model derived from a reinterpretation of the vintage seismics. The theoretical gravity response indicates a great mismatch with the observed Bouguer anomaly. The stripping approach was applied, and the stripped Bouguer anomaly indicates that the density values of the Jurassic, but especially for the Triassic, the Buntsandstein, and the upper part of the basement, were overestimated even using the density values measured in the deep geothermal borehole. This suggests that the borehole density values do not reflect the density variations occurring at larger scale. To reduce the Bouguer anomaly during stripping, a negative density contrast should be affected to the Buntsandstein layer overlaying the basement, suggesting that the part located between the Buntsandstein and the upper part of the basement presents a low-density value compared to the reference density, which is not necessarily expected and is not observed in the densities measured in the borehole. Interestingly, a correlation is found between the gravity analyses and the thermal gradient boreholes in the northern part of the study area. For two boreholes, the gravity interpretation suggests a huge density decrease in the Buntsandstein, which may arise from a combination of high-density fracturing and the important quantity of geothermal fluid significantly affecting the bulk density. Analysis of the thermal borehole data suggests that these two boreholes indicate higher geothermal potential compared with the other boreholes.

How Do Secondary Minerals in Granite Help Distinguish Paleo- from Present-Day Permeable Fracture Zones? Joint Interpretation of SWIR Spectroscopy and Geophysical Logs in the Geothermal Wells of Northern Alsace

The investigation of permeable hydrothermally altered and fractured zones and their distribution is a key issue for the understanding of fluid circulation in granitic rocks, on which the success of geothermal projects relies. Based on the use of short-wave infrared (SWIR) spectroscopy applied to rock cuttings coupled with interpretation of geophysical logs, we propose an investigation of the clay signature of fault and fracture zones (FZ) inside the granitic basement. This methodology was applied to two geothermal wells: GRT-2 from the Rittershoffen and GPK-1 from the Soultz-sous-Forêts (Soultz) geothermal sites, both located in the Upper Rhine Graben (URG). A total of 1430 SWIR spectra were acquired and analysed. Variations in the 2200 nm absorption band area are correlated with hydrothermal alteration grades. The 2200 nm absorption band area is found to reflect the illite quantity and its variations in the granitic basement. Low, stable values are observed in the unaltered granite facies, showing good reproducibility of the method, whereas scattered high values are associated with high hydrothermal alteration and FZs. Variations in the 2200 nm absorption band area were correlated with the gamma ray and electrical resistivity logs. This procedure allowed us to confirm that illite mainly controls the resistivity response except inside the permeable FZs, where the resistivity response is controlled by the geothermal brine. Thus, the architecture of these permeable FZs was described precisely by using a combination of the 2200 nm absorption band area data and the electrical resistivity log. Moreover, by correlation with other geophysical logs (temperature (T), porosity, and density), paleo-permeable and currently permeable FZs inside the reservoir were distinguished. The correlation of SWIR spectroscopy with electrical resistivity logs appears to be a robust tool for geothermal exploration in granitic reservoirs in the URG.

Modelling acid stimulation in the enhanced geothermal system of Soultz-sous-Forêts (Alsace, France)

The Soultz-sous-Forêts geothermal plant in northern Alsace (France) is under consideration for a campaign to actively stimulate the low-permeability granitic reservoir. The operation of the enhanced geothermal system requires the re-injection of fluid into the geothermal reservoir, which can cause a strong disequilibrium between the fluid and the granitic rock, as well as the possible dissolution/precipitation of minerals. These mineralogical transformations may have a significant impact on porosity, permeability and fluid pathways in the geothermal reservoir. Various studies have reported that the hydraulic connection between the injection well and the production well is quite poor. Therefore, chemical stimulations are necessary to increase the productivity and injectivity of the Soultz geothermal system.Before performing chemical stimulation in the Soultz geothermal system, a modelling approach is considered to discuss different scenarios in terms of the choice of acid and the amount and duration of acid injection. The approach used in this work is based on the geochemical code KIRMAT, which enables us to represent the geothermal reservoir using single-porosity and double-porosity models. The code evaluates the changes in the porosity and permeability of the Soultz reservoir based on the evolution of all primary and secondary minerals as a function of time in the geothermal system.The modelling results showed that chemical stimulation could significantly increase porosity and permeability; however, for both single and double-porosity models, the improved zones are very limited and are only present within a few meters of the reinjection well. The modelling results obtained from the sensitivity case studies showed the significant impact of the Darcy velocity, the initial concentration of the acid used, the duration of the injection and the initial calcite amount, especially in regard to the changes in porosity and the mineralogical transformation of the zone around the acid-injected well. Higher concentrations of the HCl solution also increase the dissolution of primary minerals and then exhibit a stronger porosity increase in the zone around the injection well, but further, precipitation reduces the positive achievements of chemical stimulation.

Social shaping of deep geothermal projects in Alsace: politics, stakeholder attitudes and local democracy

This paper examines the social, cultural and political factors that favor or disrupt deep geothermal energy projects in Alsace. The research was conducted in the Risk Governance package of the H2020 DESTRESS program, aimed at comparing public perceptions of deep geothermal energy in different contexts. The French case studies focus on two different contexts: one in Northern Alsace, where geothermal energy is fairly well accepted, and another in the Eurometropolis of Strasbourg, where some projects have raised substantial controversy. Several conceptual tools are used to understand variations in the public perceptions of geothermal projects. First, the distinction between “locally anchored” projects and “unbound” or exogenous projects account for the way these projects take shape, based on dialog or facilitated by a favorable economic and national political context, ignoring local specificities. Second, the concepts of social identity and social worlds allow us to yield insights into the acceptability of a project on a more sociological and cultural level, instead of adopting a limited risk perception focus. By considering the social and cultural contexts, we can better explain the weight that opponents place in their discourses on induced risks, local politics, or environmental issues during a technoscientific controversy. This ultimately leads us to underline the limitations of the communication models for project governance that are chiefly aimed at reassuring the local population.

Hybrid gravimetry monitoring of Soultz-sous-Forêts and Rittershoffen geothermal sites (Alsace, France)

We investigate the feasibility of the hybrid gravity technique applied to two geothermal reservoirs in northern Alsace, France, namely the Soultz-sous-Forêts and Rittershoffen sites. Soultz-sous-Forêts site is the first enhanced geothermal system demonstration site producing electricity in France. Here a geothermal fluid at 165 °C allows to produce around 1.5 MW thanks to one injection well and one production well 5 km deep. Rittershoffen site is dedicated to an industrial use and it is designed to produce 24 MWth heat power with 2 wells around 2.5 km deep. The most recent production episodes of Rittershoffen and Soultz-sous-Forêts geothermal plants have started respectively on May and June 2016. To study underground mass redistribution, time-lapse relative microgravity measurements have been done since 2014 on a network designed ad hoc for Soultz-sous-Forêts site and since 2015 for Rittershoffen site. After tide and drift correction, double differences are calculated to retrieve the gravity variation at each measuring point compared to a reference time and station. Absolute gravity measurements have been also collected at one of the reference stations. Before the beginning of the production in 2016, the stability of the Soultz-sous-Forêts reference station was monitored through the repetition of absolute measurements and continuous gravity records. In 2016, regular ties between the reference stations and the Strasbourg gravity observatory STJ9 have been done. Several superconducting gravimeters operate continuously in STJ9. Thus, we approach the concept of hybrid gravity. Vertical deformation is also controlled thanks to six continuous GPS measurements: the height changes are less than 1 cm. So we consider that our gravity variations are only due to subsurface mass changes. For the Soultz-sous-Forêts network, we notice significant changes in agreement with the position of the injection and the production wells. The maximum gravity change is 31 ± 8 μGal. On the contrary, we do not detect any similar signal for the Rittershoffen network. A simplistic model using two spherical sources located at 5 and 2 km for Soultz-sous-Forêts and Rittershoffen sites respectively shows negligeable surface gravity changes when taking into account the known injection and production flow rates.

Russula vinosoflavescens sp. nov., from deciduous forests of Northern Alsace, France

Based on morphological, molecular, and ecological data, a new species of Russula sect. Russula, found on several occasions under deciduous trees in Northern Alsace is described and illustrated as: Russula vinosoflavescens, belonging to R. subsect. Sardoninae.

Continuous and time-lapse magnetotelluric monitoring of low volume injection at Rittershoffen geothermal project, northern Alsace – France

An increasing interest in magnetotelluric monitoring of hydraulic stimulation experiments and of natural earthquakes currently requires a detailed investigation of the limits of this method. Our study contributes to this discussion with a unique quasi-continuous and long-term magnetotelluric monitoring of different injection and production experiments at the Rittershoffen geothermal site in Alsace (France). Here, we provide a first evaluation of the data and an outlook on more specific challenges.The geothermal doublet at Rittershoffen is operated through two wells GRT1 and GRT2. The magnetotelluric monitoring covers the end of drilling phase of GRT2, mostly, production from, but also injection into this well, injection into GRT1 and a circulation experiment. Magnetotelluric data were processed successfully using remote referencing. Transfer functions show particular variation pattern for different operations, i.e. an increase in uncertainty, conductivity and phase during test operation with a preferential direction sub-parallel to Shmin, i.e. perpendicular to the expected extension of the fractures controlling the reservoir. In particular fluid injection, either into GRT2 or GRT1 causes a strong decrease in resistivity by up to one order of magnitude in the YX component between about 8–25s of period. However, the comparison between the temporal distribution of the occurring anomalies and the magnetic field intensity reveals an amplifying effect on the anomalies.In conclusion, we confirm earlier findings on the influence of the magnetic field intensity on magnetotelluric monitoring and at the same time show the lower limit of monitoring in particular monitoring of injection into deep geothermal reservoirs.

Ambient noise tomography with non-uniform noise sources and low aperture networks: case study of deep geothermal reservoirs in northern Alsace, France

Ambient noise tomography with non-uniform noise sources and low aperture networks: case study of deep geothermal reservoirs in northern Alsace, France

Hedges, colluvium and lynchets along a reference toposequence (Habsheim, Alsace, France): history of erosion in a loess area

Lynchets are topographic ledges that are formed by the accumulation of colluvium behind a hedge grown perpendicular to the slope of a hillside. This colluvium constitutes a natural archive for the analysis of agricultural erosion over time, as we demonstrate in this study of a toposequence characterized by two lynchets along a loess slope. The detailed description of the soil catena coupled with OSL dating records the major erosional phases. The earliest phase begins during the late Neolithic at ca. 5,760 ± 500 yr. The upslope hedge is installed at ca. 2,710 ± 200 yr, at the very end of the Bronze Age or the beginning of the Early Iron Age and the downslope hedge is planted during the Early Middle Ages. Therefore, onwards from 2,710 ± 200 yr, erosion only affects individual plots or sections rather than the entire slope. Our dates also show that the soils on the uppermost portion of the slope had undergone a complete ablation prior to 2,710 ± 200 yr. This early erosional phase, likely Neolithic in age, is consistent with other dates obtained in the northern Alsace region.

PyGrav, a Python-based program for handling and processing relative gravity data

pyGrav is a Python-based open-source software dedicated to the complete processing of relative-gravity data. It is particularly suited for time-lapse gravity surveys where high precision is sought. Its purpose is to bind together single-task processing codes in a user-friendly interface for handy and fast treatment of raw gravity data from many stations of a network. The intuitive object-based implementation allows to easily integrate additional functions (reading/writing routines, processing schemes, data plots) related to the appropriate object (a station, a loop, or a survey). This makes pyGrav an evolving tool. Raw data can be corrected for tides and air pressure effects. The data selection step features a double table-plot graphical window with either manual or automatic selection according to specific thresholds on data channels (tilts, gravity values, gravity standard deviation, duration of measurements, etc.). Instrumental drifts and gravity residuals are obtained by least square analysis of the dataset. This first step leads to the gravity simple differences between a reference point and any point of the network. When different repetitions of the network are done, the software computes then the gravity double differences and associated errors. The program has been tested on two specific case studies: a large dataset acquired for the study of water storage changes on a small catchment in West Africa, and a dataset operated and processed by several different users for geothermal studies in northern Alsace, France. In both cases, pyGrav proved to be an efficient and easy-to-use solution for the effective processing of relative-gravity data.

How do permeable fractures in the Triassic sediments of Northern Alsace characterize the top of hydrothermal convective cells? Evidence from Soultz geothermal boreholes (France)

The thermal regime of the Upper Rhine Graben (URG) is characterized by a series of anomalies near Soultz-sous-Forêts (France), Rittershoffen (France), and Landau (Germany). These temperature anomalies are associated with groundwater circulation in fractures and faults distributed in the Cenozoic and Mesozoic sedimentary cover associated with and connected to fractures originating deep within the Paleozoic basement. The present study helps to understand the convective cell structure in order to optimize geothermal borehole trajectories. The work concentrated on a detailed interpretation of the geophysical and geological logs from Soultz geothermal wells mainly from the topographic surface to the Triassic formations, at between 800- and 1,400-m depth above the deep granitic basement. The analysis of drilling mud logging data and geophysical well logging data from the deep Soultz geothermal wells (GPK-2, GPK-3, GPK-4) reveals the occurrence of nine fracture zones situated at depths greater than 900 m in the limestones of the Muschelkalk (Middle Trias) and the sandstones of the Buntsandstein (Lower Trias). Based on indications of total or partial mud losses, these fracture zones have been classified as permeable or impermeable. Permeable fractures between circa 900-m depth and 1,400-m depth are connected to a large-scale fault and control the top of the convective cells. There is no indication of permeability in the formations above the Keuper layer, and the uppermost part of the sedimentary cover acts as a cap rock, insulating the convective regime in the Triassic sediments and the granitic basement.

Characterization of ambient seismic noise near a deep geothermal reservoir and implications for interferometric methods: a case study in northern Alsace, France

Ambient noise correlation techniques are of growing interest for imaging and monitoring deep geothermal reservoirs. They are simple to implement and can be performed continuously to follow the evolution of the reservoir at low cost. However, these methods rely on assumptions of spatial and temporal uniformity of seismic noise sources. Violating them can result in misinterpretation of seismic velocities owing to preferential noise propagation directions. Using several years of seismic data recorded around the two geothermal sites of Soultz-sous-forets and Rittershoffen in northern Alsace, France, we propose a detailed characterization of the spatial and temporal properties of the high frequency seismic noise (0.2 to 5Hz). We consider two fundamental properties of the cross correlation functions (CCFs) of ambient noise. Firstly, the reliability of the Green's function reconstruction, an important indicator for tomographic studies. Secondly, the temporal repeatability of the CCFs between 0.2 and 0.5 seconds. At periods below 1s, we observe a sharp decrease in signal to noise ratio resulting from the non uniform distribution of anthropogenic sources. At periods above 1s, we show that the high directivity of the northern Atlantic microseismic peak biases the CCFs' phase significantly. We show that nocturnal noise is the most suited for temporal analysis of the CCFs. Using nocturnal noise, we should be able to monitor temporal variations induced by the geothermal activities inside the reservoir.

Possible natural fluid pathways from gravity pseudo-tomography in the geothermal fields of Northern Alsace (Upper Rhine Graben)

BackgroundThis study aims on investigating the regional flow field of the Soultz and adjacent geothermal fields located on the western side of the central Upper Rhine Graben and thus to provide insight into the origin of the 70% of the geothermal fluid coming from the regional inflow in the deep reservoir of the Soultz site. In an integrative approach, we consolidate conceptual models on fluid flow in the central Upper Rhine Graben.MethodsBased on a 3D geological model and a new 3D temperature interpolation, we tackle the relation between tectonic structures and the occurrence of advection/convection along favourably oriented fault zones. Using sequential Butterworth filters, we study the distribution of negative residual anomalies in a pseudo-tomography down to a depth of about 6 to 8 km.ResultsWe derived N-S-striking V-shaped negative anomalies that are consistent with the orientation of fault zones revealing major temperature anomalies to their east.ConclusionsFollowing the concept of negative anomalies revealing zones of increased fracture porosity, and in agreement with fluid-chemistry, our findings suggest infiltration of meteoric water through the graben boundary fault and along preferential flow pathways that merge at intermediate depth. Up-flow of thermal water mixed most likely with brine from the deeper eastern part of the graben occurs along W-dipping typically rather steep structures.

Procedure to construct three-dimensional models of geothermal areas using seismic noise cross-correlations: application to the Soultz-sous-Forets enhanced geothermal site

The aim of this work is to assess the feasibility of the noise-cross correlation tomography method for imaging and detecting potential geothermal reservoirs even in highly urbanised areas. We tested the noise correlation method to reconstruct the shape of the Soutlz-sous- Forˆets enhanced geothermal system (northern Alsace, France). We inverted Rayleigh waves (RWs) reconstructed from cross-correlations of 15 months of ambient seismic noise recorded by a seismic array installed around the Soutlz geothermal power plant. By correlating noise records between 22 receivers, we reconstructed RWs with sufficient signal-to-noise ratio for 231 interstation paths. The reconstructed waveforms were used to estimate group velocity dispersion curves at periods between 1.0 and 5.0 s. The results were inverted for 2-D group velocity maps, and finally for a 3-D S-wave velocity model from 0 to 5.2 km depth. Our results clearly show the presence of low velocity bodies in the crystalline basement below the Soultz power plant at depth of 4-5 km, and at shallower depth (2.5-3.5 km) beneath the Rittershoffen and Woerth villages. These observations, in agreement with some previous studies, confirm that our procedure is suitable for geothermal exploration. Furthermore, the model presented here provides some suggestions to improve the existing geothermal power plant and inferences for further explorations in the area.

Structure of the low permeable naturally fractured geothermal reservoir at Soultz

The permeability of the granite geothermal reservoir of Soultz is primarily related to major fracture zones, which, in turn, are connected to dense networks of small-scale fractures. The small-scale fractures are nearly vertical and the major direction is about N0°E. This direction differs from that of the Rhine Graben, which is about N20°E to N45°E in northern Alsace. A total of 39 fracture zones, with a general strike of N160°E, have been identified in six wells between 1400 and 5000 m depth. These fracture zones are spatially concentrated in three clusters. The upper cluster at 1800–2000 m True Vertical Depth (TVD) is highly permeable. At 3000–3400 m TVD, the intermediate cluster in composed of a dense network developed in an altered matrix and forms the upper reservoir. In the lower part of the wells, the deeper cluster appears as a fractured reservoir developed within a low permeable matrix. Fracture zones represent a key element to take into account for predicting the geothermal reservoir life time submitted to various thermo-hydromechanical and chemical processes generated by hydraulic or chemical stimulations and by hydraulic circulation tests related to long-term exploitation.

The Final Crisis: Combat in Northern Alsace, January 1945

The Final Crisis: Combat in Northern Alsace, January 1945

Geochronology of the mid-German crystalline rise west of the River Rhine

The mid-German crystalline rise has its westernmost exposures at the western margin of the Rhine graben in the southern Pfalz and the northern Alsace. The outcrops are made up of granitoid rocks and minor volcano-sedimentary sequences. Radiometric ages obtained by U/Pb, Pb/Pb, Sm/Nd and Rb/Sr analyses of the igneous rocks from this area range from ∼433 to ∼325 Ma thus covering a time span from the Silurian to the end of the lower Carboniferous. Because the investigated rocks are — according to their chemical composition — largely related to subduction zone environments, the following three geodynamical scenarios are postulated, always taking subduction of oceanic crust beneath the mid-German crystalline rise into account: (a) subduction of the Rheic ocean during the Silurian from the north; (b) subduction of the Rhenohercynian ocean during the late Devonian (∼369 Ma) from the north; (c) subduction of the Saxothuringian ocean during the lower Carboniferous (∼334 Ma) from the south.

Fécondité et mortalité en Alsace entre le 18e et le 19e siècle : vers une transition démographique

Fertility and mortality in rural Alsace from the 18.cen to the 19.cen century Although there was no corresponding increase in available resources, the Lutheran village of Bosselshausen, located in northern Alsace, experienced an important population increase near the end of the 18th century. This population increase was due in large measure to a decline in the death rates of 1 to 10 years old, children even though infant and adult mortality rates remained high. In response to this additional population pressure, the population began to modify its behaviour. Mothers began to breast-feed their children for a longer period of time, a practice which supressed the birth rate to a certain extent. This finding does not, however, suggest that Bosselshausen began to undergo the demographic transition at this time or that the population of this village began to pratice birth control as we know it.