Characterizing Fracture Aperture and Transport Dynamics with Hydrogeophysics: Theoretical and Expérimental Advances

plusieurs km, les fractures peuvent servir de conduits ou de barrières aux écoulements des fluides et jouent un rôle majeur dans divers processus et applications comme l’extraction des eaux souterraines, la migration des contaminants dans les roches fracturées, le stockage souterrain des déchets nucléaires mais aussi la séquestration du dioxyde de carbone et le stockage géothermique. Les propriétés géométriques d’une fracture telles que les variations d’ouverture sur son plan influencent l’écoulement et le transport du fluide. Au sein d’une fracture, la caractérisation statistique des ouvertures est délicate puisque celles-ci présentent souvent une auto-affinitude ; une propriété qui implique des motifs similaires à plusieurs échelles spatiales. Les expériences hydrologiques classiques par leur caractère discret sur le terrain ne fournissent pas d’informations directes sur les variations d’ouvertures. Cette limitation peut en partie être surmontée par l’hydrogéophysique qui combine des méthodes géophysiques avec des expériences hydrologiques. Dans cette étude, nous présentons des avancées expérimentales et théoriques dédiées à l’utilisation du géoradar (GPR) seul et aussi combinées avec des essais push-pull afin de caractériser plus précisément l’ouverture des fractures. Sur le plan théorique, nous avons utilisé des solutions analytiques et développé un cadre de modélisation pour simuler les réflexions GPR dans le milieu fracturé. Les fractures qui le composent sont caracterisées par des ouvertures hétérogènes mais sont intégrées dans une matrice de roche uniforme. En présence de cette hétérogénéité, nous avons dé- montré que les approches classiques qui reposent sur une ouverture de fracture uniforme, conduisent à des estimations fortement biaisées de l’ouverture moyenne. Ce cadre de mo- délisation est important pour l’utilisation du GPR dans un aquifèr fracturé mais convient également à d’autres applications, telles que l’imagerie des fractures dans le béton. Sur le plan expérimental, nous avons réalisé les premières expériences permettant la visualisation de la migration d’un traceur salin lors d’un test push-pull. Les données GPR obtenues ont permis de mesurer la dynamique du panache du traceur et de décrire la nature des fractures. De plus, elles ont mis en évidence des effets de densité qui sont délétères pour les inférences de la dynamique et des processus naturels d’écoulement, comme l’écoulement ambiant. Pour remédier à ces effets, des mesures ont été réalisées en ajoutant de l’éthanol au traceur salin afin d’obtenir une flottabilité neutre tout en conservant la conductivité électrique. La comparaison des résultats pour les deux types de traceurs montrent que l’ajout d’éthanol permet d’éliminer les effets de densité. C’est pourquoi nous suggérons que des traceurs de densité neutre soient utilisés pour les expériences hydrogéophysiques. Enfin, nous introdui- sons un modèle simultané qui peut simuler les expériences push-pull et GPR. Ce modèle pourrait permettre d’inférer les propriétés d’une fracture par les données réels, ce qui serait pertinent en utilisant des méthodes Markov-chain Monte-Carlo. -- Fractures are ubiquitous in the Earth’s crust. From the scale of a few cm to several km, fractures can act as conduits or barriers to fluid flow and play a major role in several processes and applications, including groundwater extraction, contaminant migration in fractured rock, underground storage of disposed nuclear waste and sequestration of carbon dioxide, as well as geothermal heat migration. A fracture’s geometrical properties such as aperture variations along its plane influence fluid flow and transport within. Nevertheless, it is a challenging task to statistically characterize aperture variations of a single fracture because these are often self-affine; a property that implies similar patterns over several spatial scales. Classical hydrological tests are spatially sparse and do not provide direct information about aperture variations. This limitation can partly be overcome by hydrogeophysics, which combines geophysical methods with hydrological experiments. Here, we present experimental and theoretical advances on the use of ground penetrating radar (GPR) alone and also combined with push-pull tests for improved fracture aperture characterization. On the theoretical aspect we used analytical solutions to develop a modeling framework that simulates GPR reflections from fractures with heterogeneous aperture distributions, embedded in a uniform rock matrix. In the presence of aperture heterogeneity in a single fracture, we demonstrate that classical aperture-inference approaches that rely on uniform fracture properties lead to biased estimates of mean aperture. The modeling framework is suitable for GPR use in fractured rock but is also suitable for other applications such as fracture imaging in concrete. On the experimental side, we present the first experiments in which the migration of a saline tracer is imaged during a push-pull test. The GPR data are informative about the dynamics of the tracer plume and the fractures involved in the experiment, but also highlight density effects that decrease our ability to infer natural flow dynamics and processes, such as ambient flow. We address the density issue by introducing a neutrally-buoyant, yet electrically conductive tracer, which consists of ethanol mixed with a saline tracer. A comparison of results from the two types of tracer tests demonstrates that the addition of ethanol diminishes the density effect; we therefore suggest that neutrally buoyant tracers should be used in hydrogeophysics. Finally, we introduce a simultaneous modeling approach that can simulate the combined experiments. The coupled model can be applied within a Markov-chain Monte- Carlo inversion of the data from the combined experiment to infer a fracture’s geometric properties

The 3-D structure of the Somma-Vesuvius volcanic complex (Italy) inferred from new and historic gravimetric data

Existing 3-D density models of the Somma-Vesuvius volcanic complex (SVVC), Italy, largely disagree. Despite the scientific and socioeconomic importance of Vesuvius, there is no reliable 3-D density model of the SVVC. A considerable uncertainty prevails concerning the presence (or absence) of a dense body underlying the Vesuvius crater (1944 eruption) that is implied from extensive seismic investigations. We have acquired relative gravity measurements at 297 stations, including measurements in difficult-to-access areas (e.g., the first-ever measurements in the crater). In agreement with seismic investigations, the simultaneous inversion of these and historic data resolves a high-density body that extends from the surface of the Vesuvius crater down to depths that exceed 2 km. A 1.5-km radius horseshoe-shaped dense feature (open in the southwestern sector) enforces the existing model of groundwater circulation within the SVVC. Based on its volcano-tectonic evolution, we interpret volcanic structures that have never been imaged before

Heat as a tracer for understanding transport processes in fractured media: Theory and field assessment from multiscale thermal push-pull tracer tests

International audienceThe characterization and modeling of heat transfer in fractured media is particularly challenging as the existence of fractures at multiple scales induces highly localized flow patterns. From a theoretical and numerical analysis of heat transfer in simple conceptual models of fractured media, we show that flow channeling has a significant effect on the scaling of heat recovery in both space and time. The late time tailing of heat recovery under channeled flow is shown to diverge from the TðtÞ / t 21:5 behavior expected for the classical parallel plate model and follow the scaling TðtÞ / 1=tðlog tÞ 2 for a simple channel modeled as a tube. This scaling, which differs significantly from known scalings in mobile-immobile systems, is of purely geometrical origin: late time heat transfer from the matrix to a channel corresponds dimensionally to a radial diffusion process, while heat transfer from the matrix to a plate may be considered as a one-dimensional process. This phenomenon is also manifested on the spatial scaling of heat recovery as flow channeling affects the decay of the thermal breakthrough peak amplitude and the increase of the peak time with scale. These findings are supported by the results of a field experimental campaign performed on the fractured rock site of Ploemeur. The scaling of heat recovery in time and space, measured from thermal breakthrough curves measured through a series of push-pull tests at different scales, shows a clear signature of flow channeling. The whole data set can thus be successfully represented by a multichannel model parametrized by the mean channel density and aperture. These findings, which bring new insights on the effect of flow channeling on heat transfer in fractured rocks, show how heat recovery in geothermal tests may be controlled by fracture geometry. In addition, this highlights the interest of thermal push-pull tests as a complement to solute tracers tests to infer fracture aperture and geometry

Caractérisation de l'ouverture de fracture et la dynamique de transport avec l'hydrogéophysique: Avancées théoriques et expérimentales

Fractures are ubiquitous in the Earth’s crust. From the scale of a few cm to several km,fractures can act as conduits or barriers to fluid flow and play a major role in several processesand applications, including groundwater extraction, contaminant migration in fractured rock,underground storage of disposed nuclear waste and sequestration of carbon dioxide, as wellas geothermal heat migration. A fracture’s geometrical properties such as aperture variationsalong its plane influence fluid flow and transport within. Nevertheless, it is a challengingtask to statistically characterize aperture variations of a single fracture because these areoften self-affine; a property that implies similar patterns over several spatial scales. Classicalhydrological tests are spatially sparse and do not provide direct information about aperturevariations. This limitation can partly be overcome by hydrogeophysics, which combinesgeophysical methods with hydrological experiments. Here, we present experimental andtheoretical advances on the use of ground penetrating radar (GPR) alone and also combinedwith push-pull tests for improved fracture aperture characterization. On the theoreticalaspect we used analytical solutions to develop a modeling framework that simulates GPRreflections from fractures with heterogeneous aperture distributions, embedded in a uniformrock matrix. In the presence of aperture heterogeneity in a single fracture, we demonstratethat classical aperture-inference approaches that rely on uniform fracture properties leadto biased estimates of mean aperture. The modeling framework is suitable for GPR use infractured rock but is also suitable for other applications such as fracture imaging in concrete.On the experimental side, we present the first experiments in which the migration of asaline tracer is imaged during a push-pull test. The GPR data are informative about thedynamics of the tracer plume and the fractures involved in the experiment, but also highlightdensity effects that decrease our ability to infer natural flow dynamics and processes, such asambient flow. We address the density issue by introducing a neutrally-buoyant, yet electricallyconductive tracer, which consists of ethanol mixed with a saline tracer. A comparison ofresults from the two types of tracer tests demonstrates that the addition of ethanol diminishesthe density effect; we therefore suggest that neutrally buoyant tracers should be used inhydrogeophysics. Finally, we introduce a simultaneous modeling approach that can simulatethe combined experiments. The coupled model can be applied within a Markov-chain Monte-Carlo inversion of the data from the combined experiment to infer a fracture’s geometricproperties.Les fractures sont omniprésentes dans la croûte terrestre. Sur une échelle de quelques cm àplusieurs km, les fractures peuvent servir de conduits ou de barrières aux écoulements desfluides et jouent un rôle majeur dans divers processus et applications comme l’extractiondes eaux souterraines, la migration des contaminants dans les roches fracturées, le stockagesouterrain des déchets nucléaires mais aussi la séquestration du dioxyde de carbone et lestockage géothermique. Les propriétés géométriques d’une fracture telles que les variationsd’ouverture sur son plan influencent l’écoulement et le transport du fluide. Au sein d’unefracture, la caractérisation statistique des ouvertures est délicate puisque celles-ci présententsouvent une auto-affinitude ; une propriété qui implique des motifs similaires à plusieurséchelles spatiales. Les expériences hydrologiques classiques par leur caractère discret surle terrain ne fournissent pas d’informations directes sur les variations d’ouvertures. Cettelimitation peut en partie être surmontée par l’hydrogéophysique qui combine des méthodesgéophysiques avec des expériences hydrologiques. Dans cette étude, nous présentons desavancées expérimentales et théoriques dédiées à l’utilisation du géoradar (GPR) seul et aussicombinées avec des essais push-pull afin de caractériser plus précisément l’ouverture desfractures. Sur le plan théorique, nous avons utilisé des solutions analytiques et développé uncadre de modélisation pour simuler les réflexions GPR dans le milieu fracturé. Les fracturesqui le composent sont caracterisées par des ouvertures hétérogènes mais sont intégréesdans une matrice de roche uniforme. En présence de cette hétérogénéité, nous avons dé-montré que les approches classiques qui reposent sur une ouverture de fracture uniforme,conduisent à des estimations fortement biaisées de l’ouverture moyenne. Ce cadre de mo-délisation est important pour l’utilisation du GPR dans un aquifèr fracturé mais convientégalement à d’autres applications, telles que l’imagerie des fractures dans le béton. Sur leplan expérimental, nous avons réalisé les premières expériences permettant la visualisationde la migration d’un traceur salin lors d’un test push-pull. Les données GPR obtenues ontpermis de mesurer la dynamique du panache du traceur et de décrire la nature des fractures.De plus, elles ont mis en évidence des effets de densité qui sont délétères pour les inférencesde la dynamique et des processus naturels d’écoulement, comme l’écoulement ambiant.Pour remédier à ces effets, des mesures ont été réalisées en ajoutant de l’éthanol au traceursalin afin d’obtenir une flottabilité neutre tout en conservant la conductivité électrique. Lacomparaison des résultats pour les deux types de traceurs montrent que l’ajout d’éthanolpermet d’éliminer les effets de densité. C’est pourquoi nous suggérons que des traceurs dedensité neutre soient utilisés pour les expériences hydrogéophysiques. Enfin, nous introdui-sons un modèle simultané qui peut simuler les expériences push-pull et GPR. Ce modèlepourrait permettre d’inférer les propriétés d’une fracture par les don

Apparent apertures from ground penetrating radar data and their relation to heterogeneous aperture fields

Considering fractures with heterogeneous aperture distributions, we explore the reliability of constant-aperture estimates derived from ground penetrating radar (GPR) reflection data. We generate geostatistical fracture aperture realizations that are characterized by the same mean-aperture and variance, but different Hurst exponents and cut-off lengths. For each of the 16 classes of heterogeneity considered, we generate 1000 fracture realizations from which we compute GPR reflection data using our recent effective-dipole forward model. We then use each (noise-contaminated) data set individually to invert for a single ‘apparent’ aperture, that is, we assume that the fracture aperture is homogeneous. We find that the inferred ‘apparent’ apertures are only reliable when fracture heterogeneity is non-fractal (the Hurst exponent is close to 1) and the scale of the dominant aperture heterogeneities is larger than the first Fresnel zone. These results are a direct consequence of the nonlinear character of the thin-bed reflection coefficients. As fracture heterogeneity is ubiquitous and often fractal, our results suggest that robust field-based inference of fracture aperture can only be achieved by accounting for the nonlinear response of fracture heterogeneity on GPR data

Modelling and inferring fracture curvature from borehole GPR data: A case study from the Bedretto Laboratory, Switzerland

Fracture curvature has been observed from the millimetre to the kilometre scales. Nevertheless, characterizing curvature remains challenging due to data sparsity and geometric ambiguities. As a result, most numerical models often assume planar fractures to ease computations. To address this limitation, we present a novel approach for inferring fracture geometry from travel-time data of electromagnetic or seismic waves. Our model utilizes co-kriging interpolation of control points in a three-dimensional surface mesh to simulate fracture curvature effectively, resulting in an unstructured triangular grid. We then refine the fracture surface into a structured grid with equidistant elements so that both small-scale heterogeneities and large-scale curvature can be modelled. To constrain the fracture geometry, we perform a deterministic travel-time inversion to optimally place these control points. We validate our methodology with synthetic data and address its limitations. Finally, we infer the geometry of a large (more than 200 m) fracture observed in single-hole ground-penetrating radar field data. The fracture surface closely agrees with borehole televiewer observations and is also constrained far from the boreholes. Our modelling approach can be trivially adapted to multi-offset ground-penetrating radar or active seismic data.ISSN:1569-4445ISSN:1873-060

Inference of Fractured Rock Transport Properties by Joint Inversion of Push-Pull and Single-Hole Ground Penetrating Radar Data

International audienceFlow and transport characterization of fractured rock formations is very challenging and important for a multitude of applications that include groundwater extraction, nuclear waste storage and geothermal energy production. One popular hydrogeological method to study fractured rock is a push-pull test, in which injection and retrieval of a tracer is made at the same depth interval in a borehole. In theory, push-pull tests are not sensitive to changes in the heterogeneity of the tracer flow path since the retrieval at the injection location minimizes advective effects and makes the test more sensitive to time-dependent transport processes. This assumption is limiting in the presence of a natural hydraulic gradient or if non-neutrally buoyant tracers are used, but these limitations can be reduced by monitoring push-pull tests with ground penetrating radar (GPR). We present a methodology for combined modeling and inversion of a series of push-pull tests that we monitored with the single hole ground penetrating radar (GPR) method. For the GPR modeling we use a newly developed approach to simulate the GPR response in fractured rock. We coupled the GPR model to a flow-and-transport simulator that we use to define the electrical properties of the fracture filling.The combined model can cope with heterogeneous fractures of any orientation, aperture and size and allows for the effect of density driven flow (that is strong during the saline tracer tests). We use the combined simulator to create synthetic datasets for both the time-series of the GPR traces at different locations and the tracer breakthrough curves. Since the combined problem is highly non-linear and the inverse solution is ill-posed, we use stochastic inversion techniques to obtain probabilistic estimates of the parameters of interest (fracture length, orientation and aperture distribution) and assess the use of different measures to compare the simulated and experimental data

Probabilistic inference of fracture-scale flow paths and aperture distribution from hydrogeophysically-monitored tracer tests

Fracture-scale heterogeneity plays an important role in driving dispersion, mixing and heat transfer in fractured rocks. Current approaches to characterize fracture scale flow and transport processes largely rely on indirect information based on the interpretation of tracer tests. Geophysical techniques used in parallel with tracer tests can offer time-lapse images indicative of the migration of electrically-conductive tracers away from the injection location. In this study, we present a methodology to invert time-lapse ground penetrating radar reflection monitoring data acquired during a push-pull tracer test to infer fracture-scale transport patterns and aperture distribution. We do this by using a probabilistic inversion based on a Markov chain Monte Carlo algorithm. After demonstration on a synthetic dataset, we apply the new inversion method to field data. Our main findings are that the marginal distribution of local fracture apertures is well resolved and that the field site is characterized by strong flow channeling, which is consistent with interpretations of heat tracer tests in the same injection fracture