Snowy Owl
Retired in 2010, In Memoriam
Fluid transfers and reactivities
PRESENTATION OF THE TOPIC
Researches on the "fluid" compartment have literally rocketed the last few years at Geosciences with several important research projects that range from the cycle of surface waters to groundwater flows. Apart from one or two exceptions, these researches have been initiated less than five years ago by researchers moving into a new field. This development has been carried out together with a voluntarist policy by the UMR with a recruitment of young researchers that is evolving considerably. The fact that the last two CNRS recruitments have concerned this field (in 2001 and moreover, in 2002) shows the reality of the emergence of this field both at the UMR level and at the national level.
The originality of the researches in Rennes clearly lies in the variety of the approaches with a range of researches that are theoretical, digital, experimental and field-related, and of physical, chemical and even biological approaches. The study of the main processes is one of the strong points of the current projects, that is reaffirmed, and even reinforced, in this four-year project.
Details on the main scientific projects that we consider a priority are given thereafter. From a general viewpoint, they aim at solving a number of important problems of the scientific knowledge, particularly on the way to take into account the high heterogeneity (static or induced) of environments and processes,
- on the characterization of the main chemical or biological transformations, and of their results in a system where fluxes are fundamentally heterogeneous,
- on the way how to quantify and predict the slow, large-scale evolutions (geological scales), but fundamentally related to systems of which dynamics can be extremely fast (hydrodynamics, meteorology).
In response to these points, we consider these three complementary methodological approaches as a priority:
- the development of effective modeling methods adapted to these systems,
- the development of experimental approaches at several scales ? from laboratory to experimental site,
- the development of a long-term follow-up of some outstanding systems, particularly within the framework of research observation posts in environment.
The projects developed by the UMR encompass several disciplines of Earth Sciences: hydro(geo)logy, geochemistry, geophysics and even tectonics. Important collaborations are initiated towards biology, physics and chemistry. Even if interdisciplinarity is an actual fact, and even a development objective, we can tell that it is not at all a disadvantage for the coherence of the researches. There exist many collaborations, particularly thanks to the "processes" view, which allows real discussions between researchers.
Finally, researches are carried on particularly topical environment issues, like the transfer of contaminants in catchment areas, the management of the water supply, the disposal of subterranean waste or the oil resource. In this respect, they represent an important pole of the IFR CAREN, and they participate at a national scale to the development of researches on what INSU has recently called "continental surfaces", that is, the key zone at the interface between the solid Earth, the hydrosphere and the atmosphere.
People involved
These fields involve about twenty researchers with about 14 researchers working full-time (about 1/3 of the UMR). The researchers involved are:
Luc Aquilina, Philippe Boulvais, Olivier Bour, Jean de Br?mond d?Ars, Xavier Ch?tellier, Peter R. Cobbold, Alain Crave, Olivier Dauteuil, M?lanie Davranche, Philippe Davy, Aline Dia, Jean-Raynald de Dreuzy, Serge Fourcade, Pierre Gavrilenko, Dominique Gibert, G?rard Gruau, Florence Nicollin, Steve Pride, Gilles Ruffet.
8 technicians and engineers are involved at various levels in experiments (physical and biochemical), field measurements and operating of the analytical means:
Martine Le Coz-Bouhnik, Jean-Pierre Caudal, Jean Cornichet, Fr?d?ric Conil, Odile H?nin, Jean-Jacques Kermarrec, Michel Lemoine, Patrice Petitjean and Yves Qu?t?. Their contribution to this field corresponds to about 7 full-time jobs.
Scientific projects
We have separated two levels: researches on processes, and researches on the characterization of the natural environment.
1. ReSEARCHES ON "processEs"
1.1 Modeling of ohysico-chemical transfers in heterogeneous environments
The high heterogeneity of the geological environments, and the complexity of the solid/liquid interactions, represent the main difficulties to be overcome when modeling these systems in order to predict water and material fluxes. Fractured systems are particularly representative of the problems encountered, but this difficulty is found in most geological environments. Although societal and economical stakes are numerous (management of the water supply, evaluation of disposal sites of toxic waste, oil exploration, ...), so are the problems with practical and theoretical difficulties. The fine and exhaustive characterization of the environment comes up against the intrinsic difficulties of the in situ observation. From a theoretical viewpoint, taking into account a multi-scale heterogeneity is a problem that has not been solved in the current models that "homogeneize" the environment or that at best take only a fraction of the complexity into account. The challenge is therefore to build models that can be physically accepted while being characterized by a number of parameters compatible with the available data.
Our objective is fundamentaly to determine the physico-chemical laws describing the transport at a field scale, so that it is possible to reverse the local measurements, even in a strongly heterogeneous environment. The first studies have shown that the classical advection-diffusion laws cannot be used to explain field observations showing the high spatial dispersion of the solutes as well as the wide range of the arrival time. These features are due both to the heterogeneity of the velocity field resulting from the complex structures and to the chemical reactivity of the solutes. Chemical and physical effects are even coupled since the pore geometry and the transfer velocities control the contact zone of the solutes with the environment on which it can adsorb or diffuse and the residence time of the solutes determining the progress of the chemical reactions, respectively.
Our four-year objectives show too poles consisting in:
- Developing field and laboratory studies in order to best characterize the processes studied. The aim is also to obtain sufficient dataset to validate the more theoretical approaches. Within this framework, many field studies are planned on several sites including the Ploemeur site (Morbihan, France) where the aim is not only to characterize the properties of flow and dispersion of the environment by pumping tests and tracings, but also to measure the velocity distribution in situ. The velocity field is particularly important because it results from the heterogeneity and because it controls the macroscopic transport.
- Carrying on with the digital and theoretical developments that allow to relate environment models with transport properties. We are currently developing a three-dimensional digital model that is capable of simulating domains large enough to solve the problem of scale change. This project is carried out in the framework of a collaboration with the IRISA (INRIA laboratory in Rennes). We have also started to study the reactive transport in order to take into account more complex chemical and biological reactions in transfer models. The aim of these simulations is to act as a guide and a validation tool for the definition of transport laws as a function of the characteristics of the heterogeneity and the chemical reactions. Obtaining these general laws will then allow to consider the development of modeling methods taking into account the amount and uncertainty of the data. The assessment of the model uncertainty, which depends on the data quantity and quality, is determining for the assessment of the risk associated, for example, to the subterranean disposal of toxic waste.
1.2 Physico-chemical reactivities - Solution/Minerals/Bacteria interactions
Physico-chemical reactions at the solution/mineral/bacteria interfaces are the main factor controlling the chemical composition of waters, whether marine or continental. The understanding and the modeling of these reactions is therefore a major stake, with potentially important implications both in areas like the understanding and the modeling of erosion mechanisms - water considered as the medium of material transfers at the Earth?s surface - and the protection and the management of our environment - water considered as a vital resource for human beings and living beings. The main objectives of the researches that we want to develop in Rennes the next four years are:
- The study and the modeling of a number of fundamental mechanisms present at the mineral/bacterium/solution interfaces: reducing dissolution; biomineralization; adsorption of mineral and/or organic matter on the surface of bacteria;
- The study of the behaviour of elements or molecules having particularly interesting properties (Rare Earths, organic matter, ...) since they can be used as a model for the understanding of the mechanisms at the interfaces, or as markers of the physico-chemical conditions present at these interfaces (redox potential in particular);
- The integration, at a larger scale (soil profile; first- to third-order catchment area), of the elementary processes taking into account the role of the bioclimatic variability and of the heterogeneity of the physical environment on the mechanisms involved in the acquisition of the chemical composition of waters and on the fluxes of exported matter.
This research gives rise to fundamental questions in the chemistry of solutions and in the physico-chemistry of interfaces, like the role of organic matter on the dissolution of minerals or the speciation and the transport of elements in solution. It also requires the development of a laboratory experimentation, which is the only way to determine and quantify the role played by the different factors and parameters involved in the solid-solution exchanges. From this viewpoint, the recruitment on a CR2 position of Xavier Ch?telier, biophysician specialized in the interactions between bacteria and metallic oxyhydroxydes, will bring the competences necessary to the integration of the bacterium component in the experiments.
1.3 Couplings between fluids and deformation
Flow forces and slope stability
Fluids migrate through rock pores at velocities that are proportional to permeabilities and pressure gradients. While migrating, fluids transmit so-called flow forces to the solid environment. These forces are well known in soil mechanics, because they affect the slope stability, but their role in sedimentary basins is far less known. Yet, recent deep drillings in a number of basins show fluid pressures close to lithostatic values, due to the weight of the solid environment. Among the causes of such overpressures, we can mention hydraulic forces, due to the relief, the sediment compaction and the production of hydrocarbons. The consequences for the slope stability are important, for example on submarine continental margins and large deltas.
For a few years at Geosciences Rennes, we have been studying such phenomena through experimentation and through a new protocol. As a study area, we have chosen the Amazon delta in Brazil where, thanks to a cooperation with the BP company, we have access to data from deep areas (wells and sismics).
In the next 4 years, we want to develop the digital modeling of these phenomena, as well as an experimental equipment being more complete and having more pieces of apparatus.
Hydraulic fracturing under various stress and deformation regimes
In a solid environment, when pore pressure exceeds the resistance to fissure formation, a mechanism of hydraulic fracturing operates. This phenomenon, well known in rock mechanics, is the cause of the burst of deep drillings. More generally, it explains the formation of magmatic veins, the discharge of hydrocarbons from their parent rocks and the formation of metalliferous deposits. Despite the importance of the process, details are poorly understood, especially the effects of stresses and deformations at the system limits.
In order to study these phenomena, we have set an experimental protocol, allowing the injection of liquids in a cohesive granular environment. Thus, we obtain a hydraulic fracturing and we observe that the stresses and deformations at the system limits are first-order parameters.
We are expecting industrial valuations, particularly through a collaboration with Total Fina Elf and Statoil companies and within the framework of the GEODE program (Geodynamics of the Metalliferous Deposits) of the European Fundation for Science.
In the next 4 years, we want to improve the experimental equipment and to develop the complementary digital modeling.
Coupling between fluid mobility and location of the deformation.
A model commonly admitted in the literature in order to explain the localized character of the deformation, brittle but also ductile, is the ? softening ? through which the mineral destabilization during the deformation in presence of (aqueous) fluids allows the development of phyllites, of which presence highly affects the rock properties and in turn, locally facilitates the deformation. This model can successfully explain the development of shear zones in greenschist facies on granitic matrices. The example of the Vendean porphyroids, studied at Geosciences Rennes (see the activity report) shows that in relatively low temperature conditions (< 400 ?C), very large volumes of rock can be affected by very high deformation rates, in a distributed manner, by deformation mechanisms mainly involving diffusion. The original aspect of this evolution has been explained by the closed character of the fluid regime in this system. Therefore, a coupling between the fluid regime and the deformation regime is easily formulated at low temperature: fluid mobility = localized deformation; static fluid = distributed deformation.
There are two types of questions raised by this work:
- What are the extension and the validity of the deformation mode identified in porphyroids? Many examples of systems affected by a deformation have been described, wherein the presence of phyllites is explained as an evidence of the ? softening ? scenario, without real proof: these phyllites could well have been present initially in the system or not act as an active agent of the deformation.
- What are the thresholds (in terms of ratios between fluids and rocks) that define the transition of mechanisms?
- Does the behaviour described for the porphyroids result from high-pressure conditions or can it arise from lower pressure metamorphic paths?
We suggest to tackle these questions by using systems of reference evolving in retrogressive conditions with a deformation evolving from a distributed ductile regime towards a brittle regime channelled with the set of methods already successfully applied in the past: coupling between petrology, structural analysis, geochemistry and fluid inclusions.
1.4 Induced geophysical phenomena
Fluid transfers give rise to changes of the environment and secondary physical phenomena that can be measured and controlled by the methods used by geophysics. The parameter that is the most influenced by the presence of fluids is the electrical conductivity, which highly depends on water saturation and salinity. The electrokinetic phenomena associated with the flows give rise to sources of electric currents of which potential can be measured at the surface or in drillings, in order to follow the spatio-temporal evolution of the fluid movements. These phenomena occur upon passage of earth waves and allow local studies by sismo-electric methods associating sismic measurements and electromagnetic measurements.
Several methodological improvements have been done in the last few years at Geosciences Rennes in the electrical tomography, sismic endoscopy and electro-sismics areas. In the next four years, we want to undertake researches combining theoretical work and modeling to field and laboratory experiments. Natural sites showing a geophysical, hydrological or geochemical interest, of which study has started a few years ago, will of course be favoured targets for these future studies. Important laboratory equipments are now operational and will aid field experiments.
Among the sites where we plan to undertake geophysical studies, we can mention, for example:
- Hydrological sites such as Ploemeur or Kerbernez that have been studied for several years and where the geophysical tools allow to have more information on the geological structure (electrical and sismic tomography) and on the operation (self potentials, differential tomography,...).
- The Pont-P?an mining site, which has been subjected to several campaigns of electrical tomography, of electromagnetic measurements and of self potential. This site has the advantage of having a relatively well-known structure thanks to the mining activity having taken place for several centuries. This structure is sub-two-dimensional, making the measurements and the inversion methodologies much easier. This site, which has the advantage of being in the southern suburbs of Rennes, can be visited very easily and it is planned to install, on this site, several networks of permanent measurements in electrical tomography, induced polarization and self potential. Carrying out drillings would be an important contribution in order to characterize the hydraulic operation of this site mainly consisting of a small sedimentary basin feeding water to a highly fractured dolerite vein and located in the plane of a sub-vertical fault.
- The Soufri?re dome in Guadeloupe has been subjected to electric and sismic tomography experiments carried out jointly by the geophysics team from Geosciences Rennes and by a team from the LGIT (Grenoble). About 10000 measurements of the electrical resistivity and of the induced polarization have been made along 2500 meters of profiles passing through the sides and the summit of the dome. The important pluviometry, about 12 meters per year, makes the Soufri?re dome an interesting object on the volcanology viewpoint as well as on the hydrology viewpoint. Another advantage of this site concerns the erosion and the instability of the ground resulting from the high rainfall intensity. We want to carry on the 3D-imagery campaigns of the Soufri?re dome and to add networks of permanent measurements of the self potential and of other electromagnetic parameters, such as conductivity and chargeability. The extent of the project requires the cooperation of several laboratories (mainly Geosciences Rennes, LGIT Grenoble, IPG Paris, EOPG Clermont-Ferrand) and the close association, particularly from the scientific viewpoint, of the volcanologic observation post of the Guadeloupe Soufri?re.
- The ANDRA future underground laboratory is another site where we plan important geophysical experiments. The important delays of the works do not allow to indicate a precise intervention schedule for the experiments within the galleries. We plan to carry out a 3D electrical tomography on this site, thanks to a network consisting of about 1000 electrodes allowing to probe the zone affected by the damage caused by the drilling. Several geophysical experiments are planned in the same zone (REG gallery) and particularly, sismo-electric measurements carried out by Steve Pride (Geosciences Rennes) and Michel Dietrich (LGIT). Other experiments carried out by Geosciences Rennes deal with sismic endoscopy, which is a new 3D imagery method of the surrounding of a drilling, of which algorithmic and instrumental developments continue thanks to the CAREN important equipment (acoustic tank, tomographic test wells, etc.).
2 RESEARCHES ON THE water cycle and ON TRANSPORTED elements
2.1 Water cycle in catchments areas
Catchment areas can be considered only as an interface between surface environments and subterranean environments. In the surface area (soil), the difficulty encountered lies in a strong dependency between water fluxes, controlled by the physical environment, and the chemical reactivity, which depends on bio-climatic conditions. The aim of the approach carried out on experimental sites is to thoroughly follow the two aspects. This approach is undertaken in the following sites, from the functioning of the humid areas to the ground waters, via the interface of the unsaturated environment:
- Peatlands from the Carentan swamp (hydrobiochemical functioning of the peatlands as a function of the exchanges with surface waters and ground waters, project in common with ECOBIO),
- Experimental site of Kerbernez (element transfers and influence on the biochemical activity during processes of aquifer recharge, project in common with INRA and ECOBIO)
- Catchment areas from Naizin and Kerbernez (water and solute transfers and chemical reactivity of the environments, project in common with INRA)
- Catchment area of Kaluvelly (India) (origin of the salinification, rehabilitation of the "ergs" and soil influence on mineralization, project in common with Paris 6 and Tours universities)
2.2 Groundwater Resources
The management of water resources is a problem that will become more and more important in the next few years in order to answer to the increase of the demand. Our action in this area is organized around the H+ project, which has been submitted in 2002 upon the call for tenders ?Research Observation Posts in Environment? of the Ministry of Research. The main objective of H+ is to maintain and organize a network of experimental sites capable of supplying pertinent data ? including chronicles or long-term experiments ? for the understanding of the cycle of the water and of the elements transported in groundwater aquifers, in order to develop tools for managing the groundwater resources. This effective experimental tool should allow to structure and stimulate hydrogeology researches at a national level.
The H+ project currently includes four experimental sites, complementary in terms of geological environment, exploitation, and research objectives:
- The Ploemeur site: corresponds to an aquifer exploited in a fractured crystalline domain for the drinking water supply of a city of 20 000 inhabitants. This site has a good piezometric coverage (40 drillings at a depth exceeding 100 m) and a good geochemical follow-up. The major problems concern the transfer of the agriculture pollutions towards the groundwater aquifer, the possible salinification risks of this coastal aquifer, the definition of the recharge zones related to the setting of a sanitary zone of well protection, and the evolution of the resource as a function of climate fluctuations. This site, managed by the local government control of Ploemeur, also represents a strong collaboration between fundamental research and users.
- The Poitiers site: corresponds to a fractured sedimentary environment developed by a team from the UMR Hydrasa from Poitiers, within the framework of 12th CPER. This site will include forty wells having a large diameter (8 inches at a depth of 120 m) allowing all the hydraulic investigations of the aquifer and of the surface installations for the follow-up of the vertical transfers towards the ground water. Free from any exploitation constraint, the site will allow to carry out long-lasting pumping tests, and tracing experiments.
- The Cadarache site is developed in the framework of a collaboration between the ISTEEM (Montpellier) and the CEN (CEA). 290 observation drillings and important follow-up means have been developed in order to characterize and model (i) the transfer structures and processes that control the flows, (ii) the transport of chemical species in the fractured multi-layered aquifer system, and (iii) the water-rock interactions.
- The Lavalette site, about 3 km far from the campus of Montpellier 2 University, is an experimental site for geophysics and hydrodynamics in drillings. The main objective of this site is to serve as a support for the development of new methods and new geophysical tools complementary to the study of the first several hundred meters under the surface.
These sites are already or will be highly equipped in order to allow long-lasting experiments and/or to obtain information on the time constants of the evolutions of the environment (liquid, mineral and microbiological). They all represent potential international references in terms of hydrogeological experimentations.
The H+ observation post also aims at organizing a partnership between fundamental research and ultimate users represented by the research departments and operators. This is already the case for one of the sites (Ploemeur) of which the contractor is the local government control.
2.3 Paleo-circulations
The deep fluid circulation is a major issue in Earth Sciences because it deals with several unresolved questions that may potentially have a strong impact on essential aspects of the physics and the chemistry of geological environments, especially in areas directly involved with the societal demand: some natural risks (for example, seismogenesis, disposal of final waste) or exhaustible resources (energy and mineral deposits). This topic is too large to be treated as a whole at Geosciences Rennes, but a number of studies are implemented at Geosciences Rennes, that will be subject to a new effort in the next four years.
A first important topic concerns the fluid circulation at the interfaces sedimentary basins - basement (S. Fourcade, P. Cobbold, O. Galland, collaborations G2R Nancy, Leeds, SURRC...., frame activities GDR FORPRO and TRANSMET, GEODE European Program). It is known that large-scale lateral circulations can be activated in these systems in compressive or extensive contexts and lead to mineralizations that can be extremely important (MVT, uranium bodies under unconformities...). Mineralogical effects of these circulations are often impressive about level with faulted structures having an apparently very small play and they involve fluids having abnormally high temperatures in this context (compared to the projection of the geothermal gradient about level with the unconformity).
On a small number of well-identified examples, we will try to 1) understand the mechanisms of recharge in fluids, matter and heat of these particular sites with an approach combining regional geology, geochemistry and fluid inclusions; we can hope that a digital modeling can be build from this; 2) determine what is the geodynamic situation likely to favour, in general, efficient fluid circulations in systems having a pre-established fracturing (basements or basins). These issues will be addressed on examples involving issues of socioeconomical nature:
- disposal of radioactive waste: relationships between the circulation of certain fluids and the regional dynamics on the Eastern site;
- cloggings in the Variscan basement under cover (within the framework of potential granitic sites for the disposal of radioactive waste);
- gold-bearing mineralizations (Variscan and Argentinian).
http://www.geosciences.univ-rennes1.fr/ENG/themes/trans_fluid_ev.htm#Geophys
PRESENTATION OF THE TOPIC
Researches on the "fluid" compartment have literally rocketed the last few years at Geosciences with several important research projects that range from the cycle of surface waters to groundwater flows. Apart from one or two exceptions, these researches have been initiated less than five years ago by researchers moving into a new field. This development has been carried out together with a voluntarist policy by the UMR with a recruitment of young researchers that is evolving considerably. The fact that the last two CNRS recruitments have concerned this field (in 2001 and moreover, in 2002) shows the reality of the emergence of this field both at the UMR level and at the national level.
The originality of the researches in Rennes clearly lies in the variety of the approaches with a range of researches that are theoretical, digital, experimental and field-related, and of physical, chemical and even biological approaches. The study of the main processes is one of the strong points of the current projects, that is reaffirmed, and even reinforced, in this four-year project.
Details on the main scientific projects that we consider a priority are given thereafter. From a general viewpoint, they aim at solving a number of important problems of the scientific knowledge, particularly on the way to take into account the high heterogeneity (static or induced) of environments and processes,
- on the characterization of the main chemical or biological transformations, and of their results in a system where fluxes are fundamentally heterogeneous,
- on the way how to quantify and predict the slow, large-scale evolutions (geological scales), but fundamentally related to systems of which dynamics can be extremely fast (hydrodynamics, meteorology).
In response to these points, we consider these three complementary methodological approaches as a priority:
- the development of effective modeling methods adapted to these systems,
- the development of experimental approaches at several scales ? from laboratory to experimental site,
- the development of a long-term follow-up of some outstanding systems, particularly within the framework of research observation posts in environment.
The projects developed by the UMR encompass several disciplines of Earth Sciences: hydro(geo)logy, geochemistry, geophysics and even tectonics. Important collaborations are initiated towards biology, physics and chemistry. Even if interdisciplinarity is an actual fact, and even a development objective, we can tell that it is not at all a disadvantage for the coherence of the researches. There exist many collaborations, particularly thanks to the "processes" view, which allows real discussions between researchers.
Finally, researches are carried on particularly topical environment issues, like the transfer of contaminants in catchment areas, the management of the water supply, the disposal of subterranean waste or the oil resource. In this respect, they represent an important pole of the IFR CAREN, and they participate at a national scale to the development of researches on what INSU has recently called "continental surfaces", that is, the key zone at the interface between the solid Earth, the hydrosphere and the atmosphere.
People involved
These fields involve about twenty researchers with about 14 researchers working full-time (about 1/3 of the UMR). The researchers involved are:
Luc Aquilina, Philippe Boulvais, Olivier Bour, Jean de Br?mond d?Ars, Xavier Ch?tellier, Peter R. Cobbold, Alain Crave, Olivier Dauteuil, M?lanie Davranche, Philippe Davy, Aline Dia, Jean-Raynald de Dreuzy, Serge Fourcade, Pierre Gavrilenko, Dominique Gibert, G?rard Gruau, Florence Nicollin, Steve Pride, Gilles Ruffet.
8 technicians and engineers are involved at various levels in experiments (physical and biochemical), field measurements and operating of the analytical means:
Martine Le Coz-Bouhnik, Jean-Pierre Caudal, Jean Cornichet, Fr?d?ric Conil, Odile H?nin, Jean-Jacques Kermarrec, Michel Lemoine, Patrice Petitjean and Yves Qu?t?. Their contribution to this field corresponds to about 7 full-time jobs.
Scientific projects
We have separated two levels: researches on processes, and researches on the characterization of the natural environment.
1. ReSEARCHES ON "processEs"
1.1 Modeling of ohysico-chemical transfers in heterogeneous environments
The high heterogeneity of the geological environments, and the complexity of the solid/liquid interactions, represent the main difficulties to be overcome when modeling these systems in order to predict water and material fluxes. Fractured systems are particularly representative of the problems encountered, but this difficulty is found in most geological environments. Although societal and economical stakes are numerous (management of the water supply, evaluation of disposal sites of toxic waste, oil exploration, ...), so are the problems with practical and theoretical difficulties. The fine and exhaustive characterization of the environment comes up against the intrinsic difficulties of the in situ observation. From a theoretical viewpoint, taking into account a multi-scale heterogeneity is a problem that has not been solved in the current models that "homogeneize" the environment or that at best take only a fraction of the complexity into account. The challenge is therefore to build models that can be physically accepted while being characterized by a number of parameters compatible with the available data.
Our objective is fundamentaly to determine the physico-chemical laws describing the transport at a field scale, so that it is possible to reverse the local measurements, even in a strongly heterogeneous environment. The first studies have shown that the classical advection-diffusion laws cannot be used to explain field observations showing the high spatial dispersion of the solutes as well as the wide range of the arrival time. These features are due both to the heterogeneity of the velocity field resulting from the complex structures and to the chemical reactivity of the solutes. Chemical and physical effects are even coupled since the pore geometry and the transfer velocities control the contact zone of the solutes with the environment on which it can adsorb or diffuse and the residence time of the solutes determining the progress of the chemical reactions, respectively.
Our four-year objectives show too poles consisting in:
- Developing field and laboratory studies in order to best characterize the processes studied. The aim is also to obtain sufficient dataset to validate the more theoretical approaches. Within this framework, many field studies are planned on several sites including the Ploemeur site (Morbihan, France) where the aim is not only to characterize the properties of flow and dispersion of the environment by pumping tests and tracings, but also to measure the velocity distribution in situ. The velocity field is particularly important because it results from the heterogeneity and because it controls the macroscopic transport.
- Carrying on with the digital and theoretical developments that allow to relate environment models with transport properties. We are currently developing a three-dimensional digital model that is capable of simulating domains large enough to solve the problem of scale change. This project is carried out in the framework of a collaboration with the IRISA (INRIA laboratory in Rennes). We have also started to study the reactive transport in order to take into account more complex chemical and biological reactions in transfer models. The aim of these simulations is to act as a guide and a validation tool for the definition of transport laws as a function of the characteristics of the heterogeneity and the chemical reactions. Obtaining these general laws will then allow to consider the development of modeling methods taking into account the amount and uncertainty of the data. The assessment of the model uncertainty, which depends on the data quantity and quality, is determining for the assessment of the risk associated, for example, to the subterranean disposal of toxic waste.
1.2 Physico-chemical reactivities - Solution/Minerals/Bacteria interactions
Physico-chemical reactions at the solution/mineral/bacteria interfaces are the main factor controlling the chemical composition of waters, whether marine or continental. The understanding and the modeling of these reactions is therefore a major stake, with potentially important implications both in areas like the understanding and the modeling of erosion mechanisms - water considered as the medium of material transfers at the Earth?s surface - and the protection and the management of our environment - water considered as a vital resource for human beings and living beings. The main objectives of the researches that we want to develop in Rennes the next four years are:
- The study and the modeling of a number of fundamental mechanisms present at the mineral/bacterium/solution interfaces: reducing dissolution; biomineralization; adsorption of mineral and/or organic matter on the surface of bacteria;
- The study of the behaviour of elements or molecules having particularly interesting properties (Rare Earths, organic matter, ...) since they can be used as a model for the understanding of the mechanisms at the interfaces, or as markers of the physico-chemical conditions present at these interfaces (redox potential in particular);
- The integration, at a larger scale (soil profile; first- to third-order catchment area), of the elementary processes taking into account the role of the bioclimatic variability and of the heterogeneity of the physical environment on the mechanisms involved in the acquisition of the chemical composition of waters and on the fluxes of exported matter.
This research gives rise to fundamental questions in the chemistry of solutions and in the physico-chemistry of interfaces, like the role of organic matter on the dissolution of minerals or the speciation and the transport of elements in solution. It also requires the development of a laboratory experimentation, which is the only way to determine and quantify the role played by the different factors and parameters involved in the solid-solution exchanges. From this viewpoint, the recruitment on a CR2 position of Xavier Ch?telier, biophysician specialized in the interactions between bacteria and metallic oxyhydroxydes, will bring the competences necessary to the integration of the bacterium component in the experiments.
1.3 Couplings between fluids and deformation
Flow forces and slope stability
Fluids migrate through rock pores at velocities that are proportional to permeabilities and pressure gradients. While migrating, fluids transmit so-called flow forces to the solid environment. These forces are well known in soil mechanics, because they affect the slope stability, but their role in sedimentary basins is far less known. Yet, recent deep drillings in a number of basins show fluid pressures close to lithostatic values, due to the weight of the solid environment. Among the causes of such overpressures, we can mention hydraulic forces, due to the relief, the sediment compaction and the production of hydrocarbons. The consequences for the slope stability are important, for example on submarine continental margins and large deltas.
For a few years at Geosciences Rennes, we have been studying such phenomena through experimentation and through a new protocol. As a study area, we have chosen the Amazon delta in Brazil where, thanks to a cooperation with the BP company, we have access to data from deep areas (wells and sismics).
In the next 4 years, we want to develop the digital modeling of these phenomena, as well as an experimental equipment being more complete and having more pieces of apparatus.
Hydraulic fracturing under various stress and deformation regimes
In a solid environment, when pore pressure exceeds the resistance to fissure formation, a mechanism of hydraulic fracturing operates. This phenomenon, well known in rock mechanics, is the cause of the burst of deep drillings. More generally, it explains the formation of magmatic veins, the discharge of hydrocarbons from their parent rocks and the formation of metalliferous deposits. Despite the importance of the process, details are poorly understood, especially the effects of stresses and deformations at the system limits.
In order to study these phenomena, we have set an experimental protocol, allowing the injection of liquids in a cohesive granular environment. Thus, we obtain a hydraulic fracturing and we observe that the stresses and deformations at the system limits are first-order parameters.
We are expecting industrial valuations, particularly through a collaboration with Total Fina Elf and Statoil companies and within the framework of the GEODE program (Geodynamics of the Metalliferous Deposits) of the European Fundation for Science.
In the next 4 years, we want to improve the experimental equipment and to develop the complementary digital modeling.
Coupling between fluid mobility and location of the deformation.
A model commonly admitted in the literature in order to explain the localized character of the deformation, brittle but also ductile, is the ? softening ? through which the mineral destabilization during the deformation in presence of (aqueous) fluids allows the development of phyllites, of which presence highly affects the rock properties and in turn, locally facilitates the deformation. This model can successfully explain the development of shear zones in greenschist facies on granitic matrices. The example of the Vendean porphyroids, studied at Geosciences Rennes (see the activity report) shows that in relatively low temperature conditions (< 400 ?C), very large volumes of rock can be affected by very high deformation rates, in a distributed manner, by deformation mechanisms mainly involving diffusion. The original aspect of this evolution has been explained by the closed character of the fluid regime in this system. Therefore, a coupling between the fluid regime and the deformation regime is easily formulated at low temperature: fluid mobility = localized deformation; static fluid = distributed deformation.
There are two types of questions raised by this work:
- What are the extension and the validity of the deformation mode identified in porphyroids? Many examples of systems affected by a deformation have been described, wherein the presence of phyllites is explained as an evidence of the ? softening ? scenario, without real proof: these phyllites could well have been present initially in the system or not act as an active agent of the deformation.
- What are the thresholds (in terms of ratios between fluids and rocks) that define the transition of mechanisms?
- Does the behaviour described for the porphyroids result from high-pressure conditions or can it arise from lower pressure metamorphic paths?
We suggest to tackle these questions by using systems of reference evolving in retrogressive conditions with a deformation evolving from a distributed ductile regime towards a brittle regime channelled with the set of methods already successfully applied in the past: coupling between petrology, structural analysis, geochemistry and fluid inclusions.
1.4 Induced geophysical phenomena
Fluid transfers give rise to changes of the environment and secondary physical phenomena that can be measured and controlled by the methods used by geophysics. The parameter that is the most influenced by the presence of fluids is the electrical conductivity, which highly depends on water saturation and salinity. The electrokinetic phenomena associated with the flows give rise to sources of electric currents of which potential can be measured at the surface or in drillings, in order to follow the spatio-temporal evolution of the fluid movements. These phenomena occur upon passage of earth waves and allow local studies by sismo-electric methods associating sismic measurements and electromagnetic measurements.
Several methodological improvements have been done in the last few years at Geosciences Rennes in the electrical tomography, sismic endoscopy and electro-sismics areas. In the next four years, we want to undertake researches combining theoretical work and modeling to field and laboratory experiments. Natural sites showing a geophysical, hydrological or geochemical interest, of which study has started a few years ago, will of course be favoured targets for these future studies. Important laboratory equipments are now operational and will aid field experiments.
Among the sites where we plan to undertake geophysical studies, we can mention, for example:
- Hydrological sites such as Ploemeur or Kerbernez that have been studied for several years and where the geophysical tools allow to have more information on the geological structure (electrical and sismic tomography) and on the operation (self potentials, differential tomography,...).
- The Pont-P?an mining site, which has been subjected to several campaigns of electrical tomography, of electromagnetic measurements and of self potential. This site has the advantage of having a relatively well-known structure thanks to the mining activity having taken place for several centuries. This structure is sub-two-dimensional, making the measurements and the inversion methodologies much easier. This site, which has the advantage of being in the southern suburbs of Rennes, can be visited very easily and it is planned to install, on this site, several networks of permanent measurements in electrical tomography, induced polarization and self potential. Carrying out drillings would be an important contribution in order to characterize the hydraulic operation of this site mainly consisting of a small sedimentary basin feeding water to a highly fractured dolerite vein and located in the plane of a sub-vertical fault.
- The Soufri?re dome in Guadeloupe has been subjected to electric and sismic tomography experiments carried out jointly by the geophysics team from Geosciences Rennes and by a team from the LGIT (Grenoble). About 10000 measurements of the electrical resistivity and of the induced polarization have been made along 2500 meters of profiles passing through the sides and the summit of the dome. The important pluviometry, about 12 meters per year, makes the Soufri?re dome an interesting object on the volcanology viewpoint as well as on the hydrology viewpoint. Another advantage of this site concerns the erosion and the instability of the ground resulting from the high rainfall intensity. We want to carry on the 3D-imagery campaigns of the Soufri?re dome and to add networks of permanent measurements of the self potential and of other electromagnetic parameters, such as conductivity and chargeability. The extent of the project requires the cooperation of several laboratories (mainly Geosciences Rennes, LGIT Grenoble, IPG Paris, EOPG Clermont-Ferrand) and the close association, particularly from the scientific viewpoint, of the volcanologic observation post of the Guadeloupe Soufri?re.
- The ANDRA future underground laboratory is another site where we plan important geophysical experiments. The important delays of the works do not allow to indicate a precise intervention schedule for the experiments within the galleries. We plan to carry out a 3D electrical tomography on this site, thanks to a network consisting of about 1000 electrodes allowing to probe the zone affected by the damage caused by the drilling. Several geophysical experiments are planned in the same zone (REG gallery) and particularly, sismo-electric measurements carried out by Steve Pride (Geosciences Rennes) and Michel Dietrich (LGIT). Other experiments carried out by Geosciences Rennes deal with sismic endoscopy, which is a new 3D imagery method of the surrounding of a drilling, of which algorithmic and instrumental developments continue thanks to the CAREN important equipment (acoustic tank, tomographic test wells, etc.).
2 RESEARCHES ON THE water cycle and ON TRANSPORTED elements
2.1 Water cycle in catchments areas
Catchment areas can be considered only as an interface between surface environments and subterranean environments. In the surface area (soil), the difficulty encountered lies in a strong dependency between water fluxes, controlled by the physical environment, and the chemical reactivity, which depends on bio-climatic conditions. The aim of the approach carried out on experimental sites is to thoroughly follow the two aspects. This approach is undertaken in the following sites, from the functioning of the humid areas to the ground waters, via the interface of the unsaturated environment:
- Peatlands from the Carentan swamp (hydrobiochemical functioning of the peatlands as a function of the exchanges with surface waters and ground waters, project in common with ECOBIO),
- Experimental site of Kerbernez (element transfers and influence on the biochemical activity during processes of aquifer recharge, project in common with INRA and ECOBIO)
- Catchment areas from Naizin and Kerbernez (water and solute transfers and chemical reactivity of the environments, project in common with INRA)
- Catchment area of Kaluvelly (India) (origin of the salinification, rehabilitation of the "ergs" and soil influence on mineralization, project in common with Paris 6 and Tours universities)
2.2 Groundwater Resources
The management of water resources is a problem that will become more and more important in the next few years in order to answer to the increase of the demand. Our action in this area is organized around the H+ project, which has been submitted in 2002 upon the call for tenders ?Research Observation Posts in Environment? of the Ministry of Research. The main objective of H+ is to maintain and organize a network of experimental sites capable of supplying pertinent data ? including chronicles or long-term experiments ? for the understanding of the cycle of the water and of the elements transported in groundwater aquifers, in order to develop tools for managing the groundwater resources. This effective experimental tool should allow to structure and stimulate hydrogeology researches at a national level.
The H+ project currently includes four experimental sites, complementary in terms of geological environment, exploitation, and research objectives:
- The Ploemeur site: corresponds to an aquifer exploited in a fractured crystalline domain for the drinking water supply of a city of 20 000 inhabitants. This site has a good piezometric coverage (40 drillings at a depth exceeding 100 m) and a good geochemical follow-up. The major problems concern the transfer of the agriculture pollutions towards the groundwater aquifer, the possible salinification risks of this coastal aquifer, the definition of the recharge zones related to the setting of a sanitary zone of well protection, and the evolution of the resource as a function of climate fluctuations. This site, managed by the local government control of Ploemeur, also represents a strong collaboration between fundamental research and users.
- The Poitiers site: corresponds to a fractured sedimentary environment developed by a team from the UMR Hydrasa from Poitiers, within the framework of 12th CPER. This site will include forty wells having a large diameter (8 inches at a depth of 120 m) allowing all the hydraulic investigations of the aquifer and of the surface installations for the follow-up of the vertical transfers towards the ground water. Free from any exploitation constraint, the site will allow to carry out long-lasting pumping tests, and tracing experiments.
- The Cadarache site is developed in the framework of a collaboration between the ISTEEM (Montpellier) and the CEN (CEA). 290 observation drillings and important follow-up means have been developed in order to characterize and model (i) the transfer structures and processes that control the flows, (ii) the transport of chemical species in the fractured multi-layered aquifer system, and (iii) the water-rock interactions.
- The Lavalette site, about 3 km far from the campus of Montpellier 2 University, is an experimental site for geophysics and hydrodynamics in drillings. The main objective of this site is to serve as a support for the development of new methods and new geophysical tools complementary to the study of the first several hundred meters under the surface.
These sites are already or will be highly equipped in order to allow long-lasting experiments and/or to obtain information on the time constants of the evolutions of the environment (liquid, mineral and microbiological). They all represent potential international references in terms of hydrogeological experimentations.
The H+ observation post also aims at organizing a partnership between fundamental research and ultimate users represented by the research departments and operators. This is already the case for one of the sites (Ploemeur) of which the contractor is the local government control.
2.3 Paleo-circulations
The deep fluid circulation is a major issue in Earth Sciences because it deals with several unresolved questions that may potentially have a strong impact on essential aspects of the physics and the chemistry of geological environments, especially in areas directly involved with the societal demand: some natural risks (for example, seismogenesis, disposal of final waste) or exhaustible resources (energy and mineral deposits). This topic is too large to be treated as a whole at Geosciences Rennes, but a number of studies are implemented at Geosciences Rennes, that will be subject to a new effort in the next four years.
A first important topic concerns the fluid circulation at the interfaces sedimentary basins - basement (S. Fourcade, P. Cobbold, O. Galland, collaborations G2R Nancy, Leeds, SURRC...., frame activities GDR FORPRO and TRANSMET, GEODE European Program). It is known that large-scale lateral circulations can be activated in these systems in compressive or extensive contexts and lead to mineralizations that can be extremely important (MVT, uranium bodies under unconformities...). Mineralogical effects of these circulations are often impressive about level with faulted structures having an apparently very small play and they involve fluids having abnormally high temperatures in this context (compared to the projection of the geothermal gradient about level with the unconformity).
On a small number of well-identified examples, we will try to 1) understand the mechanisms of recharge in fluids, matter and heat of these particular sites with an approach combining regional geology, geochemistry and fluid inclusions; we can hope that a digital modeling can be build from this; 2) determine what is the geodynamic situation likely to favour, in general, efficient fluid circulations in systems having a pre-established fracturing (basements or basins). These issues will be addressed on examples involving issues of socioeconomical nature:
- disposal of radioactive waste: relationships between the circulation of certain fluids and the regional dynamics on the Eastern site;
- cloggings in the Variscan basement under cover (within the framework of potential granitic sites for the disposal of radioactive waste);
- gold-bearing mineralizations (Variscan and Argentinian).
http://www.geosciences.univ-rennes1.fr/ENG/themes/trans_fluid_ev.htm#Geophys