Pauline Collon

3D geomodelling combining implicit surfaces and Voronoi-based remeshing

In this paper we demonstrate how recent geomodelling techniques can be combined and used to build a 3D geological model on a real case study: the former coal mine of Merlebach (France), that is targeted to be exploited for low-temperature geothermal energy production. From geological maps, cross-sections, borehole and mine exploitation data, we build a 3D model in which are identified the rocks and infrastructures having significantly different permeabilities. First, a structural model of the main geological interfaces in our area of interest (2 horizons and 13 faults) is built with classical geomodelling techniques. Then, we propose to model by surfaces the 71 irregularly stacked, very close and very thin, sub-vertical coal beds. To ease their construction, we use an implicit method which represents 3D surfaces as isovalues of a scalar field defined in a 3D tetrahedral grid of the area. The corresponding triangulated surfaces are remeshed with a recently proposed method based on Voronoi diagrams so that the exploited parts of the coal beds, now filled by sand, can be computed. The 3D surface-based geological model, in which infrastructures can be inserted as piecewise lines, can be volumetrically meshed. It is available for download as supplemental material, as well as a volumetric grid.

A geostatistical approach to the simulation of stacked channels

Turbiditic channels evolve continuously in relation to erosion-deposition events. They are often gathered into complexes and display various stacking patterns. These patterns have a direct impact on the connectivity of sand-rich deposits. Being able to reproduce them in stochastic simulations is thus of significant importance. We propose a geometrical and descriptive approach to stochastically control the channel stacking patterns. This approach relies on the simulation of an initial channel using a Lindenmayer system. This system migrates proportionally to a migration factor through either a forward or a backward migration process. The migration factor is simulated using a sequential Gaussian simulation or a multiple-point simulation. Avulsions are performed using a Lindenmayer system, similarly to the initial channel simulation. This method makes it possible to control the connectivity between the channels by adjusting the geometry of the migrating areas. It furnishes encouraging results with both forward and backward migration processes, even if some aspects such as data conditioning still need to be explored.

3D modeling from outcrop data in a salt tectonic context: Example from the Inceyol minibasin, Sivas Basin, Turkey

AbstractWe have developed a 3D modeling strategy of the encased minibasin of Inceyol in Sivas (Turkey). The challenge lies in the combination of sparse outcrop data and the complex interpretive geometry of geologic structures that come from salt tectonics. We have succeeded in modeling the convoluted salt surface using an explicit indirect surface patch construction method followed by a manual mesh improvement. Then, we modeled the minibasin sediments with an implicit approach. The result highlighted the remarkable geometry of the convoluted salt horizon and its associated minibasin by extending in 3D the geologist’s interpretive 2D sections. This case study proved that building complex geometries is feasible with the existing tools and a good expertise in the various geomodeling techniques. Our work also underlined the need for new methods to ease the modeling of such tectonic features from sparse data. We have developed a 3D view of the model thanks to WebGL technology, as well as downloadable data to c...

Reconstruction of Channelized Systems Through a Conditioned Reverse Migration Method

Geological heterogeneities directly control underground flow. In channelized sedimentary environments, their determination is often underconstrained: it may be possible to observe the most recent channel path and the abandoned meanders on seismic or satellite images, but smaller-scale structures are generally below image resolution. In this paper, reconstruction of channelized systems is proposed with a stochastic inverse simulation reproducing the reverse migration of the system. Maps of the recent trajectories of the Mississippi river were studied to define appropriate relationships between simulation parameters. Measurements of curvature and migration vectors showed (i) no significant correlation between curvature and migration offset and (ii) correlation trends of downstream and lateral migration offsets versus the curvature at half-meander scale. The proposed reverse migration method uses these trends to build possible paleo-trajectories of the river starting from the last stage of the sequence observed from present-day (satellite or seismic) data. As abandoned meanders provide clues about the paleo-locations of the river, they are integrated time step by time step during the reverse simulation process. We applied the method to a satellite image of a fluvial system. Each of the different resulting geometries of the system honored most of the available observations and presented meandering patterns similar to the observed ones.

Stochastic simulation of channelized sedimentary bodies using a constrained L-system

Simulating realistic sedimentary bodies while conditioning all the available data is a major topic of research. We present a new method to simulate the channel morphologies resulting from the deposition processes. It relies on a formal grammar system, the Lindenmayer system, or L-system. The L-system puts together channel segments based on user-defined rules and parameters. The succession of segments is then interpreted to generate non-rational uniform B-splines representing straight to meandering channels. Constraints attract or repulse the channel from the data during the channel development. They enable to condition various data types, from well data to probability cubes or a confinement. The application to a synthetic case highlights the methods ability to manage various data while preserving at best the channel morphology.

Double-peaked breakthrough curves as a consequence of solute transport through underground lakes: a case study of the Furfooz karst system, Belgium

The existence of double-peaked breakthrough curves (BTC), which are the result of the transport of a dye tracer through underground lakes, is reported. Investigations were undertaken on the Furfooz karst system in southern Belgium. In this system, the River Lesse sinks partially into a swallow hole. The water follows a solitary conduit leading to an underground lake that is directly connected to a second underground lake. Double-peaked BTCs were detected in the resurgent water, downstream of this second lake. The report first describes field data (tracer tests in various hydrologic conditions) which point towards the double peak being linked to a nonlinear process that originates within the lakes. Complementary investigations within the lakes show a complex behavior of the dye tracer related to a specific hydrodynamic feature that leads to the separation of the solute plume. A conceptual model of the solute transport within the lakes is proposed. This model emphasizes the physical effect of the lakes on the dye flow-through process.RésuméOn rapporte l’existence de courbes de restitution à double pics (CR), résultant du transport d’un traceur artificiel (colorant) dans des lacs souterrains. Des études ont été menées sur le système karstique de Furfooz dans le Sud de la Belgique. Dans ce système, la rivière Lesse s’infiltre partiellement dans une perte. L’eau s’écoule au sein d’un conduit unique menant à un lac souterrain qui est directement connecté à un deuxième lac souterrain. Les CR à double pics ont été détectées dans une résurgence, située en aval de ce deuxième lac. Cet article décrit d’abord les données de terrain (essais de traçage dans des conditions hydrologiques diverses) qui indiquent que le double pic est lié à un processus non linéaire qui prend son origine au sein des lacs. Des études complémentaires au niveau des lacs montrent un comportement complexe du traceur artificiel associé à une hydrodynamique spécifique conduisant à la séparation du panache du soluté. Un modèle conceptuel du transport de soluté au sein des lacs est proposé. Ce modèle met l’accent sur l’effet physique des lacs sur les processus de transport du traceur artificiel.ResumenSe informa acerca de la existencia de curvas de avance de doble pico (BTC), que son el resultado del transporte de un trazador de colorantes a través de lagos subterráneos. Se realizaron investigaciones sobre el sistema karst de Furfooz en el sur de Bélgica. En este sistema, el río Lesse se hunde parcialmente en pozos kársticos. El agua sigue un conducto único que conduce a un lago subterráneo que está conectado directamente a un segundo lago subterráneo. Se detectaron BTCs de doble pico en el agua resurgente, aguas abajo de este segundo lago. El artículo describe primero los datos de campo (pruebas de trazabilidad en diversas condiciones hidrológicas) que apuntan hacia el doble pico vinculado a un proceso no lineal que se origina dentro de los lagos. Las investigaciones complementarias dentro de los lagos muestran un comportamiento complejo del trazador de colorantes relacionado con una característica hidrodinámica específica que conduce a la separación de la pluma de soluto. Se propone un modelo conceptual del transporte de soluto dentro de los lagos. Este modelo enfatiza el efecto físico de los lagos en el proceso de flujo del colorante.摘要报道了双峰突破曲线(BTC)的存在,这是通过地下湖运输染料示踪剂的结果。对比利时南部的Furfooz岩溶系统进行了调查。在这个系统中,River Lesse部分地陷入了一个燕窝。水沿着一条通向直接连接到第二个地下湖泊的地下湖的独立导管。在第二湖下游的复水中检测到双峰BTC。本文首先描述了现场数据(各种水文条件下的示踪剂测试),其指向与峰值起源于湖泊内的非线性过程相关联的双峰。湖中的互补调查显示了染料示踪剂与导致溶质羽流分离的特定流体动力特征相关的复杂行为。提出了湖泊内溶质运输的概念模型。该模型强调了湖泊对染料流通过程的物理效应。ResumoApresenta-se a existência de curvas de identificação de pico duplo como resultado do transporte de um traçador corante através de lagoas subterrâneas. As investigações foram realizadas no sistema cárstico de Furfooz ao sul da Bélgica. Neste sistema o Rio Lesse desaparece parcialmente dentro de uma cavidade rasa. A água segue por um conduto isolado em direção a uma lagoa subterrânea que está diretamente conectada a uma segunda lagoa subterrânea. Curvas de identificação de pico duplo foram detectadas em ressurgências, a jusante da segunda lagoa. O artigo explica em primeiro lugar os dados de campo (testes com traçadores em diversas condições hidrogeológicas) que apontam para o pico duplo associado a um processo não linear que se origina dentro das lagoas. Investigações complementarias dentro das lagoas mostram um comportamento complexo do traçador corante, relacionado a uma característica hidrodinâmica específica que condiciona a separação da pluma de soluto. É proposto um modelo conceitual do transporte de soluto dentro das lagoas. Este modelo enfatiza o efeito físico das lagoas no processo de fluxo do corante.

3D Geomodelling in Structurally Complex Areas - Implicit vs. Explicit representations

Since a few decades, various approaches, techniques and software have been developed to model easily and quickly the 3D structural geometry of subsurface. Among them, some use an explicit representation of geological interfaces while others are based on an implicit one.n Several works tend to promote one approach against the other. In this contribution, we suggest that both approaches are not mutually exclusive and can be combined in the same study depending on available data and on the type of geological features to be modelled. This is demonstrated through two examples that aimed at modelling geologically complex areas: the abandoned coal mine of Merlebach (Lorraine, France) and a minibasin in a salt tectonic context: Inceyol (Sivas, Turkey).

On the Chronological Ordering of Oxbow Lakes for Reverse Migration of Channels Conditioned to Subsurface Data

Channelized systems form complex sedimentary reservoirs. Their modeling calls for using conditioning data when possible. A reverse migration simulation fitting to observed data has been proposed. It permits to reconstruct the channelized paleo-system and to better understand reservoir architecture. Ancient oxbow lakes are precious witnesses of the evolution of the system. However, the global chronology of their abandonment can be hard to deduce from subsurface data. Most of the time only local relative chronologies are accessible for close oxbow lakes. The knowledge of oxbow lakes global relative abandonment ages is of first importance for the reconstruction of the paleo-system. We propose a few criteria based on the geometry of the system to facilitate the extraction of local chronologies. From these local analyses, deduction of the global chronology is possible thanks to a stochastic ordering method.

Quality Analysis of Geostatistical Simulations through their Connected Structures

Various methods have been developped to perform geostatistical simulations. Depending on the studied case, each of them can claim to obtain the best results, but it is rarely supported by an objective and quantitative analysis, especially concerning the geological structure reproduction. In this work we propose to go deeper into the assessment of realization quality through connected geobodies, focusing on the capacity to reproduce connected geological strutures. This quality assessment relies on quality indicators computed on each realization. The realizations are then compared based on a dissimilarity calculated from the indicators. The dissimilarity analysis is facilitated using multidimensional scaling (MDS) completed with heat maps. The application of this methodology to a synthetic case and associated realizations gives rise to some practical considerations. While MDS is a powerful tool to facilitate the analysis, it does not represent exactly the dissimilarities, leading to possible misinterpretations. Details considering the relationship between the realizations should preferably be analyzed on the heat map as it represents directly the dissimilarities. This connected geobody-based method appears to complete pattern histogram-based method, which less take into account pattern relationships.